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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 13685−13692

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Tough and Conductive Hybrid Hydrogels Enabling Facile Patterning Fengbo Zhu,† Ji Lin,† Zi Liang Wu,*,‡ Shaoxing Qu,† Jun Yin,*,§ Jin Qian,*,† and Qiang Zheng‡ †

Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Department of Engineering Mechanics and ‡MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China § State Key Laboratory of Fluid Power and Mechatronic Systems, Key Laboratory of 3D Printing Process and Equipment of Zhejiang Province, School of Mechanical Engineering, Zhejiang University, Hangzhou 310028, China S Supporting Information *

ABSTRACT: Conductive polymer hydrogels (CPHs) that combine the unique properties of hydrogels and electronic properties of conductors have shown their great potentials in wearable/implantable electronic devices, where materials with remarkable mechanical properties, high conductivity, and easy processability are demanding. Here, we have developed a new type of polyion complex/polyaniline (PIC/PAni) hybrid hydrogels that are tough, conductive, and can be facilely patterned. The incorporation of conductive phase (PAni) into PIC matrix through phytic acid resulted in hybrid gels with ∼65 wt % water; high conductivity while maintaining the key viscoelasticity of the tough matrix. The gel prepared from 1 M aniline (Ani) exhibited the breaking strain, fracture stress, tensile modulus, and electrical conductivity of 395%, 1.15 MPa, 5.31 MPa, and 0.7 S/m, respectively, superior to the most existing CPHs. The mechanical and electrical performance of PIC/PAni hybrid hydrogels exhibited pronounced rate-dependent and selfrecovery behaviors. The hybrid gels can effectively detect subtle human motions as strain sensors. Alternating conductive/ nonconductive patterns can be readily achieved by selective Ani polymerization using stencil masks. This facile patterning method based on PIC/PAni gels can be readily scaled up for fast fabrication of wavy gel circuits and multichannel sensor arrays, enabling real-time monitoring of the large-extent and large-area deformations with various sensitivities. KEYWORDS: conductive hydrogel, tough hydrogel, mechanical performance, strain sensor, patterning

1. INTRODUCTION The development of wearable/implantable electronic devices and bioelectronics has been marching toward hydrogel-based soft electronics, which calls for materials with superior mechanical performance, high electrical conductivity, and easy processability. Hydrogel-based materials are also rich in water and possess a high degree of flexibility/adaptivity, resembling natural tissues, and holding a great potential for biomedical applications. However, the majority of existing hydrogels are electrically nonconductive, which impedes their use as bioelectronics and soft electrodes.1−5 On the other hand, most of the existing conductors are metal-,6,7 carbon-, or silicon-based materials,8,9 which struggle to meet the specific applications in biomedical engineering for the lack of stretchability and biocompatibility. Conductive polymer hydrogels (CPHs) provide a unique class of materials that combine the adaptive properties of hydrogels and electrical conductivity,10,11 with a great potential in a variety of applications such as implantable/wearable electronic devices, bioelectronics, electrochemical supercapacitors, and so on.4,11−16 Hydrogels swollen with saline solutions have been demonstrated to enable stretchable and transparent conductors.15−17 However, this strategy inevitably requires high salt concentrations as solvents for conducting the ability and © 2018 American Chemical Society

electromechanical transduction. For example, 8 M lithium chloride solution was used to transmit electrical signals within elastomers and gels as an ionic cable.17 When being used for implantable devices, such ionic conductors may lead to severe diffusion of salt ions and impair the normal functions of organs. Also, ionic conductors cannot maintain their conductivity under aqueous conditions for the loss of ions. Other conductive polymer gels involving ionic liquids as the conductive medium encounter similar problems that hinder their use in biomedical applications.18−21 As an alternative, intrinsically conductive polymers with unique π-conjugated structures have been employed to synthesize CPHs with functional cross-linkers, as achieved in hydrogels of polyaniline (PAni) with phytic acid,10 polypyrrole (PPy) with graphene oxide,22 and poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) with Fe3+.23 Among the listed, phytic acid/ conductive polymer (PA/CP)-based hydrogels have advantageous features because their fabrication process is facile and fast, and the synthesized CPHs exhibit remarkable electrochemical properties. The key to successful fabrication of PA/ Received: January 31, 2018 Accepted: April 2, 2018 Published: April 2, 2018 13685

