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Mussel-Inspired Adhesive and Conductive Hydrogel with Long-Lasting Moisture and Extreme Temperature Tolerance Lu Han, Kezhi Liu, Menghao Wang, Kefeng Wang, Liming Fang, Haiting Chen, Jie Zhou, and Xiong Lu* devices,[1] biosensors,[2] bioactuators,[3] health recording electrodes,[4] and medical patches.[5] Traditional conductive hydrogels are generally based on a hydrated matrix of conducting polymers, such as polyaniline, polypyrrole, and poly(3,4ethylenedioxythiophene), however, their poor strength/flexibility limits their practical applications.[6] Recently, a number of tough and conductive hydrogels have been developed by incorporating conducting polymers,[7] carbon based nanomaterials,[8] and metallic nanomaterials[9] in the strong polymer networks. Among them, carbon nanotube (CNT)-incorporated nanocomposite hydrogels have tremendous potential in biomedical engineering,[10] as they are soft conductive materials with remarkable electrical and mechanical properties. Despite the success in CNT-based hydrogels, there are still challenges needed to be confronted. CNTs are easy to aggregate in hydrogel due to van der Waals attractions between the individual tubes, which weakens the conductivity and mechanical properties of the hydrogels.[11] Moreover, the weak interfacial interactions between CNTs and organic polymer matrix restrict efficient load transfer to polymers,[12] and therefore the reinforcing effect of CNTs in the nanocomposites are also limited. Modifying CNTs with functional groups is an effective approach to improve the dispersion of CNTs and strengthen interfacial interactions with polymer network, thus enhancing the physical properties of CNT-reinforced hydrogels.[13] However, most of these modification processes are carried out at a high temperature or in a harsh chemical condition that is detrimental to the structure and properties of CNTs, which is a major concern with regard to the fabrication of CNT-incorporated conductive hydrogels. In addition, almost all of the hydrogels cannot resist hot or cold environment and also do not have long-term stability.[14] High temperatures lead to drying out of the hydrogels, while low temperatures result in freezing of the hydrogels. Even under the ambient conditions, water is inevitably lost from the hydrogels by evaporation, hindering the long-term usability of the hydrogel. Note that the loss of the water causes the hydrogels to dry and hard, which seriously weaken the properties of the hydrogels, such as flexibility and stretchability. Thus, the real-world usage of hydrogels is still limited by their poor stability. It is a challenge to design a hydrogel with the properties

Conductive hydrogels are a promising class of materials to design bioelectronics for new technological interfaces with human body, which are required to work for a long-term or under extreme environment. Traditional hydrogels are limited in short-term usage under room temperature, as it is difficult to retain water under cold or hot environment. Inspired by the antifreezing/antiheating behaviors from nature, and based on mussel chemistry, an adhesive and conductive hydrogel is developed with long-lasting moisture lock-in capability and extreme temperature tolerance, which is formed in a binary-solvent system composed of water and glycerol. Polydopamine (PDA)-decorated carbon nanotubes (CNTs) are incorporated into the hydrogel, which assign conductivity to the hydrogel and serve as nanoreinforcements to enhance the mechanical properties of the hydrogel. The catechol groups on PDA and viscous glycerol endow the hydrogel with high tissue adhesiveness. Particularly, the hydrogel is thermal tolerant to maintain all the properties under extreme wide tempreature spectrum (−20 or 60 °C) or stored for a long term. In summary, this mussel-inspired hydrogel is a promising material for self-adhesive bioelectronics to detect biosignals in cold or hot environments, and also as a dressing to protect skin from injuries related to frostbites or burns.

1. Introduction Conductive hydrogels due to their tissue-like softness provide a new route to design bioelectronics for new technological interfaces with the human body, such as wearable and implantable Dr. L. Han, K. Z. Liu, M. H. Wang, Dr. J. Zhou, Prof. X. Lu Key Lab of Advanced Technologies of Materials Ministry of Education School of Materials Science and Engineering Southwest Jiaotong University Chengdu 610031, Sichuan, China E-mail: [email protected] Dr. K. F. Wang National Engineering Research Center for Biomaterials Genome Research Center for Biomaterials Sichuan University Chengdu 610064, Sichuan, China Dr. L. M. Fang, H. T. Chen Department of Polymer Science and Engineering School of Materials Science and Engineering South China University of Technology Guangzhou 510641, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201704195.

