Sustainable Collagen Blends with Different Ionic Liquids for Resistive Touch Sensing Applications

Considering the sustainable development goals to reduce environmental impact, sustainable sensors based on natural polymers are a priority as the large im plementation of these materials is required considering the Internet of Things (IoT) paradigm. In this context, the present work reports on sustainable blends based on collagen and different ionic liquids (ILs), including ([Ch][DHP], [Ch][TSI], [Ch][Seri]) and ([Emim][TFSI]), processed with varying contents and types of ILs in order to tailor the electrical response. Varying IL types and contents leads to different interactions with the collagen polymer matrix and, therefore, to varying mechanical, thermal, and electrical properties. Collagen/[Ch][Seri] samples display the most pronounced decrease of the tensile strength (3.2 ± 0.4 MPa) and an increase of the elongation at break (50.6 ± 1.5%). The best ionic conductivity value of 0.023 mS cm–1 has been obtained for the sample with 40 wt % of the IL [Ch][Seri]. The functional response of the collagen–IL films has been demonstrated on a resistive touch sensor whose response depends on the ionic conductivity, being suitable for the next generation of sustainable touch sensing devices.


■ INTRODUCTION
Smart and functional materials are of increasing interest for a variety of areas due to their ability to exhibit a functional response variation in a predetermined manner upon changes in their environment. 1−4 Their technological significance requires the development of advanced functional materials with tailored properties toward specific applications. In this context, in the last years, special attention has been paid to the development of smart and functional materials based on the combination of polymers and ionic liquids (ILs) for a wide variety of applications, ranging from sensors, actuators, energy generation and storage, filtration systems, biomedical, and environmental sensing. 5−7 The major advantage of IL−polymer-based materials relies on the absence of micro-and nanoparticles to develop a multifunctional composite. 6 In fact, ILs have gained special attention in the last decade in several fields of knowledge due to their interesting properties. 6,8,9 ILs, also known as green solvents with melting temperatures below 100°C, are entirely composed of organic cations and organic/inorganic anions with properties such as negligible vapor pressure, high ionic conductivity, nonflammability, nonvolatility, and a wide electrochemical window (between 4 and 6 V). 10−12 The applicability of ILs in a wide range of areas is intrinsically related to the simple tunability of the physical−chemical properties by varying cations and anions, 6 a high number of possible cation and anion combinations existing, 10 leading to specific functional properties, including magnetic, electrical, and optical properties, among others. 6,13,14 Thus, ILs have been combined with specific polymers like poly(L-lactic acid) (PLLA), 15,16 cellulose, 16 poly(vinylidene fluoride) (PVDF), 17−21 and poly(methyl methacrylate) (PMMA), leading to a variety of functional characteristics. 22−25 IL−polymer-based materials have been explored to develop pressure sensing materials based on PMMA/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim]-[TFSI]) with high sensitivity as revealed by a gauge factor of ∼2.73, being stable for >1300 cycles (stable operation) 26 or [Emim] [BF 4 ] incorporated in different polymeric matrices including Nafion, PVDF, and thermoplastic polyurethane (TPU), the IL/PVDF composites displaying the best sensing performance (101.20 mV MPa −1 ). 27 Further, a transparent piezoionic material has been presented based on the thermo-plastic elastomer styrene−ethylene−butylene−styrene (SEBS) and the ionic liquid 1-butyl-3-methyl-imidazolium dicyanamide ([Bmim][N(CN) 2 ]), the composite resistance varying linearly with the applied force with a pressure sensitivity of approximately 25 kΩ N −1 in a dynamic range from 0 to 10 N. 28 However, besides the great interest in the development of IL− polymer-based materials, most of the studies are based on the combination of ILs with synthetic polymers, there existing a lack of studies regarding their combination with natural polymers, 6,16 areas in which significant efforts need to be performed to develop sustainable smart and functional materials. 6 Concern for the environment has increased a special interest in natural and renewable materials in terms of sustainability. 29,30 In this view, the transition to a circular economy provides an opportunity to establish a more sustainable, efficient, and competitive economy, 31,32 with one of the aims being the valorization of byproducts. Among the different natural polymers, collagen emerges as a suitable approach for the development of IL−polymer based composites due to its natural availability, as it is the most abundant protein in the extracellular matrix of mammals and generally extracted from the bones and skin of cattle, poultry, or pigs. 33 Alternatively, collagen can also be obtained from the scales, skin, swimming bubbles, and bones of fish. 34−36 It provides a high mechanical strength to skin, bone, and other tissues. 37 In terms of chemical properties, collagen backbone is composed of three parallel polypeptide-α chains in the form of cross-linked fibrils with a triple helix structure that leads to the formation of insoluble fibers responsible for the high mechanical strength and integrity of the extracellular matrix of mammalians. The polarity of the collagen chain also provides interesting electrical properties. 38 Thus  considered control samples. All films were conditioned in a climatic chamber, ACS Angelantoni, at 25°C and 50% relative humidities before testing. Sample Characterization. Differential Scanning Calorimetry and Thermogravimetric Analysis. Differential scanning calorimetry, DSC, was carried out in a Mettler-Toledo DSC 822. Samples (3.0 ± 0.2 mg) were sealed in aluminum pans to avoid mass loss during the experiment. Filled pans were heated from 25 to 300°C at a rate of 10°C min −1 under inert atmosphere conditions (10 mL N 2 min −1 ) to avoid thermo-oxidative reactions.
