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Enhancement of the Adhesive Properties by Optimizing the Water Content in PNIPAM-Functionalized Complex Coacervates

  • Marco Dompé
    Marco Dompé
    Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
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  • Mehdi Vahdati
    Mehdi Vahdati
    Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France
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  • Froukje van Ligten
    Froukje van Ligten
    Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
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  • Francisco J. Cedano-Serrano
    Francisco J. Cedano-Serrano
    Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France
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    Orcidhttp://orcid.org/0000-0001-9792-2551
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  • Dominique Hourdet
    Dominique Hourdet
    Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France
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    Orcidhttp://orcid.org/0000-0002-0328-7014
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  • Costantino Creton
    Costantino Creton
    Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France
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    Orcidhttp://orcid.org/0000-0002-0177-9680
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  • Marco Zanetti
    Marco Zanetti
    Department of Chemistry and NIS Centre, University of Turin, Via P. Giuria 7, Torino 10125, Italy
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  • Pierangiola Bracco
    Pierangiola Bracco
    Department of Chemistry and NIS Centre, University of Turin, Via P. Giuria 7, Torino 10125, Italy
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  • Jasper van der Gucht
    Jasper van der Gucht
    Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
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    Orcidhttp://orcid.org/0000-0001-5525-8322
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  • Thomas Kodger
    Thomas Kodger
    Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
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  • Marleen Kamperman*
    Marleen Kamperman
    Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
    Polymer Science, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0002-0520-4534
Cite this: ACS Appl. Polym. Mater. 2020, 2, 4, 1722–1730
Publication Date (Web):March 20, 2020
https://doi.org/10.1021/acsapm.0c00185

Copyright © 2020 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

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Most commercially available soft tissue glues offer poor performance in the human body. We have developed an injectable adhesive whose setting mechanism is activated by a change in environmental factors, i.e., temperature and/or ionic strength. The material and setting process are inspired by the adhesive processing mechanism observed in natural maritime glues. Complex coacervation, a liquid–liquid phase separation between oppositely charged polyelectrolytes, is thought to play an important role in the processing. Complex coacervates are characterized by a high water content, which inevitably weakens the glue. Here, we aim to increase the adhesive performance by systematically tuning the water content. Among the several strategies here explored, the most effective one is the mechanical removal of water using an extruder, resulting in an increase of work of adhesion by 1 order of magnitude compared to the original formulation.

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Introduction

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Adhesive technology, despite being known and exploited since the middle Pleistocene, (1) is rarely applied when dealing with adverse environments. In medicine, for instance, surgical tissue closure still relies on conventional techniques, such as suturing and stapling, (2) which have many pitfalls, e.g., inflammatory reactions, scar formation, and stress localization. (3−5) Tissue glues represent a valid alternative to these methods, especially because of the easy handling and minimal tissue damage. (3,5,6) However, most commercially available adhesives fail to offer a proper performance in wet and dynamic environments and do not achieve the required bonding strength. (3,7)
In nature, many aquatic species (e.g., sandcastle worms and mussels) have managed to solve the challenges related to underwater adhesion by developing protein-based glues that bond strongly to a variety of surfaces even in submerged conditions. (8−10) A phenomenon directly involved in the processing and delivery of these protein-based adhesives is complex coacervation, (11,12) an associative phase separation process which requires the presence of two oppositely charged polyelectrolytes. (13) Under the proper conditions (pH, ionic strength, and mixing ratio), the polyelectrolyte solution partitions into a dilute phase (polymer-poor) and a complex coacervate phase (polymer-rich). (14) Several research groups have employed these electrostatic interactions to fabricate viscous glues, most of which set underwater due to covalent cross-linking reactions, providing the strength required to oppose detachment. (15−20)
Previously, (21−23) we have produced a complex coacervate-based adhesive which undergoes a liquid-to-solid transition in response to environmental triggers, forming exclusively physical bonds without the addition of any cross-linking agent. The glue is prepared by mixing oppositely charged polyelectrolytes grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains. PNIPAM is a water-soluble polymer which phase separates at temperatures above its lower critical solution temperature (LCST), which is around 32 °C. The adhesive can be reinforced by heating the sample above the LCST (temperature switch), by immersing the sample in a lower ionic strength medium (salt switch), or by combining the two triggers (temperature + salt switch), mimicking the conditions that the glue would experience in the human body. However, despite optimizing the performance by assessing the effect of different parameters (polymer composition, temperature, ionic strength and probe surface), we could obtain, at best, a 3-fold enhancement of the adhesive properties compared to the original formulation. (21−23)
For this adhesive system, and basically for all water-containing adhesives, the adhesive performance and the mechanical properties heavily depend on water content. For instance, commercial poly(ethylene glycol) (PEG)-based glues, such as DuraSeal and CoSeal, bind to tissues with a low adhesive strength, which is mainly attributable to the high water content, ranging from 90% to 99% w/v. (24) Another related drawback is the significant swelling in physiological conditions (>700%), (25) which can lead to medical complications and further weakening of the adhesive: DuraSeal has been reported to cause nerve compression, (26,27) while CoSeal has shown a dramatic decrease in both moduli and energy to failure (in compression mode) over a period of 3 days in physiological conditions due to water sorption. (28) To circumvent issues associated with hydrophilic polymers, relatively hydrophobic surgical glues have been proposed. For example, Karp and co-workers designed a hydrophobic prepolymer (poly(glycerol sebacate acrylate) (PGSA)) which can be cross-linked in situ by UV light: the glue showed limited swelling in physiological conditions, providing stronger adhesion than standard tissue adhesives in highly dynamic environments. (29,30)
However, it is not always desirable to increase the hydrophobicity. For instance, recently Yuk et al. developed dry double-sided tapes whose adhesion mechanism is initiated by the removal of the interfacial water present at the tissue surface: in this case, water absorption by the adhesive matrix is considered a crucial requirement to promote an intimate contact with the tissue, resulting in the quick establishment of both covalent and noncovalent interactions with the surface. (31) Additionally, water may also act as plasticizer, which can improve the adhesion performance. (32) Feldstein et al. studied how water content affects the adhesive properties of poly(N-vinylpyrrolidone)–poly(ethylene glycol) (PVP–PEG) blends. (33,34) By increasing the content of PEG, the water content increased and adhesion was enhanced, enabling the material to sustain higher deformations without considerably affecting the ultimate tensile strength. The optimal performance, in terms of peel force, was observed when the PEG concentration was increased to 36%, with the mode of failure transitioning from adhesive to cohesive, allowing fibrillation within the material. A further increase in water sorption, however, caused a dilution of the entanglement structure, excessively lowering the modulus and making the material too fluid-like, leading to a decrease in adhesion. (32) Analogous to the work of Feldstein, (32−34) we believe that the optimization of the water content and, consequently, of the polymer concentration is necessary to enhance the adhesion performance.
In the PNIPAM-based adhesive system that we recently developed, (21−23) the water content is always above 90%, strongly limiting the adhesive properties. In this work, we test several strategies to reduce and control the water content within the complex coacervate phase, with the goal of enhancing the work of adhesion. The first strategy is by changing the ionic strength: a lower salt concentration results in a higher polymer concentration in the complex coacervate phase. (14) The second strategy entails increasing the polymer concentration of the solutions when preparing the complex coacervates. The third strategy involves the use of mechanical force to squeeze the water out of the complex coacervate phase by using an extruder. Earlier research has shown that the high shear forces experienced by polyelectrolyte complexes during extrusion are effective in removing liquids trapped within pores, resulting in an enhancement of the mechanical properties. (35−37)

Results and Discussion

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The polymers used in this work are poly(acrylic acid)-grafted-poly(N-isopropylacrylamide) (PAA-g-PNIPAM) and poly(dimethylaminopropylacrylamide)-grafted-poly(N-isopropylacrylamide) (PDMAPAA-g-PNIPAM). The synthesis and characterization procedure is described in an earlier publication, (21) and in the Supporting Information the composition of the polymers used (Figures S1–S3) and of the samples (Table S2) is given.

Strategy 1: Effect of Salt Concentration

The first explored strategy to reduce the water content is to decrease the salt concentration at preparation conditions. Previous work has indeed shown that complex coacervates retain less water when prepared at low ionic strength due to the stronger electrostatic interactions. (14)Figure 1A shows the water content of the analyzed samples as a function of the added salt concentration at T = 20 °C (no change in water content is observed as a function of temperature). (21,23)

Figure 1

Figure 1. (A) Water content of complex coacervates plotted as a function of the added salt concentration for the samples analyzed in Strategy 1. (B) Frequency sweeps (G′ represented as full dots and G″ as hollow dots) for graft copolymer complex coacervates below the LCST (T = 5 °C).

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In line with previous reports, (14) complex coacervates prepared from homopolymer solutions have a lower water content at lower ionic strength. However, the complex coacervates prepared from graft copolymers show a constant water content over the whole range of salt concentrations analyzed. As observed in our previous work, (23) the constant water content might be ascribed to a different distribution of water among the domains: the excess water could be absorbed, at low ionic strength, by the PNIPAM domains and, at high ionic strength (when PNIPAM chains are more prone to dehydration), (38) by the polyelectrolyte matrix. Additionally, while homopolymer complex coacervates are transparent or slightly turbid at every salt concentration, graft copolymer complex coacervates turn white at 0.5 M NaCl and below. At low ionic strength, the formation of a microporous structure, due to the water entrapment within the material, is observed: the higher opacity is attributed to the greater dimensions of the pores, which have sizes big enough to scatter light. (39)
However, even though the water content does not change significantly upon lowering the salt concentration, the mechanical properties are strongly affected, as shown in Figure 1B. At high ionic strength (GS7P1) the material exhibits characteristics of a viscoelastic fluid (Figure 1B), with both storage (G′) and loss (G″) moduli frequency dependent. A crossover between G′ and G″ is observed at a characteristic frequency ωc, from which it is possible to calculate the relaxation time τ (1/ωc): at time scales longer than τ (ω < ωc) the polymer chains can relax the applied stress by sliding along each other, while at shorter time scales (ω > ωc) the chain movement is partially inhibited, leading to a solid-like response. When lowering the salt concentration, a liquid-to-solid transition is already observed at temperatures below the LCST of PNIPAM: both moduli display a very weak frequency dependence, with G′ always higher than G″, which indicates gel formation. This behavior is due to the stronger electrostatic interactions at lower ionic strength, which slow down the relaxation of the polyelectrolyte chains. (40) This also leads to a high complex viscosity at low ionic strength (Figure S4), which makes it difficult for the sample to be injected through a small-bore needle during application. Therefore, since the water content is independent of the salt concentration, lowering the ionic strength is not an effective strategy to increase the polymer content within the adhesive matrix.

Strategy 2: Effect of Starting Polymer Concentration

The second strategy consists in increasing the polymer concentration upon mixing. Figure 2 shows a schematic phase diagram of the graft copolymer mixture, plotting the added salt concentration as a function of the polymer concentration.

Figure 2

Figure 2. (A) Complex coacervate (CC) phase diagram. In the two-phase region a gradient from yellow to red is used to indicate the transition from a liquid dilute phase (left side) to a liquid CC phase (top right side) and finally to a solid CC phase (bottom right side). (B) Effect of polymer concentration: zoom in the liquid CC area, in which the black dots represent the analyzed samples, which phase separate into a dilute phase, shown as a light blue dot, and a complex coacervate phase, shown as a red dot. (C) Samples analyzed in strategy 2.

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In the phase diagram (Figure 2A), two regions can be recognized: a one-phase region (white color) and a two-phase region (colored with a yellow-to-red gradient) in which phase separation into a dilute phase (on the left border of the diagram) and a complex coacervate phase (on the right border) occurs. In Figure 2B, we assume that the salt concentration is the same in the dilute and complex coacervate phase, in line with other reports in the literature. (14) However, it should be mentioned that other researchers have found that salt concentrations are not the same in the two phases. (41−43) The dilute phase is always a liquid aqueous solution (as indicated by the yellow color in Figure 2) at any salt concentration, while the complex coacervate phase transitions from a solid (red) to a liquid (orange) by increasing the added NaCl concentration. When surpassing the critical salt concentration (CSC), which in this system is between 0.8 and 0.85 M NaCl, the electrostatic interactions are screened to such an extent that phase separation is prevented. At 0.7 M NaCl, a phase-separated material is obtained with a viscosity low enough to ensure injectability of the adhesive: therefore, this ionic strength is set as standard for all following experiments.
When increasing the polymer concentration while maintaining the salt concentration constant, the system (black dots in Figure 2B) always phase separates into the same two phases with the same polymer concentrations (blue and red dots in Figure 2B): the only factor that changes is the volumetric ratio between the two phases, with the complex coacervate phase percentage increasing as the initial polymer concentration increases. When the polymer concentration at mixing reaches the polymer concentration in the complex coacervate phase, the system enters a one-phase region: the polymer concentration at mixing is equal to the final one, meaning that an increase in the polymer concentration in the preparation stage leads to a final material with a higher polymer concentration. To reach a concentration that is high enough to access this region, several samples, shown in Figure 2C and indicated with black dots in Figure 2B, were prepared by varying the polymer concentration in the preparation stage. When preparing complex coacervates, it is more straightforward to express the starting polymer concentration in terms of total charged monomer concentration. However, that expression does not give a clear picture of the total amount of polymer present in solution: to facilitate the comparison with the amount of polymer present in the complex coacervate phase, those values have been transformed into total polymer percentages.
As shown in Figure 3A, over the whole range of polymer concentrations analyzed, complex coacervates with nearly the same water content (around 90%), and consequently the same polymer concentration, are obtained. Figure 3B indicates that the complex coacervate volume fraction increases linearly when increasing the polymer concentration, as expected.

Figure 3

Figure 3. Effect of the polymer concentration on (A) water content and (B) complex coacervate volume fraction. The theoretical polymer concentrations of the dilute (blue dot) and complex coacervate phase (red dot) are obtained by extrapolating the regression line to y = 100% and y = 0%, respectively.

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However, contrary to our expectations, it was not possible to enter the one-phase region (and therefore increase the polymer concentration) because of solubility issues, especially concerning the anionic graft copolymer. When mixing the polyelectrolytes at a 1:1 charge ratio, the maximum reachable total polymer content is 4.39% w/v, which, as shown in Figure 3B, is lower than the polymer concentration in the complex coacervate phase (around 5.3% w/v), obtained by extrapolating the regression line in Figure 3B to 100% (border between the two-phase and one-phase region). The value obtained with this method is comparable to the one obtained by performing a thermogravimetric analysis (TGA) on the complex coacervate phase (5.5% w/v), highlighting the validity of the phase diagram. In conclusion, increasing the polymer concentration in the preparation stage is not an effective strategy to raise the polymer content in the complex coacervate phase.

Strategy 3: Extrusion

As a third strategy, we used a mini-extruder to mechanically force water out of the adhesive. The added salt concentration and the total charged monomer concentration are 0.7 M NaCl and 0.05 M, respectively. After complex coacervate formation, the dilute phase is removed. The complex coacervate is heated above its LCST and inserted through an inlet on top of the screws in the extrusion chamber. Then, the material is submitted to an extrusion cycle at 50 °C. At first, the material is moved forward in the chamber by the shearing forces of the extruder screws; its progression is then stopped when the whole sample has passed through the screws, with the complex coacervate being solidified in an inner reservoir (Figure S5). After 3 min in the chamber, the material is removed and stored in the refrigerator overnight, and if required, the following day another cycle is performed (up to a maximum of 3). After one cycle, expelled water, separated from the material, is visible in the extruded chamber (Figure S5), indicating that this strategy is effective in reducing the water content and, therefore, in increasing the polymer concentration of the material. To study the process in more detail, the effect of the extruder frequency and of the number of extrusion cycles are investigated (Figure 4, Figures S6 and S7).

Figure 4

Figure 4. Water content as a function of (A) extruder frequency and (B) number of extrusion cycles. (C) Relaxation time plotted as a function of polymer concentration (the last point is not the real value but the relaxation time corresponding to the minimum frequency accessed in the experiment).

