Rapid Self-Recoverable Hydrogels with High Toughness and

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Applications of Polymer, Composite, and Coating Materials

Rapid Self-recoverable Hydrogels with High Toughness and Excellent Conductivity Meixiang Wang, Yong Mei Chen, Yang Gao, Chen Hu, Jian Hu, Li Tan, and Zhimao Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06567 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Applied Materials & Interfaces

Rapid

Self-recoverable

Hydrogels

with

High

Toughness and Excellent Conductivity Mei Xiang Wang†1,2, Yong Mei Chen†1,2*, Yang Gao2, Chen Hu1,2, Jian Hu2, Li Tan3,4,5*, Zhimao Yang1* 1

School of Science, State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong

University, Xi’an, 710049, China. 2State Key Laboratory for Strength and Vibration of Mechanical Structures, International Center for Applied Mechanics and School of Aerospace, Collaborative Innovation Center of Suzhou Nano Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China.

3

Department of Mechanical & Materials Engineering,

University of Nebraska, Lincoln, NE. 4Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE. 5Center for Biotechnology, University of Nebraska, Lincoln, NE. Correspondence and requests for materials should be addressed to Y.M.C. (email: [email protected]),

L.T.

(email:

[email protected]),

Z.Y.

(email:

[email protected]). [†] These authors contributed equally to this work. KEYWORDS: hydrogel, self-recovery, conductivity, pressure sensor, high toughness, high strength

ACS Paragon Plus Environment

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ABSTRACT: Hydrogels as soft and wet materials have attracted much attention in sensing and flexible electronics. However, traditional hydrogels are fragile or have unsatisfactory recovery capability, which largely limit their applications. Here, a novel hydrogen bond based sulfuric acid-poly (acrylic acid) (PAA)/ poly (vinyl alcohol) (PVA) (SPP) physical hydrogel is developed for addressing above drawbacks. Sulfuric acid serves two functions: one is to inhibit the ionization of carboxyl groups from PAA chains to form more hydrogen bonds and the other is to provide conductive ions to promote conductivity of hydrogel. Consequently, the hydrogel obtains comprehensive mechanical properties, including extremely rapid self-recovery (strain = 1, instantly self-recover; strain = 20, self-recover within 10 min), high fracture strength (3.1 MPa) and high toughness (18.7 MJ m-3). In addition, we demonstrate this hydrogel as stretchable ionic cable and pressure sensor to exhibit stable operation after repeated loadings. This work provides a new concept to synthesize physical hydrogels, which will hopefully expand applications of hydrogel in stretchable electronics.

Introduction The development of flexible electronics urgently needs soft conductors incorporating multifunctions simultaneously, such as mechanical performances (toughness, stretchability, and selfrecovery), conductivity and transparency.1-6 In particular, a stretchable conductor which keeps stable conductivity while undergoing large deformations is highly desired for realizing the nextgeneration portable and wearable electronics, like flexible circuitries, stretchable displays or energy storage devices.7-11 Unfortunately, currently used materials for conductors (e.g., metals, carbon-based materials and conductive polymers) are usually stiff or have limited stretchability,


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and especially suffer from fast deterioration in conductivity under deformations. This makes them struggle to satisfy the requirements of stretchable flexible electronics.12-14 The ionic conductors of soft tissues, such as skin, heart and brain, are inspirations of developing the soft materials for flexible electronics. Hydrogel, a kind of tissue like semi-solid ionic conductor, has attracted extensive interests in flexible electronics, because of their promising advantages such as high transparency, viscoelasticity, tunable properties and excellent biocompatibility.3,

