Skin-Inspired Gels with Toughness, Antifreezing, Conductivity, and

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Skin-inspired Gels with Toughness, Antifreezing, Conductivity and Remoldability Hao Chen, Xiuyan Ren, and Guanghui Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11032 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Skin-inspired Gels with Toughness, Anti-freezing, Conductivity and Remoldability

Hao Chen, Xiuyan Ren, Guanghui Gao*

Polymeric and Soft Materials Laboratory, School of Chemical Engineering and Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, P. R. China

Corresponding to Guanghui Gao E-mail: [email protected].

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In recent years, nature-inspired conductive hydrogels have become ideal materials for design of bioactuators, healthcare monitoring sensors and flexible wearable devices. However, conductive hydrogels are often plagued by problems such as poor mechanical property, non-reusability and narrow operating temperature range. Here, a novel skin-inspired gel is prepared via one step of blending polyvinyl alcohol, gelatin and glycerin. Due to their dermis-mimicking structure, the obtained gels possess high mechanical properties (fracture stress of 1044 kPa, fracture strain of 715%, Young's modulus of 157 kPa and toughness of 3605 kJ/m3). Especially, the gels exhibit outstanding strain sensitive electric behavior as biosensors to monitor routine movement signals of human body. Moreover, the gels with low temperature tolerance can maintain good conductivity and flexibility at -20 ºC. Interestingly, the gels are capable of being recovered and reused by heating injection, cooling molding and freezing-thawing cycles. Thus, as bionic materials, the gels have fascinating potential applications in various fields, such as human-machine interfaces, biosensors and wearable devices. Keywords: skin-inspired gels, toughness, anti-freezing, conductivity, recyclability


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Introduction Flexible electronic devices have received a myriad of interests in recent years, have been extensively used for such as sensors1–3, actuators4 and artificial muscle and skin5,6. Especially, conductive hydrogels with fast response and strain sensitivity, paving the way for the burgeoning applications human-machine interfaces7,8, human activity monitoring9–11, electronic skin12,13. There are two types of common conductive materials. One is intrinsically conductive material, such as polyaniline14–16, polypyrrole17,18, PEDOT/PSS19,20, and grapheme21–23. The limitation is expensive price and inherent color. The other one is adding conductive materials (salts and polyelectrolytes) to endow conductivity with hydrogels. For example, Sun et al. developed polyampholyte hydrogels with high stretchability, self-healing and conductivity24.. The hydrogels were potential candidates for wearable electronics, flexible electronic devices and batteries. Kim et al. prepared a highly transparent and stretchable ionic touch panel by adding lithium chloride into polyacrylamide hydrogels25. The ionic touch panel could quickly and accurately sense the touch position and realize functions such as writing and playing games. However, these types of hydrogels usually are prepared by traditional materials including acrylamide, acrylic acid and so on, which are not suitable for directly contacting the human body. The other trouble is the limited application below the freezing point. Hence, natural and synthetic materials with excellent biocompatibility become new favorite of researchers, such as polyvinyl alcohol (PVA)

26,27,

gelatin (GE)

28,29,

chitosan30, guar gum31, agar32 etc. Moreover, Glycerin is colorless, non-toxic liquid, which contains a mass of hydroxyl groups and has a low freezing point after dissolving with water. Therefore, it is often used as plasticizer and non-toxic anti-freezing agent. For instance, Wang et al. successfully prepared PVA-glycerin