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A drop in water content from 88% to 78% (Figure 4A), giving rise to an increase in viscoelastic moduli G′ and G″ (Figure S6), is observed after one extrusion cycle, independently of extruder frequency. Therefore, for the remaining measurements, the extruder frequency was set to 30/min. In contrast, increasing the number of extrusion cycles, after homogenizing the material in the fridge overnight between two successive cycles, significantly influences the complex coacervate phase properties. The water content decreases at a higher number of cycles, reaching 63% after three extrusion processes (Figure 4B).
Differently from the extruder frequency, a higher number of cycles profoundly affect the rheological behavior: the as-made material shows typical features of a viscous liquid at a temperature below the PNIPAM LCST, with both moduli frequency dependent (Figure S7A). G′ overcomes G″ at a crossover frequency ωc of 7.36 rad/s, corresponding to a relaxation time τ = 1/ωc = 0.85 s. After one extrusion cycle, the water content drops to 78%, with the complex coacervates still showing liquid behavior. However, the moduli become less frequency dependent and increase. The relaxation time increases by an order of magnitude to τ = 8.33 s. This behavior becomes more obvious at a higher number of cycles: after three extrusion cycles, the crossover frequency is not detectable in the window of frequencies analyzed, meaning that τ ≥ 62.5 s. This is evidence of a higher polymer concentration in the material which slows down chain relaxation, in accordance with the sticky Rouse model, which has been adopted to describe chain dynamics in complex coacervates (40,44) and which predicts a power-law increase of the relaxation time with the polymer volume fraction, as observed in Figure 4C.
When performing a temperature switch by heating the sample above the LCST of PNIPAM (between 10 and 35 °C, Figures S9 and S10), the collapse of the thermoresponsive chains is promoted: as a result, all samples acquire characteristics of soft solids, with G′ higher than G″, and both frequency independent (Figure S7B). G′ shows a power-law increase as a function of the polymer concentration (Figure 5A), highlighting that the extrusion process is effective in stiffening the material by increasing the polymer concentration: this is in accordance with the theory of rubber elasticity, which states that the storage modulus is proportional to the amount of elastically active chains per unit volume. (45)

Figure 5

Figure 5. Solidification upon temperature or salt switch. (A) Storage modulus (recorded at ω = 1 rad/s and T = 50 °C) as a function of polymer concentration. (B) Time sweeps after one extrusion cycle when performing a salt (S) or a combined salt and temperature (S+T) switch.

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The adhesive can also be reinforced by performing a salt switch, namely, by exposing the material to an ionic strength gradient. When the sample is exposed to a lower ionic strength environment (0.1 M NaCl), the excess salt ions diffuse out of the adhesive, strengthening the electrostatic interactions between the polyelectrolyte chains. This promotes a reinforcement of the material over time, with the moduli progressively increasing and heading toward a plateau after 1 h (Figure 5B). When applying a combined salt and temperature switch, different kinetics are observed (Figure 5B). The rheological properties increase more abruptly during the first stages of the transition due to the immediate collapse (response) of the thermoresponsive PNIPAM chains when exposed to a medium with a higher temperature than the LCST. This process is much faster than ion diffusion from a confined region. In contrast, for the salt switch, a contact time of 1 h is required for the full setting of the material. (22) At the end of the transition, the complex coacervate phase has the characteristics of a soft elastic solid, with increasing moduli for samples with a higher number of extrusion cycles (Figure S11A). The obtained values (Figure S11B) are similar to the ones obtained after a single temperature switch: this means that the total number of nodes is almost the same. However, in this case, both PNIPAM–PNIPAM and electrostatic interactions contribute to the final moduli, while when a temperature switch is applied, the formed network results mainly from the collapse of the thermoresponsive chains.
Figure 6 shows the rheological properties of the sample obtained after one extrusion cycle and of the homopolymer complex coacervate: despite having a different preparation history, the two materials have the same water content (78% for both samples).

Figure 6

Figure 6. Rheological properties of extruded and homopolymer complex coacervates: (A) frequency sweeps performed at 5 °C, (B) frequency sweep performed at 50 °C, and (C) frequency sweeps performed after a combined temperature and salt switch.

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At temperatures below PNIPAM LCST, the polymer concentration seems to dictate the rheological properties, with both samples showing a similar behavior (Figure 6A). However, the presence of PNIPAM is crucial above the LCST: when heated (Figure 6B), the extruded sample turns into a gel because of the collapse of the PNIPAM chains which abruptly slows down the relaxation processes, while in the homopolymer complex coacervates, no considerable variation is visible due to the absence of any temperature-sensitive unit. However, both samples show a transition in response to a combined temperature–salt switch, mimicking the conditions that the sample would experience in a physiological environment (Figure 6C). Both materials turn into soft gels, with G′ overcoming G″ at any analyzed frequency. However, the moduli of the homopolymer complex coacervates are more frequency dependent, meaning that the crossover frequency (not detectable because it is out of the frequency range analyzed) is anticipated to be higher and the relaxation time will therefore be lower. This indicates that despite a similar polymer concentration, the presence of the PNIPAM units allows the formation of a soft elastic network which is more effective in slowing down chain relaxation.
The underwater adhesive properties of the extruded graft samples are determined by using the setup developed by Sudre et al. (46) In Figure 7A, the effect of a combined temperature–salt switch is reported for samples that were exposed to different extrusion cycles.

Figure 7

Figure 7. (A) Underwater adhesion performance after application of a temperature–salt switch at a nominal strain rate of 0.2 s–1. (B) Plot of tan δ and tan δ/G′ recorded with a rheometer at ω = 1 rad/s (comparable to the strain rate in the tack measurement) as a function of the number of extrusion cycles.

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Compared to the unprocessed sample (0 cycles), the adhesive that has been submitted to one extrusion cycle shows a much higher adhesion energy, with both an increase in the peak stress (60 kPa) and a maximum strain (3500%). The mode of failure is the same in both samples, with the material failing cohesively, leaving residues on both retracting surfaces. The higher polymer concentration reinforces the material, enabling a higher resistance to an applied stress: this might be ascribed to a combination of increased stiffness, as observed in Figure 5A, and dissipation, characterized by the damping factor tan δ (G″/G′), which shows a 50% increase after one extrusion cycle (Figure 7B). However, a further increase in polymer concentration, obtained when performing additional extrusion cycles, leads to a decrease in the adhesive performance: despite showing a higher peak stress than the unprocessed sample, the strain at break decreases significantly. The mode of failure also changes, with the material now failing adhesively, without residues on the probe. This suggests a much less viscoelastic and more elastic gel-like behavior. In adhesion science, this is generally quantified by the ratio of the damping factor to the storage modulus, employed as a gauge of expected adhesive performance. (47) An excessively high value of G′, relative to the dissipative character of the adhesive, will prevent cavity formation in the vertical direction: (47) this means that when the material is too stiff and too elastic, i.e., a too low value of tan δ/G′ (Figure 7B), resistance to interfacial crack propagation is too low, and the material fails adhesively at low strain. (47) Therefore, an optimal polymer concentration is needed for a balance between cohesive and adhesive properties. (48)
Another requirement that an underwater adhesive should meet is the dimensional stability in submerged conditions. (25,49) When performing an extrusion process, the complex coacervate phase is brought in an out of equilibrium condition and, if submerged, could likely reabsorb the water which has been previously removed. Swelling experiments were therefore performed on the samples in conditions mimicking the physiological environment (T = 37 °C, [NaCl] = 0.1 M , pH = 7.0) over a period of 1 h to check the water uptake/release in the time scale of the experiment (Figure 8A). The unprocessed sample, with the highest water content, shows a negative swelling ratio: this is an indication of shrinking, mainly due to the collapse of the PNIPAM chains, as already observed in other work on thermoresponsive adhesives, (25) and to the contraction of the polyelectrolyte matrix at lower ionic strength. Additionally, the sample turns white (Figure S12)—evidence that water also remains trapped within the material, forming a porous structure, as already proposed in our previous work. (21−23)

Figure 8

Figure 8. (A) Swelling ratio versus the number of extrusion cycles. The dashed line represents the border between swelling (above the line) and shrinking (below the line). (B) Swelling ratio as a function of time for the sample extruded one time in physiological conditions (T = 37 °C, [NaCl] = 0.1 M, pH = 7.0).

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The swelling ratio increases as a function of extrusion cycles: the samples submitted to one extrusion cycle shows a higher dimensional stability, exhibiting a swelling ratio close to zero (Figure 8A), similar to what is observed in hydrophobic tissue adhesives. (29,30) This means that the water removed through the extrusion process is not reabsorbed by the sample (no swelling is observed over a 5 day period, Figure 8B). However, when the amount of extrusion cycles is further increased, the material swells due to water sorption (Figure S12A), which might also contribute to the decrease in the adhesive performance as seen in Figure 7, as reported for PEG-based tissue adhesives. (28) When decreasing the amount of water within the material, the adhesive, after setting, becomes less opaque (Figure S12)—evidence of a lower amount and/or smaller size pores containing trapped water within the material. (36)
Therefore, the optimal balance between adhesive and cohesive properties, together with a proper dimensional stability, leads to a large enhancement of the adhesion performance in the sample submitted to only one extrusion cycle.
A poorer adhesive performance is observed when submitting the samples only to a temperature trigger (Figure 9A). Despite obtaining similar moduli when reinforcing the material with a single or a combined trigger (Figure 5A and Figure S11B), the sample fails in an adhesive fashion at a much lower strain when applying only a temperature trigger: this could be correlated to the size of the polymers involved in the reinforcing mechanism. When just a temperature trigger is performed, only the short PNIPAM chains are collapsed, forming domains of a small size. Thereby, when applying a detaching stress, the chains can be stretched only to a small extent. By contrast, by decreasing the ionic strength, also the electrostatic interactions between the long backbones are activated. To break the sample, also these additional bonds need to be disrupted and the adhesive can be stretched to a higher extent before failure, resulting in a higher toughness. Additionally, this behavior might also be ascribed to the more viscoelastic character of the material after a combined switch, as confirmed by the slightly higher values of the damping factor and of tan δ/G′ as compared to values obtained for a single temperature switch (Figure S13A).

Figure 9

Figure 9. Underwater adhesive performance of the sample after one extrusion cycle after application of a temperature–salt switch compared to (A) the application of a single temperature switch and (B) the performance of the homopolymer complex coacervate upon a combined trigger.

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Similarly, when probing the homopolymer counterpart with the same water content in physiological conditions, we observed an adhesive failure at low strain (Figure 9B). This behavior is surprising since, as observed in Figure S13B, the homopolymer complex coacervates have a more dissipative character than the extruded sample. In this case, the linear rheology properties fail to predict the nonlinear behavior observed in the probe-tack test. The improved adhesive performance shown by the extruded sample might indicate that the presence of two types of interactions in the same material (PNIPAM nodes and electrostatic interactions) favors an increase in toughness (not detectable via linear rheology), which might be related to the variety of bond strength present in the adhesive, as observed for polyampholyte hydrogels and double networks. (50−52) Additionally, the higher (in absolute value) swelling ratio (−23.4%) reported in our previous work (22) for homopolymer complex coacervates measured in similar conditions might affect the adhesive properties: a higher amount of released water might weaken the interfacial interactions with the probe, resulting in a lower dimensional stability and a worse adhesive performance compared to the extruded samples, which show a lower swelling ratio (−9.6%).
Lastly, in Figure 10 the work of adhesion (Wadh), obtained from the area below the stress–strain curves, is plotted versus the amount of extrusion cycles.

Figure 10

Figure 10. Work of adhesion versus the amount of extrusion cycles and versus the applied trigger.

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After one extrusion cycle, when performing a combined switch, the work of adhesion increases from 3.8 to 60.6 J/m2, which is much higher than the one shown by the homopolymer counterpart (4.7 J/m2, not shown in the figure) and the one obtained after performing a temperature switch only (4.6 J/m2). When further raising the number of extrusion cycles, the work of adhesion drastically decreases, with a similar trend observed for both triggers. An optimum underwater performance is obtained when a porous structure is preserved, the adhesive and cohesive properties are well balanced, and the in situ swelling is limited.

Conclusions

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Polymer concentration strongly affects the mechanical properties of an adhesive. PNIPAM-functionalized complex coacervates exhibit a high water content, which inevitably affects the moduli and the adhesive properties of the material. Several routes have been adopted in this work to optimize the water content: among the attempted strategies, extrusion is the most effective in increasing the polymer concentration within the complex coacervate phase.
While a higher number of extrusion cycles does lead to a progressive enhancement of the dynamic moduli, the underwater Wadh initially benefits from the higher polymer concentration within the material but then drastically drops when the water content is further decreased. By carefully tuning the polymer concentration, promising underwater adhesion data is achieved, reaching an adhesive strength of 60 kPa and work of adhesion of 60 J/m2: these values are higher than those reported for commercial adhesives measured underwater, such as fibrin glues, and comparable to the highest values reported for bioinspired adhesives tested in similar conditions. (53,54) However, differently from those materials, which are already in the solid state before application or need to be solidified in situ with external agents, this glue sets immediately when released in physiological conditions, experiencing an environmentally triggered phase transition. Further studies are required to systematically address the role of the microstructure on the adhesion properties.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsapm.0c00185.

  • Experimental section, synthesis details of the polymers, additional rheology and swelling data (PDF)

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Author Information

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  • Corresponding Author
    • Marleen Kamperman - Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands;  Polymer Science, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands;  Orcidhttp://orcid.org/0000-0002-0520-4534;  Email: [email protected]
  • Authors
    • Marco Dompé - Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
    • Mehdi Vahdati - Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France
    • Froukje van Ligten - Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
    • Francisco J. Cedano-Serrano - Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France;  Orcidhttp://orcid.org/0000-0001-9792-2551
    • Dominique Hourdet - Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France;  Orcidhttp://orcid.org/0000-0002-0328-7014
    • Costantino Creton - Soft Matter Sciences and Engineering, ESPCI Paris, PSL University, Sorbonne University, CNRS, F-75005 Paris, France;  Orcidhttp://orcid.org/0000-0002-0177-9680
    • Marco Zanetti - Department of Chemistry and NIS Centre, University of Turin, Via P. Giuria 7, Torino 10125, Italy
    • Pierangiola Bracco - Department of Chemistry and NIS Centre, University of Turin, Via P. Giuria 7, Torino 10125, Italy
    • Jasper van der Gucht - Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands;  Orcidhttp://orcid.org/0000-0001-5525-8322
    • Thomas Kodger - Laboratory of Physical Chemistry and Soft Matter, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
  • Funding

    The project received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 642861. J.v.d.G. acknowledges the European Research Council for financial support (ERC Consolidator grant Softbreak).

  • Notes
    The authors declare no competing financial interest.

