Conductive Tough Hydrogels with a Staggered Ion-Coordinating

Jun 17, 2019 - By contrast, the work of extension sharply increased initially and then continued to ... (a) Tensile stress–strain curves of poly(N-a...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24598−24608

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Conductive Tough Hydrogels with a Staggered Ion-Coordinating Structure for High Self-Recovery Rate Van Tron Tran,†,‡ Md. Tariful Islam Mredha,†,‡ Suraj Kumar Pathak,† Hyungsuk Yoon,†,§ Jiaxi Cui,*,∥,⊥ and Insu Jeon*,† †

School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea ∥ INM - Leibniz Institute for New Materials, Campus D2 2, Saarbrücken 66123, Germany ⊥ Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China

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ABSTRACT: Conductive hydrogels are attracting increasing attention owing to their great potential for applications in flexible devices. For practical use, these high-water-content materials should not only show good conductivity but also be strong, stretchable, tough, and elastic. Herein, we describe a class of novel conductive tough hydrogels based on strong staggered Fe3+-carboxyl coordinating interactions. They are made from copolymers of acrylamide and N-acryloyl glutamic acid, a bidentate-based comonomer. The design of the staggered structure of Fe3+ and bidentate units is expected to enable energy dissipation and also results in a synergetic effect of two binding sites for fast self-recovery. We demonstrate that the equilibrated hydrogels with a water content of 53 wt % exhibit superior mechanical properties (e.g., highest tensile strength, 12.1 MPa; Young’s modulus, 36.1 MPa; work of extension, 42.1 MJ m−3; fracture energy, 10,691 J m−2; compressive strength, 65.1 MPa at 98% strain without a macroscopic fracture) compared to the ion-coordinated hydrogels reported to date, including elasticity at small strain, fast selfrecoverability at room temperature (∼25 °C), a high dielectric constant (k = 341−1395 at 100 kHz), and good electrical conductivity (0.0018−0.024 S cm−1). Given their extraordinary overall characteristics, we envision their potential applications in flexible electronic devices. KEYWORDS: conductive hydrogels, staggered structure, fast self-recovery, tough hydrogels, high dielectric constant

1. INTRODUCTION Conductive hydrogels are promising materials for use in flexible electronic devices, especially those used in/on the human body, owing to their biocompatibility, mechanical flexibility, good electronic properties, and easy fabrication.1−5 These materials consist of cross-linked hydrophilic polymer networks with a large amount of water molecules entrapped in them. The water component is solid-like at the macroscopic level but provides a continuous aqueous microenvironment in which various additives, including carbon-based materials,6−8 conductive polymers,9,10 metal nanoparticles,11−14 and salt15−18 can be added to tailor the macroscopic electronic properties of hydrogels. Such a structure allows functionalization of the hydrogels for different applications such as touch panels,19 energy conversion and storage components of wearable devices,20−25 flexible electronic sensors,26−28 and body monitors.29 For practical use in these applications, the conductive hydrogels should not only show good electronic properties but also be strong, stretchable, tough, and elastic.3 © 2019 American Chemical Society

However, because of the high water content, traditional conductive hydrogels are normally brittle and mechanically weak.30,31 Although several strategies have recently been developed to improve the mechanical properties of hydrogels, as exemplified by double-network,32,33 alginate/polyacrylamide,34,35 nanocomposite,36,37 dynamically cross-linked,38,39 sliding ring structure,40 and polyampholyte41 hydrogels, subtle structure design and fabrication are frequently required in these methods to realize an efficient energy dissipation mechanism and to balance different mechanical parameters. Some parameters, such as toughness and recovery rate, are difficult to consider together since efficient, reversible energy dissipation for high toughness normally leads to a low recovery rate. Therefore, the preparation of conductive hydrogels with Received: April 25, 2019 Accepted: June 17, 2019 Published: June 17, 2019 24598