hydrogels with high toughness by utilizing the effect of strong hydrogen bonds between PVA and glycerin33. Zhou et al. introduced a simple method of solvent replacement to obtain anti-freezing and anti-drying hydrogels34. These hydrogels had high mechanical properties, anti-freeze ability and water retaining capacity at a considerable range of operating temperature owing to the interaction of PVA and glycerin. More importantly, the oil-water system was a typical feature of the skin. Based on this, it was envisioned that the introduction of the oil-water system into materials with good biocompatibility could endow the gels with skin-like properties, including high mechanical strength, anti-freezing and electric sensing. In this work, the skin-inspired gels with excellent mechanical properties, anti-freezing, conductivity and recyclability were prepared. The dermis-mimicking networks were formed driven by hydrogen bonds between PVA, gelatin and glycerin, which allowed the gels with excellent mechanical properties. In addition, compared to use of organic solvents (glycerin, ethylene glycol) alone, the mixed used of glycerin and NaCl not only further increased the low temperature tolerance of the gels, but also imparted good conductivity to the gels. Thus, the gels were well suited to fabricate biosensors for monitoring the human motion such as fingers, arms, wrists, and speaking, even at -20 ºC. Besides, glycerin and NaCl on the properties of gels were also discussed. Especially, there were only non-covalent interactions, the gels could be recovered and reused by heating injection, cooling molding and freezing-thawing cycles. All these traits made PVA/GE/GL/NaCl gels having bright future for healthcare monitoring sensors, artificial skin, and human-machine interfaces.

Experimental section Materials


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Polyvinyl alcohol (PVA, polymerization degree 1799, 98~99% alcoholysis, Shanghai Aladdin Reagent Co, Ltd., Shanghai, China), Gelatin (GE, Shanghai Aladdin Reagent Co, Ltd., Shanghai, China), glycerin (GL), NaCl, LiCl, KCl, NH4Cl (Beijing Chemical Works, Beijing, China) and deionized water (18.2 MΩ cm resistivity at 25 ºC) were used in the experiment. Preparation of gels The preparation method of gels was as follows: NaCl with different qualities were dissolved in the mixed solvent of deionized water and glycerin. Then, gelatin was added into the mixed solution and stirred at 85 ºC for 0.5 h. When the gelatin was completely dissolved, PVA was added to stir at 100 ºC for 2 h. Finally, the mixed solution was transferred into 1 mL small needle tubes with diameter of 4 mm. The tubes were placed in a freezer (-20 ºC) for 12 h and thawed at ambient temperatures for 2 h. Some gels were dyed with different colors. The recipes of all gels were shown in Table 1. Table 1. Recipes for the gels PVA

gelatin

salt

glycerin

deionized water

(g)

(g)

(g)

(mL)

(mL)

6

0

0

0

34

5.4

0.6

0

0

34

5.4

0.6

0

8.5

25.5

5.4

0.6

1.5

8.5

25.5

5.4

0.6

2.5

8.5

25.5

Mechanical tests The prepared gels were measured by a universal stretching machine (SHIMADZU,



model AGS-X, 100N, Japan) at room temperature. The gel samples with 80 mm long and the 4 mm diameter cylinder were carried out by the tensile tests and tensile loading-unloading tests at the tensile speed of 100 mm min-1. Stress-strain data between t=10-20% were used to calculate the initial elastic modulus (E), while fracture energy (toughness) was estimated by the area below the stress-strain curve. The dissipated energy (hysteresis) was calculated from the area between the loading-unloading curves. For the accuracy of the test, each set of test data was the average after five trials. Rheology measurements Rheology measurements of gels were conducted with MCR302 H-PTD200 Rheometer (Anton Paar) using a parallel plate of diameter 25 mm. The frequency sweep was performed over the range of 0.1-100 rad s-1 at a fixed strain of 1%. All measurements were performed at 25±0.1 ºC controlled by a constant temperature table. ATR -FTIR spectroscopy Fourier transform infrared (FT-IR) spectra of gel samples from 4000 to 525 cm-1 were recorded on a Thermo Scientiﬁc Nicolet iS10 FT-IR spectrophotometer using attenuated total reflection (ATR) method. DSC analysis The freezing and thawing temperatures of water in the PVA/GE, PVA/GE/GL and PVA/GE/GL/NaCl gels were tested by differential scanning calorimetry (DSC 214, Netsch). The cooling cycle was executed from 40 ºC to -70 ºC at a rate of 5 ºC/min. X-ray diffraction measurement (XRD) X-ray diffraction (XRD) of gel samples was studied by X-ray diffractometer (D/Max200/PC, Rigaku, Japan). The XRD radiation was started up using Cu Kα