References

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    Despite the recent progress in and demand for wet adhesives, practical underwater adhesion remains limited or non-existent for diverse applications. Translation of mussel-inspired wet adhesion typically entails catechol functionalization of polymers and/or polyelectrolytes, and soln. processing of many complex components and steps that require optimization and stabilization. Here we reduced the complexity of a wet adhesive primer to synthetic low-mol.-wt. catecholic zwitterionic surfactants that show very strong adhesion (∼50 mJ m-2) and retain the ability to coacervate. This catecholic zwitterion adheres to diverse surfaces and self-assembles into a molecularly smooth, thin (<4 nm) and strong glue layer. The catecholic zwitterion holds particular promise as an adhesive for nanofabrication. This study significantly simplifies bio-inspired themes for wet adhesion by combining catechol with hydrophobic and electrostatic functional groups in a small mol.
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  18. 18
    Seo, S.; Das, S.; Zalicki, P. J.; Mirshafian, R.; Eisenbach, C. D.; Israelachvili, J. N.; Waite, J. H.; Ahn, B. K. Microphase Behavior and Enhanced Wet-Cohesion of Synthetic Copolyampholytes Inspired by a Mussel Foot Protein. J. Am. Chem. Soc. 2015, 137 (29), 9214– 9217,  DOI: 10.1021/jacs.5b03827
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    18
    Microphase Behavior and Enhanced Wet-Cohesion of Synthetic Copolyampholytes Inspired by a Mussel Foot Protein
    Seo, Sungbaek; Das, Saurabh; Zalicki, Piotr J.; Mirshafian, Razieh; Eisenbach, Claus D.; Israelachvili, Jacob N.; Waite, J. Herbert; Ahn, B. Kollbe
    Journal of the American Chemical Society (2015), 137 (29), 9214-9217CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Numerous attempts have been made to translate mussel adhesion to diverse synthetic platforms. However, the translation remains largely limited to the Dopa (3,4-dihydroxyphenylalanine) or catechol functionality, which continues to raise concerns about Dopa's inherent susceptibility to oxidn. Mussels have evolved adaptations to stabilize Dopa against oxidn. For example, in mussel foot protein 3 slow (mfp-3s, one of two electrophoretically distinct interfacial adhesive proteins in mussel plaques), the high proportion of hydrophobic amino acid residues in the flanking sequence around Dopa increases Dopa's oxidn. potential. In this study, copolyampholytes, which combine the catechol functionality with amphiphilic and ionic features of mfp-3s, were synthesized and formulated as coacervates for adhesive deposition on surfaces. The ratio of hydrophilic/hydrophobic as well as cationic/anionic units was varied in order to enhance coacervate formation and wet adhesion properties. Aq. solns. of two of the four mfp-3s-inspired copolymers showed coacervate-like spherical microdroplets (.vphi. ≈ 1-5 μm) at pH ∼4 (salt concn. ∼15 mM). The mfp-3s-mimetic copolymer was stable to oxidn., formed coacervates that spread evenly over mica, and strongly bonded to mica surfaces (pull-off strength: ∼17.0 mJ/m2). Increasing pH to 7 after coacervate deposition at pH 4 doubled the bonding strength to ∼32.9 mJ/m2 without oxidative crosslinking and is about 9 times higher than native mfp-3s cohesion. This study expands the scope of translating mussel adhesion from simple Dopa-functionalization to mimicking the context of the local environment around Dopa.
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  19. 19
    Zhao, Q.; Lee, D. W.; Ahn, B. K.; Seo, S.; Kaufman, Y.; Israelachvili, J. N.; Waite, J. H. Underwater contact adhesion and microarchitecture in polyelectrolyte complexes actuated by solvent exchange. Nat. Mater. 2016, 15, 407,  DOI: 10.1038/nmat4539
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    19
    Underwater contact adhesion and microarchitecture in polyelectrolyte complexes actuated by solvent exchange
    Zhao, Qiang; Lee, Dong Woog; Ahn, B. Kollbe; Seo, Sungbaek; Kaufman, Yair; Israelachvili, Jacob N.; Waite, J. Herbert
    Nature Materials (2016), 15 (4), 407-412CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Polyelectrolyte complexation is crit. to the formation and properties of many biol. and polymeric materials, and is typically initiated by aq. mixing followed by fluid-fluid phase sepn., such as coacervation. Yet little to nothing is known about how coacervates evolve into intricate solid microarchitectures. Inspired by the chem. features of the cement proteins of the sandcastle worm, here we report a versatile and strong wet-contact microporous adhesive resulting from polyelectrolyte complexation triggered by solvent exchange. After premixing a catechol-functionalized weak polyanion with a polycation in DMSO, the soln. was applied underwater to various substrates whereupon electrostatic complexation, phase inversion, and rapid setting were simultaneously actuated by water-DMSO solvent exchange. Spatial and temporal coordination of complexation, inversion and setting fostered rapid (∼25 s) and robust underwater contact adhesion (Wad ≥ 2 J m-2) of complexed catecholic polyelectrolytes to all tested surfaces including plastics, glasses, metals and biol. materials.
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  20. 20
    Zhang, C.; Huang, J.; Zhang, J.; Liu, S.; Cui, M.; An, B.; Wang, X.; Pu, J.; Zhao, T.; Fan, C.; Lu, T. K.; Zhong, C. Engineered Bacillus subtilis biofilms as living glues. Mater. Today 2019, 28, 40– 48,  DOI: 10.1016/j.mattod.2018.12.039
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  21. 21
    Dompé, M.; Cedano-Serrano, F. J.; Heckert, O.; van den Heuvel, N.; van der Gucht, J.; Tran, Y.; Hourdet, D.; Creton, C.; Kamperman, M. Thermoresponsive Complex Coacervate-Based Underwater Adhesive. Adv. Mater. 2019, 31 (21), 1808179,  DOI: 10.1002/adma.201808179
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  22. 22
    Dompe, M.; Cedano-Serrano, F. J.; Vahdati, M.; Westerveld, L.; Hourdet, D.; Creton, C.; der Gucht, J.; Kodger, T.; Kamperman, M. Underwater Adhesion of Multiresponsive Complex Coacervates. Adv. Mater. Interfaces 2020, 7, 1901785,  DOI: 10.1002/admi.201901785
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    22
    Underwater Adhesion of Multiresponsive Complex Coacervates
    Dompe, Marco; Cedano-Serrano, Francisco J.; Vahdati, Mehdi; van Westerveld, Larissa; Hourdet, Dominique; Creton, Costantino; van der Gucht, Jasper; Kodger, Thomas; Kamperman, Marleen
    Advanced Materials Interfaces (2020), 7 (4), 1901785CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Many marine organisms have developed adhesives that are able to bond under water, overcoming the challenges assocd. with wet adhesion. A key element in the processing of several natural underwater glues is complex coacervation, a liq.-liq. phase sepn. driven by complexation of oppositely charged macromols. Inspired by these examples, the development of a fully synthetic complex coacervate-based adhesive is reported with an in situ setting mechanism, which can be triggered by a change in temp. and/or a change in ionic strength. The adhesive consists of a matrix of oppositely charged polyelectrolytes that are modified with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) grafts. The adhesive, which initially starts out as a fluid complex coacervate with limited adhesion at room temp. and high ionic strength, transitions into a viscoelastic solid upon an increase in temp. and/or a decrease in the salt concn. of the environment. Consequently, the thermoresponsive chains self-assoc. into hydrophobic domains and/or the polyelectrolyte matrix contracts, without inducing any macroscopic shrinking. The presence of PNIPAM favors energy dissipation by softening the material and by allowing crack blunting. The high work of adhesion, the gelation kinetics, and the easy tunability of the system make it a potential candidate for soft tissue adhesion in physiol. environments.
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  23. 23
    Dompé, M.; Cedano-Serrano, F. J.; Vahdati, M.; Sidoli, U.; Heckert, O.; Synytska, A.; Hourdet, D.; Creton, C.; Van der Gucht, J.; Kodger, T.; Kamperman, M. Tuning the Interactions in Multiresponsive Complex Coacervate-Based Underwater Adhesives. Int. J. Mol. Sci. 2020, 21, 100,  DOI: 10.3390/ijms21010100
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    23
    Tuning the interactions in multiresponsive complex coacervate-based underwater adhesives
    Dompe, Marco; Cedano-Serrano, Francisco J.; Vahdati, Mehdi; Sidoli, Ugo; Heckert, Olaf; Synytska, Alla; Hourdet, Dominique; Creton, Costantino; Van Der Gucht, Jasper; Kodger, Thomas; Kamperman, Marleen
    International Journal of Molecular Sciences (2020), 21 (1), 100CODEN: IJMCFK; ISSN:1422-0067. (MDPI AG)
    In this work, we report the systematic investigation of a multiresponsive complex coacervate-based underwater adhesive, obtained by combining polyelectrolyte domains and thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) units. This material exhibits a transition from liq. to solid but, differently from most reactive glues, is completely held together by non-covalent interactions, i.e., electrostatic and hydrophobic. Because the solidification results in a kinetically trapped morphol., the final mech. properties strongly depend on the prepn. conditions and on the surrounding environment. A systematic study is performed to assess the effect of ionic strength and of PNIPAM content on the thermal, rheol. and adhesive properties. This study enables the optimization of polymer compn. and environmental conditions for this underwater adhesive system. The best performance with a work of adhesion of 6.5 J/m2 was found for the complex coacervates prepd. at high ionic strength (0.75 M NaCl) and at an optimal PNIPAM content around 30% mol/mol. The high ionic strength enables injectability, while the hydrated PNIPAM domains provide addnl. dissipation, without softening the material so much that it becomes too weak to resist detaching stress.
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  24. 24
    Kelmansky, R.; McAlvin, B. J.; Nyska, A.; Dohlman, J. C.; Chiang, H. H.; Hashimoto, M.; Kohane, D. S.; Mizrahi, B. Strong tissue glue with tunable elasticity. Acta Biomater. 2017, 53, 93– 99,  DOI: 10.1016/j.actbio.2017.02.009
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    24
    Strong tissue glue with tunable elasticity
    Kelmansky, Regina; McAlvin, Brian J.; Nyska, Abraham; Dohlman, Jenny C.; Chiang, Homer H.; Hashimoto, Michinao; Kohane, Daniel S.; Mizrahi, Boaz
    Acta Biomaterialia (2017), 53 (), 93-99CODEN: ABCICB; ISSN:1742-7061. (Elsevier Ltd.)
    Many bio-adhesive materials adhere weakly to tissue due to their high water content and weak structural integrity. Others provide desirable adhesive strength but suffer from rigid structure and lack of elasticity after administration. We have developed two water-free, liq. four-armed PEG pre-polymers modified with NHS or with NH2 end groups which upon mixing changed from liqs. to an elastic solid. The sealant and adhesive properties increased with the amt. of the %vol./vol. PEG4-NHS pre-polymer, and achieved adhesive properties comparable to those of cyanoacrylate glues. All mixts. showed minimal cytotoxicity in vitro. Mixts. of 90%vol./vol. PEG4-NHS were retained in the s.c. space in vivo for up to 14 days with minimal inflammation. This material's combination of desirable mech. properties and biocompatibility has potential in numerous biomedical applications. Many bio-adhesive materials adhere weakly to tissue (e.g. hydrogels) due to their high water content and weak structural integrity. Others provide desirable mech. properties but suffer from poor biocompatibility (e.g. cyanoacrylates). This study proposes a new concept for the formation of super strong and tunable tissue glues. Our bio-materials' enhanced performance is the product of new neat (without water or other solvents) liq. polymers that solidify after administration while allowing interactions with the tissue. Moreover, the elastic modulus of these materials could easily be tuned without compromising biocompatibility. This system could be an attractive alternative to sutures and staples since it can be applied more quickly, causes less pain and may require less equipment while maintaining the desired adhesion strength.
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  25. 25
    Barrett, D. G.; Bushnell, G. G.; Messersmith, P. B. Mechanically Robust, Negative-Swelling, Mussel-Inspired Tissue Adhesives. Adv. Healthcare Mater. 2013, 2 (5), 745– 755,  DOI: 10.1002/adhm.201200316
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    25
    Mechanically Robust, Negative-Swelling, Mussel-Inspired Tissue Adhesives
    Barrett, Devin G.; Bushnell, Grace G.; Messersmith, Phillip B.
    Advanced Healthcare Materials (2013), 2 (5), 745-755CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Most synthetic polymer hydrogel tissue adhesives and sealants swell considerably in physiol. conditions, which can result in mech. weakening and adverse medical complications. This paper describes the synthesis and characterization of mech. tough zero- or neg.-swelling mussel-inspired surgical adhesives based on catechol-modified amphiphilic poly(propylene oxide)-poly(ethylene oxide) block copolymers. The formation, swelling, bulk mech., and tissue adhesive properties of the resulting thermosensitive gels were characterized. Catechol oxidn. at or below room temp. rapidly resulted in a chem. crosslinked network, with subsequent warming to physiol. temp. inducing a thermal hydrophobic transition in the PPO domains and providing a mechanism for volumetric redn. and mech. toughening. The described approach can be easily adapted for other thermally sensitive block copolymers and crosslinking strategies, representing a general approach that can be employed to control swelling and enhance mech. properties of polymer hydrogels used in a medical context.
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  26. 26
    Lee, G.; Lee, C. K.; Bynevelt, M. DuraSeal-Hematoma: Concealed Hematoma Causing Spinal Cord Compression. Spine 2010, 35 (25), E1522– E1524,  DOI: 10.1097/BRS.0b013e3181edfe2c
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    Mulder, M.; Crosier, J.; Dunn, R. Cauda Equina Compression by Hydrogel Dural Sealant After a Laminotomy and Discectomy: Case Report. Spine 2009, 34 (4), E144– E148,  DOI: 10.1097/BRS.0b013e31818d5427
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    Campbell, P. K.; Bennett, S. L.; Driscoll, A.; Sawhney, A. S. Evaluation of absorbable surgical sealants: in vitro testing. https://pdfs.semanticscholar.org/0e72/159a6027168d8ecb11dcd2375ad692c30ab3.pdf, 2005.
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    Lang, N.; Pereira, M. J.; Lee, Y.; Friehs, I.; Vasilyev, N. V.; Feins, E. N.; Ablasser, K.; O’Cearbhaill, E. D.; Xu, C.; Fabozzo, A.; Padera, R.; Wasserman, S.; Freudenthal, F.; Ferreira, L. S.; Langer, R.; Karp, J. M.; del Nido, P. J. A Blood-Resistant Surgical Glue for Minimally Invasive Repair of Vessels and Heart Defects. Sci. Transl. Med. 2014, 6 (218), 218ra6  DOI: 10.1126/scitranslmed.3006557
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    Nijst, C. L. E.; Bruggeman, J. P.; Karp, J. M.; Ferreira, L.; Zumbuehl, A.; Bettinger, C. J.; Langer, R. Synthesis and Characterization of Photocurable Elastomers from Poly(glycerol-co-sebacate). Biomacromolecules 2007, 8 (10), 3067– 3073,  DOI: 10.1021/bm070423u
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    Synthesis and Characterization of Photocurable Elastomers from Poly(glycerol-co-sebacate)
    Nijst, Christiaan L. E.; Bruggeman, Joost P.; Karp, Jeffrey M.; Ferreira, Lino; Zumbuehl, Andreas; Bettinger, Christopher J.; Langer, Robert
    Biomacromolecules (2007), 8 (10), 3067-3073CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    Elastomeric networks are increasingly being investigated for a variety of biomedical applications including drug delivery and tissue engineering. However, in some cases, their prepn. requires the use of harsh processing conditions (e.g., high temp.), which limits their biomedical application. Herein, we demonstrate the ability to form elastomeric networks from poly(glycerol-co-sebacate) acrylate (PGSA) under mild conditions while preserving a wide range of phys. properties. These networks presented a Young's modulus between 0.05 and 1.38 MPa, an ultimate strength from 0.05 to 0.50 MPa, and elongation at break between 42% and 189% strain, by varying the degree of acrylation (DA) of PGSA. The in vitro enzymic and hydrolytic degrdn. of the polymer networks was dependent on the DA. The copolymn. of poly(ethylene glycol) diacrylate with PGSA allowed for an addnl. control of mech. properties and swelling ratios in an aq. environment, as well as enzymic and hydrolytic degrdn. Photocured PGSA networks demonstrated in vitro biocompatibility as judged by sufficient human primary cell adherence and subsequent proliferation into a confluent monolayer. These photocurable degradable elastomers could have potential application for the encapsulation of temp.-sensitive factors and cells for tissue engineering.
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  31. 31
    Yuk, H.; Varela, C. E.; Nabzdyk, C. S.; Mao, X.; Padera, R. F.; Roche, E. T.; Zhao, X. Dry double-sided tape for adhesion of wet tissues and devices. Nature 2019, 575 (7781), 169– 174,  DOI: 10.1038/s41586-019-1710-5
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    Dry double-sided tape for adhesion of wet tissues and devices
    Yuk, Hyunwoo; Varela, Claudia E.; Nabzdyk, Christoph S.; Mao, Xinyu; Padera, Robert F.; Roche, Ellen T.; Zhao, Xuanhe
    Nature (London, United Kingdom) (2019), 575 (7781), 169-174CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Two dry surfaces can instantly adhere upon contact with each other through intermol. forces such as hydrogen bonds, electrostatic interactions and van der Waals interactions1,2. However, such instant adhesion is challenging when wet surfaces such as body tissues are involved, because water separates the mols. of the two surfaces, preventing interactions3,4. Although tissue adhesives have potential advantages over suturing or stapling5,6, existing liq. or hydrogel tissue adhesives suffer from several limitations: weak bonding, low biol. compatibility, poor mech. match with tissues, and slow adhesion formation5-13. Here we propose an alternative tissue adhesive in the form of a dry double-sided tape (DST) made from a combination of a biopolymer (gelatin or chitosan) and crosslinked poly(acrylic acid) grafted with N-hydrosuccinimide ester. The adhesion mechanism of this DST relies on the removal of interfacial water from the tissue surface, resulting in fast temporary crosslinking to the surface. Subsequent covalent crosslinking with amine groups on the tissue surface further improves the adhesion stability and strength of the DST. In vitro mouse, in vivo rat and ex vivo porcine models show that the DST can achieve strong adhesion between diverse wet dynamic tissues and engineering solids within five seconds. The DST may be useful as a tissue adhesive and sealant, and in adhering wearable and implantable devices to wet tissues.
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    Feldstein, M. M. Molecular Nature of Pressure-Sensitive Adhesion. In Fundamentals of Pressure Sensitivity; Benedek, I., Feldstein, M. M., Eds.; Taylor & Francis Group, LLC: Boca Raton, FL, 2008.
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    Chalykh, A. A.; Chalykh, A. E.; Novikov, M. B.; Feldstein, M. M. Pressure-sensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths. J. Adhes. 2002, 78 (8), 667– 694,  DOI: 10.1080/00218460213491
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    Pressure-sensitive adhesion in the blends of poly(N-vinylpyrrolidone) and poly(ethylene glycol) of disparate chain lengths
    Chalykh, Anna A.; Chalykh, Anatoly E.; Novikov, Mikhail B.; Feldstein, Mikhail M.
    Journal of Adhesion (2002), 78 (8), 667-694CODEN: JADNAJ; ISSN:0021-8464. (Taylor & Francis Ltd.)
    The adhesive behavior of blends of high mol. wt. poly(N-vinylpyrrolidone) (PVP) and short-chain, liq. PEG was studied using a 180° peel test as a function of compn. and water vapor sorption. Pressure-sensitive adhesion appears within a narrow compn. range (∼36 wt% PEG), and it is affected by the blend hydration. Both plasticizers, PEG and water, behave as tackifiers in the blends with glassy PVP. However, PEG alone is shown to account for the occurrence of adhesion, and the tackifying effect of PEG is appreciably stronger than that of sorbed water. Blend hydration enhances adhesion for the systems that exhibit an apparently adhesive type of debonding from a std. substrate (at PEG content <36 wt%), but the same amts. of sorbed water also are capable of depressing adhesion in the PEG-overloaded blends, where a cohesive mechanism of adhesive joint failure is typical. The blend contg. 36 wt% PEG couples both the adhesive and cohesive mechanisms of bond rupture (i.e., the fibrillation of adhesive polymer under debonding force and predominantly adhesive locus of failure). Blend hydration effects on adhesion are reversible.
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    Roos, A.; Creton, C.; Novikov, M. B.; Feldstein, M. M. Viscoelasticity and tack of poly(vinyl pyrrolidone)-poly(ethylene glycol) blends. J. Polym. Sci., Part B: Polym. Phys. 2002, 40 (20), 2395– 2409,  DOI: 10.1002/polb.10279
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    34
    Viscoelasticity and tack of poly(vinyl pyrrolidone)-poly(ethylene glycol) blends
    Roos, Alexandra; Creton, Costantino; Novikov, Mikhail B.; Feldstein, Mikhail M.
    Journal of Polymer Science, Part B: Polymer Physics (2002), 40 (20), 2395-2409CODEN: JPBPEM; ISSN:0887-6266. (John Wiley & Sons, Inc.)
    The adhesive properties of blends of high mol. wt. poly(vinylpyrrolidone) (PVP) and low mol. wt. polyethylene glycol (PEG) were systematically investigated with a probe test and correlated with their viscoelastic properties. The material parameters that were varied were the PEG content (31, 36, and 41%) and the debonding rate. The 36% PEG showed the best balance of properties for a pressure-sensitive adhesive. At low debonding rates, the debonding took place through the formation of a fibrillar structure, whereas at high debonding rates, the debonding was brittle. This transition was attributed to the breakage and reformation of hydrogen bonds between PVP units and OH groups on PEG during the large strain of the polymer chains in elongation. This transition was obsd., albeit shifted in frequency, for all three compns., and the characteristic relaxation times of the hydrogen-bonded network were estd. A comparison between the tack properties of the adhesives and their linear viscoelastic properties showed a very strong decoupling between the small-strain and large-strain properties of the adhesive, which was indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen during tensile tests, was attributed to the peculiar phase behavior of the blends.
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    Fu, J.; Wang, Q.; Schlenoff, J. B. Extruded Superparamagnetic Saloplastic Polyelectrolyte Nanocomposites. ACS Appl. Mater. Interfaces 2015, 7 (1), 895– 901,  DOI: 10.1021/am5074694
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    Extruded Superparamagnetic Saloplastic Polyelectrolyte Nanocomposites
    Fu, Jingcheng; Wang, Qifeng; Schlenoff, Joseph B.
    ACS Applied Materials & Interfaces (2015), 7 (1), 895-901CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Iron oxide nanoparticles of diam. ca. 12 nm were dispersed into polyelectrolyte complexes made from poly(styrenesulfonate) and poly(diallyldimethylammonium). These nanocomposites were plasticized with salt water and extruded into dense, tough fibers. Magnetometry of these composites showed they retained the superparamagnetic properties of their constituent nanoparticles with satn. magnetization that scaled with the loading of nanoparticles. Their superparamagnetic response allowed the composites to be heated remotely by radiofrequency fields. While the modulus of fibers was unaffected by the presence of nanoparticles the toughness and tensile strength increased significantly.
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    Shamoun, R. F.; Reisch, A.; Schlenoff, J. B. Extruded Saloplastic Polyelectrolyte Complexes. Adv. Funct. Mater. 2012, 22 (9), 1923– 1931,  DOI: 10.1002/adfm.201102787
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    Extruded Saloplastic Polyelectrolyte Complexes
    Shamoun, Rabih F.; Reisch, Andreas; Schlenoff, Joseph B.
    Advanced Functional Materials (2012), 22 (9), 1923-1931CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Tough, dense polyelectrolyte complexes (PECs) with well-defined cross-sections are prepd. using a lab. extruder and plasticizing the complexes with salt water. Stoichiometric starting materials yield stoichiometric complexes of poly(diallyldimethylammonium) (PDADMA) and poly(styrene sulfonate) (PSS). As an example of this enabling technol., macroscopic tubes of PEC are produced. Microscopy images of cross-sections of rods, tape, and tubes show a pore vol. of less than 10% in the bulk of the extruded complex and fully dense material towards the surface, where the shear is greatest. Thermal gravimetric anal. reveals the expected salt content for PECs doped with NaCl, and a lack of salt for PECs rinsed in water. The fact that doped PECs are transparent suggests they are supersatd. with salt. Residual stress following extrusion is relieved by exposure to solns. of NaCl. Stress relaxation expts. show decreasing equil. moduli as a function of increasing salt doping, consistent with prior results on multilayers of the same polymers.
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    Wang, Q.; Schlenoff, J. B. Tough strained fibers of a polyelectrolyte complex: pretensioned polymers. RSC Adv. 2014, 4 (87), 46675– 46679,  DOI: 10.1039/C4RA08733J
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    Tough strained fibers of a polyelectrolyte complex: pretensioned polymers
    Wang, Qifeng; Schlenoff, Joseph B.