DOI: 10.1002/adfm.201704195

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of long-term stability, antifreezing, and antiheating to maintain these properties of the hydrogel in a wide temperature window. Introducing glycerol into hydrogels may provide a feasible method to prevent the loss of water. Glycerol is a well-known, nontoxic antifreezing agent and a nonionic kosmotrope.[15] It forms strong hydrogen bonds with water molecules that compete with hydrogen bonds in water,[16] and disrupts the formation of crystal lattices of ice at low temperatures and prevents the evaporation of water at high temperatures. Furthermore, conductive hydrogels usually do not have adhesiveness, and therefore cannot self-adhere on human skin, which limited their practical applications, especially when they are used for detecting biosignals. Currently, most conductive hydrogels are attached onto the skin with the assistance of extra adhesives, such as bandage, scotch tapes, or 3M adhesives.[17] These adhesives usually exhibit excessive adhesion to the skin and cannot be easily removed, especially when the hydrogels are applied on frail skin. Therefore, there is an increasing demand for intrinsically biocompatible and adhesive conductors, which can easily adhere to the human body and effectively transport electrical signals. Various strategies have been developed to prepare hydrogels with adhesiveness by introducing adhesive components or functional groups in the hydrogels that can strongly interact with the surrounding tissues. The most biocompatible adhesive hydrogels for tissue bonding are composed of polysaccharide, proteins, or poly(ethylene glycols).[18] Despite the success of commonly used adhesive hydrogels, the toughness and adhesion strength of these hydrogels are weak and therefore they are vulnerable to debonding. Recently, a bioinspired tough adhesive consisting an adhesive surface and a dissipative matrix was designed for diverse tissue surfaces.[19] However, this adhesive involved in situ crosslinking at the adhesion site, which was not convenient for clinical usages. Inspired by the mussel adhesion chemistry, polydopamine (PDA)-based hydrogel were developed, which exhibited high adhesiveness to various substrates regardless of the surface physicochemical properties.[20] Several mussel inspired adhesive hydrogels have been developed, including catechol-containing protein-based hydrogels[21] and catechol-modified polymer-based hydrogels,[22] and all these hydrogels showed excellent adhesion to tissues. Our previous studies also demonstrate that it is feasible to develop tough and adhesive hydrogel by incorporating catechol groups in the hydrogels.[4,23] These previous studies motivated us to design adhesive and conductive hydrogels based on mussel adhesion chemistry. Herein, we successfully achieved a mussel-inspired glycerol– water hydrogel (GW-hydrogel) with PDA-decorated CNTs as conducting nanofillers, which simultaneously possesses antifreezing and antiheating performance, long-term stability, good conductivity, super mechanical properties and tissue adhesiveness. The GW-hydrogel was obtained by elaborately designed gelation process in glycerol–water binary-solvent system. Quantum mechanics calculation was used to study interactions between glycerol–water and monomers, and the results guided us to copolymerize acrylamide (AM) and acrylic acid (AA) monomers in glycerol–water binary-solvent system to form the hydrogel. The optimized hydrogel exhibits high conductivity (8.2 S m−1), stretchability (700%), toughness (2300 J m−2), good recoverability, and tissue adhesiveness (60 kPa). The most prominent

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features of the GW-hydrogel are that it had superior antifreezing and antiheating properties to resist tempreature change and long-term stability. Thus, the GW-hydrogel can maintain all the above-mentioned properties even when cooled to −20 °C or heated to 60 °C, or subjected to long periods of storage.