DOI: 10.1021/acsami.9b06478 ACS Appl. Mater. Interfaces 2019, 11, 24598−24608

Research Article

ACS Applied Materials & Interfaces superior mechanical properties, elasticity, and fast selfrecoverability remains a challenge. Reversible ionic coordination is emerging as a powerful supramolecular interaction to build hydrogels with good mechanical and/or electrical properties. In their pioneering work, Zhou et al.42 demonstrated that strong Fe3+-carboxyl coordination can be used as the sacrificial bonds to dissipate energy in hydrogels, and this made them extremely strong (∼6 MPa) and tough (∼27 MJ m−3) while maintaining a water content of ∼60 wt %. A similar ferric ion cross-linked supramolecular poly(acrylic acid) (PAA) hydrogel electrolyte (KCl-Fe3+/PAA) was designed with 1 M KCl for flexible supercapacitors, which showed excellent conductivity (0.09 S cm−1) due to the added salts.43 The gel electrolyte with ∼60 wt % water content exhibited a tensile strength of ∼430 kPa and stretchability of ∼700%.43 Enhancing the mechanical properties appears to induce a deterioration in the conductivity, as evidenced in a very recent example (conductivity, 0.0024 S cm−1; tensile strength, 3.5 MPa; work of extension, 28.5 MJ m−3; compressive strength, 32 MPa; water content, ∼60 wt %),44 because strong ionic coordinating interaction would restrict the mobility of the ions. Polyampholyte in which ions might move along the polymer chains could be a solution to realize good mechanical properties and conductivity, as seen in the case of polyampholyte hydrogels (conductivity, ∼0.03 S cm−1; tensile strength, ∼1.3 MPa; work of extension, ∼6.7 MJ m−3; water content, ∼50 wt %).45 Strong electrostatic interactions between the polymer chains can lead to good mechanical properties, and the undialyzed KCl salts present in the hydrogels contribute to good conductivity. However, these hydrogel systems show a low self-recovery rate (self-recovery in 2 h after sustaining a tensile stress of ∼0.7 MPa) because of the time-consuming reformation of the strong cluster electrostatic interactions of the polyampholyte. To realize good mechanical properties, especially the self-recovery rate and strength, in addition to conductivity, the strong ion−ligand complex designed for energy dissipation should be dragged back to reform quickly after being dissembled under stretching. We hypothesize that a repeating unit of the polymer with two chelating sites could allow the formation of strong staggered coordination with Fe3+ because of the Fe3+tricarboxyl complex structure. In such a staggered coordination structure, the disassembled bond could reform again quickly with the assistance of neighboring bonds connected in the same repeating unit in the polymer chains, leading to a high recovery rate. Hence, the resulting conductive gels can simultaneously exhibit good mechanical and self-recovery properties. Based on this concept, herein, we report a kind of novel conductive tough hydrogel made from N-acryloyl glutamic acid, a bidentate-based monomer. As shown in Figure 1, the bidentate moieties in the repeating units not only result in stronger and more efficient Fe3+-carboxyl bonds than their monodentate counterparts but also form a staggered structure that in turn results in a synergetic effect in energy dissipation. As a result of such design, the hydrogels consisting of poly(Nacryloyl glutamic acid-co-acrylamide) [P(AGA-co-AAm)] show excellent mechanical properties, good conductivity, and especially, a high self-recovery rate. The best performing hydrogel with 53 wt % water content exhibited the highest tensile strength of 12.1 MPa, a Young’s modulus of 36.1 MPa, a fracture energy of 10,691 J m−2, a compression stress of 65.1 MPa at 98% strain without a macroscopic fracture, almost

Figure 1. Schematic representations of poly(N-acryloyl glutamic acidco-acrylamide) [P(AGA-co-AAm)] copolymer hydrogel with staggered Fe3+-carboxyl coordinating structure.

complete self-recovery in 20 min after withstanding a tensile stress of 4.3 MPa, and ∼80% self-recovery in 2.5 h after withstanding a tensile stress of 8.8 MPa at room temperature (∼25 °C). Furthermore, our hydrogels exhibit good electrical conductivity (0.0018−0.024 S cm−1) and high dielectric constants (k = 341−1395 at 100 kHz). Despite the similar water contents, the mechanical properties of our conductive hydrogels are far superior compared to those of other reported conductive hydrogels.43−45 To the best of our knowledge, this is the strongest ion-coordinated conductive hydrogel reported to date.