DOI: 10.1021/acsami.8b01873 ACS Appl. Mater. Interfaces 2018, 10, 13685−13692

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of the synthetic process of PIC/PAni hybrid gels, where PAni is cross-linked to the PIC matrix via PA molecules. (a) PIC was first plasticized by the mixture of Ani monomers, phytic acid solution, and saline water; after compression molding into thin sheets, the as-prepared hybrid gels were immersed in APS solution for the in situ polymerization of Ani monomers in the gel matrix. The as-prepared gels were immersed in a large amount of deionized water for the dialysis of counterions and extra initiator, resulting in strong, tough, and conductive PIC/PAni gels. (b) Optical images of the hydrogel samples in different mechanical states during the synthetic process. The scale bar is 1 cm.

were achieved by the formation of ionic clusters with a wide spectrum of strength, where relatively weak clusters act as dynamic and sacrificial bonds upon loading, whereas strong ones maintain the structural integrity.40−42 This tough PIC hydrogel is physically cross-linked by ionic bonds, which can be easily processed into desired shapes via sol−gel transition in compression molding or 3D printing.37−39 For conductivity, the PIC matrix consists of abundant polycations that provide anchoring sites for ionic PA molecules, which can be further cross-linked to PAni (conductive phase) through phosphoric acid groups. In the following, tough and conductive PIC/PAni hydrogels have been prepared by incorporating PAni into PIC matrix. The obtained hybrid gels with 60−70 wt % water synergize the mechanical advantages of PIC matrix and the conductive feature of PAni. For example, 1 M aniline (Ani) rendered the hybrid gel with tensile fracture stress, breaking strain and elastic modulus of 1.15 MPa, 395%, and 5.31 MPa, respectively, plus an electrical conductivity of 0.7 S/m. The conductivity can be further enhanced with elevated contents of Ani. Such mechanical and electrical performances exhibited pronounced rate-dependent and self-recovery behaviors, indicating that the PIC/PAni hybrid gels inherited the main mechanics of the PIC matrix. The present PIC/PAni hydrogels can function as strain sensors attributed to the strong electromechanical coupling within the materials, enabling the detection of subtle human motions such as throat swallowing and finger bending. Moreover, these conductive gels can be facilely patterned into structures/circuits by the stencil-aided selective polymerization of Ani monomers in the as-prepared PIC/Ani hydrogels, which may provide a new avenue toward large-scale designs of soft electronics using tough and conductive hydrogels.

CP hydrogels is the introduction of PA, a natural molecule consisting of six phosphoric acid groups, which acts as both a dopant and a cross-linker contributing to the formation of 3D interconnected conductive networks. The gelation can be completed within several minutes after a simple mixture of initiator (oxidizer) solutions and monomer/PA solutions.24−26 Stretchability and load-bearing ability are critical factors for materials used in soft bioelectronics and wearable/implantable devices.27,28 Unfortunately, most of the existing CPHs exhibited very poor mechanical properties with a typical breaking strain lower than 20% and fracture stress lower than 50 kPa because only weakly cross-linked and rigid heterocyclic backbone chains were formed during their synthesis process.25,29 To improve the mechanical performance of CPHs, the concepts of double network (DN) hydrogels and nanocomposite hydrogels have been utilized directly or indirectly to synthesize CPHs, as exemplified by PEDOT:PSS/DN hydrogel,29−32 reduced graphene oxide/polymer hydrogel,33,34 PPy-grafted chitosan/ poly(acrylic acid) hydrogel,35 and PAni/poly(N-isopropylacrylamide) hydrogel.36 Nevertheless, these CPHs after treatment still exhibited either limited breaking strength or poor stretchability because the penetrated conductive phase may impair the physical bonding of the nonconductive gel matrix.29 Moreover, the design of the hydrogel-based electronic devices calls for hydrogels that can be facilely patterned into functional and integrated patterns/circuits, which remains unexplored for tough and conductive gel systems. Here, we pursue CPHs with remarkable mechanical properties, high conductivity as well as processability, where tough hydrogel matrix, conductive medium, and strong bonding interface between the conductive medium and hydrogel matrix are the critical factors. Previously, we have developed a tough and easily processable polyion complex (PIC) hydrogel through the complexation of two oppositely charged polyelectrolytes.37−40 Superior mechanical properties 13686