2. Results and Discussion 2.1. Synthesis of the GW-Hydrogel The main obstacle to obtain the GW-hydrogel composed of glycerol–water binary solvent was that dopamine (DA) cannot be oxidized to PDA even under alkali and oxidative environment when glycerol presented in the solvent (Figure S1, Supporting Information), which would lead to the failure of gelation of the hydrogel.[24] To overcome this problem, a synthetic procedure including four sequential steps was designed to prepare the hydrogel (Figure 1). First, CNTs were added in DA/NaOH aqueous solution and DA was oxidized and polymerized on CNT surfaces to form PDA-decorated CNTs (PDA-CNTs) that can be dispersed uniformly in aqueous solution. The PDAdecorated CNTs were characterized by X-ray photoelectron spectroscopy (XPS), UV–vis spectra and Transmission electron microscopy (TEM), and the results indicated that PDA was successfully decorated on CNT surfaces after polymerizing DA in water under alkali environment (Figure S2c–d, Supporting Information). Second, AM monomers, AA monomers, N,N′methylenebis acrylamide, and ammonium persulfate were completely dissolved in the PDA-CNTs dispersion. Third, glycerol was added to form glycerol–water binary solvent. Finally, the GW-hydrogel was formed in situ by UV-initiated copolymerization of AM and AA monomers in the glycerol–water binary solvent. The hydrogel formed in pure water (W-hydrogel) was also prepared as control. The compositions of the hydrogels were listed in Tables S1 and S2 (Supporting Information). The GW-hydrogel differing from conventional hydrogels, exhibits good antifreezing/antiheating and long-term stability to resist tempreature change because of water-locking effect of glycerol in the hydrogel. Glycerol forms hydrogen bonds with water molecules that inhibits the formation of ice at low temperatures and prevents the evaporation of water at high temperatures, imparting the properties of antifreezing and antiheating to the GW-hydrogel. In addition, the glycerol–water binary solvent introduces noncovalent interactions within the polymer chains, and therefore improves the mechanical behaviors of the GW-hydrogels. The presence of CNTs makes the GW-hydrogel be conductive and CNTs also serve as nanoreinforcements that further enhance the mechanical properties of the GW-hydrogel. The surfaces of the CNTs were decorated by mussel-inspired PDA, which played multiple roles in the hydrogel. First, the catechol groups of PDA imparted good adhesiveness to the hydrogel. Second, the presence of PDA facilitated uniform distribution of the CNTs in the polymer network, strengthening the interactions between the CNTs and the polymer networks. Third, PDA interacted with the PAM-PAA polymer networks through the interactions between the catechol groups and the carboxyl groups/amino groups. Thus, the GW-hydrogels

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Figure 1. Design strategy for a mussel-inspired adhesive, tough, and conductive hydrogel with long-lasting moisture and extreme temperature tole rance based on glycerol–water binary solvent. a) Polydopamine decorating carbon nanotubes (PDA-CNTs) in water based on mussel chemistry. b) AM and AA monomers were dissolved in PDA-CNT dispersion. UV-initiated polymerization of AA and AM to form water-based (W) hydrogel in pure water in which the CNTs tended to aggregate. c) AM and AA monomers were dissolved in PDA-CNT dispersion. UV-initiated polymerization of AA and AM to form glycerol–water (GW) hydrogel in glycerol–water binary solvent. The PDA-CNTs were well dispersed in acid (PAM-co-PAA) networks. The covalent/noncovalent hybrid crosslinked network and nanoreinforcing CNTs synergistically contributed to the good mechanical properties of the GW-hydrogel. d) Strong hydrogen bonding between glycerol and water to firmly lock water molecules in the GW hydrogel. e) The GW hydrogel showed antifreezing and antiheating performance to maintain flexible, conductive, adhesive behaviors for a long term even under cold (−20 °C) and hot (60 °C) environment.

displayed multiple mechanisms for better dissipation of the energy during deformation due to the noncovalent interactions between glycerol, PDA-CNTs, and PAA-PAM chains. Consequently, the GW-hydrogels showed high toughness and recoverability even after large deformation. To gain further insight into the role of glycerol–water in the hydrogel, we carried out density functional theory (DFT) calculations with Dmol3/GGA-PBE/DNP basis set. The DFT analysis showed that the hydrogen bonding in the glycerol–water

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mixture is more stable than that in water–water (Figure S5 and Table S4, Supporting Information), which is vital to lock the water in the hydrogel. In addition, the glycerol–water mixture exhibited stronger interactions with the polymers than pure water or glycerol (Figure 2, and Figure S6 and Table S5 in the Supporting Information), demonstrating that the glycerol–water can form more stable hydrogel networks than pure water or glycerol. Furthermore, glycerol had stronger interactions with PAA than with PAM, and the interaction energy of