Thermogravimetric analysis, TGA, was carried out in a TGA/ DCS3+ Mettler-Toledo. Dynamic scans from 25 to 800°C were carried out at a constant rate of 10°C min −1 under a nitrogen atmosphere (10 mL N 2 min −1 ) to avoid thermo-oxidative reactions.
Fourier Transform Infrared Spectroscopy. Fourier transform infrared spectroscopy, FTIR, was performed by using an Alpha II Compact FTIR spectrometer equipped with an attenuated total reflectance (ATR) crystal (ZnSe). A total of 32 scans were carried out at a 4 cm −1 resolution.
Water Contact Angle. Water contact angle, WCA, measurements of the samples were performed using a DataPhysics OCA 20 contact angle system. A 3 μL droplet of distilled water was placed on the film surface to evaluate its hydrophobic or hydrophilic character. The image of the drop was captured using SCA20 software.  X-ray Diffraction. X-ray diffraction, XRD, measurements were performed at 40 kV and 40 mA with a diffraction unit (PANalytical Xpert PRO, Madrid, Spain), generating the radiation from a Cu Kα (λ = 1.5418 Å) source. Data were recorded from 2 to 50°.
Scanning Electron Microscopy. Previously to the scanning electron microscopy, SEM, measurements, films were placed on a metal stub and coated with gold using a JEOL fine-coat ion sputter JFC-1100 and argon atmosphere. Samples were observed using a Hitachi S-4800 scanning electron microscope (Hitachi, Madrid, Spain) at a 15 kV accelerating voltage.
Mechanical Properties. Bone-shaped samples (4.75 mm × 22.25 mm) were cut, and an Instron 5967 mechanical testing system (Instron, Barcelona, Spain) was used to carry out tensile tests at 1 mm min −1 , according to the ASTM D 638-03 standard.
Electrical Properties. The ionic conductivity value of the collagen blend films was evaluated with a Biologic VMP3 instrument with stainless steel disc electrodes at different temperatures from 25 to 100°C and a voltage amplitude of 10 mV in the frequency range from 1 MHz to 10 mHz. The ionic conductivity value (σ′) was calculated considering eq 1 where R b is the bulk resistance obtained from the intercept of the imaginary impedance (minimum value of Z″) with the slanted line in the real impedance (Z′) and t and A are the thickness and area of the collagen-based films, respectively. Resistive Touch Sensor Device. Figure 1 shows the schematic representation of the resistive touch sensor. Silver electrodes (Novacentrix Metalon HPS-021LV ink) were deposited with the pattern of Figure 1 on the collagen blend films by screen-printing technique, using a manual screen-printing with a mesh of 100 threads by centimeter. The collagen blend films with the deposited ink were cured at 40°C in an electric convection oven (Pselecta) for 2 h. Then, the films were placed on a poly(ethylene terephthalate) (PET, Goodfellow ES303010/21) substrate of 1 mm of thickness for mechanical stability. On top of the film, a PET separator (Dupont Teijin Melinex 506) of 100 μm of thickness was glued. A second layer of collagen blend film was placed on top of the PET separator with transparent film tape (3M Polyester Film Tape 856). Two strips of copper tape (3M 1181 6 mm) were glued to the top and bottom collagen blend films for electric connection. The pattern enables us to tailor system resistance for an easier readout of the material resistance by the electronic system.  39,40 Furthermore, the amide A band, corresponding to the N−H stretching vibration, appears at 3300 cm −1 , instead of 3400 cm −1 , indicating that the NH group is involved in hydrogen bonding. 41,42 The spectral region between 1200 and 900 cm −1 is attributed to the stretching vibrations of C−O bonds in collagen and those related to the hydroxyl groups in glycerol. The band at 1043 cm −1 is assigned to the stretching of C−O linkages in C1 and C3 of glycerol, and the band at 1110 cm −1 is related to the stretching of C−O in C2 of glycerol. 