DOI: 10.1021/acsami.8b01873 ACS Appl. Mater. Interfaces 2018, 10, 13685−13692

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Tensile stress−strain curves of the hybrid gels with various Ani concentrations. (b) Electrical resistance of the hybrid gel prepared from 1 M Ani as a function of the applied strain, where the stretch-dependent conductivity was demonstrated by the luminance variation of a red LED (stretching rate = 50 mm/min). (c) Tensile stress−strain curves and (d) relative resistance variations of the hybrid gel (prepared from 1 M Ani) at different stretching rates.

Table 1. Water Content, Electrical Conductivity (from Four-Point Method) and Mechanical Properties of the PIC/PAni Hybrid hydrogels with Different Concentrations of Ania concentration of Ani (M) 0 0.5 1 1.5 a

water content (wt %)

conductivity (S/m)

± ± ± ±

0.001 0.2 0.7 1.4

64.1 66.4 65.7 64.4

3.1 3.6 2.4 2.8

strength (MPa) 1.05 1.25 1.15 0.91

± ± ± ±

0.11 0.05 0.07 0.08

breaking strain 5.07 4.64 3.95 2.94

± ± ± ±

0.17 0.24 0.25 0.41

modulus (MPa) 2.04 3.78 5.31 8.31

± ± ± ±

0.19 0.34 0.74 0.11

0 M of Ani concentration means that the sample was only plasticized by the mixture of saline and PA.

2. RESULTS AND DISCUSSION Previously, we have demonstrated the processing of tough PIC hydrogels by simple compression molding or extrusion-based printing by utilizing the distinct strength of ionic bonds in saline water and in pure water.37,38 To endow PIC hydrogels with conducting ability, here, we incorporated intrinsically conductive polymer PAni into the PIC gel matrix. The formation of PIC/PAni hybrid hydrogels has been illustrated in Figure 1a. Ani monomers were first evenly dispersed with PA and NaCl at selective concentrations, forming the plasticizing solution. PIC precipitates were mixed with the plasticizing solution under compression molding, resulting in brown and transparent hydrogel sheets. After that, these hydrogel sheets were immersed in ammonium persulfate (APS) solution as the redox agent, and PAni was formed in situ inside the PIC matrix, as reflected by a color change from brown to dark green (Figure 1b). It can be noticed that the as-prepared gels before and after the polymerization of Ani were both mechanically weak, as evidenced by the fact that they cannot withstand their weight (Figure 1b). This weak state may be attributed to the synergetic plasticization effect of salt, PA, and APS because of ionic shielding and diminishing the physical bonding between the oppositely charged polyelectrolytes [i.e., poly(3-(methacryloylamino) propyl-trimethylammonium chloride (PMPTC) and poly(sodium p-styrenesulfonate) (PNaSS)]. To confirm this, we dialyzed out the counterions and extra initiator with a large amount of deionized water, and the resultant PIC/PAni hybrid