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wider antifreezing tempreature window than glycerol–water solutions, which was because the other components might further preclude ice crystallization. In addition, the evaporation rate of water from the glycerol–water mixed solutions decreased with the content of glycerol (Figure S9a,b, Supporting Information) at hot environment (60 °C), because glycerol was relatively nonvolatile and lowered the vapor pressure of water.[25,26] Similarly, introducing glycerol in the hydrogel also precluded the evaporation of water at hot environment (Figure S9c,d, Supporting Information). When the Gvol% was lower than 50 vol%, the GW-hydrogels were dried out under 60 °C and became hard polymer with irregularly contraction shrink (Figure 3a). In contrast, the GW-hydrogels with high content of glycerol (Gvol% > 50 vol%) were still moist and well kept their states. To investigate the performance of the GW-hydrogels after undergoing different temperatures, the hydrogels were pre-placed in a freezer with low temperature (−20 °C) and heater with high temperature (60 °C) for a day. Then, the mechanical properties, conductivity, adhesiveness of these samples in different conditions were immediately tested after the samples were taken out. The mechanical properties of the GWFigure 2. Density functional theory (DFT) optimized structures of glycerol–water and PDA interhydrogels were well preserved under harsh acting with polymer chains via multiple nonbond interactions in the glycerol–water (GW) hydrogel. environment. As shown in Figure 3b and a) Glycerol–water–(polyacrylic acid) (G-W-PAA); b) glycerol–water–polyacrylamide (G-W-PAM); Figure S10 (Supporting Information), the c) polydopamine–(polyacrylic acid) (PDA-PAA); (d) polydopamine–polyacrylamide (PDA-PAM). GW-hydrogel exhibited high flexibility to endure large deformations such as twisting, compression, or pulling, and had high resilience to quickly glycerol-PAA was twice that of glycerol-PAM (Figure S6 and recover from deformed shapes after a day’s storage at −20 Table S5, Supporting Information), prompting us to select AA or 60 °C. In contrast, the W-hydrogel was frozen at −20 °C, for copolymerization to form the hydrogel. The copolymerized and was totally dried at 60 °C and broken during twisting. PAA-PAM resulted in the hydrogel with high mechanical propThe quantified mechanical properties of the GW-hydrogels, erties (Figure S4 and Table S3, Supporting Information). The including strength, elongation, fracture toughness, and recovDFT analysis also demonstrated that PDA interacted intensively erability kept similar value after cooling or heating (Figure 3c, with the glycerol–water mixture and the PAA-PAM networks and Figures S11 and S12 in the Supporting Information), conto form multiple hydrogen bonds, as shown in Figure 1e and firming the stable mechanical properties of the GW-hydrogels Figure S7 (Supporting Information). even used in unnormal environment. The mechanical properties under hot conditions did not decay after one-day storage, which was because water can be well retained in the hydrogel 2.2. Antifreezing and Antiheating Properties of the Hydrogels after a short period of storage. As shown in Figure S9b (Supporting Information), the water loss of hydrogel was small after The GW-hydrogel exhibited good antifreezing and antiheating 1 d of storage at 60 °C. properties, which were highly dependent on the volume perThe GW-hydrogel also kept highly conductive and adhesive centage of glycerol (Gvol%) in the mixture. The phase diagram under harsh environment, which was superior to W-hydrogel. of the glycerol–water mixed solutions with different Gvol% indiAs tested by a two-probe method (Figure S13, Supporting Inforcated that the solution with Gvol% in the range of 50–75 vol% mation), the conductivity of GW-hydrogels was maintained did not freeze when the temperature was at −20 °C,[25] as well at −20 and 60 °C (Figure 3d), suggesting that the electrical shown in Figure S8 (Supporting Information). This was visupercolation path was protected to be stable inside the gels. In ally demonstrated in Figure 3a. Accordingly, the GW-hydrogels addition, the adhesion of the GW-hydrogel to porcine skin formed in glycerol–water mixed solution with Gvol% in the after freezing or heating was nearly the same as that of the range of 50–87.5 vol% well kept their states, and others were original one (Figure 3e), and the GW-hydrogels firmly adhered frozen under −20 °C. Note that the GW-hydrogels exhibited

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Figure 3. The antifreezing and antiheating performance of the GW-hydrogel. a) The effect of glycerol volume percentage (Gvol%) on the antifreezing and antiheating performance of the GW-hydrogel. The yellow part was the glycerol/water solutions stained by yellow helianthin B (0.1 wt%) after one day of cooling at −20 °C. The red “X” represents that the GW-hydrogel cannot form in pure glycerol. The compositions of the hydrogels are listed in Table S1 (Supporting Information). b) The photos of GW-hydrogel after one day of cooling at −20 °C. 1) twisted, 2) compressed, and 3) pulled.