5, 12, 15-17

Different from those hard and less stretchable electronic conductors,

the conductivity of hydrogel based ionic conductors is insensitive to stretch.3, 5 Recently, we have shown that ionic cables fabricated by chemically cross-linked polyacrylamide hydrogel swollen by saline water can transmit music signal as a line of earphone.5 Furthermore, on the basis of its good conductivity and excellent transparence, we used the hydrogel to fabricate electroluminescence devices of giant stretchability via sandwiching phosphor particles. 18 Besides deformation insensitive conductivity, self-recovery speed after repeated deformations is also a critical factor in determining the performance of flexible electronics.19 Therefore, lots of efforts have been devoted to improve the hydrogels’ comprehensive mechanical properties, in order to meet practical requirements of large deformability and fast recovery capability.20-23 Some mechanically robust hydrogels, such as double network hydrogel, tetra-armed polymer hydrogel and interpenetrating polymer network hydrogel, have been synthesized via introducing a covalently cross-linked network as sacrificial bonds to dissipate energy.24-27 Nevertheless, covalent bond largely impairs the recovery capability because of its intrinsic irreversibility.28-31 Thus, reversible non-covalent bonds (e.g., hydrogen bond, hydrophobic interaction and ionic interaction) are expected to simultaneously improve the toughness and recovery capability of hydrogels.31-33 However, most non-covalently cross-linked tough hydrogels cannot fully recover


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at room temperatures, resulting in an incomplete or time-consuming recovery process.34-37 Overall, it is still a challenge to develop a tough hydrogel with rapid self-recovery and stable conductivity. To this end, we present a facile two-step strategy, combining free radical polymerization without any covalent cross-linker and freeze-thaw process, to synthesize a hydrogen bond based sulfuric acid (H2SO4)-poly (acrylic acid) (PAA)/poly (vinyl alcohol) (PVA) (SPP) hydrogel. Here, the tough physically cross-linked SPP hydrogel with rich hydrogen bonds is realized by the presence of H2SO4 and freeze-thaw treatment, resulting in high fracture strength of 3.1 MPa and toughness of 18.7 MJ m-3, as well as rapid self-recovery properties. Moreover, H2SO4 provides enough free ions for stable conductivity, promising the SPP hydrogel potentially be used as stretchable ionic cable and capacitance based pressure sensor. Results and discussion The design strategy is important for fabricating tough, highly stretchable and rapid selfrecovery SPP hydrogel. We developed a simple and feasible two-step process to fabricate SPP hydrogel by UV polymerization of AA and freeze-thaw treatment (Figure 1A). In a general procedure, PVA polymer, AA monomer and α-ketoglutaric acid initiator were dissolved in 1 mol L-1 H2SO4 to get a homogenous mixture. After exposing the mixture to 365 nm ultraviolet (UV) light for 6 h at room temperature, primary SPP hydrogel was obtained by free radical polymerization. Subsequently, the final SPP hydrogel was obtained by freezing the primary hydrogel under -20 oC for 12 h and then thawing to room temperature (Figure 1A). In the system, the presence of H2SO4 can inhibit the ionization of carboxyl groups dangling on PAA polymer chains, providing more active sites for physical cross-links. In addition, the freeze-thaw


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process can further facilitate formation of hydrogen bonds between the polymer chains and enhance the mechanical properties. For example, the fracture strength of SPP hydrogel with freeze-thaw treatment is 3.0 MPa, which is about twice of that of the untreated sample (Figure S1). As a result, the SPP hydrogel is mechanically robust enough to undergo knotting, extensive stretching and severely compression (Figure 1B-C). A knotted dumbbell-shaped SPP hydrogel can be stretched to 12 times of its original length without any rupture, and then instantly recovered to its original state after releasing the tension (Figure 1B). A cylinder-shaped hydrogel can endure 80% compression deformation and immediately fully recovered after removing the pressure (Figure 1C). In addition, the synthesized method of this hydrogel is universal adaptability. The poly acrylamide (PAAm)/PVA hydrogel swollen in 1 mol L-1 H2SO4 solution prepared by this method can obtain fracture strength of 1.2 MPa, which is 27 times of that of PAAm/PVA hydrogel swollen in water (0.044 MPa) (Figure S2).