gels became much stiffer after reaching the equilibrium, with the enhancement of physical ionic bonding between PMPTC and PNaSS (Figure 1b). The effects of PA in the preparation process were twofold: (i) it first interacted with Ani through the protonating nitrogen groups on Ani, making the Ani monomers successfully dissolved in water; (ii) more importantly, each PA molecule provided six phosphate groups for the possible cross-linking sites to interconnect PAni with the polyanions of PIC gel matrix by forming robust ionic hydrogen bond between the phosphate group and amine/imine group,10,43,44 leading to a strong interface between the conductive medium and the hydrogel matrix. We also confirmed that pure PMPTC can form a gel with PA through cross-linking (see experimental details in the Supporting Information). The mechanical properties of the PIC/PAni hybrid hydrogels were evaluated for different concentrations of Ani using typical uniaxial tests. The contents of Ani can effectively tune the mechanical properties of the hybrid gels, as shown by the tensile stress−strain curves in Figure 2a. The PIC/PA complex, without PAni molecules, exhibited the largest breaking strain (>500%) but the lowest modulus (∼2 MPa) in the tensile tests. As the concentration of PAni increased, the hybrid gels became stiffer, whereas their breaking strain gradually decreased, which can be understood from the viewpoint that the rigid and brittle nature of PAni chains may partially impair the flexibility of PIC matrix with increasing concentration of Ani. The water contents of these hybrid gels were determined to be 60−70 13687

DOI: 10.1021/acsami.8b01873 ACS Appl. Mater. Interfaces 2018, 10, 13685−13692

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Figure 3. Self-recovery behaviors of the PIC/PAni hybrid gel (Ani concentration = 1 M). (a) Cyclic tensile curves of the gel samples after different waiting times. (b) Hysteresis ratio and residual strain of the hybrid gel as a function of the waiting time. (c) Measured resistance of the conductive gel during consecutive loading cycles with different waiting times. (d) Relative resistance variations of the conductive gel in response to cyclic straining of 5 and 10% magnitude.

their mechanical performance but also evident in their resistance variation under different loading rates. Larger resistance variations were observed at higher stretching rates (Figure 2d). At lower stretching rates, these hybrid hydrogels have more time to relax, allowing for the formation of microscopic conducting connections. However, such a window for the formation of conducting connections will be overcome by the forcible and continuous breaking of conducting points at high loading rates, leading to a significant increase in the electrical resistance. Cyclic tensile tests were also carried out to investigate the mechanical as well as electrical self-recovery behaviors of the PIC/PAni hybrid gels. For the mechanical part, pronounced hysteresis was observed from the tensile stress−strain curve for the first loading−unloading cycle, indicating vast energy dissipation in breaking the internal network of the gel (Figure 3a). The time scale of the self-recovery was provided by the observation that the loading−unloading loop after 3 h can almost reproduce the first cycle. Notable residual strain existed after the unloading phase for waiting times less than 3 h, as shown in the curve of the residual strain versus the waiting time (Figure 3b), demonstrating the viscoelastic nature of the gel deformation. With the elapsing waiting time, the residual strain gradually decreased and finally vanished after 3 h, indicating that the viscoelastic deformation was almost recovered. The consecutive loading−unloading loop gradually approached the first cycle with increased waiting time, and the loop after 3 h almost reproduced the first cycle with 91.5% hysteresis ratio, confirming the time scale of self-recovery. Figure 3b shows the dependence of hysteresis ratio and residual strain on waiting time. Similar to the PIC hydrogels,41,42 the recovery process of the present PIC/PAni hybrid gels showed a quick speed at the initial 10 min and a much slower speed afterward, which may originate from the competition between the elasticity of the gel matrix and the strength of the reformed ionic bonds during the unloading process. These self-recovery and hysteresis behaviors are quantitatively consistent with our previous data on pure PIC gel.37 Because the mechanical