2. RESULTS AND DISCUSSION 2.1. Bidentate Monomer-Based Ion-Coordinating Hydrogel. The bidentate N-acryloyl glutamic acid (AGA) monomer was synthesized and characterized (Figures S1−S4). The hydrogels are denoted as P(AGA-co-AAm)(x−y), where x and y are the feeding concentrations of monomers N-acryloyl glutamic acid and acrylamide (AAm), respectively. To prepare the hydrogels, a mixture solution of AGA and AAm was copolymerized in the presence of a thermoinitiator in a welldefined chamber made by glass plates. The resulting asprepared hydrogels were then soaked in a FeCl3 solution for 24 h. During this time, Fe3+ ions diffused from the surface to the bulk of the hydrogel to afford a fully coordinated structure (Figure S5). Free Fe3+ ions were washed out by soaking the gel in water for 24 h. This wash-out treatment caused slight shrinkage (Figure S6) owing to the formation of uniform, stronger, and staggered ion-coordinating bonds after the removal of superfluous ions.42 The tensile strength measurement was first used to evaluate the mechanical properties of the hydrogels. The tensile strength, Young’s modulus, and work of extension of the equilibrated P(AGA-co-AAm)(0.3−3) hydrogel were determined to be 7.4 MPa, 7.9 MPa, and 39.8 MJ m−3, respectively. Notably, these values are slightly higher than those obtained for the unwashed hydrogel but significantly exceed (by factors of 308, 343, and 107, respectively) those of the as-prepared hydrogel (Figure S7). This dramatic enhancement is ascribed to the strong coordination of Fe3+ ions by the dicarboxylate groups of the AGA units (Figure 1), which results in high stiffness and strength, whereas the breakage of sacrificial metal−ligand bonds during deformation allows for high stretchability and toughness. 24599

DOI: 10.1021/acsami.9b06478 ACS Appl. Mater. Interfaces 2019, 11, 24598−24608

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Tensile stress−strain curves of poly(N-acryloyl glutamic acid-co-acrylamide) [P(AGA-co-AAm)] hydrogels fabricated using variable AGA concentrations and fixed AAm and Fe3+ concentrations (3 and 0.1 M, respectively). (b) Young’s modulus and tensile strength, (c) work of extension, and (d) water content of the resulting hydrogels as functions of AGA concentration. In plots (b−d), error bars indicate mean absolute deviations (n = 3).

2.2. Effect of Fe3+ Ion Concentration and Monomer Composition. The coordinate structure of the copolymer hydrogel and hence, its mechanical properties were influenced by the concentration of Fe3+, that is, as the concentration increased from 0.02 to 0.5 M, the mechanical properties of the equilibrated P(AGA-co-AAm)(0.3−3) hydrogel first increased and then decreased (Figure S8). Thus, low Fe3+ concentrations are insufficient for stable coordinate bond formation, whereas overly high concentrations have a disruptive effect. Owing to the presence of excess ions at high Fe3+ concentrations, an inhomogeneous structure consisting of both low and high coordination numbers should be simultaneously formed in the hydrogel network, which can reduce its mechanical properties. The result is also consistent with the effect of water washing (as described in the previous section), that is, after removing the superfluous ions by water washing, a large number of high coordination structures were formed inside the hydrogel, thus increasing its strength.42 Consequently, a moderate Fe3+ concentration of 0.1 M, which resulted in the formation of a hydrogel with superior properties, was selected for further study. To understand the effects of the monomer composition, we used AGA concentrations of 0.2−0.4 M to prepare a series of hydrogels and evaluated their tensile properties (Figure 2a, Table S1). The concentration of AAm was fixed at 3 M, which afforded the optimal mechanical performance (Figure S9). Surprisingly, Young’s modulus and tensile strength of the hydrogels sharply increased with increasing AGA concentration within a narrow interval (Figure 2b). By contrast, the work of extension sharply increased initially and then continued to slowly increase at high AGA concentrations, which is ascribed to the concomitant decrease of stretchability (Figure 2c). Moreover, the hydrogel water content also sharply decreased with increasing AGA concentration (Figure 2d). These results indicate that the density of the carboxylate groups coordinated to Fe3+ strongly influences the stiffness and