Figure 1. Preparation scheme and photographs of SPP hydrogel. (A) Two-step approach to prepare hydrogen bond based physical hydrogel. A mixture of 1 mol L-1 H2SO4 containing PVA polymer, AA monomer and α-ketoglutaric acid initiator was exposed to UV light for 6 h to obtain the primary hydrogel. Then the primary hydrogel was freezed to -20 oC for 12 h and


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thawed to room temperature for obtaining final physical hydrogel. (B) Photographs of unknotted, knotted, stretched and recovered SPP hydrogel. (C) Photographs of compression process of SPP hydrogel (height: 10 mm, diameter: 7 mm). Ws = 30 wt%, RAP = 9:1 for the samples in figure B and C. To demonstrate the mechanical reinforce effect of H2SO4, the mechanical properties of SPP hydrogel was compared with the PAA/PVA hydrogel swelling in water (WPP hydrogel). As a typical example, the tensile stress of the samples containing same amount of solid (30 wt%) and weight ratio of AA/PVA (9:1) was shown in Figure 2A. It’s notable that the introducing of H2SO4 sharply improves the hydrogel’s fracture strength and strain. The fracture strength of SPP hydrogel is as high as 3.1 MPa, which is more than 2 orders larger than that of WPP hydrogel (27.0 kPa). Although the very soft WPP hydrogel generates a large strain (19), it is still inferior to SPP hydrogel which can be stretched to 26 times of its original length. In addition, to optimize the concentration of sulfuric acid, the mechanical properties of a serious of SPP hydrogels containing different concentration of sulfuric acid were measured. The results showed that the SPP hydrogel containing 1 mol L-1 sulfuric acid exhibits maximum fracture strength of 3 MPa as the concentration of sulfuric acid changing from 0 to 2 mol L-1 (Figure S3). What’s more, hydrochloric acid (HCl) can also enhance the mechanical properties of PAA/PVA hydrogel by inhibiting the ionization of carboxyl groups of PAA. The hydrogel containing 1 mol L-1 HCl shows superior mechanical properties than WPP hydrogel and its fracture strength reaches maximum value of 1.5 MPa (Figure S4). More importantly, H2SO4 significantly enhances the self-recovery property of SPP hydrogel, demonstrating by cyclic load-unload tests (Figure 2B). After testing SPP hydrogel for 100 cycles with a strain of 1, the stress-strain curves of the first and 100th cycle do not seem to change much,


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which demonstrates their extremely high repeatability. However, for the WPP hydrogel, it exhibits a large drift with an obvious residual deformation after first cycle, and the residual deformation even changes to 50 % strain after 100 cycles. Furthermore, toughness under loading-unloading cyclic tensile tests discloses the excellent self-recovery property of SPP hydrogel (Figure 2C). The toughness increases from 40 ± 1.7 kJ m-3 to 48.9 ± 1.1 kJ m-3 after 100 cycles, which may due to orientation of polymer chains in the SPP hydrogel networks during the test process.39 On the contrary, the toughness of WPP hydrogel drops obviously from 17 ± 1.1 kJ m-3 to 8 ± 0.8 kJ m-3 after 100 cycles. It is considered that H2SO4 can provide more binding sites for hydrogen bonds formation by inhibiting the ionization of carboxyl groups of PAA polymer chains. Hence, much denser hydrogen bonds in SPP hydrogel than in WPP hydrogel endows it with fast self-recovery capability and high toughness. To prove the mechanical properties of SPP hydrogels enhanced by sulfuric acid, we soaked the prepared SPP hydrogels in different pH conditions for 24 h and measured their tensile properties. The fracture strength of SPP hydrogels sharply drop from 3.0 MPa to 0.014 MPa and their dimensions severely swell as the soaked conditions changing from acid to alkaline (Figure S5). This is because the OH- from alkaline solution reacts with carboxyl groups of PAA and break hydrogen bonds in SPP hydrogel networks, leading to the hydrogels seriously swollen by water. Even been stretched as high as 20 times of its original length, the reload stress-strain curve of SPP hydrogel could recover to its original state in 10 min without any external stimuli (Figure 2D). To study the effect of stretching speed on the self-recovery velocity, we measured the recover ratio of SPP hydrogels under different stretching speed (Figure S6). The results show that the self-recovery velocity will increase as stretching speed decreasing, which is due to some hydrogen bonds in SPP hydrogels would change to high strength cross-linkers and resist the