wt %, insensitive to Ani concentration, which can be understood that PAni/PA can also form a hydrogel that traps comparable water content as PIC does. These gels with significant water contents inherited the excellent mechanical properties of PIC hydrogels, whose tensile fracture stress, breaking strain, and elastic modulus were summarized in Table 1. The incorporation of PAni into PIC matrix should endow the hybrid gels with electrical properties while maintaining the key viscoelastic features of the matrix. The electrical conductivity of the bulk PIC/PAni gel (prepared from 1 M Ani) was recorded against the imposed strain in Figure 2b. The resistance of the hybrid gel significantly increased as the imposed strain became larger, indicating a strong electromechanical coupling within the material. This trend can be readily elucidated from the fact that the applied strain can deform the hydrogel network and break part of the conducting connections, thereby increasing the apparent resistance of the bulk material. The resistance variation of the PIC/PAni gel can also be observed by a lightemitting diode (LED) connected through a hybrid gel sheet in a powered circuit, where the LED glowed weaker at increasing levels of the applied strain (Figure 2b). As the content of Ani was increased to 1.5 M, the conductivity of the PIC/PAni gels reached 1.4 S/m. To the best of our knowledge, the tough and conductive hydrogels with such a combined performance are rarely reported in the literature. These tough and conductive hybrid gels were developed based on PIC hydrogels, whose network structure is mainly cross-linked by physical bonds. Similar to the PIC counterparts,37 the present PIC/PAni hydrogels exhibited ratedependent mechanical behaviors under stretching. They exhibited higher fracture strength but lower breaking strain with elevated levels of stretching rate (Figure 2c), indicating a competition of the time scales between the internal kinetics of physical bonding/debonding and external loading rate. At lower stretching rates, the internal physical bonds have more time to reach their mechanical/kinetic equilibrium. Interestingly, such rate dependence of the hybrid gels was not only exhibited in 13688

DOI: 10.1021/acsami.8b01873 ACS Appl. Mater. Interfaces 2018, 10, 13685−13692

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ACS Applied Materials & Interfaces

Figure 4. PIC/PAni hydrogels used as wearable strain sensors to detect bodily motions. (a) Design of the PIC/PAni hydrogel sensor. (b) Real-time signals of the PIC/PAni hydrogel sensor in monitoring the swallowing motion of the throat in drinking. (c) Relative resistance variation of the PIC/ PAni hydrogel sensor in response to repetitive finger bending with various magnitudes.

properties of the hybrid gel are mainly contributed by the PIC matrix, the observed viscoelasticity should be rooted in the ionic bonding with a spectrum of strength: the relatively strong associations maintain the integrity of the material under loading, whereas the relatively weak associations behave as dynamic/reversible bonds that break and reform over time. PAni in the gel matrix endows the hybrid gel with electrical conductivity, but the PAni/PA associations are rather weak compared to the ionic bonds of PIC.10 The imposed deformation to the hydrogels would also induce the breaking of the conducting connections between PAni polymers, leading to the increase in the electrical resistance. Similarly, it was found that the PIC/PAni hydrogels can exhibit a self-recovery character in electrical resistance, that is, the resistance−strain curves approached closely to the first loading−unloading cycle after 3 h of waiting time (Figure 3c). The waiting time for selfrecovery was found to rely on the magnitude of strain cycling. In contrast to the 3 h of self-recovery in 200% straining (Figure 3c), the variation of electrical resistance in response to cyclic straining is repeatable without noticeable hysteresis at small magnitudes of 5 or 10% (Figure 3d). Because of their significant water content, our tough and conductive PIC/PAni hydrogels match well with the nature of human skin, therefore providing promising materials for the design of soft and wearable devices. To test their functionality, we designed a PIC/PAni hydrogel sensor by sealing a rectangular sheet of hybrid gel between two VHB tapes, with the attachment of two copper wires as the electrodes (Figure 4a). The obtained sensor was able to detect some subtle motions of the human body with an output of resistance change, indicating its practical potentials as wearable devices. When being attached to a human throat, the hydrogel sensor can provide real-time feedbacks in response to the periodic small strains of the throat in swallow motion (Figure 4b). When being attached to an index finger, the hydrogel sensor produced electrical signals in response to the repetitive finger bending with various magnitudes, discerning different bending angles (Figure 4c). The combined mechanical and electrical properties of the bulk PIC/PAni hydrogels have laid the foundation for the design of soft electronics, in which stretchability and conductivity are important endowments. Electronic circuits also require an alternating conductive/nonconductive pattern in space, which can be readily achieved by selective polymerization of Ani monomers in the present gel system. As depicted in Figure 5a, the as-prepared PIC/Ani/PA hydrogel membranes before Ani polymerization were covered with a poly(ethylene terephthalate) (PET) stencil mask that was prepatterned. Aided