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to the author’s finger during pulling (Figure 3b). However, the W-hydrogel totally lost its adhesion at −20 and 60 °C. The high adhesiveness of the GW-hydrogel after one day storage at 60 °C was attributed to unique glycerol–water binary system. Although water evaporation happened at 60 °C, the increase of temperature might lead to high mobility of the polymer chains and glycerol,[27] which resulted in easy interpenetration of the GW-hydrogel to the substrates. Thus, the GW-hydrogel maintained good adhesion in contrast to W-hydrogels that totally lost adhesion due to high rate of water evaporation. The combination of excellent adhesion and conductivity of the GW-hydrogel under different environments was demonstrated using a circuit with an LED array was connected by the GW-hydrogel that strongly attached to the glass and polymer surfaces (Figure S15, Supporting Information). Based on its good adhesiveness and conductivity, GW-hydrogels were designed as self-adhesive electrocardiogram (ECG) electrodes, which collected ECG signals accurately and stably even after cooling or heating (Figures S16 and S17, Supporting Information). Figure 3f shows the typical ECG waveform recorded by the GW-hydrogels after cooling at −20 °C. The ECG signal showed the distinct separated and negligible fluctuated P, QRS, and T curves, providing meaningful medical information about cardiovascular problems. The GW-hydrogels also served well as strain sensor and recorded arm movements even after a day of cooling in −20 °C (Figure 3g). The detected signals show high repeatability under different temperatures, indicating the high precision of as-prepared electronics with high tempreature insensitivity against extreme cold and hot temperature.

2.3. Long-Term Stability of the GW-Hydrogel The long-term stability of the GW-hydrogel was certified by the fact that there was no performance degradation after a prolonged storage in normal condition (25 °C) for 30 d (Figure 4a). The weight of the GW-hydrogels and the W-hydrogels were measured at different time intervals (Figure 4b). The weight of current GW-hydrogel has very minor drift (less than 2%) and it remained moist throughout the storage period, caused by effectively preventing the evaporation of the water through glycerol–water interactions. However, the W-hydrogel quickly lost its weight, close to its original water content, after the initial 5 d, and finally transformed a dried bulk polymer. The good mechanical properties, conductivity, and adhesiveness after 30 d of longterm storage in normal environment (25 °C) or at cold condition (−20 °C) are very consistent and reproducible, as shown in the Figure 4c–h. To further demonstrate the antidehydration property of the GW-hydrogel, the GW-hydrogel and the W-hydrogel were placed in a freeze-dryer for a day. The W-hydrogel was

freeze-dried and changed to dry and hard scaffold (Figure 4a), while the GW-hydrogel was still soft without losing water and maintained its properties (Figure 4c–h, and Figure S12 in the Supporting Information). In addition, the GW-hydrogel-based electrodes collected ECG signals more accurately than commercial Ag/AgCl electrodes even after 30 d of storage, indicating the good consistency and durability of GW-hydrogel-based bioelectronics for long-term use (Figure S18, Supporting Information). The long-term stability of the GW-hydrogel was attributed to the presence of glycerol in the hydrogel network, which was nonvolatile and had low vapor pressure and hygroscopicity.[28] The glycerol formed strong hydrogen bonds with water molecules that prevented the evaporation of water molecule from the GWhydrogel. Therefore, the GW-hydrogel maintained the initial gel state and retained their superior properties for a long time, as against the water-based hydrogels.