    RSC Advances (2014), 4 (87), 46675-46679CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Polyelectrolyte complexes, PECs, are formed spontaneously by the interaction of oppositely charged polyelectrolytes. When hydrated, PECs are tough, elastic, biocompatible materials, but when dry they are hard and brittle. In either form, PECs have long been considered 'unprocessable'. Here, we show that PECs are transformed from brittle to tough (when dry) by extrusion into highly strained fibers. Partial mol. strain in dry fibers may be relaxed by exposure to solns. of salt or hot water under a salt/temp. equivalence. Efficient shape memory of a complex strained in hot water is possible by cooling and recovering the original shape later in hot water.
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  38. 38
    Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. Specific Ion Effects on the Water Solubility of Macromolecules: PNIPAM and the Hofmeister Series. J. Am. Chem. Soc. 2005, 127 (41), 14505– 14510,  DOI: 10.1021/ja0546424
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    38
    Specific Ion Effects on the Water Solubility of Macromolecules: PNIPAM and the Hofmeister Series
    Zhang, Yanjie; Furyk, Steven; Bergbreiter, David E.; Cremer, Paul S.
    Journal of the American Chemical Society (2005), 127 (41), 14505-14510CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Aq. processes ranging from protein folding and enzyme turnover to colloidal ordering and macromol. pptn. are sensitive to the nature and concn. of the ions present in soln. Herein, the effect of a series of sodium salts on the lower crit. soln. temp. (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated with a temp. gradient microfluidic device under a dark-field microscope. While the ability of a particular anion to lower the LCST generally followed the Hofmeister series, anal. of solvent isotope effects and of the changes in LCST with ion concn. and identity showed multiple mechanisms were at work. In solns. contg. sufficient concns. of strongly hydrated anions, the phase transition of PNIPAM was directly correlated with the hydration entropy of the anion. On the other hand, weakly hydrated anions were salted-out through surface tension effects and displayed improved hydration by direct ion binding.
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  39. 39
    Hariri, H. H.; Lehaf, A. M.; Schlenoff, J. B. Mechanical Properties of Osmotically Stressed Polyelectrolyte Complexes and Multilayers: Water as a Plasticizer. Macromolecules 2012, 45 (23), 9364– 9372,  DOI: 10.1021/ma302055m
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    39
    Mechanical Properties of Osmotically Stressed Polyelectrolyte Complexes and Multilayers: Water as a Plasticizer
    Hariri, Haifa H.; Lehaf, Ali M.; Schlenoff, Joseph B.
    Macromolecules (Washington, DC, United States) (2012), 45 (23), 9364-9372CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Compacted, macroporous complexes of poly(styrenesulfonate) and poly(diallyldimethylammonium chloride) were dehydrated under defined osmotic stress using poly(ethylene glycol), PEG. A strong mech. response to dehydration was obsd. At the lowest osmotic stress applied, macropores within the complex were compacted, and the material became transparent. With addnl. osmotic stress, the decrease in water content with increasing stress slowed considerably, but the complex became much stiffer, the equil. modulus reaching several hundred MPa. Concurrently, the complexes became more brittle. Multilayers of the same polyelectrolytes reached equil. hydration levels much faster and also increased significantly in modulus. Using an empirical fit, the plasticizing efficiency of water was shown to be exceptionally strong.
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    Spruijt, E.; Cohen Stuart, M. A.; van der Gucht, J. Linear Viscoelasticity of Polyelectrolyte Complex Coacervates. Macromolecules 2013, 46 (4), 1633– 1641,  DOI: 10.1021/ma301730n
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    40
    Linear Viscoelasticity of Polyelectrolyte Complex Coacervates
    Spruijt, Evan; Cohen Stuart, Martien A.; van der Gucht, Jasper
    Macromolecules (Washington, DC, United States) (2013), 46 (4), 1633-1641CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Two flexible, oppositely charged polymers can form liq.-like complex coacervate phases with rich but poorly understood viscoelastic properties. They serve as an exptl. model system for many biol. and man-made materials made from oppositely charged macromols. We use rheol. to systematically study the viscoelastic properties as a function of salt concn., chain length, chain length matching, and mixing stoichiometry of model complex coacervates of poly(N,N-dimethylaminoethyl methacrylate), PDMAEMA, and poly(acrylic acid), PAA. The dynamics of making and breaking ionic bonds between the oppositely charged chains underlie all linear viscoelastic properties of the complex coacervates. We treat (clusters of) ionic bonds as sticky points and find that there is a remarkable resemblance between the relaxation spectra of these complex coacervates and the classical sticky Rouse model for single polymer systems. Salt affects all relaxation processes in the same way, giving rise to a widely applicable time-salt superposition principle. The viscoelastic properties of the complexes are very different from those of the individual components. In the complexes with a chain length mismatch, the effect of the mismatch on the viscoelastic properties is not trivial: changing the length of the polycation affects the relaxation behavior differently from changing the length of the polyanion.
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  41. 41
    Li, L.; Srivastava, S.; Andreev, M.; Marciel, A. B.; de Pablo, J. J.; Tirrell, M. V. Phase Behavior and Salt Partitioning in Polyelectrolyte Complex Coacervates. Macromolecules 2018, 51 (8), 2988– 2995,  DOI: 10.1021/acs.macromol.8b00238
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    41
    Phase Behavior and Salt Partitioning in Polyelectrolyte Complex Coacervates
    Li, Lu; Srivastava, Samanvaya; Andreev, Marat; Marciel, Amanda B.; de Pablo, Juan J.; Tirrell, Matthew V.
    Macromolecules (Washington, DC, United States) (2018), 51 (8), 2988-2995CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Polyelectrolyte complexes are omnipresent both in nature and in the technol. world, including nucleotide condensates, biol. marine adhesives, food stabilizers, encapsulants, and carriers for gene therapy. However, the true phase behavior of complexes, resulting from associative phase sepn. of oppositely charged polyelectrolytes, remains poorly understood. Here, we rely on complementary exptl. and simulation approaches to create a complete quant. description of the phase behavior of polyelectrolyte complexes that represents a significant advance in our understanding of the underlying physics of polyelectrolyte complexation. Expts. employing multiple approaches with model polyelectrolytes-oppositely charged polypeptides poly(L-lysine) and poly(D,L-glutamic acid) of matched chain lengths-led to phase diagrams with compns. of the complex and the supernatant that were in excellent agreement with simulation results. Contrary to the widely accepted theory for complexation, we found preferential partitioning of salt ions into the supernatant phase. Addnl., the salt partitioning into the supernatant phase was found to initially increase and then decrease on increasing the salt concns., manifesting as a distinct min. in the salt partition coeffs. These trends were shown by simulations to be strongly influenced by the excluded vol. interactions in the complex phase, which were not accounted for in their entirety in earlier theories. We believe the comprehensive data we present will be conducive to the development of an accurate phys. theory for polyelectrolyte complexation with predictive capabilities.
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    Perry, S. L.; Sing, C. E. PRISM-Based Theory of Complex Coacervation: Excluded Volume versus Chain Correlation. Macromolecules 2015, 48 (14), 5040– 5053,  DOI: 10.1021/acs.macromol.5b01027
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    PRISM-Based Theory of Complex Coacervation: Excluded Volume versus Chain Correlation
    Perry, Sarah L.; Sing, Charles E.
    Macromolecules (Washington, DC, United States) (2015), 48 (14), 5040-5053CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Aq. solns. of oppositely charged polyelectrolytes can undergo liq.-liq. phase sepn. into materials known as complex coacervates. These coacervates have been a subject of intense exptl. and theor. interest. Efforts to provide a phys. description of complex coacervates have led to a no. of theories that qual. (and sometimes quant.) agree with exptl. data. However, this agreement often occurs in a degeneracy of models with profoundly different starting assumptions and different levels of sophistication. Theor. difficulties in these systems are similar to those in most polyelectrolyte systems where charged species are highly correlated. These highly correlated systems can be described using liq. state (LS) integral equation theories, which surpass mean-field theories by providing information on local charge ordering. We extend these ideas to complex coacervate systems using PRISM-type theories and are able to capture effects not observable in traditional coacervate models, particularly connectivity and excluded vol. effects. We can thus bridge two traditional but incommensurate theories meant to describe complex coacervates: the Voorn-Overbeek theory and counterion release. Importantly, we hypothesize that a cancellation of connectivity and excluded vol. effects provides an explanation for the ability of Voorn-Overbeek theory to fit exptl. data despite its well-known approxns.
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    Qin, J.; Priftis, D.; Farina, R.; Perry, S. L.; Leon, L.; Whitmer, J.; Hoffmann, K.; Tirrell, M.; de Pablo, J. J. Interfacial Tension of Polyelectrolyte Complex Coacervate Phases. ACS Macro Lett. 2014, 3 (6), 565– 568,  DOI: 10.1021/mz500190w
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    Interfacial Tension of Polyelectrolyte Complex Coacervate Phases
    Qin, Jian; Priftis, Dimitrios; Farina, Robert; Perry, Sarah L.; Leon, Lorraine; Whitmer, Jonathan; Hoffmann, Kyle; Tirrell, Matthew; de Pablo, Juan J.
    ACS Macro Letters (2014), 3 (6), 565-568CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)
    We consider polyelectrolyte solns. which, under suitable conditions, phase sep. into a liq.-like coacervate phase and a coexisting supernatant phase that exhibit an extremely low interfacial tension. Such interfacial tension provides the basis for most coacervate-based applications, but little is known about it, including its dependence on mol. wt., charge d., and salt concn. By combining a Debye-Huckel treatment for electrostatic interactions with the Cahn-Hilliard theory, we derive explicit expressions for this interfacial tension. In the absence of added salts, we find that the interfacial tension scales as N-3/2(η/ηc-1)3/2 near the crit. point of the demixing transition, and that it scales as η1/2 far away from it, where N is the chain length and η measures the electrostatic interaction strength as a function of temp., dielec. const., and charge d. of the polyelectrolytes. For the case with added salts, we find that the interfacial tension scales with the salt concn. ψ as N-1/4(1-ψ/ψc)3/2 near the crit. salt concn. ψc. Our predictions are shown to be in quant. agreement with expts. and provide a means to design new materials based on polyelectrolyte complexation.
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    Spruijt, E.; Sprakel, J.; Lemmers, M.; Stuart, M. A. C.; van der Gucht, J. Relaxation Dynamics at Different Time Scales in Electrostatic Complexes: Time-Salt Superposition. Phys. Rev. Lett. 2010, 105 (20), 208301,  DOI: 10.1103/PhysRevLett.105.208301
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    Relaxation dynamics at different time scales in electrostatic complexes: time-salt superposition
    Spruijt, Evan; Sprakel, Joris; Lemmers, Marc; Stuart, Martien A. Cohen; van der Gucht, Jasper
    Physical Review Letters (2010), 105 (20), 208301/1-208301/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    In this Letter we show that in the rheol. of electrostatically assembled soft materials, salt concn. plays a similar role as temp. for polymer melts, and as strain rate for soft solids. We rescale linear and nonlinear rheol. data of a set of model electrostatic complexes at different salt concns. to access a range of time scales that is otherwise inaccessible. This provides new insights into the relaxation mechanisms of electrostatic complexes, which we rationalize in terms of a microscopic mechanism underlying salt-enhanced activated processes.
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    Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953.
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    Sudre, G.; Olanier, L.; Tran, Y.; Hourdet, D.; Creton, C. Reversible adhesion between a hydrogel and a polymer brush. Soft Matter 2012, 8 (31), 8184– 8193,  DOI: 10.1039/c2sm25868d
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    Reversible adhesion between a hydrogel and a polymer brush
    Sudre, Guillaume; Olanier, Ludovic; Tran, Yvette; Hourdet, Dominique; Creton, Costantino
    Soft Matter (2012), 8 (31), 8184-8193CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
    We have developed a new exptl. methodol. to investigate the adhesive properties of hydrogels on solid surfaces under fully immersed conditions. The method, based on contact mechanics, provides time-resolved reproducible and quant. data on the work of adhesion between a hydrogel at swelling equil. and a planar surface grafted with responsive brushes. We used poly(N,N-dimethylacrylamide) (PDMA) and polyacrylamide (PAM) as model gels and poly(acrylic acid) (PAA) as pH dependent polymer brush. The effect of pH, contact time and debonding velocity on adhesive interactions was specifically investigated. As expected from mol. interactions, we found that adhesion increased as the pH decreased and this was attributed to the formation of hydrogen bonds at the interface. Surprisingly, however, the buildup of adhesion increased slowly with the time of contact up to one hour and depended markedly on debonding velocity despite the very elastic nature of the hydrogels. Furthermore, the max. pH where adhesion was obsd. was significantly higher for the couple PAM-PAA than for the couple PDMA-PAA, in contrast with the onset of mol. interactions in dil. solns.
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  47. 47
    Wang, T.; Lei, C.-H.; Dalton, A. B.; Creton, C.; Lin, Y.; Fernando, K. A. S.; Sun, Y.-P.; Manea, M.; Asua, J. M.; Keddie, J. L. Waterborne, Nanocomposite Pressure-Sensitive Adhesives with High Tack Energy, Optical Transparency, and Electrical Conductivity. Adv. Mater. 2006, 18 (20), 2730– 2734,  DOI: 10.1002/adma.200601335
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    47
    Waterborne, nanocomposite pressure-sensitive adhesives with high tack energy, optical transparency, and electrical conductivity
    Wang, Tao; Lei, Chun-Hong; Dalton, Alan B.; Creton, Costantino; Lin, Yi; Fernando, K. A. Shiral; Sun, Ya-Ping; Manea, Mihaela; Asua, Jose M.; Keddie, Joseph L.
    Advanced Materials (Weinheim, Germany) (2006), 18 (20), 2730-2734CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Transparent and conductive pressure-sensitive adhesives are cast from aq. colloidal dispersions of poly(Bu acrylate) (P(BuA)) and functionalized carbon nanotubes (CNTs). At the percolation threshold for network formation (at only 0.3 wt % functionalized CNT), the nanotubes remarkably double the amt. of strain at adhesive failure and increase the adhesion energy by 85 % (see figure). The tack properties are explained by current models of adhesive debonding.
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    Deplace, F.; Carelli, C.; Mariot, S.; Retsos, H.; Chateauminois, A.; Ouzineb, K.; Creton, C. Fine Tuning the Adhesive Properties of a Soft Nanostructured Adhesive with Rheological Measurements. J. Adhes. 2009, 85 (1), 18– 54,  DOI: 10.1080/00218460902727381
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    Fine Tuning the Adhesive Properties of a Soft Nanostructured Adhesive with Rheological Measurements
    Deplace, F.; Carelli, C.; Mariot, S.; Retsos, H.; Chateauminois, A.; Ouzineb, K.; Creton, C.
    Journal of Adhesion (2009), 85 (1), 18-54CODEN: JADNAJ; ISSN:0021-8464. (Taylor & Francis, Inc.)
    A review. The major objective of this article is to present recent advances in the methodol. to fine tune the adhesive performance of a PSA. In addn. to the so-called Dahlquist criterion requiring a low modulus, we propose two addnl. rheol. predictors of the adhesive properties. The first one is derived from the description of the detachment of a linear elastic layer from a rigid substrate. We made an approx. extension of this anal. to the viscoelastic regime and showed that the transition from interfacial cracks to cavitation and fibrillation can be quant. predicted from the easily measurable ratio tan(δ)/G'(ω). If a fibrillar structure is formed, the nonlinear large strain properties become important. We showed that the ability of the fibrils to be stretched before final debonding can be predicted from the anal. of simple tensile tests. The softening, which occurs at intermediate strains, and, more importantly, the hardening which occurs at large strains, can be used to predict the mode of failure and the energy of adhesion. The use of this methodol. to tune the PSA structure for a specific application has been illustrated for the special case of wb-PSA made of core-shell particles, and improved adhesive properties on polyethylene surfaces have been obtained.
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    Jones, J. P.; Sima, M.; O’Hara, R. G.; Stewart, R. J. Water-Borne Endovascular Embolics Inspired by the Undersea Adhesive of Marine Sandcastle Worms. Adv. Healthcare Mater. 2016, 5 (7), 795– 801,  DOI: 10.1002/adhm.201500825
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    Water-Borne Endo-vascular Embolics Inspired by the Undersea Adhesive of Marine Sandcastle Worms
    Jones, Joshua P.; Sima, Monika; O'Hara, Ryan G.; Stewart, Russell J.
    Advanced Healthcare Materials (2016), 5 (7), 795-801CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Transcatheter embolization is used to treat vascular malformations and defects, to control bleeding, and to selectively block blood supply to tissues. Liq. embolics are used for small vessel embolization that require distal penetration. Current liq. embolic agents have serious drawbacks, mostly centered around poor handling characteristics and toxicity. In this work, a water-borne in situ setting liq. embolic agent is described that is based on electrostatically condensed, oppositely charged polyelectrolytes-complex coacervates. At high ionic strengths, the embolic coacervates are injectable fluids that can be delivered through long narrow microcatheters. At physiol. ionic strength, the embolic coacervates transition into a nonflowing solid morphol. Transcatheter embolization of rabbit renal arteries demonstrated capillary level penetration, homogeneous occlusion, and 100% devascularization of the kidney, without the embolic crossing into venous circulation. The benign water-borne compn. and setting mechanism avoids many of the problems of current liq. embolics, and provides precise temporal and spatial control during endovascular embolization.
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  50. 50
    Sun, T. L.; Kurokawa, T.; Kuroda, S.; Ihsan, A. B.; Akasaki, T.; Sato, K.; Haque, M. A.; Nakajima, T.; Gong, J. P. Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity. Nat. Mater. 2013, 12, 932,  DOI: 10.1038/nmat3713
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    Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity
    Sun, Tao Lin; Kurokawa, Takayuki; Kuroda, Shinya; Ihsan, Abu Bin; Akasaki, Taigo; Sato, Koshiro; Haque, Md. Anamul; Nakajima, Tasuku; Gong, Jian Ping
    Nature Materials (2013), 12 (10), 932-937CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biol. tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mech. properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mech. properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These phys. hydrogels of supramol. structure can be tuned to change multiple mech. properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.
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    Gong, J. P. Why are double network hydrogels so tough?. Soft Matter 2010, 6 (12), 2583– 2590,  DOI: 10.1039/b924290b
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    Why are double network hydrogels so tough?
    Gong, Jian Ping
    Soft Matter (2010), 6 (12), 2583-2590CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
    A review. Double-network (DN) gels have drawn much attention as an innovative material having both high water content (ca. 90 wt%) and high mech. strength and toughness. DN gels are characterized by a special network structure consisting of two types of polymer components with opposite phys. natures: the minor component is abundantly crosslinked polyelectrolytes (rigid skeleton) and the major component comprises of poorly crosslinked neutral polymers (ductile substance). The former and the latter components are referred to as the first network and the second network, resp., since the synthesis should be done in this order to realize high mech. strength. For DN gels synthesized under suitable conditions (choice of polymers, feed compns., atm. for reaction, etc.), they possess hardness (elastic modulus of 0.1-1.0 MPa), strength (failure tensile nominal stress 1-10 MPa, strain 1000-2000%; failure compressive nominal stress 20-60 MPa, strain 90-95%), and toughness (tearing fracture energy of 100∼1000 J m-2). These excellent mech. performances are comparable to that of rubbers and soft load-bearing bio-tissues. The mech. behaviors of DN gels are inconsistent with general mechanisms that enhance the toughness of soft polymeric materials. Thus, DN gels present an interesting and challenging problem in polymer mechanics. Extensive exptl. and theor. studies have shown that the toughening of DN gel is based on a local yielding mechanism, which has some common features with other brittle and ductile nano-composite materials, such as bones and dentins.
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    Sun, J.-Y.; Zhao, X.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly stretchable and tough hydrogels. Nature 2012, 489 (7414), 133– 136,  DOI: 10.1038/nature11409
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    Highly stretchable and tough hydrogels
    Sun, Jeong-Yun; Zhao, Xuanhe; Illeperuma, Widusha R. K.; Chaudhuri, Ovijit; Oh, Kyu Hwan; Mooney, David J.; Vlassak, Joost J.; Suo, Zhigang
    Nature (London, United Kingdom) (2012), 489 (7414), 133-136CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Hydrogels are used as scaffolds for tissue engineering, vehicles for drug delivery, actuators for optics and fluidics, and model extracellular matrixes for biol. studies. The scope of hydrogel applications, however, is often severely limited by their mech. behavior. Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels have achieved stretches in the range 10-20, but these values are markedly reduced in samples contg. notches. Most hydrogels are brittle, with fracture energies of about 10 J m-2 (ref. 8), as compared with ∼1,000 J m-2 for cartilage and ∼10,000 J m-2 for natural rubbers. Intense efforts are devoted to synthesizing hydrogels with improved mech. properties; certain synthetic gels have reached fracture energies of 100-1,000 J m-2 (refs 11, 14, 17). Here we report the synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks. Although such gels contain ∼90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of ∼9,000 J m-2. Even for samples contg. notches, a stretch of 17 is demonstrated. We attribute the gels' toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, which heals by re-zipping. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.
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    Clancy, S. K.; Sodano, A.; Cunningham, D. J.; Huang, S. S.; Zalicki, P. J.; Shin, S.; Ahn, B. K. Marine Bioinspired Underwater Contact Adhesion. Biomacromolecules 2016, 17 (5), 1869– 1874,  DOI: 10.1021/acs.biomac.6b00300
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    Marine Bioinspired Underwater Contact Adhesion
    Clancy, Sean K.; Sodano, Antonio; Cunningham, Dylan J.; Huang, Sharon S.; Zalicki, Piotr J.; Shin, Seunghan; Ahn, B. Kollbe
    Biomacromolecules (2016), 17 (5), 1869-1874CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
    Marine mussels and barnacles are sessile biofouling organisms that adhere to a no. of surfaces in wet environments and maintain remarkably strong bonds. Previous synthetic approaches to mimic biol. wet adhesive properties have focused mainly on the catechol moiety, present in mussel foot proteins (mfps), and esp. rich in the interfacial mfps, for example, mfp-3 and -5, found at the interface between the mussel plaque and substrate. Barnacles, however, do not use Dopa for their wet adhesion, but are instead rich in noncatecholic arom. residues. Due to this anomaly, we were intrigued to study the initial contact adhesion properties of copolymd. acrylate films contg. the key functionalities of barnacle cement proteins and interfacial mfps, for example, arom. (catecholic or noncatecholic), cationic, anionic, and nonpolar residues. The initial wet contact adhesion of the copolymers was measured using a probe tack testing app. with a flat-punch contact geometry. The wet contact adhesion of an optimized, bioinspired copolymer film was ∼15.0 N/cm2 in deionized water and ∼9.0 N/cm2 in artificial seawater, up to 150 times greater than com. pressure-sensitive adhesive (PSA) tapes (∼0.1 N/cm2). Furthermore, max. wet contact adhesion was obtained at ∼pH 7, suggesting viability for biomedical applications.
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  • Abstract