Figure 5. (a) Patterning of PIC/PAni hydrogels by selective polymerization of Ani monomers using stencil masks. The scale bar is 1 cm. (b) SEM images of the cross-sectional structures of the patterned hybrid gel, indicating a compact PAni layer in the hybrid gel and typical porous structure of unreacted PIC. The SEM images were taken at the exposed/polymerized location (left) and masked/ unpolymerized location (right), respectively. The scale bar is 50 μm. (c) Recovery ability of the patterned gel circuit after deformation. (d) Relative resistance variation of the PIC/PAni sensor array integrating three curvy conductive structures with different strain sensitivities. The scale bar is 1 cm.

by the masking, APS solution was dropped into the apertures to selectively polymerize the Ani monomers for the regions that were exposed. Redundant Ani monomers within the gel membranes were removed out during dialysis after the polymerization. Consequently, PAni was formed within the exposed regions that became conductive, whereas the masked regions remained nonconductive. The scanning electron microscopy (SEM) images were taken at several representative locations for the cross-sectional structures of the patterned gel membrane. The exposed/polymerized locations were coated with a compact PIC/PAni composite layer with a thickness of 24.7 ± 0.8 μm (mean ± standard deviation). During the patterning process, the APS solution was dropped into the predesigned apertures of the stencil mask to initiate the polymerization of Ani monomers, leading to patterned conductive gels. This is a diffusion-reaction process of APS solution from the top surface to the deeper part of the asprepared gels. As the initiators were consumed out by the polymerization, the diffusion-reaction process ceased, leaving the underlying gel unreacted. Therefore, two layers were formed in the hybrid hydrogel for the exposed/polymerized region (Figure 5b). In contrast, the masked/unpolymerized locations exhibited the typical porous gel structure of the unreacted PIC (Figure 5b). We also investigated the lateral/ horizontal variation in the pattern dimensions caused by the invasion of polymerization process into the regions covered by the mask, which provided an estimate of spatial resolution in 13689