43 When choline serinate is added, a decrease in the intensity of the amide(I), amide(II), and amide(III) bands is observed ( Figure 2b). These changes confirm the existence of physical interactions, such as hydrogen bonds and ionic interactions, among carboxyl, amino, and hydroxyl groups of collagen, glycerol, and choline serinate, respectively. Moreover, a slight shift in amide(I) (from 1632 to 1630 cm −1 ), amide(II) (from 1547 to 1550 cm −1 ), and amide(III) (from 1238 to 1240 cm −1 ) bands was observed at an increased concentration of [Ch][Seri], indicating stronger interactions between IL and collagen. Furthermore, the addition of choline serinate increases the intensity of the band at 1400 cm −1 due to the COO − group of [Seri] anion, and small bands attributed to the ammonium groups of choline are observed at 955 cm −144, 45 Additionally, shoulders at 1080 and 1160 cm −1 , corresponding to C−OH groups in serinate and choline, 45  The imidazolium cation bands at 2979 cm −1 , related to the ethyl chains (CH), and at 3018 cm −1 , corresponding to the ring (HC−CH and N(CH)N), are overlapped with the wide amide A band (Figure 2c). 54,55 For a better understanding of the conformational changes in the secondary structure of collagen, the amide(I) profile was analyzed since these interactions can destabilize the collagen native structure according to several reports. 56,57 Amide(I) contains three major components: a band at 1650 cm −1 , related to the α-chain/random coil conformation, and two bands corresponding to the β-sheet conformation, which appears at 1615−1630 cm −1 and at 1660−1670 cm −1 . 58 As shown in Table  3 [TFSI], which show the lowest α-helix/β-sheet ratio values. In any case, α-helix is the main collagen conformation in all hybrid films with ILs, confirming the predominant triple helix structure of collagen.
The film hydrophilic character was analyzed by the measurement of the water contact angle, WCA. As shown in Table 2 The change in collagen thermal stability caused by the incorporation of the ILs was analyzed by DSC, and the different thermograms are shown in Figure 3.
Characteristic endothermic peaks are found at 85 and 150− 250°C, in accordance with previous studies. 47,59,60 The first peak is associated with the interfibrillar fraction of water. 61 In agreement with the physical interactions identified by FTIR analysis, it is worth noting that the values associated with the first peak indicate that collagen fibers remained unchanged, although a slight decrease of the temperature and height of the peak indicates changes in the network hydration. 62 It can be observed that the denaturation temperature of the samples is lower than that of the control, which may be attributed to the plasticization effect of the IL and its adhesion to the collagen backbone, as also reflected by the mechanical property results. The second peak is related to the transition of the collagen triple helix structure into Regarding thermal stability, decomposition temperatures can change from 200 to 400°C by varying the anion/cation type, indicating that this property mainly depends on the IL structure and that specific anion and cation play an important role in determining thermal stability. Additionally, it is worth to note that evaporation may occur, so the measured mass loss is a combination of evaporation and degradation. In this work, the influence of the IL amount and anion/cation type on the thermal stability of collagen films was analyzed. Figure 3c shows the thermal degradability of the [Ch][Seri]/collagen systems with an increasing amount of [Ch][Seri]. All samples show three stages of thermal degradation. The first stage of degradation is between 50 and 100°C, associated with water loss. The second thermal degradation is associated with the evaporation of glycerol (240°C) 65 and IL degradation (190°C). 66 In the case of the control sample, the evaporation temperature of glycerol is lower than that of pure glycerol, suggesting glycerol−collagen interactions.  68 Morphological and Structural Characteristics. Taking into account that ILs can be dissolved and disturb the triple helix structure of collagen during dissolution, 69 XRD and SEM analyses were carried out to analyze the effect of ILs on the collagen structure. As for XRD analysis (Figure 4), all samples show XRD patterns compatible with nearly amorphous materials, with a broad peak around 2θ = 20°, associated with the diffuse scattering of collagen fibers.