2.4. Mechanical Properties of the GW-Hydrogels The GW-hydrogels showed excellent mechanical properties, which was achieved by tuning the glycerol volume contents (Gvol%) as well as the contents of PDA-CNTs. Considering the effect of Gvol% first, a specific Gvol% resulted in a GW-hydrogel with optimized mechanical strength, stretchability, and toughness. With increase in the Gvol%, the mechanical strength decreased (Figure 5a,b). However, the fracture strain and the fracture energy reached to maximum when the Gvol% was 50 vol%, and then decreased (Figure 5c,d). Thus, the GW-hydrogel with the Gvol% of 50 vol% displayed the optimal mechanical properties. Further increase in the Gvol% diminished the mechanical properties, because excessive amount of glycerol cannot fully dissolve the AM monomers, and thereby retarding the polymerization of poly mers (Figure S1, Supporting Information). Second, PDA-CNTs incorporation remarkably improved the mechanical properties of the GW-hydrogels, compared with that of the pristine CNTs. In the absence of PDA decoration, increasing the contents of the CNTs enhanced the mechanical strengths of the GW-hydrogels (Figure 5e,f). However, a high CNTs content (15 wt%) decreased the stretchability and the fracture toughness (Figure 5g,h) due to the aggregation of the CNTs (Figures S2a and S3, Supporting Information), which was caused by the intrinsic van der Waals attractions between nanotubes,[29] and therefore leading stress concentration and preventing load transfer to reinforce the poly mer networks.[30] In contrast, PDA-CNTs increased both the tensile strength and stretchability of the GW-hydrogels (Figure 5f,g), and at 10 wt% PDA-CNTs, the GW-hydrogels showed the highest stretchability with strain of 700% (Figure S19, Supporting Information). Especially, the enhancement in the fracture toughness was particularly pronounced, from 1520 J m−2 for

c) Comparisons of quantified mechanical properties of the GW-hydrogel and W-hydrogel after one day cooling at −20 °C or heating in 60 °C. 1) tensile strength, 2) maximum tensile strain, 3) fracture energy, and 4) compression strength under different environments. d) Comparisons of conductivity of GW-hydrogel and W-hydrogel. e) Comparisons of adhesive strength of GW-hydrogel and W-hydrogel. f) Electrocardiogram (ECG) signals detected by the GW-hydrogel-based electrodes at −20 °C; the inset in the red box is magnified view of the measured ECG wave indicating P, QRS, and T waves. g) The GW-hydrogel as strain sensor to record arm motions even after one day of cooling at −20 °C. The GW-hydrogel can be well adhered on the author’s elbow joint and the resistance is changed corresponding to the curl movement of the elbow joint, indicating the good strain dependence and recoverability of resistance. The GW-hydrogel in (b)–(g) is PDA4-CNT10-GW50-hydrogel, and its composition is listed in Table S1 (Supporting Information). The blue “X” represents that the corresponding properties of W-hydrogel were lost.

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Figure 4. The water retentiveness and long-lasting performance of GW-hydrogels. a) Comparison of GW-hydrogels and W-hydrogels after placed in air for 30 d or after 1 d of freezing dry. b) Weight change of the GW-hydrogels and W-hydrogels as a function of time. The weight of GW-hydrogels nearly did not change. W0 is the initial weight. Wt is the weight at different time. c–h) The mechanical properties, conductivity, and adhesiveness of the GW-hydrogel maintained stable even after 30 d or in harsh environment, while the W-hydrogel lost its properties. The GW-hydrogel is PDA4-CNT10-GW50-hydrogel, and its composition is listed in Table S1 (Supporting Information). The blue “X” represents that the corresponding properties of the W-hydrogel were lost.

CNT-only GW-hydrogels to 2300 J m−2 for PDA-CNT-incorporated GW-hydrogels with 10 wt% CNTs (Figure 5h), which was much higher than W-hydrogel (800 J m−2) and the commonly reported

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PAM hydrogel (100–250 J m−2).[31] This is because the addition of PDA-CNTs imparted notch insensitivity to the GW-hydrogels (Figure S20, Supporting Information).

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Figure 5. The glycerol volume percentage (Gvol%) and PDA-decorated CNT’s effect on the properties of GW-hydrogels. All the properties were obtained under room temperature. The effects of Gvol% on the mechanical properties, including a) compression strength, b) tensile strength, c) maximum tensile strain, and d) fracture energy. In these figures, the content of PDA-decorated CNTs was fixed at 10 wt%. The compositions of these GW-hydrogels are listed in Table S1 (Supporting Information). The effects of CNTs on the mechanical properties, including e) compression strength, f) tensile strength, g) maximum tensile strain, and h) fracture energy. In these figures, Gvol% was fixed at 50 vol%. CNTs were either decorated by PDA or not. The compositions of these GW-hydrogels are listed in Table S2 (Supporting Information). i) The effects of CNTs on conductivity. j) The effects of Gvol% on conductivity. k) The effects of CNTs on adhesive strength. l) The effects of Gvol% on adhesive strength.