DOI: 10.1021/acsami.8b01873 ACS Appl. Mater. Interfaces 2018, 10, 13685−13692

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resultant viscous liquids were precipitated in ethanol and dried in the oven to obtain PNaSS and PMPTC powders. The polymers were dissolved in deionized water to prepare PNaSS and PMPTC solutions with prescribed concentrations. Then, PNaSS and PMPTC solutions with equal volume were slowly dripped into 250 mL of deionized water and stirred for 30 min, resulting in compact PIC precipitates. These precipitates, with the charge ratio of PNaSS/PMPTC being 1.1:1, were made into powder after being collected and dried in the oven at 110 °C. 4.3. Preparation of PIC/PAni Conductive Hydrogels. PIC/ PAni conductive hydrogels with different concentrations of Ani were prepared as the following. NaCl (0.438 g; 7.5 mmol), PA (0.921 mL; 1 mmol), and Ani (0.458 mL; 5 mmol) were mixed evenly in 3.5 mL of deionized water, forming the plasticizing solution A. Then, 1 mL of solution A and 0.5 g of PIC precipitates (prepared from PMPTC and PNaSS) were mixed in a centrifuge tube and kept at 90 °C for 4 h. The plasticized PIC/Ani/PA gels were centrifuged (Velocity 18R Refrigerated Centrifuge) under 14 000 rpm, 10 min every 1 h for three times, to remove the bubbles inside and obtain homogeneous PIC/Ani/PA gels. The samples were kept at 90 °C for 10 min to increase their fluidity and poured onto one hot (100 °C) polytetrafluoroethylene (PTFE) plate. Compression molding was immediately performed with the viscoelastic samples at room temperature for 20 min. A brown transparent hydrogel sheet with a thickness of 0.5 mm was peeled off from the mold. The obtained hydrogel sheet was further immersed in 5 mL of 0.5 M APS solution for the polymerization of Ani monomers for 5 h. The as-prepared PIC/PAni hydrogels were washed and then immersed in a large amount of deionized water for 3 d to dialyze out the counterions and achieve swelling equilibrium; the deionized water was changed every day. 4.4. Characterization of Mechanical Properties. The tensile tests of PIC/PAni hybrid gels were performed on a commercial tensile tester (Instron 3343 Tester) at room temperature. The samples were cut from gel sheets into dumbbell-shape with an initial gauge length of 12 mm (L0) and a width of 2 mm and were loaded at a stretch velocity of 100 mm/min. The nominal stress and strain were recorded as the applied load was divided by the original cross-sectional area of the samples and the clamp displacement divided by L0. The elastic modulus, E, was calculated from the initial slope of the stress−strain curve. 4.5. Characterization of Electrical Properties. The conductivity of bulk PIC/PAni hybrid gels with different concentrations of PAni were characterized by the conventional four-point method, as performed in previous studies.35,45,46 The samples were prepared and cut into rectangular shape (with a dimension of 30 mm × 5 mm); the sheet resistance (Rs) of the sample in the static state was measured from the four-probe machine (Keithley 2460 multiple-function source meter); the conductivity σ of the sample was calculated through the relation σ = 1/(Rs t), where t is the thickness of the samples. To obtain the resistance variations of the hydrogels with imposed strain, the electrical resistance of the stretched sample was detected and recorded through a two-probe digital multimeter (Keysight Co., Ltd, 34465a), when the samples were continuously stretched at certain stretching rates. Two-sided copper tapes were used as electrodes to connect the sample ends to the probes. 4.6. Patterning PIC/Ani Hybrid Gels. The as-prepared PIC/Ani/ PA gel sheets were covered by a patterned PET mask; then, APS solution of 1.25 M was carefully dropped into the designed apertures to initiate the polymerization of Ani monomers. The reaction was carried out for 1 h, after which the mask was removed and the asprepared PIC/PAni hydrogels with the predesigned conductive patterns were immersed in deionized water for 3 days to remove extra acid, byproducts, and the counterions to achieve swelling equilibrium. The deionized water was changed every day. 4.7. Preparation of Strain Sensors Based on PIC/Ani Hybrid Gels. A piece of rectangular hybrid gel sheet with the dimension of 30 mm × 8 mm × 0.8 mm was first put on a VHB tape. Two pieces of copper ribbons were then attached to the two sides of the gel sheet as

parallelizing the conductive PAni/PIC lines/curves (see experimental details in the Supporting Information). Such a facile patterning method potentially allows for fast and low-cost fabrication of soft electronics using the present tough/conductive gels. Figure 5c shows the high stretchability and recovery ability of a gel circuit from the patterning. Furthermore, such an ease of patterning is compatible with the high-throughput manufacturing process and can be easily scaled up to produce an array configuration of gel-based devices. In Figure 5d, we integrated three wavy conductive gel lines in a three-channel PIC/PAni sensor. As the sensor array was subjected to the nominally identical stretch, the sinusoidalshaped wire decreased its amplitude through bending to accommodate the imposed deformation, whereas the straight line had to elongate the material itself. Overall, this type of multichannel sensor array based on patterned PIC/PAni gels allows strain measurements up to 400% with a variety of sensitivity to the imposed strain, which may help to monitor large-extent and large-area deformations of the joint components of soft robotics.

