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Digital Light 3D Printing of PEDOT-Based Photopolymerizable Inks for Biosensing
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ACS Applied Polymer Materials

Cite this: ACS Appl. Polym. Mater. 2022, 4, 9, 6749–6759
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https://doi.org/10.1021/acsapm.2c01170
Published August 10, 2022

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Abstract

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3D conductive materials such as polymers and hydrogels that interface between biology and electronics are actively being researched for the fabrication of bioelectronic devices. In this work, short-time (5 s) photopolymerizable conductive inks based on poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS) dispersed in an aqueous matrix formed by a vinyl resin, poly(ethylene glycol) diacrylate (PEGDA) with different molecular weights (Mn = 250, 575, and 700 Da), ethylene glycol (EG), and a photoinitiator have been optimized. These inks can be processed by Digital Light 3D Printing (DLP) leading to flexible and shape-defined conductive hydrogels and dry conductive PEDOTs, whose printability resolution increases with PEGDA molecular weight. Besides, the printed conductive PEDOT-based hydrogels are able to swell in water, exhibiting soft mechanical properties (Young’s modulus of ∼3 MPa) similar to those of skin tissues and good conductivity values (10–2 S cm–1) for biosensing. Finally, the printed conductive hydrogels were tested as bioelectrodes for human electrocardiography (ECG) and electromyography (EMG) recordings, showing a long-term activity, up to 2 weeks, and enhanced detection signals compared to commercial Ag/AgCl medical electrodes for health monitoring.

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1. Introduction

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Wearable electronic devices attract great attention in several fields covering a wide range of applications, including sensors, soft robotics, touch panels, healthcare monitoring, and artificial electronic skin. (1−6) In particular, the development of cutaneous electronic devices able to measure human biopotentials represents a noninvasive powerful tool for the assessment of physiological and pathological signals allowing detection and prevention of diseases at early stages. (7−10) In this regard, the discovery of materials with robustness, flexibility, and electrical stimulus sensitivity that create an accurate interface between biology and electronics is actively being researched. (11−13)
Conducting polymers (CPs), i.e., poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), and polyaniline (PANi), with intrinsic electronic properties and soft nature, compared with traditional electronic inorganic fillers, have been investigated for such bioelectronic applications. (14,15) Among them, PEDOT is the most widely used CP due to its high electrical conductivity, thermal stability, and biocompatibility. (16) However, its infusibility and water insolubility limit its processability to traditional two-dimensional (2D) thin films by solvent-casting or spin-coating methods. Thus, PEDOT has usually been used as a filler blended with other polymers for the fabrication of 2D PEDOT-based devices with electronic properties. In some cases, this immiscible filler tends to aggregate, resulting in the conductive paths’ percolation. This impedes the adaptive movement of the conductive network during stretching, affecting the electronic properties. (17) Moreover, 2D PEDOT-based devices are usually physically and mechanically different from biological tissues that contain large amounts of water and have soft mechanical properties, with Young’s moduli in the range of 1 kPa–1 MPa (18)
For such purpose, new PEDOT-based polymer formulations that enhance PEDOT processability by 3D printing techniques have been investigated. (19,20) Very recently, we have shown that PEDOT can be copolymerized with different biopolyesters, i.e., poly(ε-caprolactone) (PCL) and poly(lactic acid) (PLA), leading to graft copolymers, PEDOT-g-PCL and PEDOT-g-PLA, respectively, with excellent shear-thinning behavior to be processed by direct ink writing (DIW) in the melting state. (21) These 3D scaffolds showed biocompatibility in the presence of myoblasts and cardiomyocytes, as well as electroactive properties for tissue engineering applications. (21,22) Apart from that, the development of printable conductive hydrogels with enhanced flexibility and adhesion properties would be beneficial. (23,24) That is the case of PEDOT:polystyrene sulfonate (PSS) dispersions mixed with organic solvents, poly(vinyl alcohol) (PVA), or natural polymers forming shear-thinning gels (102–103 Pa s) able to be printed by DIW in a shape-defined three-dimensional (3D) structure to be employed as soft sensors or cell-laden scaffolds. (25−27) Besides, PEDOT:PSS dispersions have also been inkjet-printed for the fabrication of organic electrochemical transistors (OECTs) and conformable tattoo electrodes for electrophysiology. (28−30) Likewise, inkjet-printing was employed to print photocurable formulations of PEDOT:PSS, 2-cholinium lactate methacrylate, ethylene glycol dimethacrylate as a cross-linker, and a photoinitiator, which required a postprinting photopolymerization step to obtain the final patterns for electrocardiography (ECG), making this process time-consuming. (10) This drawback can be solved by using light-based printing technologies such as stereolithography (SLA) and digital light printing (DLP), which present high-resolution printing, ∼100 μm, as compared with DIW and inkjet-printing. (31,32) This specific feature becomes highly attractive for the fabrication of medical devices, which need a high shape fidelity. (33,34) Furthermore, as it is based on the photopolymerization of a monomer or a mixture of monomers, the design of an ink formulation able to photocure in short periods during the layer-by-layer printing of the piece plays a key role. (35) As an emerging technique in the field of conducting polymers, few works employing light-based printing are reported in the literature. In this way, SLA was used by Zhang and co-workers to photoprint PEDOT-based gelatin methacryloyl (GelMA) hydrogels that were used for neuronal differentiation. (36) PEDOT-based photopolymerizable inks have also been developed by mixing PEDOT:PSS with poly(ethylene glycol) diacrylate (PEGDA) and a photoinitiator to be processed by SLA. The dispersion of PEDOT in the photocurable PEGDA matrix was favored by acid treatment in concentrated sulfuric acid, and the 3D printed materials were used as organic electrochemical transistors (OECTs). (37−39) However, as SLA is limited to a single spot photopolymerization while printing, DLP that prints a continuous plane allows accelerating the printing process while retaining a high-quality resolution. Very recently, Shao and co-workers employed a different methodology to favor the PEDOT dispersion in the PEGDA matrix. In that case, the commercial PEDOT:PSS aqueous solution was freeze-dried and dispersed in a water/ethanol mixture. Moreover, the incorporation of silica nanoparticles into the inks allowed researchers to obtain shape-defined electrodes by DLP to be used in lithium-ion batteries (LIBs). (40) Thus, the discovery of photocurable and electroactive polymer inks that can be processed by digital light 3D printing to fabricate flexible and wearable electronic devices with tailor-made shapes remains a challenge.
Therefore, herein we present the design of photocurable and conductive polymer inks to be processed by DLP leading to shape-defined conductive hydrogels for biosensing. The polymer ink is obtained through a precise formulation based on the dispersion of PEDOT:PSS in a photocurable matrix composed of poly(ethylene glycol) diacrylate (PEGDA), an aqueous commercial resin (vinyl monomers) that favors the dispersion of PEDOT in the PEGDA matrix and enhances its conductivity in conjunction with ethylene glycol (EG) and a photoinitiator (Scheme 1). All the components were commercially available, and the 3D printable ink could be easily prepared by simple mixing without any treatment or evaporation step. The influence of the number-average molecular weight of PEGDA (Mn = 250, 575, and 700 Da) on the photopolymerization kinetics and printing resolution of the inks, as well as on the swelling ratio and mechanical properties of the printed hydrogels, is studied. Moreover, the electrical conductivity of the printed materials is measured. Finally, their application as long-term bioelectrodes in electrocardiography (ECG) and electromyography (EMG) is investigated and compared with the recording properties of commercial Ag/AgCl medical electrodes.

Scheme 1

Scheme 1. (A) Synthesis of Conductive Hydrogels by the Photopolymerization Reaction, (B) Fabrication of Shape-Defined 3D Hydrogels by DLP of the Photopolymerizable Inks and C) Pictures of the 3D Printed Pieces
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2. Materials and Methods

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2.1. Materials

Clevios PH 1000 PEDOT:PSS (1.3 wt %) aqueous solution was purchased from Heraeus. Ceramic 3Dresyn CDP-WS Water-Soluble and Fine Turner FT2 ultrafast photoinitiator were supplied by 3Dresyns. Poly(ethylene glycol) diacrylate (PEGDA, Mn = 250, 575, and 700 Da) and ethylene glycol (EG) were provided by Sigma-Aldrich.

2.2. Photopolymerizable and Conductive Hydrogels

2.2.1. Photopolymerizable Conductive Ink

The photocurable conductive ink was prepared by mixing PEDOT:PSS aqueous solution (50 wt %) with 3Dresyn CDP-WS (34 wt %), Fine Turner FT2 (4 wt %), EG (4 wt %), and PEGDA (8 wt %) of different Mn (250, 575, and 700 Da). Samples were named as PEDOTx_PEGDAn where x refers to PEDOT percentage and n to the Mn of PEGDA. The mixture was protected with aluminum paper from light and stirred for 24 h.

2.2.2. Photopolymerization Reaction Kinetics

The photopolymerization reaction conditions were studied by Fourier transform infrared spectroscopy (FTIR) and photorheology.
FTIR spectra were recorded in the attenuated total reflectance (ATR) mode in a Thermo scientific model Nicolet 6700 FTIR spectrometer, with a resolution of 8 cm–1, mirror speed of 0.3165, and 1 scan. Samples were placed in a zinc selenide glass, and ATR-FTIR spectra were recorded at room temperature every 1.16 s by exposing the photocurable inks to UV light (wavelength = 365 nm, power = 2 mW cm–2) for 30 s. The conversion was calculated with eq 1 by measuring the area of the peaks located at 953 and 977 cm–1, which correspond to the C═C out of plane bending vibrations:
(1)
where At is the area of the band at a time t and A0 is the area of the band at zero time.
Photorheological measurements were carried out at room temperature in an AR-G2 rheometer (TA Instruments) using a UV-light lamp (wavelength = 365 nm, power = 2 mW cm–2), oscillation stress of 100 Pa, and 0.1 Hz frequency. The gel point was determined by placing the samples on a glass parallel plate of 20 mm diameter, letting them stabilize for 60 s to be subsequently irradiated for 5 s, and continuing to register the elastic modulus (G′) and loss modulus (G′′) until a plateau was reached.

2.3. Digital Light 3D Printing (DLP) of Conductive Hydrogels

The photopolymerizable conductive inks were placed into the vat of the 3D printer (Phrozen Sonic Mini) and exposed to UV light (wavelength = 405 nm, power = 2 mW cm–2). Printing patterns were first designed with Autodesk Inventor 2021 software and then printed at different irradiation times (2, 5, and 10 s) and layer heights (0.05, 0.1, and 0.2 mm) and a lifting speed of 50 mm min–1. (41)

2.4. Characterization of Printed Hydrogels

2.4.1. Scanning Electron Microscopy (SEM)

Measurements were performed on a Hitachi Tabletop Microscope (TM3030 series) at a 15 kV force field, running in a point-by-point scanning mode. The samples were placed on an aluminum holder with double-sided carbon tape and introduced into the SEM chamber. Finally, SEM images were analyzed with the software ImageJ.

2.4.2. Swelling Assays

Discs of 1 mm thickness and 1 cm diameter were printed and dried at 25 °C. Subsequently, the dry discs were weighted (W0) and immersed in Milli-Q water at 25 °C. At established times, the samples were removed from Milli-Q water, externally dried with filter paper to eliminate the excess water that could remain on the surface, and weighed (Wt). The swelling percentage (Sw) was calculated according to eq 2:
(2)
Besides, the gained volume in the swelling process was also calculated by measuring the dimensional changes of diameter and thickness of the discs at each established time. Samples were analyzed in triplicate and results were expressed as mean ± standard deviation.

2.4.3. Electrical conductivity

Discs of 10 mm diameter and 0.5 mm thickness were printed and their electrical conductivity was measured in dry and swollen states using a four-point probe Ossila Sheet. The swelling was carried out in Milli-Q water at 25 °C, and the drying step at 25 °C and room atmosphere, or 85 °C in an oven. The electrical conductivity was calculated taking into account the thickness of each sample at each condition tested. Samples were analyzed in triplicate and testing three different sample areas and results were expressed as mean ± standard deviation.

2.4.4. Electrochemical characterization

Cyclic voltammetry was performed using a VMP-3 potentiostat (Biologic Science Instruments) in a three-electrode setup, employing a platinum wire as the counter electrode, Ag/AgCl as the reference electrode, and glassy carbon as the working electrode. Hydrogels were fabricated by depositing 5 μL of the ink onto the glassy carbon electrode, followed by a photopolymerization step with UV light irradiation. Finally, they were cleaned in water for 5 min. 0.1 M NaCl aqueous solution was used as the electrolyte, and it was purged with nitrogen for 15 min before the experiment. Cyclic voltammetry measurements were carried out in the potential range of −0.2 to 0.5 V vs. Ag/AgCl at 20 mV s–1 and 20, 50, 100, 150, and 200 mV s–1 for the scan rate experiment.

2.4.5. Tensile Test

Type V probes were printed. The tensile test was applied to probes in the dried and swollen state by the ASTM D638 criterion universal testing system using an Instron 5569 in a single uniaxial tensile mode (n = 5). The controlled tension force (load cell of 100 N) was applied at a rate of 1 mm min–1, and the results were plotted as stress–strain curves. Samples were analyzed in triplicate and results were expressed as mean ± standard deviation.

2.5. Electrocardiography (ECG) and Electromyography (EMG)

2.5.1. Electrodes fabrication

A Kapton sheet of 75 μm thickness was covered with 10 nm of Ti and 200 nm of gold by thermal evaporation (Alliance Concept). The coated Kapton was laser cut (LPKF protolaser S) with the desired shape (1 cm diameter and 2 × 0.2 cm connection for ECG recordings, 4 × 4 cm and 1 × 0.5 cm connection for EMG recordings). Subsequently, samples of these shapes were printed, swollen in Milli-Q water, and attached to the Au electrode with a tape, resulting in PEDOT-based electrodes. In all cases, hydrogels were used in their wet state to improve the adhesion between the hydrogel and the skin, and the hydrogel and the gold.

2.5.2. ECG acquisition and signal processing

The ECGs were recorded using a Sienna Ultimate device plugging the two PEDOT-based electrodes to compare to ECG entries. The reference electrode (RE) used was always a fresh medical electrode (Gima 33 371 universal electrodes for ECG, diameter 48 × 50 mm) located on the left bottom of the chest. The working electrode (WE) and counting electrode (CE) were located on the wrists. The signal was recorded with a hardware notch filter at 50 Hz. Before plotting the different ECGs, the signal was processed using an infinite impulse response (IIR) notch filter at 50, 100, and 150 Hz. The electrodes were stored between measurements and reused for the long-term recordings at different times.

2.5.3. EMG recording and stimulation experiments

Muscle stimulation was applied on the quadriceps using a Boston Tech WE-122 Electrotherapy Unit equipment, employing a program that sends 380 μs pulses at a frequency of 104 Hz. When a current of 10 mA was applied (visual muscle contraction was observed), the muscle activity was recorded using PEDOT-based electrodes. For contraction/relaxation tests, EMGs were recorded with a Sienna Ultimate device plugging the bottom electrode to the reference using PEDOT electrodes when a healthy volunteer contracted and relaxed the quadriceps. In all cases, signals were recorded with a hardware notch filter, and the plot signals were filtered with EDFViewer using notch Filters at 50, 100, and 150 Hz and a high-pass filter at 10 Hz of order 8. The electrodes were stored between measurements and reused for the long-term recordings at different times.

3. Results and Discussion

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3.1. Development of photopolymerizable PEDOT inks

A schematic representation of the overall fabrication process of the conductive hydrogel is shown in Scheme 1. First, the PEDOT:PSS aqueous solution that provides electrical properties to the final material was dispersed in the photocurable matrix composed of 3Dresyn CDP-WS (vinyl monomers), PEGDA as a cross-linking agent, and the photoinitiator Fine Turner FT2. The PEDOT percentage was optimized to obtain homogeneous inks and maximize the electrical conductivity. In this regard, 0.65% PEDOT was the optimal quantity that could be homogeneously dispersed within the photopolymerizable matrix without hindering the digital light printing (DLP) process, as higher quantities of PEDOT could avoid the light penetration during DLP leading to defects in the printed objects. To increase the conductivity of the final material, a little quantity of EG was added to the ink formulation. (42,43) It is well-known that the PEDOT:PSS conductivity can be enhanced by a solvent doping treatment, and ethylene glycol (EG) is commonly used for such purposes. EG screens the ionic interaction between PEDOT and PSS by forming hydrogen bonds with PSS, which results in a strong step separation between PEDOT and PSS, allowing the linear orientation of the PEDOT chains. (44,45) The incorporation of EG in the inks gave rise to a 3-fold increase of the conductivity, from 2.5 × 10–3 S cm–1 to 7.1 × 10–3 S cm–1 for PEDOT0.65_PEGDA700. It is noteworthy that the energy transfer process is also influenced by thermal conductivity. (46) In this regard, the employment of EG to align the PEDOT chains within the hydrogel network reduces the structural disorder, and more phonon-like modes are available to contribute to heat conduction. (47,48) Then, hydrogels were formed by photopolymerization of the previous photocurable mixture (PEDOT:PSS, PEGDA, vinyl resin, EG, and FT2) using UV light, leading to a cross-linked network where PEDOT is trapped. The photopolymerization kinetics were studied by ATR-FTIR (Figure S1). The peaks located at 953 and 977 cm–1, which correspond to the C═C out-of-plane bending vibrations, disappear as the photopolymerization reaction proceeds. The conversion was calculated using eq 1 by measuring the area of these peaks over time (Figure 1A). Results show a fast conversion of PEDOT0.00_PEGDA700 reaching a plateau in 5 s. The kinetics were slightly decreased when PEDOT is incorporated, due to the blue color of PEDOT that limits the light penetration whereas the control sample without PEDOT is totally transparent. However, no significant differences were observed for inks prepared with different molecular weights of PEGDA. The fast photocuring kinetics make those inks ideal candidates for DLP 3D printing. One important parameter for DLP of photopolymerizable hydrogels is their ability to rapidly reach the gel point, the transition from liquid to solid state, during printing. It is known that the free-radical photopolymerization normally reaches the gel point at 40–60% conversion. (49) This conversion is achieved at approximately 5 s for PEDOT-containing samples. Therefore, the gel point was determined by photorheology, applying a 5 s UV light irradiation (Figure 1B), and the results show that all inks reach the gel point in less than 5 s. This demonstrates the capability of the inks to achieve the gel point without being necessary to reach a full conversion, allowing researchers to obtain 3D architectures by DLP in very short times.

Figure 1

Figure 1. (A) Evolution of the conversion during the photopolymerization process of the inks PEDOT0.00_PEGDA700 (green triangles), PEDOT0.65_PEGDA250 (black squares), PEDOT0.65_PEGDA575 (red circles), and PEDOT0.65_PEGDA700 (blue triangles). (B) Rheological measurements of the elastic modulus (G′) and loss modulus (G′′) to determine the gel point when the inks are irradiated for 5 s (in the interval from 60 to 65 s).

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3.2. Digital Light Printing of PEDOT Conductive Hydrogels

Digital light printing (DLP) was used to pattern conductive hydrogels with predesigned architectures during the photopolymerization process, leading to three-dimensional shape-defined network structures (Scheme 1B–C). Optimal printing conditions were investigated by DLP of 3D patterns with reliefs and holes at different irradiation times (2, 5, and 10 s) and layer heights (0.05, 0.1, and 0.2 mm) using the photopolymerizable ink PEDOT0.65_PEGDA250 (Figure S2A and B). When low irradiation times (2 s) were employed, the printing patterns did not show the designed structure because the ink could not photocure successfully, as proven by the double bond low conversion (<30%) (Figure 1A). Increasing the exposure time to UV light up to 5 s gave rise to patterns with better printing resolution. However, an excess of photopolymerization time, while printing successfully the reliefs of the pattern, did not allow printing the hollow parts of the designed piece that were fully covered by the ink during DLP. A high cross-link density makes printed hydrogels more rigid and brittle, giving rise to totally deformed objects after printing or to the appearance of scratches, which is even more evident at lower layer thicknesses (Figure S2B–D). Therefore, 5 s of light exposure was chosen as the optimal time that affords accurate photopolymerization rates, with over 50% double bond conversion (Figure 1A), ensuring high printing resolution. Then, the printing resolution was fit by varying the layer height. It was observed that lower layer heights (0.05 and 0.1 mm) did not allow researchers to obtain shape fidelity patterns, whereas using a layer height of 0.2 mm gave rise to a shape fidelity pattern that exhibited all the features of the model piece, i.e., reliefs and holes, as was corroborated by SEM (Figure S2E and F). When the system absorbs too much light, the use of thin layer heights results in not very good resolution because the photopolymerization process is deflected to unspecified points of the printing pattern, causing imperfections in the final material. However, when larger layer heights of the material absorb the same amount of light, this deviation is corrected and the reaction occurs only at the locations specified in the printing pattern. Once the printing parameters were optimized, the influence of the molecular weight of PEGDA on the printing fidelity was studied (Figure 2A–C).

Figure 2

Figure 2. (A) Pictures of hydrogel pieces printed by DLP (0.2 mm layer height, 5 s laser irradiation). SEM images of different areas of the previously printed pieces showing (B) reliefs (dashed red areas in part A) and (C) holes (dashed blue areas in part A). Scale bar = 1 mm. (D) Picture of a porous PEDOT0.65_PEGDA700 scaffold printed by DLP (0.2 mm layer height, 5 s laser irradiation). (E) SEM image of a representative area of the previous porous scaffold (dashed red area in part D) and (F) zoom-in to visualize the porous structure and pore dimensions.

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Patterns printed with the inks formulated with high molecular weight PEGDA, PEDOT0.65_PEGDA575, and PEDOT0.65_PEGDA700 show better printing resolution and shape fidelity than those of PEDOT0.65_PEGDA250. One might think this behavior depends on the acrylate functional group concentration, which in turn depends on the length of the PEGDA chains. Nevertheless, in general, all the samples have the same concentration of acrylate groups, as the PEGDA quantity in the ink formulation is only 8%wt. Therefore, this behavior seems to be more closely correlated with the PEGDA molecular weight than the acrylate concentration, which has not been fully explained in the literature yet. (50) Consequently, the PEGDA chain length affects the printability resolution, with PEDOT0.65_PEGDA700 hydrogels becoming ideal candidates for processing by DLP. To illustrate this, different complex 3D structures, i.e., a snail and the UPV/EHU logo, were successfully printed using the selected ink, PEDOT0.65_PEGDA700 (Scheme 1C). Apart from that, the possibility to print high-resolution microporous hydrogel scaffolds with PEDOT0.65_PEGDA700 was tested (Figure 2D). Morphological analysis by SEM showed a layer-by-layer microporous structure with pore diameters of ∼130 μm (Figure 2E and F).