The excellent mechanical properties of the GW-hydrogel were attributed to the following reasons. (1) The glycerol–water binary system introduced more noncovalent interactions in the polymer networks (Figure S7, Supporting Information), and these noncovalent interactions acted as sacrificial bonds to efficiently dissipate external energies during deformation so as to improve the toughness of the GW-hydrogels considerably. (2) PDA decoration promoted CNTs dispersion to form a homogenous hydrogel. As mentioned in the Introduction section, poor dispersion of pristine CNTs in the organic polymer matrix has been a significant obstacle to reach reinforcement effect. Here we used PDA decoration to achieve uniform distribution of CNTs in a mild manner, which is a new approach to produce nanocomposite hydrogel with high mechanical properties. (3) Due to its high reactivity,[32] PDA enhanced interfacial interactions between CNTs and polymer networks,[33]

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thus being favorable for load transferring from the covalent crosslinked PAA-PAM network to CNTs. In summary, the introduction of the glycerol–water system and the PDA-decorated CNTs into the covalent crosslinked polymer network formed covalent–noncovalent hybrid crosslinked network, which have multiple mechanism to transfer load and dissipate energy and therefore made the GW-hydrogels much stronger and tougher than the W-hydrogels.

2.5. Conductivity of the GW-Hydrogels The GW-hydrogels exhibited good conductivity because of the presence of PDA-decorated CNTs and the contribution of the glycerol–water system. First, incorporation of the PDACNTs greatly enhanced the conductivity of the GW-hydrogels,

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compared with that of the pristine CNTs. Figure 5i shows that the conductivity of the GW-hydrogels increased with increase in the CNTs content, whereas excessive CNTs contents (15 wt%) did not result in higher conductivity. Notably, after the PDA decoration, the conductivity of the PDA-CNT-incorporated GWhydrogels increased almost twofold and reached a maximum value of 8.2 S m−1, which is comparable to that of previously reported conductive hydrogels.[34] Second, the conductivity of the GW-hydrogels was also affected by the Gvol% (Figure 5j), and the conductivity was highest at Gvol% of 50 vol%. However, excessive Gvol% leads to lower conductivity, which is originated from suppressed ionic conduction by replacing more water with glycerol organic solvent. The good conductivity of the GWhydrogel is attributed to the combination of PDA decoration and glycerol–water binary-solvent system, which facilitated uniform dispersion of the CNTs in the polymer network, forming a well-connected network of percolation paths.

2.6. Adhesiveness of the GW-Hydrogels The GW-hydrogels displayed high tissue adhesiveness that could directly adhere on tissue without requiring extra agents during operation. Adhesion-tensile tests were conducted to quantitatively determine the adhesion strength immediately after overlapping the GW-hydrogels between two porcine skins together. As shown in Figure 5k, the pristine CNT-incorporated GWhydrogels showed weak adhesion ability. After incorporating PDA-CNTs, the GW-hydrogels showed super adhesion and the highest adhesion strength to the porcine skin was 57 ± 5.2 kPa. The adhesive strength of the GW-hydrogels was also affected by the Gvol% (Figure 5l), and increased dramatically from 19 to 57 kPa when the Gvol% was increased from 12.5 to 50 vol%. The exceptional tissue adhesiveness of the GW-hydrogels was attributed to two aspects. First, the catechol groups of PDA resembling a bridge interacted at the interface between GWhydrogels and the substrates. This was because the reactive catechol groups on PDA exhibited high binding affinity to diverse nucleophiles (e.g., amines, thiol, and imidazole),[22b,35] and therefore the GW-hydrogel can bind to peptides and proteins on tissue surfaces. Second, the addition of glycerol increased the adhesive strength because glycerol is more viscous than water,[36] and glycerol formed stronger interfacial interactions with the substrates than water. Furthermore, glycerol increased the intermolecular interactions in the polymer network, thereby increasing the cohesion of the GW-hydrogels, as demonstrated by the DFT analysis (Figures S6 and S7, Supporting Information). Combining catechol-adhesion chemistry and viscous glycerol, multiple inherent interfacial interactions, such as hydrogen bonding, π–π and/or cation–π interactions, synergistically contributed to the strong cohesion and adhesion strength of the GW-hydrogel, consequently leading to the high adhesiveness of the hydrogel.