3. CONCLUSIONS In summary, we have developed a new type of PIC/PAni hybrid hydrogels that confer a combination of high toughness and conductivity, which can be facilely patterned as well. The hybrid hydrogels were formed by employing phytic acid molecules as the dopants and cross-linkers between the polymerized Ani chains and the as-plasticized PIC gel matrix. After reaching equilibrium, the PIC/PAni hybrid gels exhibited superior mechanical and electrical properties, with the breaking strain, fracture stress, tensile modulus, and electrical conductivity being 395%, 1.15 MPa, 5.31 MPa, and 0.7 S/m, respectively (Ani concentration = 1 M). The conductivity can be further enhanced with elevated contents of Ani. These hybrid hydrogels showed intriguing rate-dependent and selfrecovery behaviors at varying stretching rates and loading− unloading cycles, both mechanically and electrically. Because of their tunable electrical response to mechanical strain, the present PIC/PAni hybrid hydrogels can effectively detect subtle human motions such as throat swallowing and finger bending. Conductive or capacitive patterns can be readily generated by stencil-aided selective polymerization of Ani monomers in the hybrid hydrogels, and the strategy can be readily scaled up for a fast and low-cost fabrication of wavy gel circuits and multichannel arrays, monitoring large-extent and large-area deformations in real-time with various sensitivities. This work should provide a new avenue toward sophisticated designs of soft electronics at the human−device interface. 4. EXPERIMENTAL SECTION 4.1. Materials. Sodium p-styrenesulfonate (NaSS; anionic monomer, 90 wt % purity), 3-(methacryloylamino)propyl-trimethylammonium chloride (MPTC; cationic monomer, 50 wt % aqueous solution), 2-oxoglutaric acid (photoinitiator), phytic acid, (PA; 50 wt % aqueous solution) and APS were purchased from Sigma-Aldrich and used as received. Ani was purchased from Macklin Co., Ltd. NaCl was received from Sinopharm Chemical Reagent Co., Ltd. Millipore deionized water was used in all of the experiments. 4.2. Preparation of PIC Powders. The PIC powders were prepared, as described in our previous work.37,38 In brief, PNaSS and PMPTC were synthesized by polymerizing the precursor aqueous solutions of 1 M NaSS and 1 M MPTC, respectively, in the presence of 0.05 mol% (relative to the monomer) 2-oxoglutaric acid under UV light irradiation (365 nm wavelength, 7.5 mW/cm2) for 8 h. The 13690

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Research Article

ACS Applied Materials & Interfaces electrodes, after which another VHB tape was covered on the top of the sheet to seal the sensing component. 4.8. Determination of Water Content. The water content of equilibrated gels, q, was calculated according to q = (ws − wd)/ws, where ws and wd are the masses of equilibrated gels and dry gels, respectively. 4.9. SEM. The microstructure of PIC/PAni hybrid hydrogels was observed by Hitachi S4800 field-emission scanning electron microscope. The hybrid gels prepared at different concentrations of Ani were freeze-dried and cryogenically fractured in liquid nitrogen. All of the SEM samples were coated with a thin layer of gold by the sputtering method before SEM characterization. On the basis of SEM images, the thickness (mean ± standard deviation) of the compact PIC/PAni composite layer formed in the mask-aided patterning was measured from seven random locations of the exposed/polymerized region.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01873. Pure PMPTC forming a gel with PA; characterizing the ratio of PAni to PIC in the hybrid gel; quantifying the lateral/horizontal variation in the pattern dimensions; and schematic of the experiment to quantify the variation in the pattern dimensions in the horizontal/lateral direction (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.L.W.). *E-mail: [email protected] (J.Y.). *E-mail: [email protected] (J.Q.). ORCID

Zi Liang Wu: 0000-0002-1824-9563 Jun Yin: 0000-0002-1937-6812 Jin Qian: 0000-0002-3597-5460 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (91748209, 11621062, and 51773179), the Key Research and Development Program of Zhejiang Province (2017C01063), and the Fundamental Research Funds for Central Universities of China (2017FZA4029).

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DOI: 10.1021/acsami.8b01873 ACS Appl. Mater. Interfaces 2018, 10, 13685−13692