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    Figure 1

    Figure 1. (A) Water content of complex coacervates plotted as a function of the added salt concentration for the samples analyzed in Strategy 1. (B) Frequency sweeps (G′ represented as full dots and G″ as hollow dots) for graft copolymer complex coacervates below the LCST (T = 5 °C).

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    Figure 2

    Figure 2. (A) Complex coacervate (CC) phase diagram. In the two-phase region a gradient from yellow to red is used to indicate the transition from a liquid dilute phase (left side) to a liquid CC phase (top right side) and finally to a solid CC phase (bottom right side). (B) Effect of polymer concentration: zoom in the liquid CC area, in which the black dots represent the analyzed samples, which phase separate into a dilute phase, shown as a light blue dot, and a complex coacervate phase, shown as a red dot. (C) Samples analyzed in strategy 2.

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    Figure 3

    Figure 3. Effect of the polymer concentration on (A) water content and (B) complex coacervate volume fraction. The theoretical polymer concentrations of the dilute (blue dot) and complex coacervate phase (red dot) are obtained by extrapolating the regression line to y = 100% and y = 0%, respectively.

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    Figure 4

    Figure 4. Water content as a function of (A) extruder frequency and (B) number of extrusion cycles. (C) Relaxation time plotted as a function of polymer concentration (the last point is not the real value but the relaxation time corresponding to the minimum frequency accessed in the experiment).

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    Figure 5

    Figure 5. Solidification upon temperature or salt switch. (A) Storage modulus (recorded at ω = 1 rad/s and T = 50 °C) as a function of polymer concentration. (B) Time sweeps after one extrusion cycle when performing a salt (S) or a combined salt and temperature (S+T) switch.

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    Figure 6

    Figure 6. Rheological properties of extruded and homopolymer complex coacervates: (A) frequency sweeps performed at 5 °C, (B) frequency sweep performed at 50 °C, and (C) frequency sweeps performed after a combined temperature and salt switch.

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    Figure 7

    Figure 7. (A) Underwater adhesion performance after application of a temperature–salt switch at a nominal strain rate of 0.2 s–1. (B) Plot of tan δ and tan δ/G′ recorded with a rheometer at ω = 1 rad/s (comparable to the strain rate in the tack measurement) as a function of the number of extrusion cycles.

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    Figure 8

    Figure 8. (A) Swelling ratio versus the number of extrusion cycles. The dashed line represents the border between swelling (above the line) and shrinking (below the line). (B) Swelling ratio as a function of time for the sample extruded one time in physiological conditions (T = 37 °C, [NaCl] = 0.1 M, pH = 7.0).

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    Figure 9

    Figure 9. Underwater adhesive performance of the sample after one extrusion cycle after application of a temperature–salt switch compared to (A) the application of a single temperature switch and (B) the performance of the homopolymer complex coacervate upon a combined trigger.

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    Figure 10

    Figure 10. Work of adhesion versus the amount of extrusion cycles and versus the applied trigger.