2.7. In Vivo Skin Dressing Considering the advantages of good adhesiveness and the ability of antiheating or antifreezing, the GW-hydrogel was

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used to serve as an excellent wearable dressing to protect skin under harsh environment. Figure 6a shows the representative photos of skin tested covered with/without GW-hydrogel during frostbite. The frostbite was induced by attaching an extremely cold coin, previously immersed in liquid nitrogen, on the skin for 15 s. After testing, the GW-hydrogel was still moisture and tightly adhered on the skin (Figure 6b, and Figure S21 in the Supporting Information). The skin protected by the GW-hydrogels was intact and similar to the normal skin as observed by the naked eye (Figure 6c), whereas the unprotected skin immediately developed a white eschar with a hyperemic zone (Figure 6d). Histological analyses further indicated that the morphological appearance of the skin protected by the GW-hydrogel was unchanged and showed the presence of complete epidermis and intact hair follicles (Figure 6c). In contrast, the unprotected skin showed significant injury after the frostbite with ruptured and disordered collagen fibers, and damaged hair follicles in the dermis (Figure 6d). The burnt models also confirmed the excellent protection efficacy of the GW-hydrogels at 100 °C (Figure 6e,f, and Figure S21 in the Supporting Information). The above results confirmed that the advantages of the antiheating and antifreezing properties of GW-hydrogels, especially under very cold, hot, or harsh environments.

3. Concluding Remarks In summary, we demonstrated a novel strategy to design and develop antifreezing and antiheating hydrogels with longterm stability by using glycerol–water mixture as the binary solvent. The strong cooperative hydrogen bonding between glycerol and water firmly anchored the water in the polymer network, and therefore endowed the GW-hydrogel with the properties of antifreezing and antiheating, and long-term stability, which cannot be achieved using water-based hydrogels. The present strategy can be generalized to other previously reported functional hydrogels to make antifreezing and antiheating hydrogels. Following the design strategy, we also incorporated mussel-inspired polydopamine-CNTs into the hydrogel as nanoreinforcements, which not only imparted good conductivity to the GW-hydrogel but also reinforced the hydrogel network. The GW-hydrogels exhibited high toughness and excellent recoverability due to the synergetic interactions between glycerol, PDA-decorated CNTs, and PAA-PAM covalent network. The GW-hydrogel showed excellent adhesion to various substrates with diverse surface chemistry due to PDA component. The GW-hydrogels exhibit a combination of exotic properties that are superior to the common water-based hydrogels, and show great potential for practical applications in antifreezing or antiheating materials, wound dressing, bioelectronics, and electronic skin. For example, the GW-hydrogel-based electrodes can be adapted to collect biosignals in some extreme conditions, such as skiing, polar, and desert expeditions. The GW-hydrogel with long-lasting moisture can also be used as a wearable dressing to protect skin from freezing or burn injury, as demonstrated by the burnt model and the frostbitten models using the back skin of rat. In conclusion, this study paves a

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Figure 6. The frostbite and burnt model on rats’ back skin to demonstrate antifreezing and antiburning performance of adhesive GW-hydrogel. a) Schematics of creating frostbite and burnt model on rat’s back skin. Photos showing frostbite model—left: a cold coin attached on the bare skin; right: a cold coin attached on the GW-hydrogel-protected skin. b) The GW-hydrogels remained tightly adhered on the skin that had no injury after frostbiting. c) Histological microscopy images of GW-hydrogel treated skin, and d) the bare skin after frostbite (H&E staining). e) Histological microscopy images of GW-hydrogel treated skin, and f) the bare skin after burn (H&E staining). The GW-hydrogel used for animal experiment is PDA4-CNT10GW50-hydrogel, and its composition is listed in Table S1 (Supporting Information).

new path for the fabrication of antifreezing and antiheating hydrogels with multifunctionality, opening up a number of future research directions and applications.

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

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Acknowledgements L.H. and K.Z.L. contributed equally to this work. This work was financially supported by the National Key R&D Program of China (2016YFB0700802), 863 Program (2015AA034202), NSFC (81671824), Fundamental Research Funds for the Central Universities (2682016CX075), and Sichuan Province Youth Science and Technology Innovation Team (2016TD0026). The experiments were performed in accordance with protocols approved by the local ethical committee and laboratory animal administration rules of China.

Conflict of Interest The authors declare no conflict of interest.

Keywords antifreezing and antiheating, carbon nanotubes, conductive hydrogel, long-lasting moisture, mussel adhesion Received: July 25, 2017 Revised: September 9, 2017 Published online: November 28, 2017

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