strength of the hydrogels as well as their ability to effectively dissipate energy. The highest tensile strength, Young’s modulus, and work of extension, which were achieved for the P(AGA-co-AAm)(0.4−3) hydrogel with ∼53 wt % water, were 12.1 MPa, 36.1 MPa, and 42.1 MJ m−3, respectively. These values considerably exceeded those previously obtained for the monocarboxylate-based poly(acrylic acid-co-acrylamide) [P(AAc-co-AAm)] hydrogel with ∼50 wt % water (∼10 MPa, 17 ± 0.8 MPa, and 26.5 ± 4.3 MJ m−3, respectively), despite the latter gel’s reliance on both coordinate covalent (Fe3+) and covalent cross-linking.42 The above relationship between properties was also observed for P(AGA-co-AAm) hydrogels with other compositions and water contents of 60−70% (Figure 2b−d).42 2.3. Effect of Monomer Structure. To clarify the synergetic effect of the bidentate repeating units, we fabricated the monodentate-based hydrogel, that is, P(AAc-co-AAm) hydrogels with (0.4−3) and (0.8−3) compositions, as the control hydrogels for comparison (Figure 3, Table S1). The tensile strength, Young’s modulus, and work of extension of P(AGA-co-AAm)(0.4−3) exceeded those of P(AAc-co-AAm)(0.4−3) by factors of 3, 19, and 3.5, respectively. Moreover, with identical carboxylate group densities, the mechanical properties of the former gel were still much better than those of the latter. These results clearly show that bidentate monomer-based hydrogels feature stable and strong metal− ligand bonds caused by a multidentate chelating effect as well as an energy dissipation capability. Therefore, these hydrogels are stiffer, stronger, and tougher than their monodentate monomer-based counterparts. 2.4. Fracture Energy and Compressive Property. P(AGA-co-AAm)(0.3−3) with 61.4 wt % water content and P(AGA-co-AAm)(0.4−3) with 53 wt % water hydrogels were chosen for further study as they exhibited the highest stretchability (707%) and highest strength/modulus (12.1/ 36.1 MPa). Both of these hydrogels exhibit very high fracture 24600