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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtVyht7fK&md5=7eb4feb7530257f59689bb0c251f0fa4
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      Gucht, J. v. d.; Spruijt, E.; Lemmers, M.; Cohen Stuart, M. A. Polyelectrolyte complexes: Bulk phases and colloidal systems. J. Colloid Interface Sci. 2011, 361 (2), 407– 422,  DOI: 10.1016/j.jcis.2011.05.080
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    14. 14
      Spruijt, E.; Westphal, A. H.; Borst, J. W.; Cohen Stuart, M. A.; van der Gucht, J. Binodal Compositions of Polyelectrolyte Complexes. Macromolecules 2010, 43 (15), 6476– 6484,  DOI: 10.1021/ma101031t
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      14
      Binodal Compositions of Polyelectrolyte Complexes
      Spruijt, Evan; Westphal, Adrie H.; Borst, Jan Willem; Cohen Stuart, Martien A.; van der Gucht, Jasper
      Macromolecules (Washington, DC, United States) (2010), 43 (15), 6476-6484CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      When oppositely charged polyelectrolytes are mixed below a crit. salt concn., their mixts. show macroscopic phase sepn. into a dil. and a dense, polyelectrolyte complex phase. Binodal compns. of the polyelectrolyte complexes have been measured exptl. using fluorescently labeled polyelectrolytes. We used fluorescein-labeled poly(acrylic acid) (PAA) of four different chain lengths (N = 20, 50, 150, and 510) to det. the binodal compns. of polyelectrolyte complexes of PAA and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) of similar chain lengths. The water content of polyelectrolyte complexes obtained has a lower limit of about 65%, practically independent of chain length, and increases with increasing salt concn. We interpret our results on binodal compns., water content and crit. salt concn. as a function of chain length using the mean-field model of Voorn and Overbeek and find good quant. agreement with our expts. using only one adjustable parameter. We believe that such a model can be used to predict equil. concns. also for other strongly charged flexible polyelectrolytes.
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    15. 15
      Kaur, S.; Weerasekare, G. M.; Stewart, R. J. Multiphase Adhesive Coacervates Inspired by the Sandcastle Worm. ACS Appl. Mater. Interfaces 2011, 3 (4), 941– 944,  DOI: 10.1021/am200082v
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      15
      Multiphase adhesive coacervates inspired by the sandcastle worm
      Kaur, Sarbjit; Weerasekare, G. Mahika; Stewart, Russell J.
      ACS Applied Materials & Interfaces (2011), 3 (4), 941-944CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Water-borne, underwater adhesives were created by complex coacervation of synthetic copolyelectrolytes that mimic the proteins of the natural underwater adhesive of the sandcastle worm. To increase bond strengths, we created a second polymer network within cross-linked coacervate network by entrapping polyethylene glycol diacrylate (PEG-dA) monomers in the coacervate phase. Simultaneous polymn. of PEG-dA and crosslinking of the coacervate network resulted in max. shear bond strengths of ∼1.2 MPa. Approx. 40% of the entrapped PEG-dA polymd. based on attenuated total reflectance-Fourier transform IR spectroscopy. The monomer-filled coacervate had complex flow behavior, thickening at low shear rates and then thinning suddenly with a 16-fold drop in viscosity at shear rates near 6 s-1. The microscale structure of the complex coacervates resembled a three-dimensional porous network of interconnected tubules. The sharp shear thinning behavior is conceptualized as a structural reorganization between the interspersed phases of the complex coacervate. The bond strength and complex fluid behavior of the monomer-filled coacervates have important implications for medical applications of the adhesives.
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    16. 16
      Shao, H.; Stewart, R. J. Biomimetic Underwater Adhesives with Environmentally Triggered Setting Mechanisms. Adv. Mater. 2010, 22 (6), 729– 733,  DOI: 10.1002/adma.200902380
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      16
      Biomimetic Underwater Adhesives with Environmentally Triggered Setting Mechanisms
      Shao, Hui; Stewart, Russell J.
      Advanced Materials (Weinheim, Germany) (2010), 22 (6), 729-733CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The purpose of the study is to copy marine worm's mechanisms of underwater bonding to create synthetic waterborne underwater medical adhesives, and in turn, to use the synthetic adhesives to test mechanistic hypotheses about the natural adhesive. Biomimetic underwater adhesives were formulated with polyelectrolytic analogs of the natural glue proteins.
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    17. 17
      Ahn, B. K.; Das, S.; Linstadt, R.; Kaufman, Y.; Martinez-Rodriguez, N. R.; Mirshafian, R.; Kesselman, E.; Talmon, Y.; Lipshutz, B. H.; Israelachvili, J. N.; Waite, J. H. High-performance mussel-inspired adhesives of reduced complexity. Nat. Commun. 2015, 6, 8663,  DOI: 10.1038/ncomms9663
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      17
      High-performance mussel-inspired adhesives of reduced complexity
      Ahn, B. Kollbe; Das, Saurabh; Linstadt, Roscoe; Kaufman, Yair; Martinez-Rodriguez, Nadine R.; Mirshafian, Razieh; Kesselman, Ellina; Talmon, Yeshayahu; Lipshutz, Bruce H.; Israelachvili, Jacob N.; Waite, J. Herbert
      Nature Communications (2015), 6 (), 8663CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Despite the recent progress in and demand for wet adhesives, practical underwater adhesion remains limited or non-existent for diverse applications. Translation of mussel-inspired wet adhesion typically entails catechol functionalization of polymers and/or polyelectrolytes, and soln. processing of many complex components and steps that require optimization and stabilization. Here we reduced the complexity of a wet adhesive primer to synthetic low-mol.-wt. catecholic zwitterionic surfactants that show very strong adhesion (∼50 mJ m-2) and retain the ability to coacervate. This catecholic zwitterion adheres to diverse surfaces and self-assembles into a molecularly smooth, thin (<4 nm) and strong glue layer. The catecholic zwitterion holds particular promise as an adhesive for nanofabrication. This study significantly simplifies bio-inspired themes for wet adhesion by combining catechol with hydrophobic and electrostatic functional groups in a small mol.
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    18. 18
      Seo, S.; Das, S.; Zalicki, P. J.; Mirshafian, R.; Eisenbach, C. D.; Israelachvili, J. N.; Waite, J. H.; Ahn, B. K. Microphase Behavior and Enhanced Wet-Cohesion of Synthetic Copolyampholytes Inspired by a Mussel Foot Protein. J. Am. Chem. Soc. 2015, 137 (29), 9214– 9217,  DOI: 10.1021/jacs.5b03827
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      18
      Microphase Behavior and Enhanced Wet-Cohesion of Synthetic Copolyampholytes Inspired by a Mussel Foot Protein
      Seo, Sungbaek; Das, Saurabh; Zalicki, Piotr J.; Mirshafian, Razieh; Eisenbach, Claus D.; Israelachvili, Jacob N.; Waite, J. Herbert; Ahn, B. Kollbe
      Journal of the American Chemical Society (2015), 137 (29), 9214-9217CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Numerous attempts have been made to translate mussel adhesion to diverse synthetic platforms. However, the translation remains largely limited to the Dopa (3,4-dihydroxyphenylalanine) or catechol functionality, which continues to raise concerns about Dopa's inherent susceptibility to oxidn. Mussels have evolved adaptations to stabilize Dopa against oxidn. For example, in mussel foot protein 3 slow (mfp-3s, one of two electrophoretically distinct interfacial adhesive proteins in mussel plaques), the high proportion of hydrophobic amino acid residues in the flanking sequence around Dopa increases Dopa's oxidn. potential. In this study, copolyampholytes, which combine the catechol functionality with amphiphilic and ionic features of mfp-3s, were synthesized and formulated as coacervates for adhesive deposition on surfaces. The ratio of hydrophilic/hydrophobic as well as cationic/anionic units was varied in order to enhance coacervate formation and wet adhesion properties. Aq. solns. of two of the four mfp-3s-inspired copolymers showed coacervate-like spherical microdroplets (.vphi. ≈ 1-5 μm) at pH ∼4 (salt concn. ∼15 mM). The mfp-3s-mimetic copolymer was stable to oxidn., formed coacervates that spread evenly over mica, and strongly bonded to mica surfaces (pull-off strength: ∼17.0 mJ/m2). Increasing pH to 7 after coacervate deposition at pH 4 doubled the bonding strength to ∼32.9 mJ/m2 without oxidative crosslinking and is about 9 times higher than native mfp-3s cohesion. This study expands the scope of translating mussel adhesion from simple Dopa-functionalization to mimicking the context of the local environment around Dopa.
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    19. 19
      Zhao, Q.; Lee, D. W.; Ahn, B. K.; Seo, S.; Kaufman, Y.; Israelachvili, J. N.; Waite, J. H. Underwater contact adhesion and microarchitecture in polyelectrolyte complexes actuated by solvent exchange. Nat. Mater. 2016, 15, 407,  DOI: 10.1038/nmat4539
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      19
      Underwater contact adhesion and microarchitecture in polyelectrolyte complexes actuated by solvent exchange
      Zhao, Qiang; Lee, Dong Woog; Ahn, B. Kollbe; Seo, Sungbaek; Kaufman, Yair; Israelachvili, Jacob N.; Waite, J. Herbert
      Nature Materials (2016), 15 (4), 407-412CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Polyelectrolyte complexation is crit. to the formation and properties of many biol. and polymeric materials, and is typically initiated by aq. mixing followed by fluid-fluid phase sepn., such as coacervation. Yet little to nothing is known about how coacervates evolve into intricate solid microarchitectures. Inspired by the chem. features of the cement proteins of the sandcastle worm, here we report a versatile and strong wet-contact microporous adhesive resulting from polyelectrolyte complexation triggered by solvent exchange. After premixing a catechol-functionalized weak polyanion with a polycation in DMSO, the soln. was applied underwater to various substrates whereupon electrostatic complexation, phase inversion, and rapid setting were simultaneously actuated by water-DMSO solvent exchange. Spatial and temporal coordination of complexation, inversion and setting fostered rapid (∼25 s) and robust underwater contact adhesion (Wad ≥ 2 J m-2) of complexed catecholic polyelectrolytes to all tested surfaces including plastics, glasses, metals and biol. materials.
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    20. 20
      Zhang, C.; Huang, J.; Zhang, J.; Liu, S.; Cui, M.; An, B.; Wang, X.; Pu, J.; Zhao, T.; Fan, C.; Lu, T. K.; Zhong, C. Engineered Bacillus subtilis biofilms as living glues. Mater. Today 2019, 28, 40– 48,  DOI: 10.1016/j.mattod.2018.12.039
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      There is no corresponding record for this reference.
    21. 21
      Dompé, M.; Cedano-Serrano, F. J.; Heckert, O.; van den Heuvel, N.; van der Gucht, J.; Tran, Y.; Hourdet, D.; Creton, C.; Kamperman, M. Thermoresponsive Complex Coacervate-Based Underwater Adhesive. Adv. Mater. 2019, 31 (21), 1808179,  DOI: 10.1002/adma.201808179
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      There is no corresponding record for this reference.
    22. 22
      Dompe, M.; Cedano-Serrano, F. J.; Vahdati, M.; Westerveld, L.; Hourdet, D.; Creton, C.; der Gucht, J.; Kodger, T.; Kamperman, M. Underwater Adhesion of Multiresponsive Complex Coacervates. Adv. Mater. Interfaces 2020, 7, 1901785,  DOI: 10.1002/admi.201901785
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      22
      Underwater Adhesion of Multiresponsive Complex Coacervates
      Dompe, Marco; Cedano-Serrano, Francisco J.; Vahdati, Mehdi; van Westerveld, Larissa; Hourdet, Dominique; Creton, Costantino; van der Gucht, Jasper; Kodger, Thomas; Kamperman, Marleen
      Advanced Materials Interfaces (2020), 7 (4), 1901785CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Many marine organisms have developed adhesives that are able to bond under water, overcoming the challenges assocd. with wet adhesion. A key element in the processing of several natural underwater glues is complex coacervation, a liq.-liq. phase sepn. driven by complexation of oppositely charged macromols. Inspired by these examples, the development of a fully synthetic complex coacervate-based adhesive is reported with an in situ setting mechanism, which can be triggered by a change in temp. and/or a change in ionic strength. The adhesive consists of a matrix of oppositely charged polyelectrolytes that are modified with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) grafts. The adhesive, which initially starts out as a fluid complex coacervate with limited adhesion at room temp. and high ionic strength, transitions into a viscoelastic solid upon an increase in temp. and/or a decrease in the salt concn. of the environment. Consequently, the thermoresponsive chains self-assoc. into hydrophobic domains and/or the polyelectrolyte matrix contracts, without inducing any macroscopic shrinking. The presence of PNIPAM favors energy dissipation by softening the material and by allowing crack blunting. The high work of adhesion, the gelation kinetics, and the easy tunability of the system make it a potential candidate for soft tissue adhesion in physiol. environments.
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    23. 23
      Dompé, M.; Cedano-Serrano, F. J.; Vahdati, M.; Sidoli, U.; Heckert, O.; Synytska, A.; Hourdet, D.; Creton, C.; Van der Gucht, J.; Kodger, T.; Kamperman, M. Tuning the Interactions in Multiresponsive Complex Coacervate-Based Underwater Adhesives. Int. J. Mol. Sci. 2020, 21, 100,  DOI: 10.3390/ijms21010100
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      23
      Tuning the interactions in multiresponsive complex coacervate-based underwater adhesives
      Dompe, Marco; Cedano-Serrano, Francisco J.; Vahdati, Mehdi; Sidoli, Ugo; Heckert, Olaf; Synytska, Alla; Hourdet, Dominique; Creton, Costantino; Van Der Gucht, Jasper; Kodger, Thomas; Kamperman, Marleen
      International Journal of Molecular Sciences (2020), 21 (1), 100CODEN: IJMCFK; ISSN:1422-0067. (MDPI AG)
      In this work, we report the systematic investigation of a multiresponsive complex coacervate-based underwater adhesive, obtained by combining polyelectrolyte domains and thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) units. This material exhibits a transition from liq. to solid but, differently from most reactive glues, is completely held together by non-covalent interactions, i.e., electrostatic and hydrophobic. Because the solidification results in a kinetically trapped morphol., the final mech. properties strongly depend on the prepn. conditions and on the surrounding environment. A systematic study is performed to assess the effect of ionic strength and of PNIPAM content on the thermal, rheol. and adhesive properties. This study enables the optimization of polymer compn. and environmental conditions for this underwater adhesive system. The best performance with a work of adhesion of 6.5 J/m2 was found for the complex coacervates prepd. at high ionic strength (0.75 M NaCl) and at an optimal PNIPAM content around 30% mol/mol. The high ionic strength enables injectability, while the hydrated PNIPAM domains provide addnl. dissipation, without softening the material so much that it becomes too weak to resist detaching stress.
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    24. 24
      Kelmansky, R.; McAlvin, B. J.; Nyska, A.; Dohlman, J. C.; Chiang, H. H.; Hashimoto, M.; Kohane, D. S.; Mizrahi, B. Strong tissue glue with tunable elasticity. Acta Biomater. 2017, 53, 93– 99,  DOI: 10.1016/j.actbio.2017.02.009
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      24
      Strong tissue glue with tunable elasticity
      Kelmansky, Regina; McAlvin, Brian J.; Nyska, Abraham; Dohlman, Jenny C.; Chiang, Homer H.; Hashimoto, Michinao; Kohane, Daniel S.; Mizrahi, Boaz
      Acta Biomaterialia (2017), 53 (), 93-99CODEN: ABCICB; ISSN:1742-7061. (Elsevier Ltd.)
      Many bio-adhesive materials adhere weakly to tissue due to their high water content and weak structural integrity. Others provide desirable adhesive strength but suffer from rigid structure and lack of elasticity after administration. We have developed two water-free, liq. four-armed PEG pre-polymers modified with NHS or with NH2 end groups which upon mixing changed from liqs. to an elastic solid. The sealant and adhesive properties increased with the amt. of the %vol./vol. PEG4-NHS pre-polymer, and achieved adhesive properties comparable to those of cyanoacrylate glues. All mixts. showed minimal cytotoxicity in vitro. Mixts. of 90%vol./vol. PEG4-NHS were retained in the s.c. space in vivo for up to 14 days with minimal inflammation. This material's combination of desirable mech. properties and biocompatibility has potential in numerous biomedical applications. Many bio-adhesive materials adhere weakly to tissue (e.g. hydrogels) due to their high water content and weak structural integrity. Others provide desirable mech. properties but suffer from poor biocompatibility (e.g. cyanoacrylates). This study proposes a new concept for the formation of super strong and tunable tissue glues. Our bio-materials' enhanced performance is the product of new neat (without water or other solvents) liq. polymers that solidify after administration while allowing interactions with the tissue. Moreover, the elastic modulus of these materials could easily be tuned without compromising biocompatibility. This system could be an attractive alternative to sutures and staples since it can be applied more quickly, causes less pain and may require less equipment while maintaining the desired adhesion strength.
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    25. 25
      Barrett, D. G.; Bushnell, G. G.; Messersmith, P. B. Mechanically Robust, Negative-Swelling, Mussel-Inspired Tissue Adhesives. Adv. Healthcare Mater. 2013, 2 (5), 745– 755,  DOI: 10.1002/adhm.201200316
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      25
      Mechanically Robust, Negative-Swelling, Mussel-Inspired Tissue Adhesives
      Barrett, Devin G.; Bushnell, Grace G.; Messersmith, Phillip B.