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deformation under high stretching speed. In the loading process, these high strength hydrogen bonds will be broken and cannot rapidly self-recover leading to long time self-recovery.40 Remarkably, the recovery speed of SPP hydrogel is extremely fast compared with other physically cross-linked hydrogels (Figure S7). For instance, our SPP hydrogel outperforms most hydrogels in terms of recovery ratio, recovery time and stretchability. It can not only fully recover instantly under small deformations, but also self-recover within very short time under a large strain (Table S1). Whereas some reported hydrogels need external stimuli and long time to improve their recovery ratio even under very small deformations. Otherwise, they cannot fully recover. We proposed that the mechanism of improving self-recovery velocity of SPP hydrogels is high density of hydrogen bonds with intrinsic reversibility. Non-covalent interactions, like hydrogen bond, are often regarded as insufficient to construct macroscopic hydrogels with high mechanical properties. In concert to create strong hydrogels by using such reversible interactions, one possible solution is to increasing their number to work for compensating the low binding energy of non-covalent interaction. In our design, both introducing of sulfuric acid and freeze-thaw treatment can boost the formation of more hydrogen bonds among polymer chains, resulting in larger number of hydrogen bonds as cross-linkers in the hydrogel networks. When the SPP hydrogel is loaded, the curved polymer chains will be stretched, and larger number of hydrogen bonds would endure the loading process and result in excellent mechanical properties like high fracture strength and fast recovery. For the self-recovery velocity, it also depends on the density of hydrogen bonds. The hydrogen bonds can automatically re-form due to their intrinsic reversibility. The high density of hydrogen bonds has expected capability to maintain the skeletons of polymer chains as the original state, which also increase re-formation of hydrogen bonds and improve self-recovery velocity. As a result, high density of hydrogen


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bonds promoted by introducing sulfuric acid and freeze-thaw treatment could boost the selfrecovery speed. What’s more, the stability of SPP and WPP hydrogels in 5 mol L-1 urea solution also proves the introducing of sulfuric acid and freeze-thaw treatment can improve the density of hydrogen bonds by promoting the formation of hydrogen bonds (Figure S8).

Figure 2. Mechanical properties of SPP and WPP hydrogels. (A) Tensile stress-strain curves. (B) The first and 100th time stress-strain curves and (C) toughness under varied cycles (strain = 1). (D) Rapid self-recovery of SPP hydrogel at room temperature for different recover time. Ws = 30 wt%, RAP = 9:1. Error bars show standard deviation. To optimize the formulation of SPP hydrogel, weight ratio of AA to PVA (RAP) and solid content (Ws) were systematically tested (Figure 3). It’s interesting that addition of small amount


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of PVA dramatically improves the fracture strength and Young’s modulus of SPP hydrogel, while decreases fracture strain (Figure 3A). The fracture strength of SPP hydrogel markedly increases from 0.89 ± 0.06 MPa to 3.1 ± 0.15 MPa when RAP changes from 10:0 to 9:1, and stays with no obvious differences as RAP changes from 9:1 to 2:1. The toughness firstly increases from 9.1 ± 0.04 MJ m-3 to 18.7 ± 0.87 MJ m-3 and then decreases to 9.5 ± 0.07 MJ m-3, when RAP changes from 10:0 to 2:1 (Figure 3B). On the other hand, the corresponding Young’s modulus increases from 50.7 ± 7.6 kPa to 87.7 ± 1.1 kPa, while fracture strain decreases from 35.7 ± 0.1 to 14.2 ± 0.5 with RAP changing from 10:0 to 2:1 (Figure S9). Hence, RAP = 9:1 was selected as the optimal weight ratio. Then the mechanical properties under varied Ws with RAP = 9:1 were compared (Figure 3C). When Ws increases from 20 to 40 wt%, the fracture strength and toughness is 3.1 MPa and 18.7 MJ m-3 for 30 wt% samples, respectively, which are much larger than those for 20 wt% (1.5 MPa, 9.1 MJ m-3) and 40 wt% (1.6 MPa, 13.4 MJ m-3) samples (Figure 3D). In addition, the Young’s modulus increases from 21.7 ± 1.2 kPa to 191.3 ± 11.4 kPa, while fracture strain decreases from 29.7 ± 1.8 to 15.3 ± 0.3 (Figure S10). In general, the SPP hydrogel exhibits the best comprehensive mechanical properties when Ws = 30 wt%. Compared with previous reported hydrogen bond based physical hydrogels, our SPP hydrogel possesses superior mechanical properties, especially in high toughness (Figure 3E). It’s need to be noted that PVA content is very small and homogeneously dispersed in the SPP hydrogel networks, which is hard for PVA to form crystalline region during the freeze-thaw treatment. And the XRD spectrums of SPP hydrogel and PVA hydrogel also proves this point, since only two diffraction peaks of water were found in SPP hydrogel (Figure S11). In a word, by changing RAP and Ws in our system, the density of hydrogen bonds can be regulated, resulting in adjustable mechanical properties of the hydrogel.