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radiation at 60 kV, 300mA and data over an angular range of 5-60º was recorded at ambient temperature. Scanning electron microscopy The gels to be tested should be freeze-dried first and coated with gold for field emission scanning electron microscopy (FE-SEM) analysis. Due to the presence of glycerin and NaCl, the samples could not be completely freeze-dried. For the convenience of testing, the gels first were soaked in excessive deionized water for 12 hours to exchange glycerin and NaCl. Then gels were freeze-dried, and cross section was exposed under liquid nitrogen conditions to observe the structure of the gels. Electrical test The conductivity of the gels was obtained by a four-electrode AC impedance method over a frequency range of 0.1–105 Hz using electrochemical workstation (PGSTAT302N, Autolab). The conductivity (σ, S/cm) of the gel samples were calculated from the following formula: σ= L/(R*S) where L was the distance between adjacent electrodes, R was the resistance of the hydrogel and S was the cross-sectional area of the gels. A multimeter (OWON B35T, with Bluetooth and data logging function) measured resistance of the PVA/GE/NaCl gels. Next, the simultaneous resistance changes of the gel biosensor under different movements of the human body were obtained by using an electrochemical workstation (Princeton Applied Research Model 2273). The resistance change ratio (%) was calculated by the following formula: ΔR/R0=(R-R0)/R0 Where R0 and R are the resistances without and strain applied, respectively. The gauge factor (GF) is defined as GF = (ΔR/R0)/ε, where ε is the strain.



Results and Discussion Mechanism and characterization In this work, a skin-inspired PVA/GE/GL/NaCl gel was synthesized by a simple freeze-thaw method, and the mechanism was shown in Figure 1. Firstly, PVA was chosen as the main network because of its good biocompatibility and excellent mechanical properties. Then, gelatin could form an interaction with PVA to increase the flexibility of gels. Finally, the alliances of glycerin and NaCl not only imparted excellent mechanical properties, but also achieved a perfect combination between anti-freezing and electrical conductivity. On one hand, glycerin with plenty of hydroxyl groups could serve as "bridges" to connect PVA chains for improving mechanical properties of gels. On the other hand, the hydrogen bonds between glycerin and water prevented the formation of ice crystals, endowing gels with anti-freezing property35. Additionally, the salt was dissolved in water and combined with water molecules to form hydration shells (the salting out effect), resulting in strong interaction of PVA and glycerin36. Therefore, the mechanical properties were improved remarkably. It is known that the colligative properties of the salt could effectively decrease the freezing point of the aqueous phase37, further increasing anti-freeze capacity of gels. Since ions had the ability to take out water based on the salting out effect, the interaction between PVA and glycerin would be enhanced, leading to the increase of mechanical properties of gels. In addition, both glycerin and salt inhibited the growth of ice crystals, and the gel exhibited excellent frost resistance. At the same time, the presence of salts also endowed the gel with good electrical conductivity. Moreover, the gels could be remolded by freezing-thawing cycles based on the dynamic interactions. The corresponding morphologies of PVA/GE, PVA/GE/GL and PVA/GE/GL/NaCl gels were shown in Figure S1.


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Figure 1. a) The PVA/GE/GL/NaCl gels were composed of PVA, gelatin, inorganic ion and glycerin to simulate human skin; and b) corresponding structure and interaction. To better verify the interaction in the system, FTIR and XRD tests were carried out in the experiment. In Figure 2a, the FTIR spectra of PVA gels showed characteristic stretching bands of O-H at 3277 cm-1 and C-O at 1088 cm-1. The two characteristic stretching bands of PVA/GE gels moved to 3270 cm-1 and 1085 cm-1, because hydrogen bonds were formed between PVA and gelatin. Comparing with the effect of hydrogen bonds between PVA chains, the effect of hydrogen bonds between PVA and glycerin was stronger33. As a result, the characteristic stretching bands of PVA/GE/GL gels continued to move toward the lower wave number (3260 cm-1 for O-H and 1036 cm-1 for C-O). For the PVA/GE/GL/NaCl gels, the solvation effect of salt ions weakened the effect between glycerin and water, and more free glycerin caused the characteristic stretch bands of O-H and C-O moving to 3285 cm-1 and 1036