3.3. Characterization of the Printed PEDOT Conductive Hydrogel

An important feature of the hydrogel scaffolds to be employed for bioelectronics is their ability to retain their shape in the swollen state. (51) The printed patterns showed a fast swelling rate (Sw) during the first hour, reaching a plateau within 2 h and retaining its structure without breaking (Figure 3A). Hydrogels, which are formed by free-radical-mediated cross-linking at the PEGDA chain ends creating a covalent network, are able to hold water in the cross-linked matrix due to the presence of hydrophilic PEGDA and the resin backbone, which form hydrogen bonds with water, at the same time that the cross-linking points of the polymer network prevent its disaggregation. Thus, the mesh size of the hydrogels decreases with the decrease of the PEGDA chain length, which indeed increases the cross-link density and decreases their swelling ability. (52,53) The presence of PEDOT in the ink formulations decreases the cross-linking density and increases the hydrophobicity, which results in a lower swelling from Sw = 230 ± 19% for PEDOT0.00_PEGDA700 to Sw = 176 ± 6% for PEDOT0.65_PEGDA700. Among samples containing PEDOT, hydrogels formed by shorter PEGDA chains, e.g., PEDOT0.65_PEGDA250, possess a higher cross-link density than those formed by longer PEGDA chains, e.g., PEDOT0.65_PEGDA575 and PEDOT0.65_PEGDA700. Therefore, PEDOT0.65_PEGDA250 hydrogels possess the lowest swelling ability (Sw = 112 ± 11%), as shown in Figure 3A. Besides, it was also proven that the printed hydrogels were swollen homogeneously in the three dimensions, reaching a maximum swelling state at around 38% diameter and thickness (Figure S3).

Figure 3

Figure 3. (A) Photographs of the hydrogel PEDOT0.65_PEGDA575 just printed (t = 0 h) and during its swelling in water (top). Swelling curves of the printed hydrogels: PEDOT0.00_PEGDA700 (green triangles), PEDOT0.65_PEGDA250 (black squares), PEDOT0.65_PEGDA575 (red circles), and PEDOT0.65_PEGDA700 (blue triangles) in water (bottom). (B) Photographs of the printed hydrogels and electrical conductivity of PEDOT0.00_PEGDA700 (green bars), PEDOT0.65_PEGDA250 (black bars), PEDOT0.65_PEGDA575 (red bars), and PEDOT0.65_PEGDA700 (blue bars). (C) Lab-made electrical circuit to conduct the electrical current from the battery to the light bulb, passing through the printed UPV/EHU logo with a PEDOT0.65_PEGDA700 hydrogel, as a visual proof-of-concept of the hydrogel conductivity. The dashed white circle highlights the printed logo, and the dashed yellow rectangle, the localization of the light bulb switched on. (D) Cyclic voltammograms of the 3D printed hydrogels, PEDOT0.00_PEGDA700 (green curve), PEDOT0.65_PEGDA250 (black curve), PEDOT0.65_PEGDA575 (red curve), and PEDOT0.65_PEGDA700 (blue curve), over a glassy carbon electrode (purple curve) in 0.1 M NaCl aqueous solution at 20 mV s–1.

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As the presence of water clearly influences the conductive properties of the printed materials, Figure 3B shows the electrical conductivity values of the synthesized hydrogels in the dry and swollen states. Immediately after printing, PEDOT-containing samples show conductivity values of 10–2 S cm–1 for PEDOT0.65_PEGDA250 and 10–3 S cm–1 for PEDOT0.65_PEGDA575 and PEDOT0.65_PEGDA700. This happens because the PEDOT0.65_PEGDA250 hydrogel absorbs less water than the other two gels (Figure 3A), and hence, PEDOT molecules are more concentrated and ordered in PEDOT0.65_PEGDA250.
After drying at 25 °C, the hydrogel scaffolds increase their electronic conductivity until 10–1 S cm–1 in all cases, as there is no water in the structure, leading to a better reorganization of the PEDOT chains. In this state, PEDOT acts as a very good electrical conductor. The annealing of the hydrogels at 85 °C did not produce any significant increase in the conductivity as compared with hydrogels dried at 25 °C (Figure S4). When samples are swollen after drying, their conductivity decreases 1 order of magnitude in comparison with their dry state but remains higher than those just after printing, while they retain their flexibility when they are stretched with tweezers as shown in the photographs. It is worth noting that the PEDOT0.00_PEGDA700 sample used as control, without PEDOT in its structure, only shows low conductivity values in its swollen state due to the ionic conductivity and it does not show electrical conductivity in its dry state, as expected, proving the key role of PEDOT within the ink formulation for conductive purposes. As a visual proof-of-concept of hydrogel conductivity, a lab-made electrical circuit that incorporates the printed hydrogel shaping the UPV/EHU logo was built (Figure 3C). When the battery is connected, the printed scaffold is able to transfer the electrical current to the LED switching it on (see Video S1). The electroactive behavior of our hydrogels has also been tested by cyclic voltammetry in 0.1 M NaCl aqueous solution (Figure 3D and Figure S5). The cyclic voltammograms (CV) of the hydrogels containing PEDOT show a capacitive behavior, with a broad anodic peak at 0.4 V and a broad cathodic peak at 0.15 V, not observed in the case of the hydrogel without PEDOT. The electroactivity of the hydrogels follows the same tendency as the conductivity measurements of the hydrogels after printing (Figure 3B), being higher in the case of hydrogels formed with a lower molecular weight of PEGDA. Moreover, the CVs of PEDOT0.65_PEGDA700 at different scan rates show a proportional increase of the anodic and cathodic currents with the scan rate, which means that the redox process is not limited by diffusion and the whole hydrogel is involved in the electrochemical process (Figure S5).
The mechanical properties of printed hydrogels in the form of type V probes were determined by measuring the Young’s modulus and elongation at break in the dried and swollen states (Figure 4A–B). In the dried state, the control hydrogel PEDOT0.00_PEGDA700 possesses the highest Young’s modulus , 90.2 ± 3.9 MPa, and elongation at break, 115% (Figure 4C), due to its high photopolymerization reaction conversion previously discussed. The presence of PEDOT within the hydrogel formulation, PEDOT0.65_PEGDA700, decreases the Young’s modulus up to 2.7 ± 0.2 MPa as well as the elongation at break (35%). This can be attributed to the fact that the black color of the PEDOT dispersion hinders the light penetration through the ink during the photopolymerization process, decreasing the cross-linking degree between the chains of PEGDA and the resin. In addition to this, it was observed that the Young’s modulus increases as the molecular weight of PEGDA decreases, reaching values of 38.3 ± 6.6 MPa for PEDOT0.65_PEGDA250, whereas the elongation at break decreases up to 18%. As above-mentioned, hydrogels formed by shorter PEGDA chains, e.g., PEDOT0.65_PEGDA250, possess a higher cross-link density than those formed by longer PEGDA chains, e.g., PEDOT0.65_PEGDA575 and PEDOT0.65_PEGDA700, exhibiting a lower swelling ability (Figure 3A). As the cross-link density increases, hydrogels become stiffer, possessing a higher Young’s modulus, and more brittle, exhibiting a lower elongation at break, as shown in Figure 4C. Conversely, in the swollen state, all hydrogels have lower and similar stiffness values, in the range of 2–3 MPa (Figure 4D), due to the hydrated state and the water held within their network. In terms of mechanical properties, these results are comparable to the Young’s modulus of the forearm skin, 1.03 ± 0.06 MPa, where electrodes are placed for ECG recordings. (54) Therefore, taking into account that the conductivity and mechanical properties of the printed conductive hydrogels in their wet state are not highly influenced by the hydrogel formulation and that PEDOT0.65_PEGDA700 hydrogels possess the highest printing fidelity for the fabrication of shape-defined electrodes, those are selected for further bioelectronics tests.

Figure 4

Figure 4. Stress–strain curves of type V probes’ printed hydrogels: PEDOT0.00_PEGDA700 (green curve), PEDOT0.65_PEGDA250 (black curve), PEDOT0.65_PEGDA575 (red curve), and PEDOT0.65_PEGDA700 (blue curve) in the (A) dry and (B) wet states. The Young’s modulus and elongation at break were obtained from the strain–stress curves of gels in the (C) dry and (D) wet states.

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3.4. Testing of 3D Printed PEDOT Hydrogels in Electrocardiography (ECG) and Electromyography (EMG) Recordings

The 3D printed conductive hydrogels (Figure 5A and Table S1) were tested as cutaneous electrodes for health monitoring on a healthy volunteer. First, they were employed for electrocardiogram (ECG) recording, a medical test used in preventive medicine to detect heart problems by measuring the electrical activity generated by the heart contraction. For such purpose, the PEDOT0.65_PEGDA700 hydrogel in the wet state was used as working electrode (WE) and counter electrode (CE) for the ECG recordings, whereas an Ag/AgCl medical electrode was employed as reference electrode (RE) (Figure 5B). The results show that the electrocardiogram (ECG) recorded when PEDOT0.65_PEGDA700 hydrogel is used (black curve) displays more intense signals than the standard medical electrode (red curve) (Figure 5C) as well as a good detection of PQRST waveforms, physiological signals that provide valuable information for diagnosis and rehabilitation (Figure 5D). The signal-to-noise ratio of the electrodes has also been determined (Figure S6). The plot contains the extracted noise from the raw data with notch filters at 50, 100, and 150 Hz as well as a low-pass filter at 150 Hz and a high-pass filter at 0.05 Hz, as recommended by the American Heart Association. As can be observed, the signal-to-noise ratio of the electrodes built up with our hydrogels is much lower than this one of the standard medical electrodes, at the same time that the QSR signal is much higher with our electrodes. (55) Besides, the PEDOT0.65_PEGDA700 bioelectrode showed long-term stability by reproducing the high-quality ECG signals after 2 weeks, which is indicative of the material stability over time. In this regard, the stability was corroborated by the degradation assay carried out by weight loss and SEM (Figure S7). The printed hydrogels in a wet state kept their initial mass for up to 2 weeks and did not experience significant changes in the surface morphology during this period.

Figure 5

Figure 5. (A) Pictures of the digital light 3D printed PEDOT0.65_PEGDA700 hydrogels, with a round shape for ECG and square shape for EMG, and schematic representation of the built-up electrodes. (B) Scheme of the ECG performed. (C) Comparison of ECG signals using a medical electrode and synthesized PEDOT0.65_PEGDA700 hydrogel. (D) ECG signals recorded with PEDOT-based hydrogel over time (t = 0, 1, and 2 weeks (w)). (E) Scheme of external muscle electrostimulation and muscle activity recording experiments. (F) EMG response of PEDOT0.65_PEGDA700 electrode after electrostimulation of a female volunteer. (G) Evolution over time (t = 0, 1, and 2 weeks (w)) of the EMG signals generated by contraction/relaxation of the quadriceps.

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Apart from that, the printed PEDOT0.65_PEGDA700 hydrogel was further tested for electromyography (EMG) recording, a measurement of the electrical activity produced by skeletal muscles (Figure 5E). Two different experiments were carried out using the PEDOT0.65_PEGDA700 hydrogel as WE: EMG recording when an external electrostimulation is applied to the quadriceps and muscle activity when the quadriceps are relaxed and contracted. For external electrostimulation, a current of 10 mA was applied (380 μs pulses, frequency = 104 Hz) to the quadriceps of a healthy volunteer, and the muscle activity was recorded (Figure 5F). It can be clearly observed when the muscle is being stimulated with very good signal quality and intensity. On the other hand, the PEDOT0.65_PEGDA700 hydrogel was used to monitor muscle contraction/relaxation when a healthy volunteer contracted and relaxed their quadriceps in alternating 2 s cycles (Figure 5G). The results demonstrate that the developed bioelectrode can monitor the muscle contraction and relaxation steps with accurate intensity signals. Furthermore, the bioelectrode also exhibits a long-term activity, showing reproducible signals for at least 2 weeks. Overall, these results reveal that the printed conductive hydrogels show a long-term activity and stability to be used as electrodes for physiological recording and health monitoring.

4. Conclusion

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Photopolymerizable and conductive PEDOT-based inks have been formulated to be employed for additive manufacturing of 3D scaffolds by DLP. All the components of the ink were commercially available, and the 3D printable ink could be easily prepared by simple mixing without any treatment or evaporation step. The photopolymerization reaction was very fast, achieving the gel point in less than 5 s, which makes the designed inks ideal candidates to be processed using commercial 3D printers. The molecular weight of the PEGDA employed within the ink formulation empirically affected the resolution printability, with the PEDOT0.65_PEGDA700 hydrogel showing the highest printing fidelity. Moreover, the capacity of the printed hydrogels to hold water confers them soft mechanical properties, with Young’s moduli in the order of 3 MPa, which makes them ideal candidates for tissue engineering applications. In addition to this, the printed hydrogels exhibited high electrical conductivity, between 10–1 and 10–2 S cm–1, enabling them to be employed as bioelectrodes for long-term ECG and EMG recordings with enhanced detection signals compared to commercial Ag/AgCl medical electrodes. Overall, these findings suggest the developed 3D printed hydrogels present great potential as flexible wearable electronic devices for a variety of biological and engineering applications.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsapm.2c01170.

  • Video of the lab-made electrical circuit while running (MP4)

  • ATR-FTIR spectra during the photopolymerization process; pictures and SEM images for printing process optimization; evolution of the diameter and thickness of hydrogels during swelling; mechanical properties of hydrogels in the dry and wet states; degradation assay (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Naroa Lopez-Larrea - POLYMAT, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018 San Sebastián, Spain;  Orcidhttps://orcid.org/0000-0003-3472-259X
    • Antonio Dominguez-Alfaro - POLYMAT, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018 San Sebastián, Spain;  Orcidhttps://orcid.org/0000-0002-3215-9732
    • Nuria Alegret - Carbon Bionanotechnology Group, Center for Cooperative Research in Biomaterials (CIC biomaGUNE), Basque Research and Technology Alliance (BRTA), 20014 San Sebastian, Spain;  IIS Biodonostia, Neurosciences Area, Group of Neuromuscular Diseases, Paseo Dr. Begiristain s/n, 20014 San Sebastian, Spain;  Orcidhttps://orcid.org/0000-0002-8329-4459
    • Isabel del Agua - Panaxium SAS, Aix-en-Provence 13100, France
    • Bastien Marchiori - Panaxium SAS, Aix-en-Provence 13100, France
  • Author Contributions

    The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

  • Funding

    This work was supported by Marie Sklodowska-Curie Research and Innovation Staff Exchanges (RISE) under grant agreement No. 823989 “IONBIKE”.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank Juan Segurola as representative of Resyner Technologies S.L. for his advice and manufacture of corresponding products.