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Figure 3. The influences of composition on the behavior of the hydrogel. (A, C) Stress-strain curves and (B, D) fracture strength and toughness of hydrogels with various Ws and R. (E) Fracture strength versus toughness charts for comparison of various hydrogen bond based physical hydrogels, such as supramolecular polymer hydrogel,42 melamine-enhanced PVA


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hydrogel,43 (PAAm-co-PAA)/PVA hydrogel,41 PVA−tannic acid hydrogel,44 PAAm-PVA hydrogel,45 poly(ethylene glycol)-2-ureido-4[1H]-pyrimidinone hydrogel, 46 and our work. Ws is 30 wt% for A and B, and RAP is 9:1 for C and D. Error bars show standard deviation. The mechanical performances discussed above have demonstrated that SPP hydrogel is excellent candidate for using in flexible electronics. As H2SO4 provides enough ions for conducting, the electrical properties of this hydrogel and its potential applications were further studied. It’s worth noting that the resistivity of the hydrogel based ionic conductor is almost identical as the hydrogel is stretched (Figure 4A). When the resistivity of a conductor is independent of deformations, the resistance (R) of this conductor can be expressed as R/R0 = λ2, where R0 is the original resistance of the conductor, and R is the resistance after the conductor is stretched to λ times of its original length. In our experiments, R and R0 of SPP hydrogel under various stretch was measured (Table S2). And the resulted resistance-stretch curve of the hydrogel closely approximates with aforementioned prediction, confirming that the SPP hydrogel has stable resistivity under deformations. We cut the electrical cable of a commercial loudspeaker (LENRUE, S33) into two segments, and connected them using the hydrogel based ionic cable. The input port of the electrical cable was inserted to a personal computer (PC), and the output port was connected to a loudspeaker. When a music signal is generated by the PC, it can be transmitted through ionic cable, and finally be broadcasted by the loudspeaker (Figure 4B). Note that the performance of this ionic cable maintains stable, even when the cable is repeated stretched from 1 to 5 times of its original length (Movie S1). Moreover, the SPP hydrogel based ionic conductor can light the LED with no change in light intensity after being loaded-unloaded 100 cycles with a strain of 1 (Figure S12). This verifies the excellent selfrecovery capability and good conductivity of SPP hydrogel.


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Figure 4. Stretchable ionic cable. (A) Electrical resistivity of SPP hydrogel was measured as function of stretch. Insert: our results (solid line) were close to the theoretical curve (dash line) of resistance change ratio versus stretch. (B) (1) Schematic of the experimental setup and (2, 3) the ionic cable is highly stretchable and still works even when it is stretched up to nearly 4 times its original length. Ws = 30 wt%, RAP = 9:1. In addition, benefit from the properties of SPP hydrogel, it also shows great potential in fabrication of pressure sensors. The one with capacitance mechanism can be easily fabricated by stacking the SPP hydrogel with two Al electrodes (Figure 5). The hydrogel was sandwiched with two electrodes which were semi-sphered Al electrode and flat Al electrode (Figure 5A). Here semi-sphered Al electrode was used to provide a changeable contact area between the electrode and hydrogel. When the electrodes were biased, a thin layer of charged ions from hydrogel would be attracted to the nanometer vicinity of the electrodes (Figure 5B). The gap between this layer of ions and that opposite charges from the electrodes defines the thickness of the electrical double layer (EDL). Due to a small value of EDL thickness, capacitors made with this mechanism have 3~5 orders of magnitude higher capacitance than dielectric ones.15 This will