cm-1. The crystalline reflection of PVA, PVA/GE, PVA/GE/GL, PVE/GE/GL/NaCl gels were in the same 2θ range of 18-21º, which suggested these gels had the same crystallite structure (Figure 2b). It had known that hydrogen bonds were formed between gelatin and PVA, which inhibited the interaction between PVA chains. Hence, the crystallinity of PVA/GE gels was smaller than that of PVA gels. The main reason for the smaller crystallinity of PVA/GE/GL gels was based on the interaction of PVA and glycerin, which further impaired the effect between PVA chains. For PVA/GE/GL/NaCl gels, the salting out effect and the decrease of solution freezing point induced PVA molecules long staying time in the liquid phase. As a consequence, they could reorganize themselves and interact for long time, resulting in high crystallization domains36,38,39. Via fitting calculation, the crystallinity of PVA, PVA/GE, PVA/GE/GL, PVE/GE/GL/NaCl gels was 31%, 24%, 10% and 15%, respectively. In addition, the salting out effect also leaded to high interaction between PVA and glycerin. Therefore, the interaction of PVA-PVA and PVA-glycerin was the main reason for improving the mechanical properties of gels.

Figure 2. a) FT-IR and b) XRD patterns spectra of PVA, PVA/GE, PVA/GE/GL and PVA/GE/GL/NaCl gels. Mechanical properties Tensile properties of PVA/GE/GL gels and PVA/GE/GL/NaCl gels were compared


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in Figures 3a, 3b. The fracture stress, strain, elastic modulus and toughness of PVA/GE/GL gels were 97 kPa, 274%, 16.8 kPa and 157 kJ/m3, respectively. For PVA/GE/GL/NaCl gels, their tensile strength reached 1044 kPa, which was about 5 times higher than that of PVA/GE/GL gels. And the corresponding fracture strain of 715% was 3 times longer than that of PVA/GE/GL gels, elastic modulus and toughness were increased to 119 kPa and 3605 kJ/m3, which were about 7 times and 23 times than those of PVA/GE/GL gels, respectively. The significantly enhanced mechanical properties testified that the interaction of PVA-PVA and the interaction of PVA-glycerin were enhanced due to the salting out effect. Furthermore, Young’s modulus and toughness are important parameters of soft materials. Generally, the Young's modulus and toughness of soft artificial tissues are 10 to 1750 kPa and 0.1 to 12 MJ/m3, respectively40–43. Therefore, the PVA/GE/GL/NaCl gels were satisfied to manufacture artificial tissues because their suitable mechanical performance. To demonstrate effective energy dissipation, the loading-unloading curves of gels were compared in Figure 3c. PVA/GE/GL/NaCl gels showed a more obvious hysteresis loop than PVA/GE/GL gels, indicating that the increasing interaction between PVA and PVA, and between PVA and glycerin.



Figure 3. a) Tensile stress-strain curves of PVA/GE/GL and PVA/GE/GL/NaCl gels; b) corresponding elastic modulus and toughness; c) loading-unloading cycles of PVA/GE/GL, PVA/GE/GL/NaCl gels; d) corresponding dissipated fracture energy and total fracture energy. Moreover, the effect of salt contents and types on mechanical properties was also investigated. The same amount of salts (LiCl, NH4Cl, KCl, NaCl) was added into PVA/GE/GL gels. Because Li+, NH4+, K+ and Na+ had different salting out effects, resulting in different mechanical properties via the interaction of PVA and glycerin (Figure 4a, 4b). Subsequently, the mechanical properties were measured by changing the content of NaCl. The result showed that the mass of NaCl was lower than 6.8%, the mechanical properties increased with the increase of NaCl (Figure 4c, 4d). However, PVA would precipitate when the excessive NaCl (>6.8%) was added to gels. As a result, the optimal mechanical properties of the gels would be acquired after adding NaCl with mass of 6.8%. The same results were obtained by the rheological tests (Figure S2). In addition, the effect of other salts on the mechanical strength as also measured in Figure S3.