DOI: 10.1021/acsami.9b06478 ACS Appl. Mater. Interfaces 2019, 11, 24598−24608

Research Article

ACS Applied Materials & Interfaces

which prevents their ultimate macroscopic failure, allowing still sustaining of the load after these points.48 Such an internal fracture of the gels is recoverable, so the largely compressed gel after 98% compression can regain >90% of its original thickness after 7 h of immersion in water (Figure S12). In terms of strength and stiffness, our hydrogels are superior to other reported tough hydrogels in terms of both tension and compression (Figure S13) and additionally, feature good stability in water (Figure S14). 2.5. Elasticity, Hysteresis, and Fast Self-Recovery. In the initial low strain region, our hydrogels exhibited excellent elasticities, which are similar to that of natural rubber.49 This is because the bidentate carboxylate group leads to the formation of strong ion coordinate bonds in the gel network, which is difficult to break upon small deformation. At 3% strain, both the P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) hydrogels showed similar loading−unloading pathways with negligible hysteresis and no residual strain for up to 10 tension and compression cycles (Figure 5a and Figure S15a). The maximum strengths (0.4−1.1 MPa) of these hydrogels remained nearly constant for up to 10 cycles, which is indicative of high resistance to long-term repeated loading in the low strain region. For comparison, we have also performed a loading−unloading test on a monocarboxylate-based P(AAcco-AAm)(0.8−3) hydrogel, which was prepared with the same carboxylate group density as that of the bidentate-based P(AGA-co-AAm)(0.4−3) hydrogel. Interestingly, unlike bidentate-based gels (Figure 5a), the P(AAc-co-AAm)(0.8−3) hydrogel exhibited a small hysteresis and residual strain at 3% strain (Figure 5b). This indicated that the ion coordinate bond in the monodentate-based gel is weaker than that of the bidentate counterparts and hence, can be dissociated upon small deformation and exhibits hysteresis. The excellent elastomeric properties were further demonstrated by dropping a ball prepared using our hydrogel and a commercial rubber ball from a height of 50 cm (Movie S1). Following the first bounce, the hydrogel ball reached ∼95% of the initial height, whereas the commercial ball only reached ∼80%. Moreover, the former ball featured a longer bouncing time than the latter. In the high strain region (30%), our hydrogels showed hysteresis, which confirmed that energy dissipation was achieved via the rupture of sacrificial coordinate covalent bonds (Figure 5c and Figure S15b). The presence of some residual strain (∼5−8%) after the cyclic test at 30% strain indicates that some permanent deformation occurred. However, the deformation can be recovered after waiting for a short time without any application of external heating. Despite the large tensile and compressive stresses at this strain, the stress, hysteresis, and residual strain of the P(AGA-coAAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) hydrogels completely recovered (at room temperature) in 5 and 20 min, respectively, during the tensile cyclic test (Figure 5c), whereas only 4 and 8 min, respectively, were required for recovery during the compressive cyclic test (Figure S15b). In sharp contrast, at 30% tensile strain, the monodentate-based P(AAcco-AAm)(0.8−3) required 1 h for recovery despite holding much lower strength (2.5 MPa) compared to its bidentate counterparts (4.3 MPa) (Figure 5d). This comparison was also valid at further higher strain. After the application of 100% tensile strain, the large hysteresis of P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) hydrogels recovered by ∼90 and ∼80% in 2 and 2.5 h, respectively (Figure 5e). On the other hand, the hysteresis of the monodentate-based P(AAc-co-

Figure 3. Mechanical properties of poly(N-acryloyl glutamic acid-coacrylamide) [P(AGA-co-AAm)] and poly(acrylic acid-co-acrylamide) [P(AAc-co-AAm)] hydrogels. (a) Tensile stress−strain curves of hydrogels prepared using different AGA and AAc concentrations and the corresponding (b) tensile strength, (c) Young’s modulus, and (d) work of extension. In plots (b−d), error bars indicate mean absolute deviations (n = 3).

energies (8.9 and 10.7 kJ m−2, respectively), as measured by a previously reported tensile fracture test procedure34,46 (Figure 4a and Figure S10). The fracture energy obtained by the

Figure 4. (a) Fracture energies of poly(N-acryloyl glutamic acid-coacrylamide) [P(AGA-co-AAm)](0.3−3) and P(AGA-co-AAm)(0.4− 3) hydrogels determined using notched and unnotched samples. (b) Compressive stress−strain curves of P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) hydrogels. The true stress was calculated by multiplying the nominal stress value by a factor of (1 + engineering strain); here, the sign of the engineering strain is negative for compression.