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help to easily capture the signal of device. Because of this mechanism, the original capacitance of our pressure sensor is 26.0 nF and it shows high response to static pressure (Figure 5C). For instance, the capacitance increases from 153.6 nF to 325.9 nF when pressure changes from 0.71 kPa to 2.47 kPa. The increase in capacitance of the sensor arises from the addition of contact area between the semi-sphered Al electrode and hydrogel. Besides, the sensor can self-recover to its original value without time-delay by removing the pressure. The sensitivity of this pressure sensor is as high as 121.1 nF kPa-1, which is superior to most pressure sensors (Figure S13). Similarly, the sensor also shows excellent performance to dynamic pressure (Figure 5D). Its response increases gradually from 1.53 µF to 7.59 µF, when the finger bends from 0o to 90o. Significantly, the original capacitance in static pressure (26 nF) is much smaller than that (1.53 µF) of dynamic pressure. This is because a preload is applied on the sensor when attached it to the finger. Furthermore, the pressure sensor was loaded 10 times as the finger bending from 0o to 90o and its response have to significant difference (Figure S14). Nevertheless, even under this condition, the capacitance of the sensor still can recover to its original state instantly, which also reveals the excellent self-recovery property of SPP hydrogel. In fact, the hydrogels with acid would have the issue of chemical corrosion when they are equipped with body. As far as we know encapsulation is a common and necessary process in wearable electronics to prevent evaporation of the water in hydrogels. Thus, the SPP hydrogel should be encapsulated by some soft materials like polyethylene (PE) and very high bond (VHB) which are robust in protecting from acid and suitable for the encapsulation. To prevent the chemical corrosion and water evaporation of SPP hydrogel, the SPP hydrogel based pressure sensor is encapsulated by plastic wrap (polyethylene, PE). After encapsulation treatment, the stability and lifetime of the device can be improved since the hydrogel is stable. For example, the


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water content of SPP hydrogel samples with or without encapsulating by PE were recorded (Figure 15). It’s notable that the water content of the encapsulated hydrogel almost does not change after 24 h at room temperature, while the without one sharply drops to 31 wt%.

Figure 5. SPP hydrogel based pressure sensor. (A) Photographs of the pressure sensor. SPP hydrogel was sandwiched between two Al electrodes (semi-sphered and flat). (B) When a voltage is applied between the two electrodes, a thin layer of charged ions from hydrogel will be attracted to the nanometer vicinity of the electrodes to form EDL. And the capacitance will increase once a pressure is applied on it. (C) Stable and high response to static pressure. (D) Response to the dynamic finger bending. The thickness, length and width of SPP hydrogel is 2 × 10 × 8 mm3, respectively. The length and width for Al electrode is 12 mm and 12 mm, respectively. Ws = 30 wt%, RAP = 9:1.


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Summary In this work, we synthesized a hydrogen bond based H2SO4-PAA/PVA physical hydrogel by a facile two-step method for the first time. H2SO4 in this hydrogel plays two roles: one is to promote the formation of hydrogen bonds by inhibiting the ionization of carboxyl groups on PAA chains and the other is to endow SPP hydrogel with conductive property. This hydrogel shows maximum fracture strength and toughness of 3.1 MPa and 18.7 MJ m-3, respectively. Reversible hydrogen bonds are the sole cross-linkers of SPP hydrogel allowing it extremely rapid self-recovery capability without external stimuli. On account of above properties, SPP hydrogel can be used as stretchable ionic cable without any difference in performance during repeated stretch process. Moreover, a SPP hydrogel based pressure sensor is designed which performs stable and high response to static and dynamic pressure. We believe this tough, rapid self-recovery and conductive hydrogel will expand the potential applications of hydrogels in the field of flexible electronics.

ASSOCIATED CONTENT Supporting Information. Materials and experimental sections; fracture strength, Young’s modulus and fracture strain of various SPP hydrogel, sensitivity of SPP hydrogel based pressure sensor and resistance of the SPP hydrogel based ionic cable. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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*E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (Grant Nos. 11674263), International Science and Technology Cooperation Program supported by Ministry of Science and Technology of China and Shaanxi Province (2013KW14-02), the Fundamental Research Funds for the Central Universities, the Program for the Key Science and Technology Innovative Team of Shaanxi Province (Grant No. 2013KCT-05).

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