References

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    Water Soluble Cationic Poly(3,4-Ethylenedioxythiophene) PEDOT-N as a Versatile Conducting Polymer for Bioelectronics
    Minudri, Daniela; Mantione, Daniele; Dominguez-Alfaro, Antonio; Moya, Sergio; Maza, Eliana; Bellacanzone, Christian; Antognazza, Maria Rosa; Mecerreyes, David
    Advanced Electronic Materials (2020), 6 (10), 2000510CODEN: AEMDBW; ISSN:2199-160X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Poly(3,4-ethylenedioxythiophene) (PEDOT) is the most popular conducting polymer in the emerging field of bioelectronics. Besides its excellent properties and com. availability, its success is due to the aq. processability of its anionically stabilized solns. or dispersions. In this work, a water sol. version of PEDOT is shown, which is cationically stabilized. This work reports the chem. oxidative (co)polymn. of EDOT-ammonium deriv. leading to PEDOT-N (co)polymers. PEDOT-N shows the typical features of PEDOT such as UV absorbance, bipolaron band, elec. cond., electrochem. behavior, and film formation ability. Furthermore, the PEDOT-N films show good biocompatibility in the presence of the human embryonic kidney-293 cell line. The water soly. of PEDOT-N and its cationic nature allows its processability in the form of thin films obtained by the layer-by-layer technique or as conducting hydrogels.
  17. 17
    Lei, Z.; Wu, P. A highly transparent and ultra-stretchable conductor with stable conductivity during large deformation. Nat. Commun. 2019, 10 (1), 3429,  DOI: 10.1038/s41467-019-11364-w
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    17
    A highly transparent and ultra-stretchable conductor with stable conductivity during large deformation
    Lei Zhouyue; Wu Peiyi; Lei Zhouyue; Wu Peiyi
    Nature communications (2019), 10 (1), 3429 ISSN:.
    Intrinsically stretchable conductors have undergone rapid development in the past few years and a variety of strategies have been established to improve their electro-mechanical properties. However, ranging from electronically to ionically conductive materials, they are usually vulnerable either to large deformation or at high/low temperatures, mainly due to the fact that conductive domains are generally incompatible with neighboring elastic networks. This is a problem that is usually overlooked and remains challenging to address. Here, we introduce synergistic effect between conductive zwitterionic nanochannels and dynamic hydrogen-bonding networks to break the limitations. The conductor is highly transparent (>90% transmittance), ultra-stretchable (>10,000% strain), high-modulus (>2 MPa Young's modulus), self-healing, and capable of maintaining stable conductivity during large deformation and at different temperatures. Transparent integrated systems are further demonstrated via 3D printing of its precursor and could achieve diverse sensory capabilities towards strain, temperature, humidity, etc., and even recognition of different liquids.
  18. 18
    Lu, B.; Yuk, H.; Lin, S.; Jian, N.; Qu, K.; Xu, J.; Zhao, X. Pure PEDOT:PSS hydrogels. Nat. Commun. 2019, 10 (1), 1043,  DOI: 10.1038/s41467-019-09003-5
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    Pure PEDOT:PSS hydrogels
    Lu Baoyang; Jian Nannan; Qu Kai; Xu Jingkun; Lu Baoyang; Yuk Hyunwoo; Lin Shaoting; Zhao Xuanhe; Xu Jingkun; Zhao Xuanhe
    Nature communications (2019), 10 (1), 1043 ISSN:.
    Hydrogels of conducting polymers, particularly poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), provide a promising electrical interface with biological tissues for sensing and stimulation, owing to their favorable electrical and mechanical properties. While existing methods mostly blend PEDOT:PSS with other compositions such as non-conductive polymers, the blending can compromise resultant hydrogels' mechanical and/or electrical properties. Here, we show that designing interconnected networks of PEDOT:PSS nanofibrils via a simple method can yield high-performance pure PEDOT:PSS hydrogels. The method involves mixing volatile additive dimethyl sulfoxide (DMSO) into aqueous PEDOT:PSS solutions followed by controlled dry-annealing and rehydration. The resultant hydrogels exhibit a set of properties highly desirable for bioelectronic applications, including high electrical conductivity (~20 S cm(-1) in PBS, ~40 S cm(-1) in deionized water), high stretchability (> 35% strain), low Young's modulus (~2 MPa), superior mechanical, electrical and electrochemical stability, and tunable isotropic/anisotropic swelling in wet physiological environments.
  19. 19
    Criado-Gonzalez, M.; Dominguez-Alfaro, A.; Lopez-Larrea, N.; Alegret, N.; Mecerreyes, D. Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities. ACS Appl. Polym. Mater. 2021, 3 (6), 2865– 2883,  DOI: 10.1021/acsapm.1c00252
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    19
    Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities
    Criado-Gonzalez, Miryam; Dominguez-Alfaro, Antonio; Lopez-Larrea, Naroa; Alegret, Nuria; Mecerreyes, David
    ACS Applied Polymer Materials (2021), 3 (6), 2865-2883CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
    A review. Conducting polymers (CPs) have been attracting great attention in the development of (bio)electronic devices. Most of the current devices are rigid two-dimensional systems and possess uncontrollable geometries and architectures that lead to poor mech. properties presenting ion/electronic diffusion limitations. The goal of the article is to provide an overview about the additive manufg. (AM) of conducting polymers, which is of paramount importance for the design of future wearable three-dimensional (3D) (bio)electronic devices. Among different 3D printing AM techniques, inkjet, extrusion, electrohydrodynamic, and light-based printing have been mainly used. This review article collects examples of 3D printing of conducting polymers such as poly(3,4-ethylene-dioxythiophene), polypyrrole, and polyaniline. It also shows examples of AM of these polymers combined with other polymers and/or conducting fillers such as carbon nanotubes, graphene, and silver nanowires. Afterward, the foremost applications of CPs processed by 3D printing techniques in the biomedical and energy fields, i.e., wearable electronics, sensors, soft robotics for human motion, or health monitoring devices, among others, will be discussed.
  20. 20
    Zhao, Q.; Liu, J.; Wu, Z.; Xu, X.; Ma, H.; Hou, J.; Xu, Q.; Yang, R.; Zhang, K.; Zhang, M.; Yang, H.; Peng, W.; Liu, X.; Zhang, C.; Xu, J.; Lu, B. Robust PEDOT:PSS-based hydrogel for highly efficient interfacial solar water purification. Chem. Eng. J. 2022, 442, 136284,  DOI: 10.1016/j.cej.2022.136284
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    Robust PEDOT:PSS-based hydrogel for highly efficient interfacial solar water purification
    Zhao, Qi; Liu, Juyang; Wu, Zhixin; Xu, Xinye; Ma, Hude; Hou, Jian; Xu, Qiaoli; Yang, Ruping; Zhang, Kaiyue; Zhang, Mengmeng; Yang, Hanjun; Peng, Wenshan; Liu, Ximei; Zhang, Chengchen; Xu, Jingkun; Lu, Baoyang
    Chemical Engineering Journal (Amsterdam, Netherlands) (2022), 442 (Part_1), 136284CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    Solar water purifn. has been attracting considerable attention due to its promising desalination and purifn. applications to alleviate water scarcity and pollution. However, despite recent rapid progress, rational design and development of robust and highly efficient interfacial photothermal materials sufficing for large scale real usage is still a crit. challenge to the science and technol. of this field. Herein, we develop a mech. robust nanocomposite hydrogel with intriguing evapn. performance by simple blending and crosslinking of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) nanofibrils and poly(vinyl alc.) (PVA) viscous soln. As-prepd. PEDOT:PSS-PVA hydrogel exhibits an excellent light absorption ratio of ∼ 99.7% in a wide wavelength range of 250 ∼ 2500 nm and displays unprecedented high energy efficiency of ∼ 98.0% with a fast evapn. rate of ∼ 2.84 kg m-2 h-1 under one sun irradn. A 60-day continuous test simulating the real seawater evapn. environment shows superior long-term stability of such hydrogel with an av. energy efficiency of ∼ 93.3%. To harness these advantageous properties, we further demonstrate the scalable hydrogel prodn. and fabricate a solar water evaporator equipment for seawater desalination and wastewater purifn., from which the acquired water can meet the drinking requirements set by the World Health Organization.
  21. 21
    Dominguez-Alfaro, A.; Gabirondo, E.; Alegret, N.; De León-Almazán, C. M.; Hernandez, R.; Vallejo-Illarramendi, A.; Prato, M.; Mecerreyes, D. 3D Printable Conducting and Biocompatible PEDOT-graft-PLA Copolymers by Direct Ink Writing. Macromol. Rapid Commun. 2021, 42 (12), 2100100,  DOI: 10.1002/marc.202100100
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    3D Printable Conducting and Biocompatible PEDOT-graft-PLA Copolymers by Direct Ink Writing
    Dominguez-Alfaro, Antonio; Gabirondo, Elena; Alegret, Nuria; De Leon-Almazan, Claudia Maria; Hernandez, Robert; Vallejo-Illarramendi, Ainara; Prato, Maurizio; Mecerreyes, David
    Macromolecular Rapid Communications (2021), 42 (12), 2100100CODEN: MRCOE3; ISSN:1022-1336. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Tailor-made polymers are needed to fully exploit the possibilities of additive manufg., constructing complex, and functional devices in areas such as bioelectronics. In this paper, the synthesis of a conducting and biocompatible graft copolymer which can be 3D printed using direct melting extrusion methods is shown. For this purpose, graft copolymers composed by conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and a biocompatible polymer polylactide (PLA) are designed. The PEDOT-g-PLA copolymers are synthesized by chem. oxidative polymn. between 3,4-ethylenedioxythiophene and PLA macromonomers. PEDOT-g-PLA copolymers with different compns. are obtained and fully characterized. The rheol. characterization indicates that copolymers contg. below 20 wt% of PEDOT show the right complex viscosity values suitable for direct ink writing (DIW). The 3D printing tests using the DIW methodol. allows printing different parts with different shapes with high resoln. (200 μm). The conductive and biocompatible printed patterns of PEDOT-g-PLA show excellent cell growth and maturation of neonatal cardiac myocytes cocultured with fibroblasts.
  22. 22
    Dominguez-Alfaro, A.; Criado-Gonzalez, M.; Gabirondo, E.; Lasa-Fernández, H.; Olmedo-Martínez, J. L.; Casado, N.; Alegret, N.; Müller, A. J.; Sardon, H.; Vallejo-Illarramendi, A.; Mecerreyes, D. Electroactive 3D printable poly(3,4-ethylenedioxythiophene)-graft-poly(ε-caprolactone) copolymers as scaffolds for muscle cell alignment. Polym. Chem. 2022, 13 (1), 109– 120,  DOI: 10.1039/D1PY01185E
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    Electroactive 3D printable poly(3,4-ethylenedioxythiophene)-graft-poly(ε-caprolactone) copolymers as scaffolds for muscle cell alignment
    Dominguez-Alfaro, Antonio; Criado-Gonzalez, Miryam; Gabirondo, Elena; Lasa-Fernandez, Haizpea; Olmedo-Martinez, Jorge L.; Casado, Nerea; Alegret, Nuria; Muller, Alejandro J.; Sardon, Haritz; Vallejo-Illarramendi, Ainara; Mecerreyes, David
    Polymer Chemistry (2022), 13 (1), 109-120CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    The development of tailor-made polymers to build artificial three-dimensional scaffolds to repair damaged skin tissues is gaining increasing attention in the bioelectronics field. Poly(3,4-ethylene dioxythiophene) (PEDOT) is the gold std. conducting polymer for the bioelectronics field due to its high cond., thermal stability, and biocompatibility; however, it is insol. and infusible, which limits its processability into three dimensional scaffolds. Here, poly(3,4-ethylendioxythiophene)-graft-poly(ε-caprolactone) copolymers, PEDOT-g-PCL, with different mol. wts. and PEDOT compns., were synthesized by chem. oxidative polymn. to enhance the processability of PEDOT. First, the chem. structure and compn. of the copolymers were characterized by NMR, IR spectroscopy, and thermogravimetric anal. Then, the additive manufg. of PEDOT-g-PCL copolymers by direct ink writing was evaluated by rheol. and 3D printing assays. The morphol. of the printed patterns was further characterized by SEM and the cond. by the four-point probe. Finally, the employment of these printed patterns to induce muscle cells alignment was tested, proving the ability of PEDOT-g-PCL patterns to produce myotubes differentiation.
  23. 23
    Yuk, H.; Lu, B.; Zhao, X. Hydrogel bioelectronics. Chem. Soc. Rev. 2019, 48 (6), 1642– 1667,  DOI: 10.1039/C8CS00595H
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    Hydrogel bioelectronics
    Yuk, Hyunwoo; Lu, Baoyang; Zhao, Xuanhe
    Chemical Society Reviews (2019), 48 (6), 1642-1667CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Bioelectronic interfacing with the human body including elec. stimulation and recording of neural activities is the basis of the rapidly growing field of neural science and engineering, diagnostics, therapy, and wearable and implantable devices. Owing to intrinsic dissimilarities between soft, wet, and living biol. tissues and rigid, dry, and synthetic electronic systems, the development of more compatible, effective, and stable interfaces between these two different realms has been one of the most daunting challenges in science and technol. Recently, hydrogels have emerged as a promising material candidate for the next-generation bioelectronic interfaces, due to their similarities to biol. tissues and versatility in elec., mech., and biofunctional engineering. In this review, we discuss (i) the fundamental mechanisms of tissue-electrode interactions, (ii) hydrogels' unique advantages in bioelec. interfacing with the human body, (iii) the recent progress in hydrogel developments for bioelectronics, and (iv) rational guidelines for the design of future hydrogel bioelectronics. Advances in hydrogel bioelectronics will usher unprecedented opportunities toward ever-close integration of biol. and electronics, potentially blurring the boundary between humans and machines.
  24. 24
    Yang, Q.; Hu, Z.; Rogers, J. A. Functional Hydrogel Interface Materials for Advanced Bioelectronic Devices. Acc. Mater. Res. 2021, 2 (11), 1010– 1023,  DOI: 10.1021/accountsmr.1c00142
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    Functional Hydrogel Interface Materials for Advanced Bioelectronic Devices
    Yang, Quansan; Hu, Ziying; Rogers, John A.
    Accounts of Materials Research (2021), 2 (11), 1010-1023CODEN: AMRCDA; ISSN:2643-6728. (American Chemical Society)
    A review. A frontier area of modern research focuses on emerging classes of implantable bioelectronic devices with unique modes of operation that are relevant both to research studies and to medical practice. These advanced technologies have the potential to enable revolutionary diagnostic and therapeutic capabilities relevant to a wide spectrum of disorders, where seamless integration onto the surfaces of vital organs allows for accurate sensing, stimulation, or even concurrent sensing and stimulation. Materials for tissue-like interfaces, such as hydrogels, that enable soft mech. coupling and multifunctional, bidirectional exchange between these technol. platforms and living systems are critically important. Functional hydrogels offer significant promise in this context, as illustrated in recent demonstrations of interlayers that support optical, mech., elec., optical, thermal, and biochem. modes of interaction, with chronic biocompatibility and stable function in live animal models. This Account highlights recent progress in hydrogel materials that serve as interfaces between bioelectronics systems and soft tissues to facilitate implantation and to support sensing and stimulation. The content includes materials concepts, compns., chemistries, and structures that allow for bioelectronic integration. Use as interfacial adhesives and as surface coatings to support mech., elec., optical, thermal, and/or chem. coupling highlight the broad range of options. The Account begins with hydrogels that exploit advanced chemistries to control internal hemorrhage, prevent bacterial infections, and to suppress foreign body responses. Subsequent sections summarize strategies to exploit the mechanics of hydrogels, such as their mech., tunable modulus, lubricating surfaces, and interface adhesion properties, to facilitate interactions between bioelectronic and biol. systems. Discussions of functional characteristics begin with the elec. cond. of different types of conductive hydrogels and their long-time stability, with applications in bioelectronic sensing and stimulation. Following sections focus on optical, thermal, and chem. properties, also in the context of device operation. A final passage on chem. outlines recently developed photocurable and bioresorbable hydrogel adhesives that support multifunctional interfaces to soft biol. tissues. The concluding paragraphs highlight remaining challenges and opportunities for research in hydrogel materials science for advanced bioelectronic devices.
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    Yuk, H.; Lu, B.; Lin, S.; Qu, K.; Xu, J.; Luo, J.; Zhao, X. 3D printing of conducting polymers. Nat. Commun. 2020, 11 (1), 1604,  DOI: 10.1038/s41467-020-15316-7
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    3D printing of conducting polymers
    Yuk, Hyunwoo; Lu, Baoyang; Lin, Shen; Qu, Kai; Xu, Jingkun; Luo, Jianhong; Zhao, Xuanhe
    Nature Communications (2020), 11 (1), 1604CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Conducting polymers are promising material candidates in diverse applications including energy storage, flexible electronics, and bioelectronics. However, the fabrication of conducting polymers has mostly relied on conventional approaches such as ink-jet printing, screen printing, and electron-beam lithog., whose limitations have hampered rapid innovations and broad applications of conducting polymers. Here we introduce a high-performance 3D printable conducting polymer ink based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) for 3D printing of conducting polymers. The resultant superior printability enables facile fabrication of conducting polymers into high resoln. and high aspect ratio microstructures, which can be integrated with other materials such as insulating elastomers via multi-material 3D printing. The 3D-printed conducting polymers can also be converted into highly conductive and soft hydrogel microstructures. We further demonstrate fast and streamlined fabrications of various conducting polymer devices, such as a soft neural probe capable of in vivo single-unit recording.
  26. 26
    Spencer, A. R.; Shirzaei Sani, E.; Soucy, J. R.; Corbet, C. C.; Primbetova, A.; Koppes, R. A.; Annabi, N. Bioprinting of a Cell-Laden Conductive Hydrogel Composite. ACS Appl. Mater. Interfaces 2019, 11 (34), 30518– 30533,  DOI: 10.1021/acsami.9b07353
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    Bioprinting of a Cell-Laden Conductive Hydrogel Composite
    Spencer, Andrew R.; Shirzaei Sani, Ehsan; Soucy, Jonathan R.; Corbet, Carolyn C.; Primbetova, Asel; Koppes, Ryan A.; Annabi, Nasim
    ACS Applied Materials & Interfaces (2019), 11 (34), 30518-30533CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Bioprinting has gained significant attention for creating biomimetic tissue constructs with potential to be used in biomedical applications such as drug screening or regenerative medicine. Ideally, biomaterials used for three-dimensional (3D) bioprinting should match the mech., hydrostatic, bioelec., and physicochem. properties of the native tissues. However, many materials with these tissue-like properties are not compatible with printing techniques without modifying their compns. In addn., integration of cell-laden biomaterials with bioprinting methodologies that preserve their physicochem. properties remains a challenge. In this work, a biocompatible conductive hydrogel composed of gelatin methacryloyl (GelMA) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was synthesized and bioprinted to form complex, 3D cell-laden structures. The biofabricated conductive hydrogels were formed by an initial crosslinking step of the PEDOT:PSS with bivalent calcium ions and a secondary photopolymn. step with visible light to cross-link the GelMA component. These modifications enabled tuning the mech. properties of the hydrogels, with Young's moduli ranging from ∼40-150 kPa, as well as tunable cond. by varying the concn. of PEDOT:PSS. In addn., the hydrogels degraded in vivo with no substantial inflammatory responses as demonstrated by haematoxylin and eosin (H&E) and immunofluorescent staining of s.c. implanted samples in Wistar rats. The parameters for forming a slurry of microgel particles to support 3D bioprinting of the engineered cell-laden hydrogel were optimized to form constructs with improved resoln. High cytocompatibility and cell spreading were demonstrated in both wet-spinning and 3D bioprinting of cell-laden hydrogels with the new conductive hydrogel-based bioink and printing methodol. The synergy of an advanced fabrication method and conductive hydrogel presented here is promising for engineering complex conductive and cell-laden structures.
  27. 27
    Shen, Z.; Zhang, Z.; Zhang, N.; Li, J.; Zhou, P.; Hu, F.; Rong, Y.; Lu, B.; Gu, G. High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines. Adv. Mater. 2022, 2203650,  DOI: 10.1002/adma.202203650
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    High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines
    Shen, Zequn; Zhang, Zhilin; Zhang, Ningbin; Li, Jinhao; Zhou, Peiwei; Hu, Faqi; Rong, Yu; Lu, Baoyang; Gu, Guoying
    Advanced Materials (Weinheim, Germany) (2022), 34 (32), 2203650CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Highly stretchable strain sensors based on conducting polymer hydrogel are rapidly emerging as a promising candidate toward diverse wearable skins and sensing devices for soft machines. However, due to the intrinsic limitations of low stretchability and large hysteresis, existing strain sensors cannot fully exploit their potential when used in wearable or robotic systems. Here, a conducting polymer hydrogel strain sensor exhibiting both ultimate strain (300%) and negligible hysteresis (<1.5%) is presented. This is achieved through a unique microphase semisepd. network design by compositing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) nanofibers with poly(vinyl alc.) (PVA) and facile fabrication by combining 3D printing and successive freeze-thawing. The overall superior performances of the strain sensor including stretchability, linearity, cyclic stability, and robustness against mech. twisting and pressing are systematically characterized. The integration and application of such strain sensor with electronic skins are further demonstrated to measure various physiol. signals, identify hand gestures, enable a soft gripper for objection recognition, and remote control of an industrial robot. This work may offer both promising conducting polymer hydrogels with enhanced sensing functionalities and tech. platforms toward stretchable electronic skins and intelligent robotic systems.
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    Mangoma, T. N.; Yamamoto, S.; Malliaras, G. G.; Daly, R. Hybrid 3D/Inkjet-Printed Organic Neuromorphic Transistors. Adv. Mater. Technol. 2022, 7, 2000798,  DOI: 10.1002/admt.202000798
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    Hybrid three-dimensional/inkjet-printed organic neuromorphic transistors
    Mangoma, Tanyaradzwa N.; Yamamoto, Shunsuke; Malliaras, George G.; Daly, Ronan
    Advanced Materials Technologies (Weinheim, Germany) (2022), 7 (2), 2000798CODEN: AMTDCM; ISSN:2365-709X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Org. electrochem. transistors (OECTs) are proving essential in bioelectronics and printed electronics applications, with their simple structure, ease of tunability, biocompatibility, and suitability for different routes to fabrication. OECTs are also being explored as neuromorphic devices, where they emulate characteristics of biol. neural networks through co-location of information storage and processing on the same unit, overcoming the von Neumann performance bottleneck. To achieve the long-term vision of translating to inexpensive, low-power computational devices, fabrication needs to be feasible with adaptable, scalable digital techniques. Here, a hybrid direct-write additive manufg. approach to fabricating OECTs is shown. 3D printing of com. available printing filament is combined to deliver conducting and insulating layers, with inkjet printing of semiconducting thin films to create OECTs. These printed OECTs show depletion mode operation paired-pulse depression behavior and evidence of adaptation to support their translation to neuromorphic devices. These results show that a hybrid of accessible and design-flexible AM techniques can be used to rapidly fabricate devices that exhibit good OECT and neuromorphic performances.
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    Bihar, E.; Roberts, T.; Zhang, Y.; Ismailova, E.; Hervé, T.; Malliaras, G. G.; De Graaf, J. B.; Inal, S.; Saadaoui, M. Fully printed all-polymer tattoo/textile electronics for electromyography. Flex. Print. Electron. 2018, 3 (3), 034004,  DOI: 10.1088/2058-8585/aadb56
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    Bihar, E.; Roberts, T.; Saadaoui, M.; Hervé, T.; De Graaf, J. B.; Malliaras, G. G. Inkjet-Printed PEDOT:PSS Electrodes on Paper for Electrocardiography. Adv. Healthc. Mater. 2017, 6 (6), 1601167,  DOI: 10.1002/adhm.201601167
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    Zhang, J.; Hu, Q.; Wang, S.; Tao, J.; Gou, M. Digital Light Processing Based Three-dimensional Printing for Medical Applications. Int. J. Bioprinting 2020, 6 (1), 620,  DOI: 10.18063/ijb.v6i1.242
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    Shahzadi, L.; Maya, F.; Breadmore, M. C.; Thickett, S. C. Functional Materials for DLP-SLA 3D Printing Using Thiol-Acrylate Chemistry: Resin Design and Postprint Applications. ACS Appl. Polym. Mater. 2022, 4 (5), 3896– 3907,  DOI: 10.1021/acsapm.2c00358
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    Functional Materials for DLP-SLA 3D Printing Using Thiol-Acrylate Chemistry: Resin Design and Postprint Applications
    Shahzadi, Lubna; Maya, Fernando; Breadmore, Michael C.; Thickett, Stuart C.
    ACS Applied Polymer Materials (2022), 4 (5), 3896-3907CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
    Three-dimensional (3D) printing techniques have greatly simplified prototype manufg. and complex design. However, most com. available stereolithog. (SLA) material components are based on (meth)acrylate-based resin systems that have several disadvantages assocd. with their use, such as inhibition of polymn. by oxygen, solvent resistance, and the inability to modify surfaces post printing. Polymn. via a thiol-acrylate mechanism can help overcome many of these drawbacks; however, these systems are less studied in the context of SLA 3D printing. In this work, we report on the design and optimization of thiol-acrylate resin formulations with a view toward effectively controlling the polymn. depth of the cured polymer layer. Four different photoblockers were studied and the use of 1,3-bis(4-methoxyphenyl)propane-1,3-dione enabled optically transparent and colorless printed objects with good resoln. to be realized. Fully enclosed microchannels with diams. as low as 250 μm were successfully printed using this approach. Taking advantage of ready postprinting surface modification of thiol-acrylate polymers, various hydrophilic, hydrophobic, and fluorescent polymer chains were successfully grafted to the object surface via reversible addn.-fragmentation chain transfer (RAFT) polymn. Free thiol groups at the surface of off-stoichiometric resin formulations were also used to immobilize gold nanoparticles for the catalytic conversion of 4-nitrophenol to 4-aminophenol. The tunability of these thiol-acrylate resins for SLA 3D printing and feasible postprint surface modifications make them attractive candidates for com. applications.
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    Melchels, F. P. W.; Feijen, J.; Grijpma, D. W. A review on stereolithography and its applications in biomedical engineering. Biomaterials 2010, 31 (24), 6121– 6130,  DOI: 10.1016/j.biomaterials.2010.04.050
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    A review on stereolithography and its applications in biomedical engineering
    Melchels, Ferry P. W.; Feijen, Jan; Grijpma, Dirk W.
    Biomaterials (2010), 31 (24), 6121-6130CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
    A review. Stereolithog. is a solid free form technique (SFF) that was introduced in the late 1980s. Although many other techniques have been developed since then, stereolithog. remains one of the most powerful and versatile of all SFF techniques. It has the highest fabrication accuracy and an increasing no. of materials that can be processed is becoming available. In this paper we discuss the characteristic features of the stereolithog. technique and compare it to other SFF techniques. The biomedical applications of stereolithog. are reviewed, as well as the biodegradable resin materials that have been developed for use with stereolithog. Finally, an overview of the application of stereolithog. in prepg. porous structures for tissue engineering is given.
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    Nestler, N.; Wesemann, C.; Spies, B. C.; Beuer, F.; Bumann, A. Dimensional accuracy of extrusion- and photopolymerization-based 3D printers: In vitro study comparing printed casts. J. Prosthet. Dent. 2021, 125 (1), 103– 110,  DOI: 10.1016/j.prosdent.2019.11.011
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    Dimensional accuracy of extrusion- and photopolymerization-based 3D printers: In vitro study comparing printed casts
    Nestler, Norbert; Wesemann, Christian; Spies, Benedikt C.; Beuer, Florian; Bumann, Axel
    Journal of Prosthetic Dentistry (2021), 125 (1), 103-110CODEN: JPDEAT; ISSN:0022-3913. (Elsevier Inc.)
    Reliable studies comparing the accuracy of complete-arch casts from 3D printers are scarce. The purpose of this in vitro study was to investigate the accuracy of casts printed by using various extrusion- and photopolymn.-based printers. A master file was sent to 5 printer manufacturers and distributors to print 37 identical casts. This file consisted of a standardized data set of a maxillary cast in std. tessellation language (STL) format comprising 5 ref. points for the measurement of 3 distances that served as ref. for all measurements: intermolar width (IMW), intercanine width (ICW), and dental arch length (AL). The digital measurement of the master file obtained by using a surveying software program (Convince Premium 2012) was used as the control. Two extrusion-based (M2 and Ultimaker 2+) and 3 photopolymn.-based printers (Form 2, Asiga MAX UV, and myrev140) were compared. The casts were measured by using a multisensory coordinate measuring machine (O-Inspect 422). The values were then compared with those of the master file. The Mann-Whitney U test and Levene tests were used to det. significant differences in the trueness and precision (accuracy) of the measured distances. The deviations from the master file at all 3 distances for the included printers ranged between 12 μm and 240 μm (trueness), with an interquartile range (IQR) between 17 μm and 388 μm (precision). Asiga MAX UV displayed the highest accuracy, considering all the distances, and Ultimaker 2+ demonstrated comparable accuracy for shorter distances (IMW and ICW). Although myrev140 operated with high precision, it displayed high deviations from the master file. Similarly, although Form 2 exhibited high IQR, it did not deviate significantly from the master file in the longest range (AL). M2 performed consistently. Both extrusion-based and photopolymn.-based printers were accurate. In general, inexpensive printers were no less accurate than more expensive ones.