      Advanced Healthcare Materials (2013), 2 (5), 745-755CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Most synthetic polymer hydrogel tissue adhesives and sealants swell considerably in physiol. conditions, which can result in mech. weakening and adverse medical complications. This paper describes the synthesis and characterization of mech. tough zero- or neg.-swelling mussel-inspired surgical adhesives based on catechol-modified amphiphilic poly(propylene oxide)-poly(ethylene oxide) block copolymers. The formation, swelling, bulk mech., and tissue adhesive properties of the resulting thermosensitive gels were characterized. Catechol oxidn. at or below room temp. rapidly resulted in a chem. crosslinked network, with subsequent warming to physiol. temp. inducing a thermal hydrophobic transition in the PPO domains and providing a mechanism for volumetric redn. and mech. toughening. The described approach can be easily adapted for other thermally sensitive block copolymers and crosslinking strategies, representing a general approach that can be employed to control swelling and enhance mech. properties of polymer hydrogels used in a medical context.
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    26. 26
      Lee, G.; Lee, C. K.; Bynevelt, M. DuraSeal-Hematoma: Concealed Hematoma Causing Spinal Cord Compression. Spine 2010, 35 (25), E1522– E1524,  DOI: 10.1097/BRS.0b013e3181edfe2c
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      There is no corresponding record for this reference.
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      Mulder, M.; Crosier, J.; Dunn, R. Cauda Equina Compression by Hydrogel Dural Sealant After a Laminotomy and Discectomy: Case Report. Spine 2009, 34 (4), E144– E148,  DOI: 10.1097/BRS.0b013e31818d5427
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    28. 28
      Campbell, P. K.; Bennett, S. L.; Driscoll, A.; Sawhney, A. S. Evaluation of absorbable surgical sealants: in vitro testing. https://pdfs.semanticscholar.org/0e72/159a6027168d8ecb11dcd2375ad692c30ab3.pdf, 2005.
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    29. 29
      Lang, N.; Pereira, M. J.; Lee, Y.; Friehs, I.; Vasilyev, N. V.; Feins, E. N.; Ablasser, K.; O’Cearbhaill, E. D.; Xu, C.; Fabozzo, A.; Padera, R.; Wasserman, S.; Freudenthal, F.; Ferreira, L. S.; Langer, R.; Karp, J. M.; del Nido, P. J. A Blood-Resistant Surgical Glue for Minimally Invasive Repair of Vessels and Heart Defects. Sci. Transl. Med. 2014, 6 (218), 218ra6  DOI: 10.1126/scitranslmed.3006557
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      There is no corresponding record for this reference.
    30. 30
      Nijst, C. L. E.; Bruggeman, J. P.; Karp, J. M.; Ferreira, L.; Zumbuehl, A.; Bettinger, C. J.; Langer, R. Synthesis and Characterization of Photocurable Elastomers from Poly(glycerol-co-sebacate). Biomacromolecules 2007, 8 (10), 3067– 3073,  DOI: 10.1021/bm070423u
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      30
      Synthesis and Characterization of Photocurable Elastomers from Poly(glycerol-co-sebacate)
      Nijst, Christiaan L. E.; Bruggeman, Joost P.; Karp, Jeffrey M.; Ferreira, Lino; Zumbuehl, Andreas; Bettinger, Christopher J.; Langer, Robert
      Biomacromolecules (2007), 8 (10), 3067-3073CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
      Elastomeric networks are increasingly being investigated for a variety of biomedical applications including drug delivery and tissue engineering. However, in some cases, their prepn. requires the use of harsh processing conditions (e.g., high temp.), which limits their biomedical application. Herein, we demonstrate the ability to form elastomeric networks from poly(glycerol-co-sebacate) acrylate (PGSA) under mild conditions while preserving a wide range of phys. properties. These networks presented a Young's modulus between 0.05 and 1.38 MPa, an ultimate strength from 0.05 to 0.50 MPa, and elongation at break between 42% and 189% strain, by varying the degree of acrylation (DA) of PGSA. The in vitro enzymic and hydrolytic degrdn. of the polymer networks was dependent on the DA. The copolymn. of poly(ethylene glycol) diacrylate with PGSA allowed for an addnl. control of mech. properties and swelling ratios in an aq. environment, as well as enzymic and hydrolytic degrdn. Photocured PGSA networks demonstrated in vitro biocompatibility as judged by sufficient human primary cell adherence and subsequent proliferation into a confluent monolayer. These photocurable degradable elastomers could have potential application for the encapsulation of temp.-sensitive factors and cells for tissue engineering.
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    31. 31
      Yuk, H.; Varela, C. E.; Nabzdyk, C. S.; Mao, X.; Padera, R. F.; Roche, E. T.; Zhao, X. Dry double-sided tape for adhesion of wet tissues and devices. Nature 2019, 575 (7781), 169– 174,  DOI: 10.1038/s41586-019-1710-5
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      31
      Dry double-sided tape for adhesion of wet tissues and devices
      Yuk, Hyunwoo; Varela, Claudia E.; Nabzdyk, Christoph S.; Mao, Xinyu; Padera, Robert F.; Roche, Ellen T.; Zhao, Xuanhe
      Nature (London, United Kingdom) (2019), 575 (7781), 169-174CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Two dry surfaces can instantly adhere upon contact with each other through intermol. forces such as hydrogen bonds, electrostatic interactions and van der Waals interactions1,2. However, such instant adhesion is challenging when wet surfaces such as body tissues are involved, because water separates the mols. of the two surfaces, preventing interactions3,4. Although tissue adhesives have potential advantages over suturing or stapling5,6, existing liq. or hydrogel tissue adhesives suffer from several limitations: weak bonding, low biol. compatibility, poor mech. match with tissues, and slow adhesion formation5-13. Here we propose an alternative tissue adhesive in the form of a dry double-sided tape (DST) made from a combination of a biopolymer (gelatin or chitosan) and crosslinked poly(acrylic acid) grafted with N-hydrosuccinimide ester. The adhesion mechanism of this DST relies on the removal of interfacial water from the tissue surface, resulting in fast temporary crosslinking to the surface. Subsequent covalent crosslinking with amine groups on the tissue surface further improves the adhesion stability and strength of the DST. In vitro mouse, in vivo rat and ex vivo porcine models show that the DST can achieve strong adhesion between diverse wet dynamic tissues and engineering solids within five seconds. The DST may be useful as a tissue adhesive and sealant, and in adhering wearable and implantable devices to wet tissues.
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    32. 32
      Feldstein, M. M. Molecular Nature of Pressure-Sensitive Adhesion. In Fundamentals of Pressure Sensitivity; Benedek, I., Feldstein, M. M., Eds.; Taylor & Francis Group, LLC: Boca Raton, FL, 2008.
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      Chalykh, A. A.; Chalykh, A. E.; Novikov, M. B.; Feldstein, M. M. Pressure-sensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths. J. Adhes. 2002, 78 (8), 667– 694,  DOI: 10.1080/00218460213491
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      Pressure-sensitive adhesion in the blends of poly(N-vinylpyrrolidone) and poly(ethylene glycol) of disparate chain lengths
      Chalykh, Anna A.; Chalykh, Anatoly E.; Novikov, Mikhail B.; Feldstein, Mikhail M.
      Journal of Adhesion (2002), 78 (8), 667-694CODEN: JADNAJ; ISSN:0021-8464. (Taylor & Francis Ltd.)
      The adhesive behavior of blends of high mol. wt. poly(N-vinylpyrrolidone) (PVP) and short-chain, liq. PEG was studied using a 180° peel test as a function of compn. and water vapor sorption. Pressure-sensitive adhesion appears within a narrow compn. range (∼36 wt% PEG), and it is affected by the blend hydration. Both plasticizers, PEG and water, behave as tackifiers in the blends with glassy PVP. However, PEG alone is shown to account for the occurrence of adhesion, and the tackifying effect of PEG is appreciably stronger than that of sorbed water. Blend hydration enhances adhesion for the systems that exhibit an apparently adhesive type of debonding from a std. substrate (at PEG content <36 wt%), but the same amts. of sorbed water also are capable of depressing adhesion in the PEG-overloaded blends, where a cohesive mechanism of adhesive joint failure is typical. The blend contg. 36 wt% PEG couples both the adhesive and cohesive mechanisms of bond rupture (i.e., the fibrillation of adhesive polymer under debonding force and predominantly adhesive locus of failure). Blend hydration effects on adhesion are reversible.
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    34. 34
      Roos, A.; Creton, C.; Novikov, M. B.; Feldstein, M. M. Viscoelasticity and tack of poly(vinyl pyrrolidone)-poly(ethylene glycol) blends. J. Polym. Sci., Part B: Polym. Phys. 2002, 40 (20), 2395– 2409,  DOI: 10.1002/polb.10279
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      34
      Viscoelasticity and tack of poly(vinyl pyrrolidone)-poly(ethylene glycol) blends
      Roos, Alexandra; Creton, Costantino; Novikov, Mikhail B.; Feldstein, Mikhail M.
      Journal of Polymer Science, Part B: Polymer Physics (2002), 40 (20), 2395-2409CODEN: JPBPEM; ISSN:0887-6266. (John Wiley & Sons, Inc.)
      The adhesive properties of blends of high mol. wt. poly(vinylpyrrolidone) (PVP) and low mol. wt. polyethylene glycol (PEG) were systematically investigated with a probe test and correlated with their viscoelastic properties. The material parameters that were varied were the PEG content (31, 36, and 41%) and the debonding rate. The 36% PEG showed the best balance of properties for a pressure-sensitive adhesive. At low debonding rates, the debonding took place through the formation of a fibrillar structure, whereas at high debonding rates, the debonding was brittle. This transition was attributed to the breakage and reformation of hydrogen bonds between PVP units and OH groups on PEG during the large strain of the polymer chains in elongation. This transition was obsd., albeit shifted in frequency, for all three compns., and the characteristic relaxation times of the hydrogen-bonded network were estd. A comparison between the tack properties of the adhesives and their linear viscoelastic properties showed a very strong decoupling between the small-strain and large-strain properties of the adhesive, which was indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen during tensile tests, was attributed to the peculiar phase behavior of the blends.
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    35. 35
      Fu, J.; Wang, Q.; Schlenoff, J. B. Extruded Superparamagnetic Saloplastic Polyelectrolyte Nanocomposites. ACS Appl. Mater. Interfaces 2015, 7 (1), 895– 901,  DOI: 10.1021/am5074694
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      35
      Extruded Superparamagnetic Saloplastic Polyelectrolyte Nanocomposites
      Fu, Jingcheng; Wang, Qifeng; Schlenoff, Joseph B.
      ACS Applied Materials & Interfaces (2015), 7 (1), 895-901CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Iron oxide nanoparticles of diam. ca. 12 nm were dispersed into polyelectrolyte complexes made from poly(styrenesulfonate) and poly(diallyldimethylammonium). These nanocomposites were plasticized with salt water and extruded into dense, tough fibers. Magnetometry of these composites showed they retained the superparamagnetic properties of their constituent nanoparticles with satn. magnetization that scaled with the loading of nanoparticles. Their superparamagnetic response allowed the composites to be heated remotely by radiofrequency fields. While the modulus of fibers was unaffected by the presence of nanoparticles the toughness and tensile strength increased significantly.
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    36. 36
      Shamoun, R. F.; Reisch, A.; Schlenoff, J. B. Extruded Saloplastic Polyelectrolyte Complexes. Adv. Funct. Mater. 2012, 22 (9), 1923– 1931,  DOI: 10.1002/adfm.201102787
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      Extruded Saloplastic Polyelectrolyte Complexes
      Shamoun, Rabih F.; Reisch, Andreas; Schlenoff, Joseph B.
      Advanced Functional Materials (2012), 22 (9), 1923-1931CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Tough, dense polyelectrolyte complexes (PECs) with well-defined cross-sections are prepd. using a lab. extruder and plasticizing the complexes with salt water. Stoichiometric starting materials yield stoichiometric complexes of poly(diallyldimethylammonium) (PDADMA) and poly(styrene sulfonate) (PSS). As an example of this enabling technol., macroscopic tubes of PEC are produced. Microscopy images of cross-sections of rods, tape, and tubes show a pore vol. of less than 10% in the bulk of the extruded complex and fully dense material towards the surface, where the shear is greatest. Thermal gravimetric anal. reveals the expected salt content for PECs doped with NaCl, and a lack of salt for PECs rinsed in water. The fact that doped PECs are transparent suggests they are supersatd. with salt. Residual stress following extrusion is relieved by exposure to solns. of NaCl. Stress relaxation expts. show decreasing equil. moduli as a function of increasing salt doping, consistent with prior results on multilayers of the same polymers.
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    37. 37
      Wang, Q.; Schlenoff, J. B. Tough strained fibers of a polyelectrolyte complex: pretensioned polymers. RSC Adv. 2014, 4 (87), 46675– 46679,  DOI: 10.1039/C4RA08733J
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      Tough strained fibers of a polyelectrolyte complex: pretensioned polymers
      Wang, Qifeng; Schlenoff, Joseph B.
      RSC Advances (2014), 4 (87), 46675-46679CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Polyelectrolyte complexes, PECs, are formed spontaneously by the interaction of oppositely charged polyelectrolytes. When hydrated, PECs are tough, elastic, biocompatible materials, but when dry they are hard and brittle. In either form, PECs have long been considered 'unprocessable'. Here, we show that PECs are transformed from brittle to tough (when dry) by extrusion into highly strained fibers. Partial mol. strain in dry fibers may be relaxed by exposure to solns. of salt or hot water under a salt/temp. equivalence. Efficient shape memory of a complex strained in hot water is possible by cooling and recovering the original shape later in hot water.
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    38. 38
      Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. Specific Ion Effects on the Water Solubility of Macromolecules: PNIPAM and the Hofmeister Series. J. Am. Chem. Soc. 2005, 127 (41), 14505– 14510,  DOI: 10.1021/ja0546424
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      Specific Ion Effects on the Water Solubility of Macromolecules: PNIPAM and the Hofmeister Series
      Zhang, Yanjie; Furyk, Steven; Bergbreiter, David E.; Cremer, Paul S.
      Journal of the American Chemical Society (2005), 127 (41), 14505-14510CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Aq. processes ranging from protein folding and enzyme turnover to colloidal ordering and macromol. pptn. are sensitive to the nature and concn. of the ions present in soln. Herein, the effect of a series of sodium salts on the lower crit. soln. temp. (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated with a temp. gradient microfluidic device under a dark-field microscope. While the ability of a particular anion to lower the LCST generally followed the Hofmeister series, anal. of solvent isotope effects and of the changes in LCST with ion concn. and identity showed multiple mechanisms were at work. In solns. contg. sufficient concns. of strongly hydrated anions, the phase transition of PNIPAM was directly correlated with the hydration entropy of the anion. On the other hand, weakly hydrated anions were salted-out through surface tension effects and displayed improved hydration by direct ion binding.
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    39. 39
      Hariri, H. H.; Lehaf, A. M.; Schlenoff, J. B. Mechanical Properties of Osmotically Stressed Polyelectrolyte Complexes and Multilayers: Water as a Plasticizer. Macromolecules 2012, 45 (23), 9364– 9372,  DOI: 10.1021/ma302055m
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      39
      Mechanical Properties of Osmotically Stressed Polyelectrolyte Complexes and Multilayers: Water as a Plasticizer
      Hariri, Haifa H.; Lehaf, Ali M.; Schlenoff, Joseph B.
      Macromolecules (Washington, DC, United States) (2012), 45 (23), 9364-9372CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Compacted, macroporous complexes of poly(styrenesulfonate) and poly(diallyldimethylammonium chloride) were dehydrated under defined osmotic stress using poly(ethylene glycol), PEG. A strong mech. response to dehydration was obsd. At the lowest osmotic stress applied, macropores within the complex were compacted, and the material became transparent. With addnl. osmotic stress, the decrease in water content with increasing stress slowed considerably, but the complex became much stiffer, the equil. modulus reaching several hundred MPa. Concurrently, the complexes became more brittle. Multilayers of the same polyelectrolytes reached equil. hydration levels much faster and also increased significantly in modulus. Using an empirical fit, the plasticizing efficiency of water was shown to be exceptionally strong.
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    40. 40
      Spruijt, E.; Cohen Stuart, M. A.; van der Gucht, J. Linear Viscoelasticity of Polyelectrolyte Complex Coacervates. Macromolecules 2013, 46 (4), 1633– 1641,  DOI: 10.1021/ma301730n
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      40
      Linear Viscoelasticity of Polyelectrolyte Complex Coacervates
      Spruijt, Evan; Cohen Stuart, Martien A.; van der Gucht, Jasper