trouser tear test also revealed similar results (9.9 and 11 kJ m−2, respectively) (Figure S11). Such energies, which are similar to that of natural rubber, have previously only been achieved for soft and low-strength hydrogels.34,47 Moreover, the hydrogels exhibit extraordinary compression properties, for example, at 98% strain, the P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) hydrogels withstood loads of 53.3 ± 1.1 and 67.2 ± 2.8 MPa, respectively, without any apparent macroscopic fracture (Figure 4b, Table S1). However, the corresponding true stress versus strain curves of the P(AGA-coAAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) hydrogels pass through the maxima at 78 and 72% strain, respectively, which correspond to nominal stress values of 21.6 and 25.2 MPa, respectively. These stress points are likely due to the microscopic fracture of the cross-linking points in the gels, 24601

DOI: 10.1021/acsami.9b06478 ACS Appl. Mater. Interfaces 2019, 11, 24598−24608

Research Article

ACS Applied Materials & Interfaces

Figure 5. Self-recovery of poly(N-acryloyl glutamic acid-co-acrylamide) [P(AGA-co-AAm)](0.3−3), P(AGA-co-AAm)(0.4−3), and poly(acrylic acid-co-acrylamide) [P(AAc-co-AAm)(0.8−3)] hydrogels at different tensile strains (3, 30, and 100%). (a, b) Stress−strain curves for 10 consecutive tensile loading−unloading cycles (waiting time: 1 min) at 3% strain of (a) P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) and (b) P(AAc-co-AAm)(0.8−3). (c, d) Stress−strain curves for tensile loading−unloading cycles at 30% strain with different waiting times of (c) P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) and (d) P(AAc-co-AAm)(0.8−3). (e, f) Stress−strain curves for tensile loading−unloading cycles at 100% strain for (e) P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) and (f) P(AAc-co-AAm)(0.8−3) with different waiting times.

AAm)(0.8−3) hydrogel recovered by only ∼70% even after 5 h (Figure 5f). This is true despite the fact that, at 100% strain, the bidentate-based P(AGA-co-AAm)(0.4−3) holds a massive 8.8 MPa stress, whereas the identical carboxylate-containing P(AAc-co-AAm)(0.8−3) hydrogel holds only 5 MPa stress. The fast recoverability of bidentate-based hydrogels was attributed to the staggered structure in which the disassembled metal ion and ligand could be dragged back by the assistance of their neighbor coordinating bonds and then reassemble rapidly (Figure 1). Figure 6a shows that the P(AGA-co-AAm)(0.3−3) hydrogel quickly regained its initial shape after one tensile or compressive loading−unloading cycle at 30% strain. After one tensile or compressive loading−unloading cycle at 50% strain, the hydrogel regained its initial shape in just 6 or 5 min (underwater), respectively. The superior self-recoverability of the strongest hydrogel, P(AGA-co-AAm)(0.4−3), was demonstrated by comparing our data with those reported for other rapidly self-recoverable hydrogels (Figure 6b). Despite withstanding very high tensile loads, our hydrogel recovered within a very short time owing to its staggered ion-coordinating structure.

The recovery of our hydrogels was further examined by inducing serious fatigue via 100 consecutive loading− unloading cycles at 30% strain (Figure 7). As a result, the tensile strength decreased by a factor of almost 2 after 100 cycles, and almost 50% of the applied strain could not be recovered. However, the P(AGA-co-AAm)(0.3−3) and P(AGA-co-AAm)(0.4−3) hydrogels completely recovered after 1.5 and 3 h, respectively. The excellent load-bearing and selfrecoverability properties were demonstrated by the fact that a 20 × 20 × 3 mm3 hydrogel sample could withstand a quarter of a load of 1800 kg car without a fracture and could immediately recover its initial size when the load was removed (Movie S2). 2.6. Electrical Properties. The presence of ion-mediated dynamic coordinate covalent interactions in our hydrogels resulted in good electrical conductivity, as exemplified by the fact that an electrical circuit comprising P(AGA-co-AAm)(0.3− 3) could brightly light up a light-emitting diode (LED) when voltages of greater than 10 V were applied (Figure 8a). The resistance (R, Ω) of our hydrogel was quantitatively evaluated as depicted in Figure S16, and the resistivity (ρ, Ω m) and conductivity (σ, S cm−1) were calculated as 5.8 ± 0.2 Ω m and 0.0017 ± 0.0001 S cm−1, respectively, using eqs 1 and 2 24602