  35. 35
    Bagheri, A.; Jin, J. Photopolymerization in 3D Printing. ACS Appl. Polym. Mater. 2019, 1 (4), 593– 611,  DOI: 10.1021/acsapm.8b00165
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    Photopolymerization in 3D Printing
    Bagheri, Ali; Jin, Jianyong
    ACS Applied Polymer Materials (2019), 1 (4), 593-611CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
    A review. The field of 3D printing is continuing its rapid development in both academic and industrial research environments. The development of 3D printing technologies has opened new implementations in rapid prototyping, tooling, dentistry, microfluidics, biomedical devices, tissue engineering, drug delivery, etc. Among different 3D printing techniques, photopolymn.-based process (such as stereolithog. and digital light processing) offers flexibility over the final properties of the 3D printed materials (such as optical, chem., and mech. properties) using versatile polymer chem. The strategy behind the 3D photopolymn. is based on using monomers/oligomers in liq. state (in the presence of photoinitiators) that can be photopolymd. (via radical or cationic mechanism) upon exposure to light source of different wavelengths (depending on the photoinitiator system). An overview of recent evolutions in the field of photopolymn.-based 3D printing and highlights of novel 3D printable photopolymers is provided herein. Challenges that limit the use of conventional photopolymers (i.e., initiation under UV light) together with prospective solns. such as incorporation of photosensitive initiators with red-shifted absorptions are also discussed in detail. This review also spotlights recent progress on the use of controlled living radical photopolymn. techniques (i.e., reversible addn.-fragmentation chain-transfer polymn.) in 3D printing, which will pave the way for widespread growth of new generations of 3D materials with living features and possibility for postprinting modifications.
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    Heo, D. N.; Lee, S.-J.; Timsina, R.; Qiu, X.; Castro, N. J.; Zhang, L. G. Development of 3D printable conductive hydrogel with crystallized PEDOT:PSS for neural tissue engineering. Mater. Sci. Eng., C 2019, 99, 582– 590,  DOI: 10.1016/j.msec.2019.02.008
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    Development of 3D printable conductive hydrogel with crystallized PEDOT:PSS for neural tissue engineering
    Heo, Dong Nyoung; Lee, Se-Jun; Timsina, Raju; Qiu, Xiangyun; Castro, Nathan J.; Zhang, Lijie Grace
    Materials Science & Engineering, C: Materials for Biological Applications (2019), 99 (), 582-590CODEN: MSCEEE; ISSN:0928-4931. (Elsevier B.V.)
    Bioelectronic devices enable efficient and effective communication between medical devices and human tissue in order to directly treat patients with various neurol. disorders. Due to the mech. similarity to human tissue, hydrogel-based electronic devices are considered to be promising for biol. signal recording and stimulation of living tissues. Here, we report the first three-dimensionally (3D) printable conductive hydrogel that can be photocrosslinked while retaining high elec. cond. In addn., we prepd. dorsal root ganglion (DRG) cell-encapsulated gelatin methacryloyl (GelMA) hydrogels which were integrated with the 3D printed conductive structure and evaluated for efficiency neural differentiation under elec. stimulation (ES). For enhanced elec. cond., a poly(3,4-ethylenedioxythiophene) (PEDOT): polystyrene sulfonate (PSS) aq. soln. was freeze-dried and mixed with polyethylene glycol diacrylate (PEGDA) as the photocurable polymer base. Next, the conductive hydrogel was patterned on the substrate by using a table-top stereolithog. (SLA) 3D printer. The fabricated hydrogel was characterized for electrochem. cond. After printing with the PEDOT:PSS conductive soln., the patterned hydrogel exhibited decreased printing diams. with increasing of PEDOT:PSS concn. Also, the resultant conductive hydrogel had significantly increased electrochem. properties with increasing PEDOT:PSS concn. The 3D printed conductive hydrogel provides excellent structural support to systematically transfer the ES toward encapsulated DRG cells for enhanced neuronal differentiation. The results from this study indicate that the conductive hydrogel can be useful as a 3D printing material for elec. applications.
  37. 37
    Bertana, V.; Scordo, G.; Parmeggiani, M.; Scaltrito, L.; Ferrero, S.; Gomez, M. G.; Cocuzza, M.; Vurro, D.; D’Angelo, P.; Iannotta, S.; Pirri, C. F.; Marasso, S. L. Rapid prototyping of 3D Organic Electrochemical Transistors by composite photocurable resin. Sci. Rep. 2020, 10 (1), 13335,  DOI: 10.1038/s41598-020-70365-8
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    Rapid prototyping of 3D Organic Electrochemical Transistors by composite photocurable resin
    Bertana, Valentina; Scordo, Giorgio; Parmeggiani, Matteo; Scaltrito, Luciano; Ferrero, Sergio; Gomez, Manuel Gomez; Cocuzza, Matteo; Vurro, Davide; D'Angelo, Pasquale; Iannotta, Salvatore; Pirri, Candido F.; Marasso, Simone L.
    Scientific Reports (2020), 10 (1), 13335CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
    Rapid Prototyping (RP) promises to induce a revolutionary impact on how the objects can be produced and used in industrial manufg. as well as in everyday life. Over the time a std. technique as the 3D Stereolithog. (SL) has become a fundamental technol. for RP and Additive Manufg. (AM), since it enables the fabrication of the 3D objects from a cost-effective photocurable resin. Efforts to obtain devices more complex than just a mere aesthetic simulacre, have been spent with uncertain results. The multidisciplinary nature of such manufg. technique furtherly hinders the route to the fabrication of complex devices. A good knowledge of the bases of material science and engineering is required to deal with SL technol., characterization and testing aspects. In this framework, our study aims to reveal a new approach to obtain RP of complex devices, namely Org. Electro-Chem. Transistors (OECTs), by SL technique exploiting a resin composite based on the conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and the photo curable Poly(ethylene glycol) diacrylate (PEGDA). A comprehensive study is presented, starting from the optimization of composite resin and characterization of its electrochem. properties, up to the 3D OECTs printing and testing. Relevant performances in biosensing for dopamine (DA) detection using the 3D OECTs are reported and discussed too.
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    Scordo, G.; Bertana, V.; Scaltrito, L.; Ferrero, S.; Cocuzza, M.; Marasso, S. L.; Romano, S.; Sesana, R.; Catania, F.; Pirri, C. F. A novel highly electrically conductive composite resin for stereolithography. Mater. Today Commun. 2019, 19, 12– 17,  DOI: 10.1016/j.mtcomm.2018.12.017
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    A novel highly electrically conductive composite resin for stereolithography
    Scordo, Giorgio; Bertana, Valentina; Scaltrito, Luciano; Ferrero, Sergio; Cocuzza, Matteo; Marasso, Simone L.; Romano, Stefano; Sesana, Raffaella; Catania, Felice; Pirri, Candido F.
    Materials Today Communications (2019), 19 (), 12-17CODEN: MTCAC7; ISSN:2352-4928. (Elsevier Ltd.)
    The use of 3D printing for scientific and industrial applications is rapidly increasing and, simultaneously, is growing the interest toward printable smart materials. It is known that using a conductive filler, e.g. nanoparticles, metal powders or composite polymers, it is possible to produce a more elec. conductive printable material. The PEDOT:PSS polymer has been used extensively, from bioelectronics to energy storage devices, for its cond. and biocompatibility. In this work, a highly elec. conductive resin for Stereolithog. was developed using a dispersion of PEDOT particles, extd. from Clevios PH1000 through a simple sepn. process, in a PEGDA matrix. The resin compn. was optimized in terms of photoinitiator, surfactant and filler concn. Furthermore, optimal printing parameters were detd. for this composite resin, obtaining a printed object with appreciable elec. cond. (0.05 S cm-1) and mech. properties (Young modulus 21 MPa).
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    Bertana, V.; Scordo, G.; Manachino, M.; Romano, S.; Gomez, M. G.; Marasso, S. L.; Ferrero, S.; Cocuzza, M.; Pirri, C. F.; Scaltrito, L. 3D Printed Active Objects based on the Promising PEDOT: PSS Resin: Investigation of their Integration inside an Electronic Circuit. Int. J. Eng. Res. Technol. 2020, 13, 462,  DOI: 10.37624/IJERT/13.3.2020.462-469
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    Ye, X.; Wang, C.; Wang, L.; Lu, B.; Gao, F.; Shao, D. DLP printing of a flexible micropattern Si/PEDOT:PSS/PEG electrode for lithium-ion batteries. Chem. Commun. 2022, 55, 7642– 7645,  DOI: 10.1039/D2CC01626E
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    Ahn, D.; Stevens, L. M.; Zhou, K.; Page, Z. A. Rapid High-Resolution Visible Light 3D Printing. ACS Cent. Sci. 2020, 6 (9), 1555– 1563,  DOI: 10.1021/acscentsci.0c00929
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    Rapid High-Resolution Visible Light 3D Printing
    Ahn, Dowon; Stevens, Lynn M.; Zhou, Kevin; Page, Zachariah A.
    ACS Central Science (2020), 6 (9), 1555-1563CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Light-driven 3D printing to convert liq. resins into solid objects (i.e., photocuring) has traditionally been dominated by engineering disciplines, yielding the fastest build speeds and highest resoln. of any additive manufg. process. However, the reliance on high-energy UV/violet light limits the materials scope due to degrdn. and attenuation (e.g., absorption and/or scattering). Chem. innovation to shift the spectrum into more mild and tunable visible wavelengths promises to improve compatibility and expand the repertoire of accessible objects, including those contg. biol. compds., nanocomposites, and multimaterial structures. Photochem. at these longer wavelengths currently suffers from slow reaction times precluding its utility. Herein, novel panchromatic photopolymer resins were developed and applied for the first time to realize rapid high-resoln. visible light 3D printing. The combination of electron-deficient and electron-rich coinitiators was crit. to overcoming the speed-limited photocuring with visible light. Furthermore, azo-dyes were identified as vital resin components to confine curing to irradn. zones, improving spatial resoln. A unique screening method was used to streamline optimization (e.g., exposure time and azo-dye loading) and correlate resin compn. to resoln., cure rate, and mech. performance. Ultimately, a versatile and general visible-light-based printing method was shown to afford (1) stiff and soft objects with feature sizes <100μm, (2) build speeds up to 45 mm/h, and (3) mech. isotropy, rivaling modern UV-based 3D printing technol. and providing a foundation from which bio- and composite-printing can emerge. Liq. resins that rapidly harden upon exposure to low-energy visible light are developed and applied to high-resoln. projection-based 3D printing across the color spectrum.
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    Gueye, M. N.; Carella, A.; Faure-Vincent, J.; Demadrille, R.; Simonato, J.-P. Progress in understanding structure and transport properties of PEDOT-based materials: A critical review. Prog. Mater. Sci. 2020, 108, 100616,  DOI: 10.1016/j.pmatsci.2019.100616
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    Progress in understanding structure and transport properties of PEDOT-based materials: A critical review
    Gueye, Magatte N.; Carella, Alexandre; Faure-Vincent, Jerome; Demadrille, Renaud; Simonato, Jean-Pierre
    Progress in Materials Science (2020), 108 (), 100616CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
    Since the late '80s, a highly stable conductive polymer has been developed, that is poly(3,4-ethylene dioxythiophene), also known as PEDOT. Its increasing cond. throughout the years combined with its intrinsic stability have aroused great attention both in the academic and industrial fields. The growing importance of PEDOT, can be easily acknowledged through the numerous applications in thermoelectricity, photovoltaics, lighting, sensing, tech. coatings, transparent electrodes, bioelectronics, and so forth. Although its high elec. cond. is strongly established in the literature, the wide range of data shows that disorder, as the limiting factor in charges' transport, hinders the design of materials with optimal performances. The aim of this article is to review and discuss recent progresses dealing with the elec. cond. and transport properties in PEDOT materials, with special attention on morphol. and structural features. Particular emphasis is given to the com. PEDOT:PSS as well as other PEDOT-based materials stabilized with smaller counter-anions. It appears that the elec. cond. and the transport mechanisms are closely related to the fabrication process, the crystallinity of the material and the choice of the counter-anions. With the tunable elec. properties, new functionalities appear accessible and add up to the already existing applications that are concisely highlighted.
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    Donoval, M.; Micjan, M.; Novota, M.; Nevrela, J.; Kovacova, S.; Pavuk, M.; Juhasz, P.; Jagelka, M.; Kovac, J.; Jakabovic, J.; Cigan, M.; Weis, M. Relation between secondary doping and phase separation in PEDOT:PSS films. Appl. Surf. Sci. 2017, 395, 86– 91,  DOI: 10.1016/j.apsusc.2016.05.076
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    Relation between secondary doping and phase separation in PEDOT:PSS films
    Donoval, Martin; Micjan, Michal; Novota, Miroslav; Nevrela, Juraj; Kovacova, Sona; Pavuk, Milan; Juhasz, Peter; Jagelka, Martin; Kovac, Jaroslav, Jr.; Jakabovic, Jan; Cigan, Marek; Weis, Martin
    Applied Surface Science (2017), 395 (), 86-91CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Conductive copolymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) has been proposed as an alternative to transparent conductive oxides because of its flexibility, transparency, and low-cost prodn. Four different secondary dopants, namely N,N-dimethylformamide, ethyleneglycol, sorbitol, and DMSO, have been used to improve the cond. The relation between the structure changes and cond. enhancement is studied in detail. Atomic force microscopy study of the thin film surface reveals the phase sepn. of PEDOT and PSS. We demonstrate that secondary doping induces the phase sepn. as well as the cond. enhancement.
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    Wei, T.-C.; Chen, S.-H.; Chen, C.-Y. Highly conductive PEDOT:PSS film made with ethylene-glycol addition and heated-stir treatment for enhanced photovoltaic performances. Mater. Chem. Front. 2020, 4 (11), 3302– 3309,  DOI: 10.1039/D0QM00529K
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    Highly conductive PEDOT:PSS film made with ethylene-glycol addition and heated-stir treatment for enhanced photovoltaic performances
    Wei, Ta-Cheng; Chen, Shih-Hsiu; Chen, Chia-Yun
    Materials Chemistry Frontiers (2020), 4 (11), 3302-3309CODEN: MCFAC5; ISSN:2052-1537. (Royal Society of Chemistry)
    Highly conductive and stable PEDOT:PSS films were prepd. by ethylene-glycol (EG) addn. and heated-stir treatment that enhanced the elec. cond. up to 1228.4 S cm-1. Anal. of the long-term figure-of-merit (FoM) performances of EG-treated films with respect to variations in stirring temps. indicated that the highest value of 78.5 was realized by the facile heated-stir treatment at 90°C. The underlying mechanism was validated by the synergetic effect of reordering linear PEDOT chains and the effective removal of water via the supplement of thermal energy that considerably displayed the improved film cond. based on the formation of interconnected PEDOT structures. Furthermore, via combining treated PEDOT:PSS films and Si nanowires, high-performance hybrid solar cells were realized. This was accomplished by reduced charge-transfer resistance and sound heterojunction characteristics that facilitated the charge sepn. and eventual charge collection by the electrode, thus contributing to the enhancement efficiency. The optimal heated-stir treatment at 90°C led to improved cell conversion efficiency achieving 12.2%, which was 1.3 times higher than the result based on the conventional room-temp. process.
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    Liu, J.; Zhu, Z.; Zhou, W.; Liu, P.; Liu, P.; Liu, G.; Xu, J.; Jiang, Q.; Jiang, F. Flexible metal-free hybrid hydrogel thermoelectric fibers. J. Mater. Sci. 2020, 55 (19), 8376– 8387,  DOI: 10.1007/s10853-020-04382-3
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    Flexible metal-free hybrid hydrogel thermoelectric fibers
    Liu, Jing; Zhu, Zhengyou; Zhou, Weiqiang; Liu, Peipei; Liu, Peng; Liu, Guoqiang; Xu, Jingkun; Jiang, Qinglin; Jiang, Fengxing
    Journal of Materials Science (2020), 55 (19), 8376-8387CODEN: JMTSAS; ISSN:0022-2461. (Springer)
    Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) fiber has been developed as a more promising candidate compared with bulk and film to achieve wearable thermoelec. energy harvesting. Single-walled carbon nanotubes (SWCNTs) with nanostructures are considered as an effective conductive filter for the further improvement in the thermoelec. (TE) performance of PEDOT:PSS fibers. However, the previous research primarily focused on PEDOT:PSS/SWCNT films instead of fibers. In this study, PEDOT:PSS/SWCNT hybrid fibers were synthesized via gelation process, which presents a 30% enhancement of the elec. cond. with negligible changes in Seebeck coeff. Moreover, there was a significant increase in the Young's modulus in accordance with the addn. of an appropriate amt. of SWCNTs. Thereafter, the as-prepd. hybrid fibers were treated using ethylene glycol (EG) to further optimize the TE performance. Moreover, the effect of the treatment time and temp. was systematically investigated. The EG treatment resulted in a significant improvement in the elec. cond. without a significant decrease in the Seebeck coeff. Furthermore, the hybrid fibers were subject to EG treatment at elevated temp., whose optimal power factor was approx. 30% higher than that of the EG-treated PEDOT:PSS/SWCNT fibers at 25°. This indicates that the solvent treatment at higher temp. improves the TE performance of hybrid fibers. The findings of this study can serve as a guide for the prepn. of flexible and metal-free hybrid fiber with enhanced TE performance and Young's modulus.
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    Selvam, C.; Lal, D. M.; Harish, S. Thermal conductivity enhancement of ethylene glycol and water with graphene nanoplatelets. Thermochim. Acta 2016, 642, 32– 38,  DOI: 10.1016/j.tca.2016.09.002
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    Thermal conductivity enhancement of ethylene glycol and water with graphene nanoplatelets
    Selvam, C.; Lal, D. Mohan; Harish, Sivasankaran
    Thermochimica Acta (2016), 642 (), 32-38CODEN: THACAS; ISSN:0040-6031. (Elsevier B.V.)
    In the present work, we report the effective thermal cond. of ethylene glycol and water with graphene nanoplatelets. Sodium deoxycholate, a bile salt was used as the surfactant to prep. stable nanofluid dispersions. Stability tests were performed using UV-vis absorption spectrometry and zeta potential to monitor the stability of the prepd. nanofluids as a function of time. Thermal cond. measurements were carried out using transient hot wire technique. Thermal cond. of the nanofluids significantly increases with respect to graphene loading. Maximum thermal cond. enhancements of ∼21% and ∼16% at a loading of 0.5 vol% was obtained for the nanofluids with graphene nanoplatelets seeded in ethylene glycol and water resp. Anal. of exptl. results with Maxwell-Garnett type effective medium theory reveal that despite the high thermal cond. of graphene, interfacial thermal resistance between graphene and the surrounding base fluid limits the thermal cond. enhancement significantly. The interfacial thermal resistance between graphene sheet and ethylene glycol was found to be 2.2 × 10-8 m2 KW-1 while between graphene sheet and water was found to be 1.5 × 10-8 m2 KW-1 resp. for the present nanofluids.
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    Zhou, J.; Lin, S.; Zeng, H.; Liu, J.; Li, B.; Xu, Y.; Zhao, X.; Chen, G. Dynamic intermolecular interactions through hydrogen bonding of water promote heat conduction in hydrogels. Mater. Horiz. 2020, 7 (11), 2936– 2943,  DOI: 10.1039/D0MH00735H
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    Dynamic intermolecular interactions through hydrogen bonding of water promote heat conduction in hydrogels
    Zhou, Jiawei; Lin, Shaoting; Zeng, Hongxia; Liu, Ji; Li, Buxuan; Xu, Yanfei; Zhao, Xuanhe; Chen, Gang
    Materials Horizons (2020), 7 (11), 2936-2943CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
    Heat-conducting polymers provide a new opportunity to tackle thermal management challenges in advanced technologies such as wearable electronics and soft robotics. One strategy to enhance heat conduction in amorphous polymers has been tuning their intermol. interactions. These intermol. forces are often static in nature as the participating mols. are anchored on the polymer chains. In this work, using hydrogel as a model system, we demonstrate how dynamic intermol. forces, which break and re-form constantly, can also enhance thermal transport. Utilizing calorimetric and spectroscopic measurements, we show that this arises from the hydrogen bonds formed between water and nearby polymer chains, which enhances the inter-chain heat transfer efficiency. This mechanism may potentially allow the design of heat-conducting polymers with self-healing or adaptability functionalities.
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    Xu, S.; Cai, S.; Liu, Z. Thermal Conductivity of Polyacrylamide Hydrogels at the Nanoscale. ACS Appl. Mater. Interfaces 2018, 10 (42), 36352– 36360,  DOI: 10.1021/acsami.8b09891
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    Thermal Conductivity of Polyacrylamide Hydrogels at the Nanoscale
    Xu, Shuai; Cai, Shengqiang; Liu, Zishun
    ACS Applied Materials & Interfaces (2018), 10 (42), 36352-36360CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    A polymer network can imbibe copious amts. of water and swell, and the resulting state is known as a hydrogel. In many potential applications of hydrogels, such as stretchable conductors, ionic cables, and neuroprostheses, the thermal conductivities of hydrogels should be understood clearly. In the present work, we build mol. dynamics (MD) models of random cross-linked polyacrylamide hydrogels with different water vol. fractions through a reaction method. On the basis of these models, thermal conductivities of hydrogels at the nanoscale are investigated by a none-equil. MD method. This work reveals that when the water fraction of hydrogels is under 85%, the thermal cond. increases with the water fraction, and can be even higher than the thermal conductivities of both pure polymer networks and pure water because of the influence of the interface between polymer networks and water. However, when the water fraction in hydrogels is bigger than 85%, its thermal cond. will decrease and get close to the water's cond. Accordingly, to explain this abnormal phenomenon, a 2-order-3-phase theor. model is proposed by considering hydrogel as a 3-phase composite. It can be found that the proposed theory can predict results which agree quite well with our simulated results.
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    Nguyen, L. H.; Koerner, H.; Lederer, K. Gel point determination for the copolymerization system of cardanyl acrylate and styrene and its critical conversion. J. Appl. Polym. Sci. 2003, 89 (9), 2385– 2390,  DOI: 10.1002/app.12279
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    Gel point determination for the copolymerization system of cardanyl acrylate and styrene and its critical conversion
    Nguyen, Le Huong; Koerner, Hilmar; Lederer, Klaus
    Journal of Applied Polymer Science (2003), 89 (9), 2385-2390CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
    The copolymn. of cardanyl acrylate (CA) and styrene (S) with Me Et ketone peroxide (MEKP) and cobalt salt as the initiator was studied by means of rheometry and isothermal DSC. The gel point of the CA-S reaction system was detd. by rheol. measurements at 50 and 70°C. In addn., rheol. measurements provide very useful information on the evolution of the phys. properties of the system during curing, such as viscosity and shear modulus. We demonstrate that isothermal DSC expts., using the Avrami model, can be used to obtain a more detailed description of the curing process.
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    Debroy, D.; Oakey, J.; Li, D. Interfacially-mediated oxygen inhibition for precise and continuous poly(ethylene glycol) diacrylate (PEGDA) particle fabrication. J. Colloid Interface Sci. 2018, 510, 334– 344,  DOI: 10.1016/j.jcis.2017.09.081
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    Interfacially-mediated oxygen inhibition for precise and continuous poly(ethylene glycol) diacrylate (PEGDA) particle fabrication
    Debroy, Daniel; Oakey, John; Li, Dongmei
    Journal of Colloid and Interface Science (2018), 510 (), 334-344CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    Poly(ethylene glycol) diacrylate (PEGDA)-based hydrogels have been engineered for multiple biomedical applications, including drug delivery, cell delivery, and tissue engineering. Miniaturization of these materials to nano- and microscale particles is a subject of intense activity and promises to extend their range of applicability. Generally, these efforts have been frustrated by the O-induced inhibition of chain growth polymn., an effect exacerbated as target length scales are reduced. A method which exploits undesirable O-inhibited photopolymn. to produce size-controlled PEGDA hydrogel particles is reported. The role of initial soln. compn. in detg. the relative particle:droplet size ratio is reported, and was detd. to contribute, through its effect on polymn. rate and O diffusivity. Facile control of photopolymn. kinetics by UV light intensity and/or exposure time, allowed PEGDA particles to be produced with dimensions independent of parent spherical droplets formed by conventional microfluidic emulsification.
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    Zhu, J.; Marchant, R. E. Design properties of hydrogel tissue-engineering scaffolds. Expert Rev. Med. Devices 2011, 8 (5), 607– 626,  DOI: 10.1586/erd.11.27
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    Design properties of hydrogel tissue-engineering scaffolds
    Zhu, Junmin; Marchant, Roger E.
    Expert Review of Medical Devices (2011), 8 (5), 607-626CODEN: ERMDDX; ISSN:1743-4440. (Expert Reviews Ltd.)
    A review. This article summarizes the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels are attractive scaffolding materials owing to their highly swollen network structure, ability to encapsulate cells and bioactive mols., and efficient mass transfer. Various polymers, including natural, synthetic and natural/synthetic hybrid polymers, have been used to make hydrogels via chem. or phys. crosslinking. Recently, bioactive synthetic hydrogels have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mech. properties, as well as an extracellular matrix-like microenvironment for cell growth and tissue formation. This article addresses various strategies that have been explored to design synthetic hydrogels with extracellular matrix-mimetic bioactive properties, such as cell adhesion, proteolytic degrdn. and growth factor-binding.
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    McAvoy, K.; Jones, D.; Thakur, R. R. S. Synthesis and Characterisation of Photocrosslinked poly(ethylene glycol) diacrylate Implants for Sustained Ocular Drug Delivery. Pharm. Res. 2018, 35 (2), 36,  DOI: 10.1007/s11095-017-2298-9
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    Synthesis and Characterisation of Photocrosslinked poly(ethylene glycol) diacrylate Implants for Sustained Ocular Drug Delivery
    McAvoy Kathryn; Jones David; Thakur Raghu Raj Singh; Thakur Raghu Raj Singh
    Pharmaceutical research (2018), 35 (2), 36 ISSN:.
    PURPOSE: To investigate the sustained ocular delivery of small and large drug molecules from photocrosslinked poly(ethylene glycol) diacrylate (PEGDA) implants with varying pore forming agents. METHODS: Triamcinolone acetonide and ovalbumin loaded photocrosslinked PEGDA implants, with or without pore-forming agents, were fabricated and characterised for chemical, mechanical, swelling, network parameters, as well as drug release and biocompatibility. HPLC-based analytical methods were employed for analysis of two molecules; ELISA was used to demonstrate bioactivity of ovalbumin. RESULTS: Regardless of PEGDA molecular weight or pore former composition all implants loaded with triamcinolone acetonide released significantly faster than those loaded with ovalbumin. Higher molecular weight PEGDA systems (700 Da) resulted in faster drug release of triamcinolone acetonide than their 250 Da counterpart. All ovalbumin released over the 56-day time period was found to be bioactive. Increasing PEGDA molecular weight resulted in increased system swelling, decreased crosslink density (Ve), increased polymer-water interaction parameter (χ), increased average molecular weight between crosslinks (Mc) and increased mesh size (ε). SEM studies showed the porosity of implants increased with increasing PEGDA molecular weight. Biocompatibility showed both PEGDA molecular weight implants were non-toxic when exposed to retinal epithelial cells over a 7-day period. CONCLUSION: Photocrosslinked PEGDA implant based systems are capable of controlled drug release of both small and large drug molecules through adaptations in the polymer system network. We are currently continuing evaluation of these systems as potential sustained drug delivery devices.
  53. 53
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    Preparation and characterization of crosslinked poly(ethylene glycol) diacrylate hydrogels as fouling-resistant membrane coating materials
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  54. 54
    Griffin, M. F.; Leung, B. C.; Premakumar, Y.; Szarko, M.; Butler, P. E. Comparison of the mechanical properties of different skin sites for auricular and nasal reconstruction. J. Otolaryngol. - Head Neck Surg. 2017, 46 (1), 33,  DOI: 10.1186/s40463-017-0210-6
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    Comparison of the mechanical properties of different skin sites for auricular and nasal reconstruction
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  • Abstract