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Figure 4. Influence of salt contents and types on the mechanical properties of PVA/GE/GL gels: a) and c) stress-strain curves; b) and d) elastic modulus and toughness. Mechanical strength and remoldability The excellent mechanical properties of PVA/GE/GL/NaCl gels were demonstrated in Figure 5a, the gel tube with only 4 mm diameter could easily lift up a 2.5-kilogram autoclave. The gel pieces with holes, knots and distortions also could withstand substantial deformation (Figure 5b). Furthermore, the remoldability and reusability of gels also were investigated. In Figure 5c, the gels were cut into small pieces and putted them into a large needle tube. Then the needle tube was heated at 100 °C until gels melted completely and wrote Chinese character "water". The gels were formed again via a simple freeze-thaw cycle. After that, the LED bulb was connected to the separated gels and was not lit, then the gels were put together and the small bulb was lit. It was interesting that the gels could be remolded by freezing-thawing cycles and the mechanical properties remained essentially unchanged after remolding in Figure S4.



Figure 5. The PVA/GE/GL/NaCl gels presented extraordinary mechanical properties: a) lifting up an autoclave of ~2.5 kg; b) twisting, knotting and holing; c) remolding by heating, injecting and freezing-thawing cycles. Conductive properties The salt ions are introduced into hydrogels to provide electrical conductivity. Figure 6a showed that conductivity of PVA/GE/GL/NaCl gels increased with the increase of NaCl. The curve of resistance change ratios versus strain was revealed in Figure 6b. The resistance change ratio increased as the strain increased. Figure 6c illuminated that the gauge factor (GF) of gels increased with increasing strain, indicating high sensitivity of gels. Generally speaking, the pressure strength of human movements is in the range of 0-100 kPa44,45. It could be seen from the inset image of Figure 6c that gels had stable and high GF in the range of deformation of 0-73% (tensile strength 0 to 100 kPa). What’s more, gels were stretched by using cyclic uniaxial force at strain of 73% for 300 cycles, showing the resistance change ratios with good repeatability and stability. Thus, they could be used for long-time and


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multiple-time.

These

objective

results

suggested

that

strain

sensitive

PVA/GE/GL/NaCl gels with good repeatable and stable resistance change ratios could be used to fabricate biosensors for monitoring human movements.

Figure 6. Electromechanical properties of PVA/GE/GL/NaCl gels. a) the change of conductivity with the different contents of NaCl; b) resistance change ratios of gels versus strain. Insert: the change of corresponding resistance recorded on the smartphone via Bluetooth; c) Gauge factor of gels versus strain; d) resistance change ratios of gels under continuous stretching and releasing (from 0 to 73% strain, 300 cycles). Biosensor based on the gel The PVA/GE/GL/NaCl gels could be acted as biosensors to monitor human activities in real time. Figures 7a-7d recorded the corresponding resistance change ratios (ΔR/R0) when the gels were integrated into arm, finger, wrist and throat under different stimulus. When the arm and wrist were bent from 0° to 90°, resistance change ratios with good repeatability and stability could be produced (Figure 7a, 7c).



Even the speed of the bend in wrists changed, resistance change ratios also remained substantially state (Figure 7c). Moreover, multi-angle bending (0°, 30°, 90°, 120°) of fingers could also produce accurate and stable resistance change ratios (Figure 7b). It was worth mentioning that the gel biosensors were very sensitive to the subtle vibration of the vocal cords (Figure 7d). When the experimenter said the words, such as "China", "CCUT", "Hydrogel", "I Love You", the biosensors attaching to the human throat would generate corresponding signals which corresponded to the treble and bass areas of above words, subsequently these signals were converted into electrical signals of the corresponding frequency. Also, these electrical signals were consistent with the sound waves which were recorded simultaneously by the smartphone.