      Macromolecules (Washington, DC, United States) (2013), 46 (4), 1633-1641CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Two flexible, oppositely charged polymers can form liq.-like complex coacervate phases with rich but poorly understood viscoelastic properties. They serve as an exptl. model system for many biol. and man-made materials made from oppositely charged macromols. We use rheol. to systematically study the viscoelastic properties as a function of salt concn., chain length, chain length matching, and mixing stoichiometry of model complex coacervates of poly(N,N-dimethylaminoethyl methacrylate), PDMAEMA, and poly(acrylic acid), PAA. The dynamics of making and breaking ionic bonds between the oppositely charged chains underlie all linear viscoelastic properties of the complex coacervates. We treat (clusters of) ionic bonds as sticky points and find that there is a remarkable resemblance between the relaxation spectra of these complex coacervates and the classical sticky Rouse model for single polymer systems. Salt affects all relaxation processes in the same way, giving rise to a widely applicable time-salt superposition principle. The viscoelastic properties of the complexes are very different from those of the individual components. In the complexes with a chain length mismatch, the effect of the mismatch on the viscoelastic properties is not trivial: changing the length of the polycation affects the relaxation behavior differently from changing the length of the polyanion.
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    41. 41
      Li, L.; Srivastava, S.; Andreev, M.; Marciel, A. B.; de Pablo, J. J.; Tirrell, M. V. Phase Behavior and Salt Partitioning in Polyelectrolyte Complex Coacervates. Macromolecules 2018, 51 (8), 2988– 2995,  DOI: 10.1021/acs.macromol.8b00238
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      41
      Phase Behavior and Salt Partitioning in Polyelectrolyte Complex Coacervates
      Li, Lu; Srivastava, Samanvaya; Andreev, Marat; Marciel, Amanda B.; de Pablo, Juan J.; Tirrell, Matthew V.
      Macromolecules (Washington, DC, United States) (2018), 51 (8), 2988-2995CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Polyelectrolyte complexes are omnipresent both in nature and in the technol. world, including nucleotide condensates, biol. marine adhesives, food stabilizers, encapsulants, and carriers for gene therapy. However, the true phase behavior of complexes, resulting from associative phase sepn. of oppositely charged polyelectrolytes, remains poorly understood. Here, we rely on complementary exptl. and simulation approaches to create a complete quant. description of the phase behavior of polyelectrolyte complexes that represents a significant advance in our understanding of the underlying physics of polyelectrolyte complexation. Expts. employing multiple approaches with model polyelectrolytes-oppositely charged polypeptides poly(L-lysine) and poly(D,L-glutamic acid) of matched chain lengths-led to phase diagrams with compns. of the complex and the supernatant that were in excellent agreement with simulation results. Contrary to the widely accepted theory for complexation, we found preferential partitioning of salt ions into the supernatant phase. Addnl., the salt partitioning into the supernatant phase was found to initially increase and then decrease on increasing the salt concns., manifesting as a distinct min. in the salt partition coeffs. These trends were shown by simulations to be strongly influenced by the excluded vol. interactions in the complex phase, which were not accounted for in their entirety in earlier theories. We believe the comprehensive data we present will be conducive to the development of an accurate phys. theory for polyelectrolyte complexation with predictive capabilities.
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    42. 42
      Perry, S. L.; Sing, C. E. PRISM-Based Theory of Complex Coacervation: Excluded Volume versus Chain Correlation. Macromolecules 2015, 48 (14), 5040– 5053,  DOI: 10.1021/acs.macromol.5b01027
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      PRISM-Based Theory of Complex Coacervation: Excluded Volume versus Chain Correlation
      Perry, Sarah L.; Sing, Charles E.
      Macromolecules (Washington, DC, United States) (2015), 48 (14), 5040-5053CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Aq. solns. of oppositely charged polyelectrolytes can undergo liq.-liq. phase sepn. into materials known as complex coacervates. These coacervates have been a subject of intense exptl. and theor. interest. Efforts to provide a phys. description of complex coacervates have led to a no. of theories that qual. (and sometimes quant.) agree with exptl. data. However, this agreement often occurs in a degeneracy of models with profoundly different starting assumptions and different levels of sophistication. Theor. difficulties in these systems are similar to those in most polyelectrolyte systems where charged species are highly correlated. These highly correlated systems can be described using liq. state (LS) integral equation theories, which surpass mean-field theories by providing information on local charge ordering. We extend these ideas to complex coacervate systems using PRISM-type theories and are able to capture effects not observable in traditional coacervate models, particularly connectivity and excluded vol. effects. We can thus bridge two traditional but incommensurate theories meant to describe complex coacervates: the Voorn-Overbeek theory and counterion release. Importantly, we hypothesize that a cancellation of connectivity and excluded vol. effects provides an explanation for the ability of Voorn-Overbeek theory to fit exptl. data despite its well-known approxns.
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    43. 43
      Qin, J.; Priftis, D.; Farina, R.; Perry, S. L.; Leon, L.; Whitmer, J.; Hoffmann, K.; Tirrell, M.; de Pablo, J. J. Interfacial Tension of Polyelectrolyte Complex Coacervate Phases. ACS Macro Lett. 2014, 3 (6), 565– 568,  DOI: 10.1021/mz500190w
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      Interfacial Tension of Polyelectrolyte Complex Coacervate Phases
      Qin, Jian; Priftis, Dimitrios; Farina, Robert; Perry, Sarah L.; Leon, Lorraine; Whitmer, Jonathan; Hoffmann, Kyle; Tirrell, Matthew; de Pablo, Juan J.
      ACS Macro Letters (2014), 3 (6), 565-568CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)
      We consider polyelectrolyte solns. which, under suitable conditions, phase sep. into a liq.-like coacervate phase and a coexisting supernatant phase that exhibit an extremely low interfacial tension. Such interfacial tension provides the basis for most coacervate-based applications, but little is known about it, including its dependence on mol. wt., charge d., and salt concn. By combining a Debye-Huckel treatment for electrostatic interactions with the Cahn-Hilliard theory, we derive explicit expressions for this interfacial tension. In the absence of added salts, we find that the interfacial tension scales as N-3/2(η/ηc-1)3/2 near the crit. point of the demixing transition, and that it scales as η1/2 far away from it, where N is the chain length and η measures the electrostatic interaction strength as a function of temp., dielec. const., and charge d. of the polyelectrolytes. For the case with added salts, we find that the interfacial tension scales with the salt concn. ψ as N-1/4(1-ψ/ψc)3/2 near the crit. salt concn. ψc. Our predictions are shown to be in quant. agreement with expts. and provide a means to design new materials based on polyelectrolyte complexation.
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      Spruijt, E.; Sprakel, J.; Lemmers, M.; Stuart, M. A. C.; van der Gucht, J. Relaxation Dynamics at Different Time Scales in Electrostatic Complexes: Time-Salt Superposition. Phys. Rev. Lett. 2010, 105 (20), 208301,  DOI: 10.1103/PhysRevLett.105.208301
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      Relaxation dynamics at different time scales in electrostatic complexes: time-salt superposition
      Spruijt, Evan; Sprakel, Joris; Lemmers, Marc; Stuart, Martien A. Cohen; van der Gucht, Jasper
      Physical Review Letters (2010), 105 (20), 208301/1-208301/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      In this Letter we show that in the rheol. of electrostatically assembled soft materials, salt concn. plays a similar role as temp. for polymer melts, and as strain rate for soft solids. We rescale linear and nonlinear rheol. data of a set of model electrostatic complexes at different salt concns. to access a range of time scales that is otherwise inaccessible. This provides new insights into the relaxation mechanisms of electrostatic complexes, which we rationalize in terms of a microscopic mechanism underlying salt-enhanced activated processes.
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      Sudre, G.; Olanier, L.; Tran, Y.; Hourdet, D.; Creton, C. Reversible adhesion between a hydrogel and a polymer brush. Soft Matter 2012, 8 (31), 8184– 8193,  DOI: 10.1039/c2sm25868d
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      Reversible adhesion between a hydrogel and a polymer brush
      Sudre, Guillaume; Olanier, Ludovic; Tran, Yvette; Hourdet, Dominique; Creton, Costantino
      Soft Matter (2012), 8 (31), 8184-8193CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
      We have developed a new exptl. methodol. to investigate the adhesive properties of hydrogels on solid surfaces under fully immersed conditions. The method, based on contact mechanics, provides time-resolved reproducible and quant. data on the work of adhesion between a hydrogel at swelling equil. and a planar surface grafted with responsive brushes. We used poly(N,N-dimethylacrylamide) (PDMA) and polyacrylamide (PAM) as model gels and poly(acrylic acid) (PAA) as pH dependent polymer brush. The effect of pH, contact time and debonding velocity on adhesive interactions was specifically investigated. As expected from mol. interactions, we found that adhesion increased as the pH decreased and this was attributed to the formation of hydrogen bonds at the interface. Surprisingly, however, the buildup of adhesion increased slowly with the time of contact up to one hour and depended markedly on debonding velocity despite the very elastic nature of the hydrogels. Furthermore, the max. pH where adhesion was obsd. was significantly higher for the couple PAM-PAA than for the couple PDMA-PAA, in contrast with the onset of mol. interactions in dil. solns.
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    47. 47
      Wang, T.; Lei, C.-H.; Dalton, A. B.; Creton, C.; Lin, Y.; Fernando, K. A. S.; Sun, Y.-P.; Manea, M.; Asua, J. M.; Keddie, J. L. Waterborne, Nanocomposite Pressure-Sensitive Adhesives with High Tack Energy, Optical Transparency, and Electrical Conductivity. Adv. Mater. 2006, 18 (20), 2730– 2734,  DOI: 10.1002/adma.200601335
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      47
      Waterborne, nanocomposite pressure-sensitive adhesives with high tack energy, optical transparency, and electrical conductivity
      Wang, Tao; Lei, Chun-Hong; Dalton, Alan B.; Creton, Costantino; Lin, Yi; Fernando, K. A. Shiral; Sun, Ya-Ping; Manea, Mihaela; Asua, Jose M.; Keddie, Joseph L.
      Advanced Materials (Weinheim, Germany) (2006), 18 (20), 2730-2734CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Transparent and conductive pressure-sensitive adhesives are cast from aq. colloidal dispersions of poly(Bu acrylate) (P(BuA)) and functionalized carbon nanotubes (CNTs). At the percolation threshold for network formation (at only 0.3 wt % functionalized CNT), the nanotubes remarkably double the amt. of strain at adhesive failure and increase the adhesion energy by 85 % (see figure). The tack properties are explained by current models of adhesive debonding.
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    48. 48
      Deplace, F.; Carelli, C.; Mariot, S.; Retsos, H.; Chateauminois, A.; Ouzineb, K.; Creton, C. Fine Tuning the Adhesive Properties of a Soft Nanostructured Adhesive with Rheological Measurements. J. Adhes. 2009, 85 (1), 18– 54,  DOI: 10.1080/00218460902727381
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      Fine Tuning the Adhesive Properties of a Soft Nanostructured Adhesive with Rheological Measurements
      Deplace, F.; Carelli, C.; Mariot, S.; Retsos, H.; Chateauminois, A.; Ouzineb, K.; Creton, C.
      Journal of Adhesion (2009), 85 (1), 18-54CODEN: JADNAJ; ISSN:0021-8464. (Taylor & Francis, Inc.)
      A review. The major objective of this article is to present recent advances in the methodol. to fine tune the adhesive performance of a PSA. In addn. to the so-called Dahlquist criterion requiring a low modulus, we propose two addnl. rheol. predictors of the adhesive properties. The first one is derived from the description of the detachment of a linear elastic layer from a rigid substrate. We made an approx. extension of this anal. to the viscoelastic regime and showed that the transition from interfacial cracks to cavitation and fibrillation can be quant. predicted from the easily measurable ratio tan(δ)/G'(ω). If a fibrillar structure is formed, the nonlinear large strain properties become important. We showed that the ability of the fibrils to be stretched before final debonding can be predicted from the anal. of simple tensile tests. The softening, which occurs at intermediate strains, and, more importantly, the hardening which occurs at large strains, can be used to predict the mode of failure and the energy of adhesion. The use of this methodol. to tune the PSA structure for a specific application has been illustrated for the special case of wb-PSA made of core-shell particles, and improved adhesive properties on polyethylene surfaces have been obtained.
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    49. 49
      Jones, J. P.; Sima, M.; O’Hara, R. G.; Stewart, R. J. Water-Borne Endovascular Embolics Inspired by the Undersea Adhesive of Marine Sandcastle Worms. Adv. Healthcare Mater. 2016, 5 (7), 795– 801,  DOI: 10.1002/adhm.201500825
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      Water-Borne Endo-vascular Embolics Inspired by the Undersea Adhesive of Marine Sandcastle Worms
      Jones, Joshua P.; Sima, Monika; O'Hara, Ryan G.; Stewart, Russell J.
      Advanced Healthcare Materials (2016), 5 (7), 795-801CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Transcatheter embolization is used to treat vascular malformations and defects, to control bleeding, and to selectively block blood supply to tissues. Liq. embolics are used for small vessel embolization that require distal penetration. Current liq. embolic agents have serious drawbacks, mostly centered around poor handling characteristics and toxicity. In this work, a water-borne in situ setting liq. embolic agent is described that is based on electrostatically condensed, oppositely charged polyelectrolytes-complex coacervates. At high ionic strengths, the embolic coacervates are injectable fluids that can be delivered through long narrow microcatheters. At physiol. ionic strength, the embolic coacervates transition into a nonflowing solid morphol. Transcatheter embolization of rabbit renal arteries demonstrated capillary level penetration, homogeneous occlusion, and 100% devascularization of the kidney, without the embolic crossing into venous circulation. The benign water-borne compn. and setting mechanism avoids many of the problems of current liq. embolics, and provides precise temporal and spatial control during endovascular embolization.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhtlantb8%253D&md5=fc29fbcf7ab3a66860690e49f8d255fc
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      Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity
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      Nature Materials (2013), 12 (10), 932-937CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biol. tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mech. properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mech. properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These phys. hydrogels of supramol. structure can be tuned to change multiple mech. properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.
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      A review. Double-network (DN) gels have drawn much attention as an innovative material having both high water content (ca. 90 wt%) and high mech. strength and toughness. DN gels are characterized by a special network structure consisting of two types of polymer components with opposite phys. natures: the minor component is abundantly crosslinked polyelectrolytes (rigid skeleton) and the major component comprises of poorly crosslinked neutral polymers (ductile substance). The former and the latter components are referred to as the first network and the second network, resp., since the synthesis should be done in this order to realize high mech. strength. For DN gels synthesized under suitable conditions (choice of polymers, feed compns., atm. for reaction, etc.), they possess hardness (elastic modulus of 0.1-1.0 MPa), strength (failure tensile nominal stress 1-10 MPa, strain 1000-2000%; failure compressive nominal stress 20-60 MPa, strain 90-95%), and toughness (tearing fracture energy of 100∼1000 J m-2). These excellent mech. performances are comparable to that of rubbers and soft load-bearing bio-tissues. The mech. behaviors of DN gels are inconsistent with general mechanisms that enhance the toughness of soft polymeric materials. Thus, DN gels present an interesting and challenging problem in polymer mechanics. Extensive exptl. and theor. studies have shown that the toughening of DN gel is based on a local yielding mechanism, which has some common features with other brittle and ductile nano-composite materials, such as bones and dentins.
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      Hydrogels are used as scaffolds for tissue engineering, vehicles for drug delivery, actuators for optics and fluidics, and model extracellular matrixes for biol. studies. The scope of hydrogel applications, however, is often severely limited by their mech. behavior. Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels have achieved stretches in the range 10-20, but these values are markedly reduced in samples contg. notches. Most hydrogels are brittle, with fracture energies of about 10 J m-2 (ref. 8), as compared with ∼1,000 J m-2 for cartilage and ∼10,000 J m-2 for natural rubbers. Intense efforts are devoted to synthesizing hydrogels with improved mech. properties; certain synthetic gels have reached fracture energies of 100-1,000 J m-2 (refs 11, 14, 17). Here we report the synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks. Although such gels contain ∼90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of ∼9,000 J m-2. Even for samples contg. notches, a stretch of 17 is demonstrated. We attribute the gels' toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, which heals by re-zipping. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.
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