The peak around 2θ = 7°represents the lateral packing distance between collagen chains and is related to the triple helix structure of collagen. 70,71 It is observed that almost all samples show similar XRD patterns, indicating the prevalence of the collagen structural order. When the [Ch][Seri] content increases, the peak intensity at 2θ = 7°decreases, suggesting the decrease of the structural order in collagen, but no significant differences were observed in the peak at 2θ = 20°. Additionally, a sharp peak appears at around 17°in the films with 20 and 40 wt % [Ch][Seri] (Figure 4a), corresponding to the serine crystals observed on the surface of those films, leading to a heterogeneous system. 64 In the same way, the addition of  in the structural order of collagen, while the broad band practically does not change. 40[Ch][TFSI] is the film that shows the most similar XRD pattern to that of the control film ( Figure  4b).
Representative SEM micrographs of the surface and cross section of the samples are shown in Figure 5.
The cross-sectional images show that all films are characterized by a compact and homogeneous dense fibrillar structure, even if the images of 40

[Ch][TFSI] and 40[EMIM][TFSI]
show some differences. In particular, the differences observed in 40[EMIM][TFSI] may be due to the fact that the collagen matrix exudates the excess of IL that covered the surface and does not allow the collagen fibrillar structure to be observed. [TFSI] samples also undergoes changes compared to the surface of the control film. On the one hand, cracks can be observed, which may be due to the hydrophobic character of those ILs, as confirmed by the WCA analysis. On the other hand, the monticules observed in the cross-sectional images may suggest a partial exudation of ILs from the polymeric matrix, which may be due to the immiscibility between the components of the samples. 72 Mechanical Properties. Mechanical properties of films are largely associated with the distribution and density of intermolecular and intramolecular interactions in the network, so the effect of the ILs on the mechanical properties is shown in Table 4. It is found that the tensile strength decreases and elongation at break increases when [Ch][Seri] is added to collagen, in particular, for the films with the highest filler content, as shown in Figure 6.
These results are attributed to the presence of the IL, typically having a plasticizing effect on the polymer films and to the presence of water molecules, which enhance chain mobility, since the hygroscopic character of this IL increases the water Also, for the same IL amount (40 wt %), the overall mechanical properties depend on the IL type. For the [Ch] cation, it is observed that the mechanical properties increase for IL with a low viscosity value due to the anion size. This behavior is also observed for the [EMIM] cation.
Electrical Properties. The electrical properties of the films were studied by impedance spectroscopy in the temperature range from 25 to 100°C and the frequency range from 1 MHz to 10 mHz, the results being presented in the Nyquist plots in Figure 7a, which depend on the IL amount and type. The Nyquist plots allow us to identify processes with different time constants, the shape of the curves providing information about the possible electrical conduction mechanisms.
The Nyquist plots represented in Figure 7a show three characteristic parts defined by a semicircle located in the highfrequency range that corresponds to the charge-transfer process (bulk material properties), a transition controlled by the diffusion of counterions, and straight line for lower frequencies that is related to the diffusion process at the film/electrode interface. 76,77 This behavior represented in Figure 7 depends on the IL content and type. For [Ch][Seri] IL, it is observed that the semicircle represented in Figure 7a decreases with increasing IL content, i.e., the intercepts on the Z′, real component, axis are dependent on the IL content. This decrease in the resistive part (ionic resistance calculated in the Z′ axis) is due to a decrease of the bulk resistance, i.e., an increase in the number of free charge carrier and their mobility. Furthermore, for the same IL content (40 wt %), it is observed that the semicircle depends on the IL type, which is related to the IL viscosity and ionic conductivity value. The inset of Figure 7a shows the Nyquist plot as a function of temperature for 40 [Ch][Seri], the behavior being similar for the other films. It is observed that the semicircle decreases with increasing temperature due to the thermally activated increasing mobility of the ions, leading to a decrease in the electrical resistance. 78 The ionic conductivity value as a function of temperature is shown in Figure 7b for the different collagen−IL films. It is observed that the ionic conductivity value increases with temperature due to a hopping conduction mechanism between local structural relaxations and segmental motions of the polymer−IL complexes. Further, increasing temperature leads to faster internal mode motions of the polymer chains, which also improves the electrical conductivity. 79 At room temperature, the best ionic conductivity value of 0.023 mS cm −1 is obtained for the 40[Ch][Seri] sample. Furthermore, the ionic 13.1 ± 0.9 36.8 ± 0.7 a Two means followed by the same letter in the same column are not significantly (P > 0.05) different from Tukey's multiple range test. N = 5 was the minimum number of replications.     Touch Sensor Response. Considering the electrical properties of the collagen−IL films, a digital multimeter Agilent 34401A was used to measure the resistance variation of the touch sensor device (Figure 1) in intervals of 10 mm with a constant applied pressure of 100 g, as shown in Figure 8a. These results demonstrate a suitable linearity between the resistance variation and the position for most of the samples, the response depending on the IL type, as can be seen in Figure 8b−e, as it is correlated with the ionic conductivity of the samples.