Figure 7. Resistance change ratio of the gel sensors attached to a) arm; b) finger; c) elbow; d) throat. Anti-freeze properties Conventional nature-inspired hydrogels froze at low temperature and lost elasticity, limiting their applications. Thus, it is necessary to investigate the anti-freezing ability


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of gels. PVA/GE, PVA/GE/GL and PVA/GE/GL/NaCl gels were compared in Figure 8. Firstly, they were placed in a -20 ºC refrigerator for 12 h and immediately taken out to test their mechanical properties. It was found that PVA/GE gels were completely frozen, and brittle fracture easily happened when bending (Figure 8a). Some ice crystals could be observed in PVA/GE/GL gels, while they still could be bent with force without arising break (Figure 8c). In contrast, there were no any ice crystals in the PVA/GE/GL/NaCl gels, which could be easily bent (Figure 8e). The freezing and thawing temperature of the gels was determined by DSC analysis. As shown in Figure 8b, the PVA/GE gels showed sharp peaks at -10 ºC and 0 ºC, which were belonging to freezing and thawing temperatures of gels. For PVA/GE/GL gels, narrower peaks appeared at -50.6 ºC and -19.6 ºC (Figure 8d). It was due to inhibiting crystallization of water by adding alcohols, which was responsible for freezing point of water drastically decreasing for PVA/GE/GL gels. Surprisingly, it could be seen from Figure 8f that the freezing and thawing temperatures of PVA/GE/GL/NaCl gels dropped to -63.7 ºC and -26.5 ºC, respectively, which was due to the common role of salt and glycerin.



Figure 8. a) anti-freeze capacity of PVA/GE gels; b) DSC thermograms of PVA/GE gels; c) anti-freeze capacity of PVA/GE/GL gels; d) DSC thermograms of PVA/GE/GL gels; e) anti-freeze capacity of PVA/GE/GL/NaCl gels; f) DSC thermograms of PVA/GE/GL/NaCl gels In order to demonstrate the conductivity of PVA/GE/GL/NaCl gels at the low temperature, the related experiments were conducted. The conductivity of the gel was measured ranging from -20 ºC to 25 ºC, and it was found that the gels still had good conductivity at -20 ºC. Besides, since the gels had good frost resistance and were stretchable at low temperature, they could be utilized as sensors to detect the human body motions in a sub-zero condition (Figure 9a, 9b). Moreover, the LED bulb was lit using gels as a wire in a -20 ºC refrigerator, then gels were prepared into a pen shape and a painting was successfully drawn on drawing app of the smart phone using gel pen (Figure 9c, 9d).


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Figure 9. a) The conductivity of PVA/GE/GL/NaCl gels ranging from -20 ºC to 25 ºC; b) resistance change ratios of the gel sensors integrated to the finger at -20 ºC; c) lightening the LED bulb by using PVA/GE/GL/NaCl gels as wires at -20 ºC; d) drawing a picture by using the PVA/GE/GL/NaCl gel pen.

Conclusion In summary, the skin-inspired PVA/GE/GL/NaCl gels were successfully prepared. The dermis-mimicking structure was formed through self-assembled by hydrogen bonds between PVA, gelatin and glycerin, which provided excellent mechanical properties. Additionally, the gels could maintain good flexibility and conductive performance even at -20 ºC owing to restraining the growth of ice crystals by adding glycerin and NaCl. In particular, a gel biosensor was manufactured for quickly, accurately and stably detecting the motion signals of the human body including both macro-scale and tiny human activities (such as fingers, arms, wrists, and speaking). In addition, the gels also had attractive remoldability and reusability. It was conceivable that the skin-inspired gels would present an unpredictable prospect in various fields,



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such as health recording electrodes, biosensors and wearable devices.

Supporting Information Supporting Information is available free of charge on the ACS Publication website: SEM image of the gels (S1); rheological test of the gels (S2); the mechanical strength of gels with different salts (S3); the mechanical properties of remodeled gels (S4).

Acknowledgements This research was supported by grants from National Natural Science Foundation of China (Nos. 51703012 and 51873024), Science and Technology Department of Jilin Province (No. 20180101207JC), and Jilin Provincial Development and Reform Commission (No. 2019C052-2).

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TOC Tough, anti-freezing, conductive and remoldable gels for strain sensing 82x44mm (300 x 300 DPI)


Skin-Inspired Gels with Toughness, Antifreezing, Conductivity, and

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