DOI: 10.1021/acsami.9b06478 ACS Appl. Mater. Interfaces 2019, 11, 24598−24608

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a) Photographs demonstrating the excellent self-recovery of equilibrated poly(N-acryloyl glutamic acid-co-acrylamide) [P(AGAco-AAm)](0.3−3) hydrogels after one cycle at 30 and 50% strain under tension (left) and compression (right). (b) After the application of tensile load, recoveries of the dissipated energy (at room temperature) and the corresponding recovery times of our P(AGA-co-AAm)(0.4−3) hydrogel (this study) were compared with P(AAc-co-AAm)(0.8−3) and previously reported hydrogels (inorganic−organic hybrid hydrogel (P(AAm-co-LMA)-HLPs),50 dual physically cross-linked hydrogel (DPC gel),51 hybrid ionic−covalent hydrogel (DN-Cit gel),52 dual cross-linked gel (P(AAc-co-AAm)/ Fe3+),42 and dual cross-linked physical hydrogel (DP gel)53). The applied strain (ε) of different hydrogels was described on the figure.

Figure 8. (a) LED illumination using a fully flexible electrical circuit comprising the poly(N-acryloyl glutamic acid-co-acrylamide) [P(AGA-co-AAm)](0.3−3) hydrogel (thickness = 3 mm). (b) Electromagnetic property of the P(AGA-co-AAm)(0.3−3) hydrogel bar (diameter = 10 mm). (c) Dielectric constants of fully and partially coordinated P(AGA-co-AAm)(0.3−3) hydrogels. (d) Dielectric constants of various flexible materials (poly(dimethylsiloxane) (PDMS),55 silicone/lead magnesium niobate/lead titanate (silicone/ PMN-PT), 56 multiwalled carbon nanotubes/polypropylene (MWCNTs@PPy),57 chitosan/gellan gum (A-gellan),58 and poly(vinyl alcohol)−poly(acrylic acid)/Pb(Zr0.52Ti0.48)O3 (PZT) hydrogel (PVA-PAA/PZT)59) compared with those of our hydrogels. In plot (c), error bars indicate mean absolute deviations (n = 3).

After equilibration in 1 M KCl solution, the conductivity of this Fe3+-containing gel increased further (0.023 ± 0.001 S cm−1), reaching the same level as that of previously reported highly conductive hydrogels.43,45 Due to the good conductivity, our gel exhibits electromagnetic properties (Figure 8b and Movie S3). For example, a water-equilibrated hydrogel bar placed in a Cu wire coil became an electromagnet when the two ends of the coil were connected to a 1.5 V battery. To evaluate the dielectric properties of the hydrogels, we used a 5 × 5 × 5 mm3 fully coordinated P(AGA-coAAm)(0.3−3) sample prepared by 60 h of incubation in 0.1 M Fe3+ followed by 36 h of soaking in water. The hydrogel capacitance was measured at frequencies of 100−800 kHz, and the corresponding dielectric constants were calculated (Figure 8c). The hydrogel showed a much higher dielectric constant than previously reported flexible dielectric materials (hydrogels or elastomers; Figure 8d).55−59 Thus, the hydrogel was used to design a force sensor with a high load-sensing capability (0−20 kgf; Figure 9a). As capacitance is known to depend on the (input loaddetermined) distance between capacitor electrodes,60 we recorded initial and final hydrogel capacitances (C0 and C, respectively) under different loading conditions (Movie S4). The (C − C0)/C0 ratio sharply increased with increasing applied force (Figure 9b) owing to the ability of the hydrogel to sustain large loads (inset, Figure 9b). The designed force sensor exhibited an excellent response capability at high input loads, although the high strength and stiffness of the hydrogel resulted in insensitivity to low input loads (