The developed touch sensing device was connected to a microcontroller (Arduino UNO) through a voltage divider, allowing the measurement of the prototype as a sliding sensor or digital potentiometer (Figure 9a). The sliding sensor changes the resistance by pressing on the film, reducing the contact resistance between the top and bottom collagen−IL films (Figure 8a), the resistance variation being different for each position. Depending on the position of the finger (0−70 mm), the percentage of the bar varies from 0 to 100% in relation to the resistance variation (Figure 9b). A Python application was developed in order to show the sliding application in a progress bar. The microcontroller sends the received digital data from the analog-to-digital converter (ADC) through the universal serial bus to the computer, where the Python application receives the digital data. The data was converted to a percentage variation, as shown in Figure 9b for 40[Ch][Seri].
In order to maintain the last value when the finger is removed, the application only varies the bar chart percentage if the variation of the resistance is below a threshold. If it is not and the variation is too large, the algorithm assumes that the finger was removed from the slider and maintains the last position (see the Video on the Supporting Information).
Considering the actual interest in sustainability and environment-friendly materials, the proposed sensor exhibits lower environmental impact than recently reported pressure sensing materials 80 since most of them are based on the combination of synthetic matrix, such as polyimide (PI), 81 poly-(dimethylsiloxane) (PDMS), 82 perfluoroalkoxy alkane (PFA), 83 poly(vinylidene difluoride) (PVDF), 84 and fillers like carbon nanotubes (CNTs), 85,86 metal nanowires (NWs), 87 and metal nanoparticles (NPs). 88 In terms of green chemistry, the primary goal of green techniques and technologies is to reduce the adverse effects of contaminants on the environment and living beings. In this context, although there are studies based on collagen for pressure sensing applications, the chemicals used in the processing of those composites are highly polluting to the environment. 89 [Seri] were prepared using a green channel as sustainable, nontoxic, and biodegradable ILs. 92 Aside from the potential benefits of environment-friendly products and processes, other challenges, such as the useful lifetime (life-cycle costs), impurities, impact of water content, thermal stability, aging, IL losses as well as recyclability data, are essential for the proper evaluation of the viability of ILs in commercial applications such as sustainable materials for multifunctional devices. Ionic liquids are expensive when compared to the mainly used components of electronics. 93 Due to economic and environmental issues, the recovery and purification of ILs are essential. 94 Moreover, the presence of impurities and water excess in some of the ionic liquids can lead to interference in the proper performance. 95  [TFSI] are purified grade. Although no effect of impurities on the characterization of the materials has been observed, the performance of the sensors should be further tested 96 in realoperating conditions and in actual prototype device implementation. 95 In this context, it is shown that IL−collagen films can be used as resistive touch sensors and that their electrical response can also be adjusted properly by varying the IL content and type, being suitable for the next generation of sustainable electronic devices. , and IL contents from 0 to 40 wt % have been developed for resistive touch sensing applications. It is observed that different α-helix/β-sheet ratio values have been obtained for the different samples, indicating different physical interactions between the ILs and collagen matrix. This behavior is also confirmed by the FTIR and XRD analyses. The IL content and type do not affect the collagen triple helix structure that is responsible for the stability of collagen molecules. Regarding mechanical properties, ILs act as plasticizers for the collagen matrix. In addition, the IL content and type improve the ionic conductivity value, the best room-temperature ionic conductivity value of 0.023 mS cm −1 being obtained for the 40[Ch][Seri] sample. Considering the electrical response, the collagen−IL films have been implemented in a resistive touch sensing device, showing an excellent response, that can be tailored depending on the IL type. This work demonstrates that collagen−IL films are suitable for resistive touch sensing applications, being also suitable for the new generation of sustainable electronic materials based on biopolymers. ■ ASSOCIATED CONTENT
Application of the sliding sensor showing different percentage stages depending on the position of the finger (MP4)