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    Scheme 1

    Scheme 1. (A) Synthesis of Conductive Hydrogels by the Photopolymerization Reaction, (B) Fabrication of Shape-Defined 3D Hydrogels by DLP of the Photopolymerizable Inks and C) Pictures of the 3D Printed Pieces
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    Figure 1

    Figure 1. (A) Evolution of the conversion during the photopolymerization process of the inks PEDOT0.00_PEGDA700 (green triangles), PEDOT0.65_PEGDA250 (black squares), PEDOT0.65_PEGDA575 (red circles), and PEDOT0.65_PEGDA700 (blue triangles). (B) Rheological measurements of the elastic modulus (G′) and loss modulus (G′′) to determine the gel point when the inks are irradiated for 5 s (in the interval from 60 to 65 s).

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    Figure 2

    Figure 2. (A) Pictures of hydrogel pieces printed by DLP (0.2 mm layer height, 5 s laser irradiation). SEM images of different areas of the previously printed pieces showing (B) reliefs (dashed red areas in part A) and (C) holes (dashed blue areas in part A). Scale bar = 1 mm. (D) Picture of a porous PEDOT0.65_PEGDA700 scaffold printed by DLP (0.2 mm layer height, 5 s laser irradiation). (E) SEM image of a representative area of the previous porous scaffold (dashed red area in part D) and (F) zoom-in to visualize the porous structure and pore dimensions.

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    Figure 3

    Figure 3. (A) Photographs of the hydrogel PEDOT0.65_PEGDA575 just printed (t = 0 h) and during its swelling in water (top). Swelling curves of the printed hydrogels: PEDOT0.00_PEGDA700 (green triangles), PEDOT0.65_PEGDA250 (black squares), PEDOT0.65_PEGDA575 (red circles), and PEDOT0.65_PEGDA700 (blue triangles) in water (bottom). (B) Photographs of the printed hydrogels and electrical conductivity of PEDOT0.00_PEGDA700 (green bars), PEDOT0.65_PEGDA250 (black bars), PEDOT0.65_PEGDA575 (red bars), and PEDOT0.65_PEGDA700 (blue bars). (C) Lab-made electrical circuit to conduct the electrical current from the battery to the light bulb, passing through the printed UPV/EHU logo with a PEDOT0.65_PEGDA700 hydrogel, as a visual proof-of-concept of the hydrogel conductivity. The dashed white circle highlights the printed logo, and the dashed yellow rectangle, the localization of the light bulb switched on. (D) Cyclic voltammograms of the 3D printed hydrogels, PEDOT0.00_PEGDA700 (green curve), PEDOT0.65_PEGDA250 (black curve), PEDOT0.65_PEGDA575 (red curve), and PEDOT0.65_PEGDA700 (blue curve), over a glassy carbon electrode (purple curve) in 0.1 M NaCl aqueous solution at 20 mV s–1.

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    Figure 4

    Figure 4. Stress–strain curves of type V probes’ printed hydrogels: PEDOT0.00_PEGDA700 (green curve), PEDOT0.65_PEGDA250 (black curve), PEDOT0.65_PEGDA575 (red curve), and PEDOT0.65_PEGDA700 (blue curve) in the (A) dry and (B) wet states. The Young’s modulus and elongation at break were obtained from the strain–stress curves of gels in the (C) dry and (D) wet states.

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    Figure 5

    Figure 5. (A) Pictures of the digital light 3D printed PEDOT0.65_PEGDA700 hydrogels, with a round shape for ECG and square shape for EMG, and schematic representation of the built-up electrodes. (B) Scheme of the ECG performed. (C) Comparison of ECG signals using a medical electrode and synthesized PEDOT0.65_PEGDA700 hydrogel. (D) ECG signals recorded with PEDOT-based hydrogel over time (t = 0, 1, and 2 weeks (w)). (E) Scheme of external muscle electrostimulation and muscle activity recording experiments. (F) EMG response of PEDOT0.65_PEGDA700 electrode after electrostimulation of a female volunteer. (G) Evolution over time (t = 0, 1, and 2 weeks (w)) of the EMG signals generated by contraction/relaxation of the quadriceps.

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  • References

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      There is increasing demand for direct in situ metabolite monitoring from cell cultures and in vivo using implantable devices. Electrochem. biosensors are commonly preferred due to their low-cost, high sensitivity, and low complexity. Metabolite detection, however, in cultured cells or sensitive tissue is rarely shown. Commonly, glucose sensing occurs indirectly by measuring the concn. of hydrogen peroxide, which is a byproduct of the conversion of glucose by glucose oxidase. However, continuous prodn. of hydrogen peroxide in cell media with high glucose is toxic to adjacent cells or tissue. This challenge is overcome through a novel, stacked enzyme configuration. A primary enzyme is used to provide analyte sensitivity, along with a secondary enzyme which converts H2O2 back to O2. The secondary enzyme is functionalized as the outermost layer of the device. Thus, prodn. of H2O2 remains local to the sensor and its concn. in the extracellular environment does not increase. This "biostack" is integrated with org. electrochem. transistors to demonstrate sensors that monitor glucose concn. in cell cultures in situ. The "biostack" renders the sensors nontoxic for cells and provides highly sensitive and stable detection of metabolites.
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      Skin-inspired electronics: an emerging paradigm
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      A review. This account covers recent important advances in skin-inspired electronics, from basic material developments to device components and proof-of-concept demonstrations for integrated bioelectronics applications. Addnl. highlighted are examples of skin-inspired electronics for three major applications: prosthetic e-skins, wearable electronics, and implantable electronics.
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      Transparent, Antifreezing, Ionic Conductive Carboxymethyl Chitosan Hydrogels as Multifunctional Sensors
      Zhao, Caimei; Gong, Xinhu; Shen, Lan; Wang, Yang; Zhang, Chaoqun
      ACS Applied Polymer Materials (2022), 4 (5), 4025-4034CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
      By a simple strategy of immersion in a CaCl2 soln., carboxymethyl chitosan hydrogels exhibited ultralow-temp. freezing resistance below -50 °C. In addn., the introduction of electrolyte ions endowed the hydrogels with elec. cond., showing stable and reversible sensitivity to human activity, such as finger bending, pressing, and pharyngeal swallowing. The conductive carboxymethyl chitosan hydrogels could even be assembled into a two-dimensional integrated array of contact sensors, which successfully perceived the contour and pressure distribution of an object with a certain resoln. These transparent biol.-based antifreezing conductive hydrogels are promising to find applications in integrated wearable sensing devices under extremely low temp. environments.
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      Ferrari, L. M.; Ismailov, U.; Badier, J.-M.; Greco, F.; Ismailova, E. Conducting polymer tattoo electrodes in clinical electro- and magneto-encephalography. npj Flex Electron. 2020, 4 (1), 4,  DOI: 10.1038/s41528-020-0067-z
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      Wu, H.; Yang, G.; Zhu, K.; Liu, S.; Guo, W.; Jiang, Z.; Li, Z. Materials, Devices, and Systems of On-Skin Electrodes for Electrophysiological Monitoring and Human-Machine Interfaces. Adv. Sci. 2021, 8 (2), 2001938,  DOI: 10.1002/advs.202001938
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      Materials, Devices, and Systems of On-Skin Electrodes for Electrophysiological Monitoring and Human-Machine Interfaces
      Wu, Hao; Yang, Ganguang; Zhu, Kanhao; Liu, Shaoyu; Guo, Wei; Jiang, Zhuo; Li, Zhuo
      Advanced Science (Weinheim, Germany) (2021), 8 (2), 2001938CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. On-skin electrodes function as an ideal platform for collecting high-quality electrophysiol. (EP) signals due to their unique characteristics, such as stretchability, conformal interfaces with skin, biocompatibility, and wearable comfort. The past decade has witnessed great advancements in performance optimization and function extension of on-skin electrodes. With continuous development and great promise for practical applications, on-skin electrodes are playing an increasingly important role in EP monitoring and human-machine interfaces (HMI). In this review, the latest progress in the development of on-skin electrodes and their integrated system is summarized. Desirable features of on-skin electrodes are briefly discussed from the perspective of performances. Then, recent advances in the development of electrode materials, followed by the anal. of strategies and methods to enhance adhesion and breathability of on-skin electrodes are examd. In addn., representative integrated electrode systems and practical applications of on-skin electrodes in healthcare monitoring and HMI are introduced in detail. It is concluded with the discussion of key challenges and opportunities for on-skin electrodes and their integrated systems.
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      Isik, M.; Lonjaret, T.; Sardon, H.; Marcilla, R.; Herve, T.; Malliaras, G. G.; Ismailova, E.; Mecerreyes, D. Cholinium-based ion gels as solid electrolytes for long-term cutaneous electrophysiology. J. Mater. Chem. C 2015, 3 (34), 8942– 8948,  DOI: 10.1039/C5TC01888A
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      Cholinium-based ion gels as solid electrolytes for long-term cutaneous electrophysiology
      Isik, Mehmet; Lonjaret, Thomas; Sardon, Haritz; Marcilla, Rebeca; Herve, Thierry; Malliaras, George G.; Ismailova, Esma; Mecerreyes, David
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2015), 3 (34), 8942-8948CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Cholinium-based bio-ion gels were prepd. by photopolymn. of poly(cholinium lactate methacrylate) network within cholinium lactate ionic liq. The rheol. and thermal properties as well as ionic cond. of ion gels of different compns. were measured. As indicated by rheol. measurements, the ion gels show the properties of gel materials which become soft by increasing the amt. of free ionic liq. Cholinium ion gels with various compn. of free ionic liq. vs. methacrylic network show glass transitions between -40° and -70 °C and thermal stability up to 200 °C. The ionic cond. of these gels increases from 10-8 to 10-3 S cm-1 at 20 °C by varying the amt. of free ionic liq. between 0 and 60 wt%, resp. Low glass transition temp. and enhanced ionic cond. make the cholinium-based ion gels good candidates to be used as a solid electrolytic interface between the skin and an electrode. The ion gels decrease the impedance with the human skin to levels that are similar to com. Ag/AgCl electrodes. Accurate physiol. signals such as electrocardiog. (ECG) were recorded with ion gels assisted electrodes for a long period of time (up to 72 h) with a remarkable stability. The low toxicity and superior ambient stability of cholinium ionic liqs. and ion gels make these materials highly attractive for long-term cutaneous electrophysiol. and other biomedical applications.
    10. 10
      Bihar, E.; Roberts, T.; Ismailova, E.; Saadaoui, M.; Isik, M.; Sanchez-Sanchez, A.; Mecerreyes, D.; Hervé, T.; De Graaf, J. B.; Malliaras, G. G. Fully Printed Electrodes on Stretchable Textiles for Long-Term Electrophysiology. Adv. Mater. Technol. 2017, 2 (4), 1600251,  DOI: 10.1002/admt.201600251
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      Mehrali, M.; Bagherifard, S.; Akbari, M.; Thakur, A.; Mirani, B.; Mehrali, M.; Hasany, M.; Orive, G.; Das, P.; Emneus, J.; Andresen, T. L.; Dolatshahi-Pirouz, A. Blending Electronics with the Human Body: A Pathway toward a Cybernetic Future. Adv. Sci. 2018, 5 (10), 1700931,  DOI: 10.1002/advs.201700931
      11
      Blending Electronics with the Human Body: A Pathway toward a Cybernetic Future
      Mehrali Mehdi; Thakur Ashish; Hasany Masoud; Andresen Thomas L; Dolatshahi-Pirouz Alireza; Bagherifard Sara; Akbari Mohsen; Mirani Bahram; Akbari Mohsen; Mirani Bahram; Akbari Mohsen; Mirani Bahram; Mehrali Mohammad; Orive Gorka; Orive Gorka; Orive Gorka; Das Paramita; Emneus Jenny
      Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2018), 5 (10), 1700931 ISSN:2198-3844.
      At the crossroads of chemistry, electronics, mechanical engineering, polymer science, biology, tissue engineering, computer science, and materials science, electrical devices are currently being engineered that blend directly within organs and tissues. These sophisticated devices are mediators, recorders, and stimulators of electricity with the capacity to monitor important electrophysiological events, replace disabled body parts, or even stimulate tissues to overcome their current limitations. They are therefore capable of leading humanity forward into the age of cyborgs, a time in which human biology can be hacked at will to yield beings with abilities beyond their natural capabilities. The resulting advances have been made possible by the emergence of conformal and soft electronic materials that can readily integrate with the curvilinear, dynamic, delicate, and flexible human body. This article discusses the recent rapid pace of development in the field of cybernetics with special emphasis on the important role that flexible and electrically active materials have played therein.
    12. 12
      Lee, W.; Kim, D.; Rivnay, J.; Matsuhisa, N.; Lonjaret, T.; Yokota, T.; Yawo, H.; Sekino, M.; Malliaras, G. G.; Someya, T. Integration of Organic Electrochemical and Field-Effect Transistors for Ultraflexible, High Temporal Resolution Electrophysiology Arrays. Adv. Mater. 2016, 28 (44), 9722– 9728,  DOI: 10.1002/adma.201602237
      12
      Integration of Organic Electrochemical and Field-Effect Transistors for Ultraflexible, High Temporal Resolution Electrophysiology Arrays
      Lee, Wonryung; Kim, Dongmin; Rivnay, Jonathan; Matsuhisa, Naoji; Lonjaret, Thomas; Yokota, Tomoyuki; Yawo, Hiromu; Sekino, Masaki; Malliaras, George G.; Someya, Takao
      Advanced Materials (Weinheim, Germany) (2016), 28 (44), 9722-9728CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This article describes about electrophysiol. array that exhibits a 4 mm2 spatial resoln. and a 3 kHz temporal resoln. by integrating the OECTs and the OFETs. The low temp. process for crosslinking the OECT channel, resulting in a redn. in the thermal degrdn. of the OFET during the fabrication process.
    13. 13
      Zhang, N.; Zhao, G.; Gao, F.; Wang, Y.; Wang, W.; Bai, L.; Chen, H.; Yang, H.; Yang, L. Wearable Flexible Sensors for Human Motion Detection with Self-Healing, Tough Guar Gum-Hydrogels of GO-P4VPBA/PDA Janus Nanosheets. ACS Appl. Polym. Mater. 2022, 4 (5), 3394– 3407,  DOI: 10.1021/acsapm.2c00028
      13
      Wearable Flexible Sensors for Human Motion Detection with Self-Healing, Tough Guar Gum-Hydrogels of GO-P4VPBA/PDA Janus Nanosheets
      Zhang, Nan; Zhao, Guangqi; Gao, Feng; Wang, Yanan; Wang, Wenxiang; Bai, Liangjiu; Chen, Hou; Yang, Huawei; Yang, Lixia
      ACS Applied Polymer Materials (2022), 4 (5), 3394-3407CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
      Flexible and wearable sensors based on nanocomposite hydrogels have been used to monitor human physiol. signals. However, it is still a challenge to develop flexible sensors using self-healing hydrogels with the properties of biocompatibility and flexibility. In this manuscript, Janus nanosheets were implanted into guar gum (GG)/poly(vinyl alc.) (PVA) 3-dimensional network structure. The obtained flexible sensor with nanocomposite hydrogels had outstanding flexibility, high sensitivity, and excellent durability. In typical oil-in-water (O/W) Pickering emulsion, GO-poly(4-vinylphenylboronic acid)/polydopamine Janus nanosheets (JNs) were surface-initiated with 4-vinylphenylboronic acid (4VPBA) on the side of GO by RAFT polymn. and self-polymeriztion of dopamine (DA) on the other side by mussel-inspired chem., resp. The JNs hydrogels had the preferable mech. strength (1.0 MPa) and self-healing efficiency (93.1%) in the presence of reversible interaction. The resistive-type hydrogels sensor with these JNs hydrogels exhibited high sensitivity (gauge factor (GF) = 12.5) and antifatigue sensing performance (100% strain, 600 cycles). The sensor could monitor different human movements, which includes both large-scale (wrist bending, elbow bending, and running) and small-scale (cough vibrations, pulse rates, and finger bending) motion precisely. These nanocomposite hydrogels will provide strategies for wearable flexible sensors with superior stability and repeatability.
    14. 14
      Donahue, M. J.; Sanchez-Sanchez, A.; Inal, S.; Qu, J.; Owens, R. M.; Mecerreyes, D.; Malliaras, G. G.; Martin, D. C. Tailoring PEDOT properties for applications in bioelectronics. Mater. Sci. Eng. R Rep. 2020, 140, 100546,  DOI: 10.1016/j.mser.2020.100546
      There is no corresponding record for this reference.
    15. 15
      Fan, X.; Nie, W.; Tsai, H.; Wang, N.; Huang, H.; Cheng, Y.; Wen, R.; Ma, L.; Yan, F.; Xia, Y. PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications. Adv. Sci. 2019, 6 (19), 1900813,  DOI: 10.1002/advs.201900813
      15
      PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications
      Fan, Xi; Nie, Wanyi; Tsai, Hsinhan; Wang, Naixiang; Huang, Huihui; Cheng, Yajun; Wen, Rongjiang; Ma, Liujia; Yan, Feng; Xia, Yonggao
      Advanced Science (Weinheim, Germany) (2019), 6 (19), 1900813CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Substantial effort has been devoted to both scientific and technol. developments of wearable, flexible, semitransparent, and sensing electronics (e.g., org./perovskite photovoltaics, org. thin-film transistors, and medical sensors) in the past decade. The key to realizing those functionalities is essentially the fabrication of conductive electrodes with desirable mech. properties. Conductive polymers (CPs) of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have emerged to be the most promising flexible electrode materials over rigid metallic oxides and play a crit. role in these unprecedented devices as transparent electrodes, hole transport layers, interconnectors, electroactive layers, or motion-sensing conductors. Here, the current status of research on PEDOT:PSS is summarized including various approaches to boosting the elec. cond. and mech. compliance and stability, directly linked to the underlying mechanism of the performance enhancements. Along with the basic principles, the most cutting edge-progresses in devices with PEDOT:PSS are highlighted. Meanwhile, the advantages and plausible problems of the CPs and as-fabricated devices are pointed out. Finally, new perspectives are given for CP modifications and device fabrications. This work stresses the importance of developing CP films and reveals their crit. role in the evolution of these next-generation devices featuring wearable, deformable, printable, ultrathin, and see-through characteristics.
    16. 16
      Minudri, D.; Mantione, D.; Dominguez-Alfaro, A.; Moya, S.; Maza, E.; Bellacanzone, C.; Antognazza, M. R.; Mecerreyes, D. Water Soluble Cationic Poly(3,4-Ethylenedioxythiophene) PEDOT-N as a Versatile Conducting Polymer for Bioelectronics. Adv. Electron. Mater. 2020, 6 (10), 2000510,  DOI: 10.1002/aelm.202000510
      16
      Water Soluble Cationic Poly(3,4-Ethylenedioxythiophene) PEDOT-N as a Versatile Conducting Polymer for Bioelectronics
      Minudri, Daniela; Mantione, Daniele; Dominguez-Alfaro, Antonio; Moya, Sergio; Maza, Eliana; Bellacanzone, Christian; Antognazza, Maria Rosa; Mecerreyes, David
      Advanced Electronic Materials (2020), 6 (10), 2000510CODEN: AEMDBW; ISSN:2199-160X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Poly(3,4-ethylenedioxythiophene) (PEDOT) is the most popular conducting polymer in the emerging field of bioelectronics. Besides its excellent properties and com. availability, its success is due to the aq. processability of its anionically stabilized solns. or dispersions. In this work, a water sol. version of PEDOT is shown, which is cationically stabilized. This work reports the chem. oxidative (co)polymn. of EDOT-ammonium deriv. leading to PEDOT-N (co)polymers. PEDOT-N shows the typical features of PEDOT such as UV absorbance, bipolaron band, elec. cond., electrochem. behavior, and film formation ability. Furthermore, the PEDOT-N films show good biocompatibility in the presence of the human embryonic kidney-293 cell line. The water soly. of PEDOT-N and its cationic nature allows its processability in the form of thin films obtained by the layer-by-layer technique or as conducting hydrogels.
    17. 17
      Lei, Z.; Wu, P. A highly transparent and ultra-stretchable conductor with stable conductivity during large deformation. Nat. Commun. 2019, 10 (1), 3429,  DOI: 10.1038/s41467-019-11364-w
      17
      A highly transparent and ultra-stretchable conductor with stable conductivity during large deformation
      Lei Zhouyue; Wu Peiyi; Lei Zhouyue; Wu Peiyi
      Nature communications (2019), 10 (1), 3429 ISSN:.
      Intrinsically stretchable conductors have undergone rapid development in the past few years and a variety of strategies have been established to improve their electro-mechanical properties. However, ranging from electronically to ionically conductive materials, they are usually vulnerable either to large deformation or at high/low temperatures, mainly due to the fact that conductive domains are generally incompatible with neighboring elastic networks. This is a problem that is usually overlooked and remains challenging to address. Here, we introduce synergistic effect between conductive zwitterionic nanochannels and dynamic hydrogen-bonding networks to break the limitations. The conductor is highly transparent (>90% transmittance), ultra-stretchable (>10,000% strain), high-modulus (>2 MPa Young's modulus), self-healing, and capable of maintaining stable conductivity during large deformation and at different temperatures. Transparent integrated systems are further demonstrated via 3D printing of its precursor and could achieve diverse sensory capabilities towards strain, temperature, humidity, etc., and even recognition of different liquids.
    18. 18
      Lu, B.; Yuk, H.; Lin, S.; Jian, N.; Qu, K.; Xu, J.; Zhao, X. Pure PEDOT:PSS hydrogels. Nat. Commun. 2019, 10 (1), 1043,  DOI: 10.1038/s41467-019-09003-5
      18
      Pure PEDOT:PSS hydrogels
      Lu Baoyang; Jian Nannan; Qu Kai; Xu Jingkun; Lu Baoyang; Yuk Hyunwoo; Lin Shaoting; Zhao Xuanhe; Xu Jingkun; Zhao Xuanhe
      Nature communications (2019), 10 (1), 1043 ISSN:.
      Hydrogels of conducting polymers, particularly poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), provide a promising electrical interface with biological tissues for sensing and stimulation, owing to their favorable electrical and mechanical properties. While existing methods mostly blend PEDOT:PSS with other compositions such as non-conductive polymers, the blending can compromise resultant hydrogels' mechanical and/or electrical properties. Here, we show that designing interconnected networks of PEDOT:PSS nanofibrils via a simple method can yield high-performance pure PEDOT:PSS hydrogels. The method involves mixing volatile additive dimethyl sulfoxide (DMSO) into aqueous PEDOT:PSS solutions followed by controlled dry-annealing and rehydration. The resultant hydrogels exhibit a set of properties highly desirable for bioelectronic applications, including high electrical conductivity (~20 S cm(-1) in PBS, ~40 S cm(-1) in deionized water), high stretchability (> 35% strain), low Young's modulus (~2 MPa), superior mechanical, electrical and electrochemical stability, and tunable isotropic/anisotropic swelling in wet physiological environments.
    19. 19
      Criado-Gonzalez, M.; Dominguez-Alfaro, A.; Lopez-Larrea, N.; Alegret, N.; Mecerreyes, D. Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities. ACS Appl. Polym. Mater. 2021, 3 (6), 2865– 2883,  DOI: 10.1021/acsapm.1c00252
      19
      Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities
      Criado-Gonzalez, Miryam; Dominguez-Alfaro, Antonio; Lopez-Larrea, Naroa; Alegret, Nuria; Mecerreyes, David
      ACS Applied Polymer Materials (2021), 3 (6), 2865-2883CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
      A review. Conducting polymers (CPs) have been attracting great attention in the development of (bio)electronic devices. Most of the current devices are rigid two-dimensional systems and possess uncontrollable geometries and architectures that lead to poor mech. properties presenting ion/electronic diffusion limitations. The goal of the article is to provide an overview about the additive manufg. (AM) of conducting polymers, which is of paramount importance for the design of future wearable three-dimensional (3D) (bio)electronic devices. Among different 3D printing AM techniques, inkjet, extrusion, electrohydrodynamic, and light-based printing have been mainly used. This review article collects examples of 3D printing of conducting polymers such as poly(3,4-ethylene-dioxythiophene), polypyrrole, and polyaniline. It also shows examples of AM of these polymers combined with other polymers and/or conducting fillers such as carbon nanotubes, graphene, and silver nanowires. Afterward, the foremost applications of CPs processed by 3D printing techniques in the biomedical and energy fields, i.e., wearable electronics, sensors, soft robotics for human motion, or health monitoring devices, among others, will be discussed.
    20. 20
      Zhao, Q.; Liu, J.; Wu, Z.; Xu, X.; Ma, H.; Hou, J.; Xu, Q.; Yang, R.; Zhang, K.; Zhang, M.; Yang, H.; Peng, W.; Liu, X.; Zhang, C.; Xu, J.; Lu, B. Robust PEDOT:PSS-based hydrogel for highly efficient interfacial solar water purification. Chem. Eng. J. 2022, 442, 136284,  DOI: 10.1016/j.cej.2022.136284
      20
      Robust PEDOT:PSS-based hydrogel for highly efficient interfacial solar water purification
      Zhao, Qi; Liu, Juyang; Wu, Zhixin; Xu, Xinye; Ma, Hude; Hou, Jian; Xu, Qiaoli; Yang, Ruping; Zhang, Kaiyue; Zhang, Mengmeng; Yang, Hanjun; Peng, Wenshan; Liu, Ximei; Zhang, Chengchen; Xu, Jingkun; Lu, Baoyang
      Chemical Engineering Journal (Amsterdam, Netherlands) (2022), 442 (Part_1), 136284CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      Solar water purifn. has been attracting considerable attention due to its promising desalination and purifn. applications to alleviate water scarcity and pollution. However, despite recent rapid progress, rational design and development of robust and highly efficient interfacial photothermal materials sufficing for large scale real usage is still a crit. challenge to the science and technol. of this field. Herein, we develop a mech. robust nanocomposite hydrogel with intriguing evapn. performance by simple blending and crosslinking of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) nanofibrils and poly(vinyl alc.) (PVA) viscous soln. As-prepd. PEDOT:PSS-PVA hydrogel exhibits an excellent light absorption ratio of ∼ 99.7% in a wide wavelength range of 250 ∼ 2500 nm and displays unprecedented high energy efficiency of ∼ 98.0% with a fast evapn. rate of ∼ 2.84 kg m-2 h-1 under one sun irradn. A 60-day continuous test simulating the real seawater evapn. environment shows superior long-term stability of such hydrogel with an av. energy efficiency of ∼ 93.3%. To harness these advantageous properties, we further demonstrate the scalable hydrogel prodn. and fabricate a solar water evaporator equipment for seawater desalination and wastewater purifn., from which the acquired water can meet the drinking requirements set by the World Health Organization.
    21. 21
      Dominguez-Alfaro, A.; Gabirondo, E.; Alegret, N.; De León-Almazán, C. M.; Hernandez, R.; Vallejo-Illarramendi, A.; Prato, M.; Mecerreyes, D. 3D Printable Conducting and Biocompatible PEDOT-graft-PLA Copolymers by Direct Ink Writing. Macromol. Rapid Commun. 2021, 42 (12), 2100100,  DOI: 10.1002/marc.202100100
      21
      3D Printable Conducting and Biocompatible PEDOT-graft-PLA Copolymers by Direct Ink Writing
      Dominguez-Alfaro, Antonio; Gabirondo, Elena; Alegret, Nuria; De Leon-Almazan, Claudia Maria; Hernandez, Robert; Vallejo-Illarramendi, Ainara; Prato, Maurizio; Mecerreyes, David
      Macromolecular Rapid Communications (2021), 42 (12), 2100100CODEN: MRCOE3; ISSN:1022-1336. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Tailor-made polymers are needed to fully exploit the possibilities of additive manufg., constructing complex, and functional devices in areas such as bioelectronics. In this paper, the synthesis of a conducting and biocompatible graft copolymer which can be 3D printed using direct melting extrusion methods is shown. For this purpose, graft copolymers composed by conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and a biocompatible polymer polylactide (PLA) are designed. The PEDOT-g-PLA copolymers are synthesized by chem. oxidative polymn. between 3,4-ethylenedioxythiophene and PLA macromonomers. PEDOT-g-PLA copolymers with different compns. are obtained and fully characterized. The rheol. characterization indicates that copolymers contg. below 20 wt% of PEDOT show the right complex viscosity values suitable for direct ink writing (DIW). The 3D printing tests using the DIW methodol. allows printing different parts with different shapes with high resoln. (200 μm). The conductive and biocompatible printed patterns of PEDOT-g-PLA show excellent cell growth and maturation of neonatal cardiac myocytes cocultured with fibroblasts.
    22. 22
      Dominguez-Alfaro, A.; Criado-Gonzalez, M.; Gabirondo, E.; Lasa-Fernández, H.; Olmedo-Martínez, J. L.; Casado, N.; Alegret, N.; Müller, A. J.; Sardon, H.; Vallejo-Illarramendi, A.; Mecerreyes, D. Electroactive 3D printable poly(3,4-ethylenedioxythiophene)-graft-poly(ε-caprolactone) copolymers as scaffolds for muscle cell alignment. Polym. Chem. 2022, 13 (1), 109– 120,  DOI: 10.1039/D1PY01185E
      22
      Electroactive 3D printable poly(3,4-ethylenedioxythiophene)-graft-poly(ε-caprolactone) copolymers as scaffolds for muscle cell alignment
      Dominguez-Alfaro, Antonio; Criado-Gonzalez, Miryam; Gabirondo, Elena; Lasa-Fernandez, Haizpea; Olmedo-Martinez, Jorge L.; Casado, Nerea; Alegret, Nuria; Muller, Alejandro J.; Sardon, Haritz; Vallejo-Illarramendi, Ainara; Mecerreyes, David
      Polymer Chemistry (2022), 13 (1), 109-120CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      The development of tailor-made polymers to build artificial three-dimensional scaffolds to repair damaged skin tissues is gaining increasing attention in the bioelectronics field. Poly(3,4-ethylene dioxythiophene) (PEDOT) is the gold std. conducting polymer for the bioelectronics field due to its high cond., thermal stability, and biocompatibility; however, it is insol. and infusible, which limits its processability into three dimensional scaffolds. Here, poly(3,4-ethylendioxythiophene)-graft-poly(ε-caprolactone) copolymers, PEDOT-g-PCL, with different mol. wts. and PEDOT compns., were synthesized by chem. oxidative polymn. to enhance the processability of PEDOT. First, the chem. structure and compn. of the copolymers were characterized by NMR, IR spectroscopy, and thermogravimetric anal. Then, the additive manufg. of PEDOT-g-PCL copolymers by direct ink writing was evaluated by rheol. and 3D printing assays. The morphol. of the printed patterns was further characterized by SEM and the cond. by the four-point probe. Finally, the employment of these printed patterns to induce muscle cells alignment was tested, proving the ability of PEDOT-g-PCL patterns to produce myotubes differentiation.
    23. 23
      Yuk, H.; Lu, B.; Zhao, X. Hydrogel bioelectronics. Chem. Soc. Rev. 2019, 48 (6), 1642– 1667,  DOI: 10.1039/C8CS00595H
      23
      Hydrogel bioelectronics
      Yuk, Hyunwoo; Lu, Baoyang; Zhao, Xuanhe
      Chemical Society Reviews (2019), 48 (6), 1642-1667CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Bioelectronic interfacing with the human body including elec. stimulation and recording of neural activities is the basis of the rapidly growing field of neural science and engineering, diagnostics, therapy, and wearable and implantable devices. Owing to intrinsic dissimilarities between soft, wet, and living biol. tissues and rigid, dry, and synthetic electronic systems, the development of more compatible, effective, and stable interfaces between these two different realms has been one of the most daunting challenges in science and technol. Recently, hydrogels have emerged as a promising material candidate for the next-generation bioelectronic interfaces, due to their similarities to biol. tissues and versatility in elec., mech., and biofunctional engineering. In this review, we discuss (i) the fundamental mechanisms of tissue-electrode interactions, (ii) hydrogels' unique advantages in bioelec. interfacing with the human body, (iii) the recent progress in hydrogel developments for bioelectronics, and (iv) rational guidelines for the design of future hydrogel bioelectronics. Advances in hydrogel bioelectronics will usher unprecedented opportunities toward ever-close integration of biol. and electronics, potentially blurring the boundary between humans and machines.
    24. 24
      Yang, Q.; Hu, Z.; Rogers, J. A. Functional Hydrogel Interface Materials for Advanced Bioelectronic Devices. Acc. Mater. Res. 2021, 2 (11), 1010– 1023,  DOI: 10.1021/accountsmr.1c00142
      24
      Functional Hydrogel Interface Materials for Advanced Bioelectronic Devices
      Yang, Quansan; Hu, Ziying; Rogers, John A.
      Accounts of Materials Research (2021), 2 (11), 1010-1023CODEN: AMRCDA; ISSN:2643-6728. (American Chemical Society)
      A review. A frontier area of modern research focuses on emerging classes of implantable bioelectronic devices with unique modes of operation that are relevant both to research studies and to medical practice. These advanced technologies have the potential to enable revolutionary diagnostic and therapeutic capabilities relevant to a wide spectrum of disorders, where seamless integration onto the surfaces of vital organs allows for accurate sensing, stimulation, or even concurrent sensing and stimulation. Materials for tissue-like interfaces, such as hydrogels, that enable soft mech. coupling and multifunctional, bidirectional exchange between these technol. platforms and living systems are critically important. Functional hydrogels offer significant promise in this context, as illustrated in recent demonstrations of interlayers that support optical, mech., elec., optical, thermal, and biochem. modes of interaction, with chronic biocompatibility and stable function in live animal models. This Account highlights recent progress in hydrogel materials that serve as interfaces between bioelectronics systems and soft tissues to facilitate implantation and to support sensing and stimulation. The content includes materials concepts, compns., chemistries, and structures that allow for bioelectronic integration. Use as interfacial adhesives and as surface coatings to support mech., elec., optical, thermal, and/or chem. coupling highlight the broad range of options. The Account begins with hydrogels that exploit advanced chemistries to control internal hemorrhage, prevent bacterial infections, and to suppress foreign body responses. Subsequent sections summarize strategies to exploit the mechanics of hydrogels, such as their mech., tunable modulus, lubricating surfaces, and interface adhesion properties, to facilitate interactions between bioelectronic and biol. systems. Discussions of functional characteristics begin with the elec. cond. of different types of conductive hydrogels and their long-time stability, with applications in bioelectronic sensing and stimulation. Following sections focus on optical, thermal, and chem. properties, also in the context of device operation. A final passage on chem. outlines recently developed photocurable and bioresorbable hydrogel adhesives that support multifunctional interfaces to soft biol. tissues. The concluding paragraphs highlight remaining challenges and opportunities for research in hydrogel materials science for advanced bioelectronic devices.
    25. 25
      Yuk, H.; Lu, B.; Lin, S.; Qu, K.; Xu, J.; Luo, J.; Zhao, X. 3D printing of conducting polymers. Nat. Commun. 2020, 11 (1), 1604,  DOI: 10.1038/s41467-020-15316-7
      25
      3D printing of conducting polymers
      Yuk, Hyunwoo; Lu, Baoyang; Lin, Shen; Qu, Kai; Xu, Jingkun; Luo, Jianhong; Zhao, Xuanhe
      Nature Communications (2020), 11 (1), 1604CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Conducting polymers are promising material candidates in diverse applications including energy storage, flexible electronics, and bioelectronics. However, the fabrication of conducting polymers has mostly relied on conventional approaches such as ink-jet printing, screen printing, and electron-beam lithog., whose limitations have hampered rapid innovations and broad applications of conducting polymers. Here we introduce a high-performance 3D printable conducting polymer ink based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) for 3D printing of conducting polymers. The resultant superior printability enables facile fabrication of conducting polymers into high resoln. and high aspect ratio microstructures, which can be integrated with other materials such as insulating elastomers via multi-material 3D printing. The 3D-printed conducting polymers can also be converted into highly conductive and soft hydrogel microstructures. We further demonstrate fast and streamlined fabrications of various conducting polymer devices, such as a soft neural probe capable of in vivo single-unit recording.
    26. 26
      Spencer, A. R.; Shirzaei Sani, E.; Soucy, J. R.; Corbet, C. C.; Primbetova, A.; Koppes, R. A.; Annabi, N. Bioprinting of a Cell-Laden Conductive Hydrogel Composite. ACS Appl. Mater. Interfaces 2019, 11 (34), 30518– 30533,  DOI: 10.1021/acsami.9b07353
      26
      Bioprinting of a Cell-Laden Conductive Hydrogel Composite
      Spencer, Andrew R.; Shirzaei Sani, Ehsan; Soucy, Jonathan R.; Corbet, Carolyn C.; Primbetova, Asel; Koppes, Ryan A.; Annabi, Nasim
      ACS Applied Materials & Interfaces (2019), 11 (34), 30518-30533CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Bioprinting has gained significant attention for creating biomimetic tissue constructs with potential to be used in biomedical applications such as drug screening or regenerative medicine. Ideally, biomaterials used for three-dimensional (3D) bioprinting should match the mech., hydrostatic, bioelec., and physicochem. properties of the native tissues. However, many materials with these tissue-like properties are not compatible with printing techniques without modifying their compns. In addn., integration of cell-laden biomaterials with bioprinting methodologies that preserve their physicochem. properties remains a challenge. In this work, a biocompatible conductive hydrogel composed of gelatin methacryloyl (GelMA) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was synthesized and bioprinted to form complex, 3D cell-laden structures. The biofabricated conductive hydrogels were formed by an initial crosslinking step of the PEDOT:PSS with bivalent calcium ions and a secondary photopolymn. step with visible light to cross-link the GelMA component. These modifications enabled tuning the mech. properties of the hydrogels, with Young's moduli ranging from ∼40-150 kPa, as well as tunable cond. by varying the concn. of PEDOT:PSS. In addn., the hydrogels degraded in vivo with no substantial inflammatory responses as demonstrated by haematoxylin and eosin (H&E) and immunofluorescent staining of s.c. implanted samples in Wistar rats. The parameters for forming a slurry of microgel particles to support 3D bioprinting of the engineered cell-laden hydrogel were optimized to form constructs with improved resoln. High cytocompatibility and cell spreading were demonstrated in both wet-spinning and 3D bioprinting of cell-laden hydrogels with the new conductive hydrogel-based bioink and printing methodol. The synergy of an advanced fabrication method and conductive hydrogel presented here is promising for engineering complex conductive and cell-laden structures.
    27. 27
      Shen, Z.; Zhang, Z.; Zhang, N.; Li, J.; Zhou, P.; Hu, F.; Rong, Y.; Lu, B.; Gu, G. High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines. Adv. Mater. 2022, 2203650,  DOI: 10.1002/adma.202203650
      27
      High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines
      Shen, Zequn; Zhang, Zhilin; Zhang, Ningbin; Li, Jinhao; Zhou, Peiwei; Hu, Faqi; Rong, Yu; Lu, Baoyang; Gu, Guoying
      Advanced Materials (Weinheim, Germany) (2022), 34 (32), 2203650CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Highly stretchable strain sensors based on conducting polymer hydrogel are rapidly emerging as a promising candidate toward diverse wearable skins and sensing devices for soft machines. However, due to the intrinsic limitations of low stretchability and large hysteresis, existing strain sensors cannot fully exploit their potential when used in wearable or robotic systems. Here, a conducting polymer hydrogel strain sensor exhibiting both ultimate strain (300%) and negligible hysteresis (<1.5%) is presented. This is achieved through a unique microphase semisepd. network design by compositing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) nanofibers with poly(vinyl alc.) (PVA) and facile fabrication by combining 3D printing and successive freeze-thawing. The overall superior performances of the strain sensor including stretchability, linearity, cyclic stability, and robustness against mech. twisting and pressing are systematically characterized. The integration and application of such strain sensor with electronic skins are further demonstrated to measure various physiol. signals, identify hand gestures, enable a soft gripper for objection recognition, and remote control of an industrial robot. This work may offer both promising conducting polymer hydrogels with enhanced sensing functionalities and tech. platforms toward stretchable electronic skins and intelligent robotic systems.
    28. 28
      Mangoma, T. N.; Yamamoto, S.; Malliaras, G. G.; Daly, R. Hybrid 3D/Inkjet-Printed Organic Neuromorphic Transistors. Adv. Mater. Technol. 2022, 7, 2000798,  DOI: 10.1002/admt.202000798
      28
      Hybrid three-dimensional/inkjet-printed organic neuromorphic transistors
      Mangoma, Tanyaradzwa N.; Yamamoto, Shunsuke; Malliaras, George G.; Daly, Ronan
      Advanced Materials Technologies (Weinheim, Germany) (2022), 7 (2), 2000798CODEN: AMTDCM; ISSN:2365-709X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Org. electrochem. transistors (OECTs) are proving essential in bioelectronics and printed electronics applications, with their simple structure, ease of tunability, biocompatibility, and suitability for different routes to fabrication. OECTs are also being explored as neuromorphic devices, where they emulate characteristics of biol. neural networks through co-location of information storage and processing on the same unit, overcoming the von Neumann performance bottleneck. To achieve the long-term vision of translating to inexpensive, low-power computational devices, fabrication needs to be feasible with adaptable, scalable digital techniques. Here, a hybrid direct-write additive manufg. approach to fabricating OECTs is shown. 3D printing of com. available printing filament is combined to deliver conducting and insulating layers, with inkjet printing of semiconducting thin films to create OECTs. These printed OECTs show depletion mode operation paired-pulse depression behavior and evidence of adaptation to support their translation to neuromorphic devices. These results show that a hybrid of accessible and design-flexible AM techniques can be used to rapidly fabricate devices that exhibit good OECT and neuromorphic performances.
    29. 29
      Bihar, E.; Roberts, T.; Zhang, Y.; Ismailova, E.; Hervé, T.; Malliaras, G. G.; De Graaf, J. B.; Inal, S.; Saadaoui, M. Fully printed all-polymer tattoo/textile electronics for electromyography. Flex. Print. Electron. 2018, 3 (3), 034004,  DOI: 10.1088/2058-8585/aadb56
      There is no corresponding record for this reference.
    30. 30
      Bihar, E.; Roberts, T.; Saadaoui, M.; Hervé, T.; De Graaf, J. B.; Malliaras, G. G. Inkjet-Printed PEDOT:PSS Electrodes on Paper for Electrocardiography. Adv. Healthc. Mater. 2017, 6 (6), 1601167,  DOI: 10.1002/adhm.201601167
      There is no corresponding record for this reference.
    31. 31
      Zhang, J.; Hu, Q.; Wang, S.; Tao, J.; Gou, M. Digital Light Processing Based Three-dimensional Printing for Medical Applications. Int. J. Bioprinting 2020, 6 (1), 620,  DOI: 10.18063/ijb.v6i1.242
      There is no corresponding record for this reference.
    32. 32
      Shahzadi, L.; Maya, F.; Breadmore, M. C.; Thickett, S. C. Functional Materials for DLP-SLA 3D Printing Using Thiol-Acrylate Chemistry: Resin Design and Postprint Applications. ACS Appl. Polym. Mater. 2022, 4 (5), 3896– 3907,  DOI: 10.1021/acsapm.2c00358
      32
      Functional Materials for DLP-SLA 3D Printing Using Thiol-Acrylate Chemistry: Resin Design and Postprint Applications
      Shahzadi, Lubna; Maya, Fernando; Breadmore, Michael C.; Thickett, Stuart C.
      ACS Applied Polymer Materials (2022), 4 (5), 3896-3907CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
      Three-dimensional (3D) printing techniques have greatly simplified prototype manufg. and complex design. However, most com. available stereolithog. (SLA) material components are based on (meth)acrylate-based resin systems that have several disadvantages assocd. with their use, such as inhibition of polymn. by oxygen, solvent resistance, and the inability to modify surfaces post printing. Polymn. via a thiol-acrylate mechanism can help overcome many of these drawbacks; however, these systems are less studied in the context of SLA 3D printing. In this work, we report on the design and optimization of thiol-acrylate resin formulations with a view toward effectively controlling the polymn. depth of the cured polymer layer. Four different photoblockers were studied and the use of 1,3-bis(4-methoxyphenyl)propane-1,3-dione enabled optically transparent and colorless printed objects with good resoln. to be realized. Fully enclosed microchannels with diams. as low as 250 μm were successfully printed using this approach. Taking advantage of ready postprinting surface modification of thiol-acrylate polymers, various hydrophilic, hydrophobic, and fluorescent polymer chains were successfully grafted to the object surface via reversible addn.-fragmentation chain transfer (RAFT) polymn. Free thiol groups at the surface of off-stoichiometric resin formulations were also used to immobilize gold nanoparticles for the catalytic conversion of 4-nitrophenol to 4-aminophenol. The tunability of these thiol-acrylate resins for SLA 3D printing and feasible postprint surface modifications make them attractive candidates for com. applications.
    33. 33
      Melchels, F. P. W.; Feijen, J.; Grijpma, D. W. A review on stereolithography and its applications in biomedical engineering. Biomaterials 2010, 31 (24), 6121– 6130,  DOI: 10.1016/j.biomaterials.2010.04.050
      33
      A review on stereolithography and its applications in biomedical engineering
      Melchels, Ferry P. W.; Feijen, Jan; Grijpma, Dirk W.
      Biomaterials (2010), 31 (24), 6121-6130CODEN: BIMADU; ISSN:0142-9612. (Elsevier Ltd.)
      A review. Stereolithog. is a solid free form technique (SFF) that was introduced in the late 1980s. Although many other techniques have been developed since then, stereolithog. remains one of the most powerful and versatile of all SFF techniques. It has the highest fabrication accuracy and an increasing no. of materials that can be processed is becoming available. In this paper we discuss the characteristic features of the stereolithog. technique and compare it to other SFF techniques. The biomedical applications of stereolithog. are reviewed, as well as the biodegradable resin materials that have been developed for use with stereolithog. Finally, an overview of the application of stereolithog. in prepg. porous structures for tissue engineering is given.
    34. 34
      Nestler, N.; Wesemann, C.; Spies, B. C.; Beuer, F.; Bumann, A. Dimensional accuracy of extrusion- and photopolymerization-based 3D printers: In vitro study comparing printed casts. J. Prosthet. Dent. 2021, 125 (1), 103– 110,  DOI: 10.1016/j.prosdent.2019.11.011
      34
      Dimensional accuracy of extrusion- and photopolymerization-based 3D printers: In vitro study comparing printed casts
      Nestler, Norbert; Wesemann, Christian; Spies, Benedikt C.; Beuer, Florian; Bumann, Axel
      Journal of Prosthetic Dentistry (2021), 125 (1), 103-110CODEN: JPDEAT; ISSN:0022-3913. (Elsevier Inc.)
      Reliable studies comparing the accuracy of complete-arch casts from 3D printers are scarce. The purpose of this in vitro study was to investigate the accuracy of casts printed by using various extrusion- and photopolymn.-based printers. A master file was sent to 5 printer manufacturers and distributors to print 37 identical casts. This file consisted of a standardized data set of a maxillary cast in std. tessellation language (STL) format comprising 5 ref. points for the measurement of 3 distances that served as ref. for all measurements: intermolar width (IMW), intercanine width (ICW), and dental arch length (AL). The digital measurement of the master file obtained by using a surveying software program (Convince Premium 2012) was used as the control. Two extrusion-based (M2 and Ultimaker 2+) and 3 photopolymn.-based printers (Form 2, Asiga MAX UV, and myrev140) were compared. The casts were measured by using a multisensory coordinate measuring machine (O-Inspect 422). The values were then compared with those of the master file. The Mann-Whitney U test and Levene tests were used to det. significant differences in the trueness and precision (accuracy) of the measured distances. The deviations from the master file at all 3 distances for the included printers ranged between 12 μm and 240 μm (trueness), with an interquartile range (IQR) between 17 μm and 388 μm (precision). Asiga MAX UV displayed the highest accuracy, considering all the distances, and Ultimaker 2+ demonstrated comparable accuracy for shorter distances (IMW and ICW). Although myrev140 operated with high precision, it displayed high deviations from the master file. Similarly, although Form 2 exhibited high IQR, it did not deviate significantly from the master file in the longest range (AL). M2 performed consistently. Both extrusion-based and photopolymn.-based printers were accurate. In general, inexpensive printers were no less accurate than more expensive ones.
    35. 35
      Bagheri, A.; Jin, J. Photopolymerization in 3D Printing. ACS Appl. Polym. Mater. 2019, 1 (4), 593– 611,  DOI: 10.1021/acsapm.8b00165
      35
      Photopolymerization in 3D Printing
      Bagheri, Ali; Jin, Jianyong
      ACS Applied Polymer Materials (2019), 1 (4), 593-611CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
      A review. The field of 3D printing is continuing its rapid development in both academic and industrial research environments. The development of 3D printing technologies has opened new implementations in rapid prototyping, tooling, dentistry, microfluidics, biomedical devices, tissue engineering, drug delivery, etc. Among different 3D printing techniques, photopolymn.-based process (such as stereolithog. and digital light processing) offers flexibility over the final properties of the 3D printed materials (such as optical, chem., and mech. properties) using versatile polymer chem. The strategy behind the 3D photopolymn. is based on using monomers/oligomers in liq. state (in the presence of photoinitiators) that can be photopolymd. (via radical or cationic mechanism) upon exposure to light source of different wavelengths (depending on the photoinitiator system). An overview of recent evolutions in the field of photopolymn.-based 3D printing and highlights of novel 3D printable photopolymers is provided herein. Challenges that limit the use of conventional photopolymers (i.e., initiation under UV light) together with prospective solns. such as incorporation of photosensitive initiators with red-shifted absorptions are also discussed in detail. This review also spotlights recent progress on the use of controlled living radical photopolymn. techniques (i.e., reversible addn.-fragmentation chain-transfer polymn.) in 3D printing, which will pave the way for widespread growth of new generations of 3D materials with living features and possibility for postprinting modifications.
    36. 36
      Heo, D. N.; Lee, S.-J.; Timsina, R.; Qiu, X.; Castro, N. J.; Zhang, L. G. Development of 3D printable conductive hydrogel with crystallized PEDOT:PSS for neural tissue engineering. Mater. Sci. Eng., C 2019, 99, 582– 590,  DOI: 10.1016/j.msec.2019.02.008
      36
      Development of 3D printable conductive hydrogel with crystallized PEDOT:PSS for neural tissue engineering
      Heo, Dong Nyoung; Lee, Se-Jun; Timsina, Raju; Qiu, Xiangyun; Castro, Nathan J.; Zhang, Lijie Grace
      Materials Science & Engineering, C: Materials for Biological Applications (2019), 99 (), 582-590CODEN: MSCEEE; ISSN:0928-4931. (Elsevier B.V.)
      Bioelectronic devices enable efficient and effective communication between medical devices and human tissue in order to directly treat patients with various neurol. disorders. Due to the mech. similarity to human tissue, hydrogel-based electronic devices are considered to be promising for biol. signal recording and stimulation of living tissues. Here, we report the first three-dimensionally (3D) printable conductive hydrogel that can be photocrosslinked while retaining high elec. cond. In addn., we prepd. dorsal root ganglion (DRG) cell-encapsulated gelatin methacryloyl (GelMA) hydrogels which were integrated with the 3D printed conductive structure and evaluated for efficiency neural differentiation under elec. stimulation (ES). For enhanced elec. cond., a poly(3,4-ethylenedioxythiophene) (PEDOT): polystyrene sulfonate (PSS) aq. soln. was freeze-dried and mixed with polyethylene glycol diacrylate (PEGDA) as the photocurable polymer base. Next, the conductive hydrogel was patterned on the substrate by using a table-top stereolithog. (SLA) 3D printer. The fabricated hydrogel was characterized for electrochem. cond. After printing with the PEDOT:PSS conductive soln., the patterned hydrogel exhibited decreased printing diams. with increasing of PEDOT:PSS concn. Also, the resultant conductive hydrogel had significantly increased electrochem. properties with increasing PEDOT:PSS concn. The 3D printed conductive hydrogel provides excellent structural support to systematically transfer the ES toward encapsulated DRG cells for enhanced neuronal differentiation. The results from this study indicate that the conductive hydrogel can be useful as a 3D printing material for elec. applications.
    37. 37
      Bertana, V.; Scordo, G.; Parmeggiani, M.; Scaltrito, L.; Ferrero, S.; Gomez, M. G.; Cocuzza, M.; Vurro, D.; D’Angelo, P.; Iannotta, S.; Pirri, C. F.; Marasso, S. L. Rapid prototyping of 3D Organic Electrochemical Transistors by composite photocurable resin. Sci. Rep. 2020, 10 (1), 13335,  DOI: 10.1038/s41598-020-70365-8
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      Rapid prototyping of 3D Organic Electrochemical Transistors by composite photocurable resin
      Bertana, Valentina; Scordo, Giorgio; Parmeggiani, Matteo; Scaltrito, Luciano; Ferrero, Sergio; Gomez, Manuel Gomez; Cocuzza, Matteo; Vurro, Davide; D'Angelo, Pasquale; Iannotta, Salvatore; Pirri, Candido F.; Marasso, Simone L.
      Scientific Reports (2020), 10 (1), 13335CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
      Rapid Prototyping (RP) promises to induce a revolutionary impact on how the objects can be produced and used in industrial manufg. as well as in everyday life. Over the time a std. technique as the 3D Stereolithog. (SL) has become a fundamental technol. for RP and Additive Manufg. (AM), since it enables the fabrication of the 3D objects from a cost-effective photocurable resin. Efforts to obtain devices more complex than just a mere aesthetic simulacre, have been spent with uncertain results. The multidisciplinary nature of such manufg. technique furtherly hinders the route to the fabrication of complex devices. A good knowledge of the bases of material science and engineering is required to deal with SL technol., characterization and testing aspects. In this framework, our study aims to reveal a new approach to obtain RP of complex devices, namely Org. Electro-Chem. Transistors (OECTs), by SL technique exploiting a resin composite based on the conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and the photo curable Poly(ethylene glycol) diacrylate (PEGDA). A comprehensive study is presented, starting from the optimization of composite resin and characterization of its electrochem. properties, up to the 3D OECTs printing and testing. Relevant performances in biosensing for dopamine (DA) detection using the 3D OECTs are reported and discussed too.
    38. 38
      Scordo, G.; Bertana, V.; Scaltrito, L.; Ferrero, S.; Cocuzza, M.; Marasso, S. L.; Romano, S.; Sesana, R.; Catania, F.; Pirri, C. F. A novel highly electrically conductive composite resin for stereolithography. Mater. Today Commun. 2019, 19, 12– 17,  DOI: 10.1016/j.mtcomm.2018.12.017
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      A novel highly electrically conductive composite resin for stereolithography
      Scordo, Giorgio; Bertana, Valentina; Scaltrito, Luciano; Ferrero, Sergio; Cocuzza, Matteo; Marasso, Simone L.; Romano, Stefano; Sesana, Raffaella; Catania, Felice; Pirri, Candido F.
      Materials Today Communications (2019), 19 (), 12-17CODEN: MTCAC7; ISSN:2352-4928. (Elsevier Ltd.)
      The use of 3D printing for scientific and industrial applications is rapidly increasing and, simultaneously, is growing the interest toward printable smart materials. It is known that using a conductive filler, e.g. nanoparticles, metal powders or composite polymers, it is possible to produce a more elec. conductive printable material. The PEDOT:PSS polymer has been used extensively, from bioelectronics to energy storage devices, for its cond. and biocompatibility. In this work, a highly elec. conductive resin for Stereolithog. was developed using a dispersion of PEDOT particles, extd. from Clevios PH1000 through a simple sepn. process, in a PEGDA matrix. The resin compn. was optimized in terms of photoinitiator, surfactant and filler concn. Furthermore, optimal printing parameters were detd. for this composite resin, obtaining a printed object with appreciable elec. cond. (0.05 S cm-1) and mech. properties (Young modulus 21 MPa).
    39. 39
      Bertana, V.; Scordo, G.; Manachino, M.; Romano, S.; Gomez, M. G.; Marasso, S. L.; Ferrero, S.; Cocuzza, M.; Pirri, C. F.; Scaltrito, L. 3D Printed Active Objects based on the Promising PEDOT: PSS Resin: Investigation of their Integration inside an Electronic Circuit. Int. J. Eng. Res. Technol. 2020, 13, 462,  DOI: 10.37624/IJERT/13.3.2020.462-469
      There is no corresponding record for this reference.
    40. 40
      Ye, X.; Wang, C.; Wang, L.; Lu, B.; Gao, F.; Shao, D. DLP printing of a flexible micropattern Si/PEDOT:PSS/PEG electrode for lithium-ion batteries. Chem. Commun. 2022, 55, 7642– 7645,  DOI: 10.1039/D2CC01626E
      There is no corresponding record for this reference.
    41. 41
      Ahn, D.; Stevens, L. M.; Zhou, K.; Page, Z. A. Rapid High-Resolution Visible Light 3D Printing. ACS Cent. Sci. 2020, 6 (9), 1555– 1563,  DOI: 10.1021/acscentsci.0c00929
      41
      Rapid High-Resolution Visible Light 3D Printing
      Ahn, Dowon; Stevens, Lynn M.; Zhou, Kevin; Page, Zachariah A.
      ACS Central Science (2020), 6 (9), 1555-1563CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      Light-driven 3D printing to convert liq. resins into solid objects (i.e., photocuring) has traditionally been dominated by engineering disciplines, yielding the fastest build speeds and highest resoln. of any additive manufg. process. However, the reliance on high-energy UV/violet light limits the materials scope due to degrdn. and attenuation (e.g., absorption and/or scattering). Chem. innovation to shift the spectrum into more mild and tunable visible wavelengths promises to improve compatibility and expand the repertoire of accessible objects, including those contg. biol. compds., nanocomposites, and multimaterial structures. Photochem. at these longer wavelengths currently suffers from slow reaction times precluding its utility. Herein, novel panchromatic photopolymer resins were developed and applied for the first time to realize rapid high-resoln. visible light 3D printing. The combination of electron-deficient and electron-rich coinitiators was crit. to overcoming the speed-limited photocuring with visible light. Furthermore, azo-dyes were identified as vital resin components to confine curing to irradn. zones, improving spatial resoln. A unique screening method was used to streamline optimization (e.g., exposure time and azo-dye loading) and correlate resin compn. to resoln., cure rate, and mech. performance. Ultimately, a versatile and general visible-light-based printing method was shown to afford (1) stiff and soft objects with feature sizes <100μm, (2) build speeds up to 45 mm/h, and (3) mech. isotropy, rivaling modern UV-based 3D printing technol. and providing a foundation from which bio- and composite-printing can emerge. Liq. resins that rapidly harden upon exposure to low-energy visible light are developed and applied to high-resoln. projection-based 3D printing across the color spectrum.
    42. 42
      Gueye, M. N.; Carella, A.; Faure-Vincent, J.; Demadrille, R.; Simonato, J.-P. Progress in understanding structure and transport properties of PEDOT-based materials: A critical review. Prog. Mater. Sci. 2020, 108, 100616,  DOI: 10.1016/j.pmatsci.2019.100616
      42
      Progress in understanding structure and transport properties of PEDOT-based materials: A critical review
      Gueye, Magatte N.; Carella, Alexandre; Faure-Vincent, Jerome; Demadrille, Renaud; Simonato, Jean-Pierre
      Progress in Materials Science (2020), 108 (), 100616CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
      Since the late '80s, a highly stable conductive polymer has been developed, that is poly(3,4-ethylene dioxythiophene), also known as PEDOT. Its increasing cond. throughout the years combined with its intrinsic stability have aroused great attention both in the academic and industrial fields. The growing importance of PEDOT, can be easily acknowledged through the numerous applications in thermoelectricity, photovoltaics, lighting, sensing, tech. coatings, transparent electrodes, bioelectronics, and so forth. Although its high elec. cond. is strongly established in the literature, the wide range of data shows that disorder, as the limiting factor in charges' transport, hinders the design of materials with optimal performances. The aim of this article is to review and discuss recent progresses dealing with the elec. cond. and transport properties in PEDOT materials, with special attention on morphol. and structural features. Particular emphasis is given to the com. PEDOT:PSS as well as other PEDOT-based materials stabilized with smaller counter-anions. It appears that the elec. cond. and the transport mechanisms are closely related to the fabrication process, the crystallinity of the material and the choice of the counter-anions. With the tunable elec. properties, new functionalities appear accessible and add up to the already existing applications that are concisely highlighted.
    43. 43
      Donoval, M.; Micjan, M.; Novota, M.; Nevrela, J.; Kovacova, S.; Pavuk, M.; Juhasz, P.; Jagelka, M.; Kovac, J.; Jakabovic, J.; Cigan, M.; Weis, M. Relation between secondary doping and phase separation in PEDOT:PSS films. Appl. Surf. Sci. 2017, 395, 86– 91,  DOI: 10.1016/j.apsusc.2016.05.076
      43
      Relation between secondary doping and phase separation in PEDOT:PSS films
      Donoval, Martin; Micjan, Michal; Novota, Miroslav; Nevrela, Juraj; Kovacova, Sona; Pavuk, Milan; Juhasz, Peter; Jagelka, Martin; Kovac, Jaroslav, Jr.; Jakabovic, Jan; Cigan, Marek; Weis, Martin
      Applied Surface Science (2017), 395 (), 86-91CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Conductive copolymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) has been proposed as an alternative to transparent conductive oxides because of its flexibility, transparency, and low-cost prodn. Four different secondary dopants, namely N,N-dimethylformamide, ethyleneglycol, sorbitol, and DMSO, have been used to improve the cond. The relation between the structure changes and cond. enhancement is studied in detail. Atomic force microscopy study of the thin film surface reveals the phase sepn. of PEDOT and PSS. We demonstrate that secondary doping induces the phase sepn. as well as the cond. enhancement.
    44. 44
      Wei, T.-C.; Chen, S.-H.; Chen, C.-Y. Highly conductive PEDOT:PSS film made with ethylene-glycol addition and heated-stir treatment for enhanced photovoltaic performances. Mater. Chem. Front. 2020, 4 (11), 3302– 3309,  DOI: 10.1039/D0QM00529K
      44
      Highly conductive PEDOT:PSS film made with ethylene-glycol addition and heated-stir treatment for enhanced photovoltaic performances
      Wei, Ta-Cheng; Chen, Shih-Hsiu; Chen, Chia-Yun
      Materials Chemistry Frontiers (2020), 4 (11), 3302-3309CODEN: MCFAC5; ISSN:2052-1537. (Royal Society of Chemistry)
      Highly conductive and stable PEDOT:PSS films were prepd. by ethylene-glycol (EG) addn. and heated-stir treatment that enhanced the elec. cond. up to 1228.4 S cm-1. Anal. of the long-term figure-of-merit (FoM) performances of EG-treated films with respect to variations in stirring temps. indicated that the highest value of 78.5 was realized by the facile heated-stir treatment at 90°C. The underlying mechanism was validated by the synergetic effect of reordering linear PEDOT chains and the effective removal of water via the supplement of thermal energy that considerably displayed the improved film cond. based on the formation of interconnected PEDOT structures. Furthermore, via combining treated PEDOT:PSS films and Si nanowires, high-performance hybrid solar cells were realized. This was accomplished by reduced charge-transfer resistance and sound heterojunction characteristics that facilitated the charge sepn. and eventual charge collection by the electrode, thus contributing to the enhancement efficiency. The optimal heated-stir treatment at 90°C led to improved cell conversion efficiency achieving 12.2%, which was 1.3 times higher than the result based on the conventional room-temp. process.
    45. 45
      Liu, J.; Zhu, Z.; Zhou, W.; Liu, P.; Liu, P.; Liu, G.; Xu, J.; Jiang, Q.; Jiang, F. Flexible metal-free hybrid hydrogel thermoelectric fibers. J. Mater. Sci. 2020, 55 (19), 8376– 8387,  DOI: 10.1007/s10853-020-04382-3
      45
      Flexible metal-free hybrid hydrogel thermoelectric fibers
      Liu, Jing; Zhu, Zhengyou; Zhou, Weiqiang; Liu, Peipei; Liu, Peng; Liu, Guoqiang; Xu, Jingkun; Jiang, Qinglin; Jiang, Fengxing
      Journal of Materials Science (2020), 55 (19), 8376-8387CODEN: JMTSAS; ISSN:0022-2461. (Springer)
      Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) fiber has been developed as a more promising candidate compared with bulk and film to achieve wearable thermoelec. energy harvesting. Single-walled carbon nanotubes (SWCNTs) with nanostructures are considered as an effective conductive filter for the further improvement in the thermoelec. (TE) performance of PEDOT:PSS fibers. However, the previous research primarily focused on PEDOT:PSS/SWCNT films instead of fibers. In this study, PEDOT:PSS/SWCNT hybrid fibers were synthesized via gelation process, which presents a 30% enhancement of the elec. cond. with negligible changes in Seebeck coeff. Moreover, there was a significant increase in the Young's modulus in accordance with the addn. of an appropriate amt. of SWCNTs. Thereafter, the as-prepd. hybrid fibers were treated using ethylene glycol (EG) to further optimize the TE performance. Moreover, the effect of the treatment time and temp. was systematically investigated. The EG treatment resulted in a significant improvement in the elec. cond. without a significant decrease in the Seebeck coeff. Furthermore, the hybrid fibers were subject to EG treatment at elevated temp., whose optimal power factor was approx. 30% higher than that of the EG-treated PEDOT:PSS/SWCNT fibers at 25°. This indicates that the solvent treatment at higher temp. improves the TE performance of hybrid fibers. The findings of this study can serve as a guide for the prepn. of flexible and metal-free hybrid fiber with enhanced TE performance and Young's modulus.
    46. 46
      Selvam, C.; Lal, D. M.; Harish, S. Thermal conductivity enhancement of ethylene glycol and water with graphene nanoplatelets. Thermochim. Acta 2016, 642, 32– 38,  DOI: 10.1016/j.tca.2016.09.002
      46
      Thermal conductivity enhancement of ethylene glycol and water with graphene nanoplatelets
      Selvam, C.; Lal, D. Mohan; Harish, Sivasankaran
      Thermochimica Acta (2016), 642 (), 32-38CODEN: THACAS; ISSN:0040-6031. (Elsevier B.V.)
      In the present work, we report the effective thermal cond. of ethylene glycol and water with graphene nanoplatelets. Sodium deoxycholate, a bile salt was used as the surfactant to prep. stable nanofluid dispersions. Stability tests were performed using UV-vis absorption spectrometry and zeta potential to monitor the stability of the prepd. nanofluids as a function of time. Thermal cond. measurements were carried out using transient hot wire technique. Thermal cond. of the nanofluids significantly increases with respect to graphene loading. Maximum thermal cond. enhancements of ∼21% and ∼16% at a loading of 0.5 vol% was obtained for the nanofluids with graphene nanoplatelets seeded in ethylene glycol and water resp. Anal. of exptl. results with Maxwell-Garnett type effective medium theory reveal that despite the high thermal cond. of graphene, interfacial thermal resistance between graphene and the surrounding base fluid limits the thermal cond. enhancement significantly. The interfacial thermal resistance between graphene sheet and ethylene glycol was found to be 2.2 × 10-8 m2 KW-1 while between graphene sheet and water was found to be 1.5 × 10-8 m2 KW-1 resp. for the present nanofluids.
    47. 47
      Zhou, J.; Lin, S.; Zeng, H.; Liu, J.; Li, B.; Xu, Y.; Zhao, X.; Chen, G. Dynamic intermolecular interactions through hydrogen bonding of water promote heat conduction in hydrogels. Mater. Horiz. 2020, 7 (11), 2936– 2943,  DOI: 10.1039/D0MH00735H
      47
      Dynamic intermolecular interactions through hydrogen bonding of water promote heat conduction in hydrogels
      Zhou, Jiawei; Lin, Shaoting; Zeng, Hongxia; Liu, Ji; Li, Buxuan; Xu, Yanfei; Zhao, Xuanhe; Chen, Gang
      Materials Horizons (2020), 7 (11), 2936-2943CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
      Heat-conducting polymers provide a new opportunity to tackle thermal management challenges in advanced technologies such as wearable electronics and soft robotics. One strategy to enhance heat conduction in amorphous polymers has been tuning their intermol. interactions. These intermol. forces are often static in nature as the participating mols. are anchored on the polymer chains. In this work, using hydrogel as a model system, we demonstrate how dynamic intermol. forces, which break and re-form constantly, can also enhance thermal transport. Utilizing calorimetric and spectroscopic measurements, we show that this arises from the hydrogen bonds formed between water and nearby polymer chains, which enhances the inter-chain heat transfer efficiency. This mechanism may potentially allow the design of heat-conducting polymers with self-healing or adaptability functionalities.
    48. 48
      Xu, S.; Cai, S.; Liu, Z. Thermal Conductivity of Polyacrylamide Hydrogels at the Nanoscale. ACS Appl. Mater. Interfaces 2018, 10 (42), 36352– 36360,  DOI: 10.1021/acsami.8b09891
      48
      Thermal Conductivity of Polyacrylamide Hydrogels at the Nanoscale
      Xu, Shuai; Cai, Shengqiang; Liu, Zishun
      ACS Applied Materials & Interfaces (2018), 10 (42), 36352-36360CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      A polymer network can imbibe copious amts. of water and swell, and the resulting state is known as a hydrogel. In many potential applications of hydrogels, such as stretchable conductors, ionic cables, and neuroprostheses, the thermal conductivities of hydrogels should be understood clearly. In the present work, we build mol. dynamics (MD) models of random cross-linked polyacrylamide hydrogels with different water vol. fractions through a reaction method. On the basis of these models, thermal conductivities of hydrogels at the nanoscale are investigated by a none-equil. MD method. This work reveals that when the water fraction of hydrogels is under 85%, the thermal cond. increases with the water fraction, and can be even higher than the thermal conductivities of both pure polymer networks and pure water because of the influence of the interface between polymer networks and water. However, when the water fraction in hydrogels is bigger than 85%, its thermal cond. will decrease and get close to the water's cond. Accordingly, to explain this abnormal phenomenon, a 2-order-3-phase theor. model is proposed by considering hydrogel as a 3-phase composite. It can be found that the proposed theory can predict results which agree quite well with our simulated results.
    49. 49
      Nguyen, L. H.; Koerner, H.; Lederer, K. Gel point determination for the copolymerization system of cardanyl acrylate and styrene and its critical conversion. J. Appl. Polym. Sci. 2003, 89 (9), 2385– 2390,  DOI: 10.1002/app.12279
      49
      Gel point determination for the copolymerization system of cardanyl acrylate and styrene and its critical conversion
      Nguyen, Le Huong; Koerner, Hilmar; Lederer, Klaus
      Journal of Applied Polymer Science (2003), 89 (9), 2385-2390CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
      The copolymn. of cardanyl acrylate (CA) and styrene (S) with Me Et ketone peroxide (MEKP) and cobalt salt as the initiator was studied by means of rheometry and isothermal DSC. The gel point of the CA-S reaction system was detd. by rheol. measurements at 50 and 70°C. In addn., rheol. measurements provide very useful information on the evolution of the phys. properties of the system during curing, such as viscosity and shear modulus. We demonstrate that isothermal DSC expts., using the Avrami model, can be used to obtain a more detailed description of the curing process.
    50. 50
      Debroy, D.; Oakey, J.; Li, D. Interfacially-mediated oxygen inhibition for precise and continuous poly(ethylene glycol) diacrylate (PEGDA) particle fabrication. J. Colloid Interface Sci. 2018, 510, 334– 344,  DOI: 10.1016/j.jcis.2017.09.081
      50
      Interfacially-mediated oxygen inhibition for precise and continuous poly(ethylene glycol) diacrylate (PEGDA) particle fabrication
      Debroy, Daniel; Oakey, John; Li, Dongmei
      Journal of Colloid and Interface Science (2018), 510 (), 334-344CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Poly(ethylene glycol) diacrylate (PEGDA)-based hydrogels have been engineered for multiple biomedical applications, including drug delivery, cell delivery, and tissue engineering. Miniaturization of these materials to nano- and microscale particles is a subject of intense activity and promises to extend their range of applicability. Generally, these efforts have been frustrated by the O-induced inhibition of chain growth polymn., an effect exacerbated as target length scales are reduced. A method which exploits undesirable O-inhibited photopolymn. to produce size-controlled PEGDA hydrogel particles is reported. The role of initial soln. compn. in detg. the relative particle:droplet size ratio is reported, and was detd. to contribute, through its effect on polymn. rate and O diffusivity. Facile control of photopolymn. kinetics by UV light intensity and/or exposure time, allowed PEGDA particles to be produced with dimensions independent of parent spherical droplets formed by conventional microfluidic emulsification.
    51. 51
      Zhu, J.; Marchant, R. E. Design properties of hydrogel tissue-engineering scaffolds. Expert Rev. Med. Devices 2011, 8 (5), 607– 626,  DOI: 10.1586/erd.11.27
      51
      Design properties of hydrogel tissue-engineering scaffolds
      Zhu, Junmin; Marchant, Roger E.
      Expert Review of Medical Devices (2011), 8 (5), 607-626CODEN: ERMDDX; ISSN:1743-4440. (Expert Reviews Ltd.)
      A review. This article summarizes the recent progress in the design and synthesis of hydrogels as tissue-engineering scaffolds. Hydrogels are attractive scaffolding materials owing to their highly swollen network structure, ability to encapsulate cells and bioactive mols., and efficient mass transfer. Various polymers, including natural, synthetic and natural/synthetic hybrid polymers, have been used to make hydrogels via chem. or phys. crosslinking. Recently, bioactive synthetic hydrogels have emerged as promising scaffolds because they can provide molecularly tailored biofunctions and adjustable mech. properties, as well as an extracellular matrix-like microenvironment for cell growth and tissue formation. This article addresses various strategies that have been explored to design synthetic hydrogels with extracellular matrix-mimetic bioactive properties, such as cell adhesion, proteolytic degrdn. and growth factor-binding.
    52. 52
      McAvoy, K.; Jones, D.; Thakur, R. R. S. Synthesis and Characterisation of Photocrosslinked poly(ethylene glycol) diacrylate Implants for Sustained Ocular Drug Delivery. Pharm. Res. 2018, 35 (2), 36,  DOI: 10.1007/s11095-017-2298-9
      52
      Synthesis and Characterisation of Photocrosslinked poly(ethylene glycol) diacrylate Implants for Sustained Ocular Drug Delivery
      McAvoy Kathryn; Jones David; Thakur Raghu Raj Singh; Thakur Raghu Raj Singh
      Pharmaceutical research (2018), 35 (2), 36 ISSN:.
      PURPOSE: To investigate the sustained ocular delivery of small and large drug molecules from photocrosslinked poly(ethylene glycol) diacrylate (PEGDA) implants with varying pore forming agents. METHODS: Triamcinolone acetonide and ovalbumin loaded photocrosslinked PEGDA implants, with or without pore-forming agents, were fabricated and characterised for chemical, mechanical, swelling, network parameters, as well as drug release and biocompatibility. HPLC-based analytical methods were employed for analysis of two molecules; ELISA was used to demonstrate bioactivity of ovalbumin. RESULTS: Regardless of PEGDA molecular weight or pore former composition all implants loaded with triamcinolone acetonide released significantly faster than those loaded with ovalbumin. Higher molecular weight PEGDA systems (700 Da) resulted in faster drug release of triamcinolone acetonide than their 250 Da counterpart. All ovalbumin released over the 56-day time period was found to be bioactive. Increasing PEGDA molecular weight resulted in increased system swelling, decreased crosslink density (Ve), increased polymer-water interaction parameter (χ), increased average molecular weight between crosslinks (Mc) and increased mesh size (ε). SEM studies showed the porosity of implants increased with increasing PEGDA molecular weight. Biocompatibility showed both PEGDA molecular weight implants were non-toxic when exposed to retinal epithelial cells over a 7-day period. CONCLUSION: Photocrosslinked PEGDA implant based systems are capable of controlled drug release of both small and large drug molecules through adaptations in the polymer system network. We are currently continuing evaluation of these systems as potential sustained drug delivery devices.
    53. 53
      Ju, H.; McCloskey, B. D.; Sagle, A. C.; Kusuma, V. A.; Freeman, B. D. Preparation and characterization of crosslinked poly(ethylene glycol) diacrylate hydrogels as fouling-resistant membrane coating materials. J. Membr. Sci. 2009, 330 (1), 180– 188,  DOI: 10.1016/j.memsci.2008.12.054
      53
      Preparation and characterization of crosslinked poly(ethylene glycol) diacrylate hydrogels as fouling-resistant membrane coating materials
      Ju, Hao; McCloskey, Bryan D.; Sagle, Alyson C.; Kusuma, Victor A.; Freeman, Benny D.
      Journal of Membrane Science (2009), 330 (1+2), 180-188CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      A family of crosslinked poly(ethylene glycol) diacrylate (XLPEGDA) materials was synthesized via free-radical photopolymn. of poly(ethylene glycol) diacrylate (PEGDA) solns. in water. These materials are potential fouling-resistant coatings for ultrafiltration (UF) membranes. PEGDA chain length (n = 10-45, where n is the av. no. of ethylene oxide units in the PEGDA mol.) and water content in the prepolymn. mixt. (0-80 wt.%) were varied, resulting in XLPEGDA materials with water permeability values ranging from 0.5 to 150 L μm/(m2 h bar). Generally, water permeability increased with increasing prepolymn. water content and with increasing PEGDA chain length. Moreover, water permeability exhibits a strong correlation with equil. water uptake. However, solute rejection, probed using poly(ethylene glycol)s of well defined molar mass, decreased with increasing prepolymn. water content and increasing PEGDA chain length. That is, there is a tradeoff between water permeability and sepn. properties. Finally, the fouling resistance of XLPEGDA materials was characterized via contact angle measurements and static protein adhesion expts. From these results, XLPEGDA surfaces are more hydrophilic in samples prepd. at higher prepolymn. water content or with longer PEGDA chains, and the more hydrophilic surfaces generally exhibit less BSA accumulation.
    54. 54
      Griffin, M. F.; Leung, B. C.; Premakumar, Y.; Szarko, M.; Butler, P. E. Comparison of the mechanical properties of different skin sites for auricular and nasal reconstruction. J. Otolaryngol. - Head Neck Surg. 2017, 46 (1), 33,  DOI: 10.1186/s40463-017-0210-6
      54
      Comparison of the mechanical properties of different skin sites for auricular and nasal reconstruction
      Griffin M F; Leung B C; Butler P E; Griffin M F; Premakumar Y; Szarko M; Griffin M F; Leung B C; Butler P E
      Journal of otolaryngology - head & neck surgery = Le Journal d'oto-rhino-laryngologie et de chirurgie cervico-faciale (2017), 46 (1), 33 ISSN:.
      BACKGROUND: Autologous and synthetic nasal and auricular frameworks require skin coverage. The surgeon's decides on the appropriate skin coverage for reconstruction based on colour matching, subcutaneous tissue thickness, expertise and experience. One of the major complications of placing subcutaneous implants is the risk of extrusion (migration through the skin) and infection. However, knowledge of lessening the differential between the soft tissue and the framework can have important implications for extrusion. This study compared the mechanical properties of the skin commonly used as skin sites for the coverage in auricular and nasal reconstruction. METHODS: Using ten fresh human cadavers, the tensile Young's Modulus of the skin from the forehead, forearm, temporoparietal, post-auricular and submandibular neck was assessed. The relaxation rate and absolute relaxation level was also assessed after 90 min of relaxation. RESULTS: The submandibular skin showed the greatest Young's elastic modulus in tension of all regions (1.28 MPa ±0.06) and forearm showed the lowest (1.03 MPa ±0.06). The forehead demonstrated greater relaxation rates among the different skin regions (7.8 MPa(-07) ± 0.1). The forearm showed the lowest rate of relaxation (4.74 MPa(-07) ± 0.1). The forearm (0.04 MPa ±0.004) and submandibular neck skin (0.04 MPa ±0.005) showed similar absolute levels of relaxation, which were significantly greater than the other skin regions (p < 0.05). CONCLUSIONS: This study provides an understanding into the biomechanical properties of the skin of different sites allowing surgeons to consider this parameter when trying to identify the optimal skin coverage in nasal and auricular reconstruction.
    55. 55
      Bailey, J. J.; Berson, A. S.; Garson, A.; Horan, L. G.; Macfarlane, P. W.; Mortara, D. W.; Zywietz, C. Recommendations for standardization and specifications in automated electrocardiography: bandwidth and digital signal processing. A report for health professionals by an ad hoc writing group of the Committee on Electrocardiography and Cardiac Electrophysiology of the Council on Clinical Cardiology, American Heart Association. Circulation 1990, 81 (2), 730– 739,  DOI: 10.1161/01.CIR.81.2.730
      55
      Recommendations for standardization and specifications in automated electrocardiography: bandwidth and digital signal processing. A report for health professionals by an ad hoc writing group of the Committee on Electrocardiography and Cardiac Electrophysiology of the Council on Clinical Cardiology, American Heart Association
      Bailey J J; Berson A S; Garson A Jr; Horan L G; Macfarlane P W; Mortara D W; Zywietz C
      Circulation (1990), 81 (2), 730-9 ISSN:0009-7322.
      There is no expanded citation for this reference.

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    • Video of the lab-made electrical circuit while running (MP4)

    • ATR-FTIR spectra during the photopolymerization process; pictures and SEM images for printing process optimization; evolution of the diameter and thickness of hydrogels during swelling; mechanical properties of hydrogels in the dry and wet states; degradation assay (PDF)


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