Conductive Organohydrogels with Ultrastretchability, Antifreezing, Self

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Applications of Polymer, Composite, and Coating Materials

Conductive organohydrogels with ultra-stretchability, anti-freezing, self-healing and adhesive properties for motion detection and signal transmission Yongqi Yang, Lin Guan, Xinyao Li, Zijian Gao, xiuyan ren, and Guanghui Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17440 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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ACS Applied Materials & Interfaces

Conductive organohydrogels with ultra-stretchability, anti-freezing, selfhealing and adhesive properties for motion detection and signal transmission Yongqi Yang, Lin Guan, Xinyao Li, Zijian Gao, 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 Authors: Xiuyan Ren and Guanghui Gao E-mail: [email protected]; [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 Conductive hydrogels had demonstrated significant prospect in the field of wearable devices. However, it was a huge limitation for hydrogels due to freezing after the temperature falls below zero. Here, a novel conductive organohydrogel was developed by introducing polyelectrolyte and glycerol into hydrogels. The gel exhibited excellent elongation, selfhealing and self-adhesive performance for various materials. Moreover, the gel could withstand the low temperature of -20°C for 24 h without freezing and still maintain good conductivity and self-healing properties. As a result, the sample could be applied for motion detection and signal transmission. For example, it can respond to finger movements and transmit network signals like network cables. Therefore, it was envisioned that the effective design strategy for conductive organohydrogel with anti-freezing, toughness, self-healing and self-adhesive properties would provide the wide applications of flexible wearable devices.

Keywords: organohydrogel; conductivity; anti-freezing; self-adhesive; flexible devices


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Introduction The emergence and development of wearable devices had provided new directions for many fields, especially artificial intelligence systems and wearable healthcare devices.1 Currently, skin-attached wearable devices are widely expected due to their ability to adhere well to the skin and enable health monitoring accurately. However, the wearable equipment is still limited to the conventional form, such as glasses. The key reason is that the critical devices cannot be flexible and stretchable, so that the device cannot be deformed and adhered to the skin. Therefore, flexible, stretchable, and conductive materials are highly expected in the wide applications.2–6 Hydrogels exhibits excellent flexibility and stretchability as promising candidates for wearable devices. Especially, conductive hydrogels could be applied for motion sensing. Currently, there are two main types of conductive hydrogels. One is to add intrinsically conductive materials to conventional hydrogels, such as metal nanowires7,8, carbon nanotubes9– 11,

graphene12–14, polypyrrole15,16, polyaniline17–19, and PEDOT/PSS20,21. Another approach is

to achieve the conductivity of the hydrogels using the conductive properties of ions. The hydrogels are usually prepared by using polyelectrolytes22–25 or adding salts (typically, lithium chloride)26–28. Since the tensile forces may cause the destruction of the hydrogels and future invalidate conductive hydrogel-based devices, it is necessary to design hydrogels with self-healing property. Current self-healing property is primarily achieved through dynamic covalent crosslinking and non-covalent crosslinking.29 The dynamic covalent crosslinking methods include Schiff bases30–33, oxime34–36, acylhydrazone37–39, disulfide bonds37,40,41, and borate esters34,42–44. Non-covalent crosslinking mainly relies on host-guest interactions45–48, hydrophobic association49,50, electrostatic interactions51–53, metal-ligand interactions54–56 and hydrogen bonding57–59. Polyelectrolyte-based hydrogels have rich positive and negative



charges inside, which impart wide dynamic crosslinking while imparting conductivity to the hydrogels. This feature provides self-healing properties that is helpful to extend the life of the wearable device. Conductive hydrogels need to be remained stable when used as devices in wearable equipment. For example, they should not lose water at room temperature or higher, otherwise its electrical conductivity will be changed. Current technology has been able to completely prevent water loss by wrapping elastomers (such as PDMS or EcoFlex) on the surface of the hydrogels.26,60 However, conductive hydrogels still have an inevitable defect: in cold environments, water in the hydrogels will be frozen, which will disable the conductive hydrogel-based devices. Therefore, a series of strategies for preventing the hydrogels from freezing at low temperatures are employed. Among them, the strategies of water-organic solvent mixture proved to be simple and effective. The mixed solvents, such as dimethyl sulfoxide61, ethylene glycol62,63, and glycerol10,64 with water or oil-water mixture65, can significantly reduce the freezing point, which may avoid the hydrogels from being frozen at low temperature. Especially, glycerol, as a non-toxic, highly viscous organic solvent, is commonly used in the manufacture of cosmetics, foods and medicines. In addition, glycerol is miscible with water in any ratio and can reduce the freezing point of the mixed solvent to a minimum of -46.5 °C 66, which is advantageous for preparing gels with anti-freeze properties and applying it to wearable devices. In this investigation, a series of gels were prepared by radical polymerization under a waterglycerol mixed solution with sodium methacrylate (MAANa) and [2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (DMC) as monomers. They were crosslinked by electrostatic interaction and disulfide bonds chemical crosslinking. The mechanical properties of gels had been studied, exhibiting significant tensile properties and cyclic tensile stability. The conductivity of the gels was investigated by an electrochemical workstation. Its electrical


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conductivity could be regulated by changes in glycerol content. They also exhibited antifreezing properties and would not be frozen even after freezing for 24 h at -20 °C. Conductivity at different temperatures had also been explored. These gels also had fast self-healing properties and high healing efficiency. The healing efficiency at different temperatures was also investigated. In addition, the adhesion properties of the gels had also been studied. They could be adhered tightly to the surface of a variety of materials, including skin, especially for Polytetrafluoroethylene (PTFE). Finally, the gels were as models to show functions of conductivity, motion sensing and signal transmitting, demonstrating potential applications in the field of wearable devices. It was believed that the design strategy of low temperature resistance conductive gels would extend the direction of flexible stretchable devices. Experimental section Materials: Acrylamide (AAm, 99.0%), sodium methacrylate (MAANa, 98%), [2(methacryloyloxy)ethyl] trimethyl ammonium chloride (DMC, 75% aqueous), potassium persulfate (KPS, 99.5%), and N,N,N′,N′-tetramethylethylenediamine (TMEDA, 99.5%) were supplied by Shanghai Aladdin Reagent Co. Ltd. Cystamine dihydrochloride was supplied by Sigma-Aldrich Co. Ltd. Glycerol and dimethyl sulfoxide (DMSO) were supplied by Sinopharm Chemical Reagent Co. Ltd. Deionized water (18.2 MΩ cm resistivity at 25 °C) was used in the experiment. Synthesis of N,N’-bis(acryloyl) cystamine (Cy): The synthesis process of disulfide bond-based chemical crosslinker was derived from previous work67. Cystamine dihydrochloride (2.25g, 10 mmol) was dissolved in a mixture of 15 mL of 3.5M NaOH and 10 mL chloroform. This solution was heated to 50 °C, and 5 mL of chloroform containing 20 mmol of acryloyl chloride was added dropwise under constant stirring over 15 min while the reaction temperature was maintained at 50°C. After separating the phases while still warm, the aqueous phase was discarded. The remaining organic phase was cooled down to room temperature, and the product



precipitated directly from the solution. The white solid was recovered by filtration and recrystallized from chloroform to give a white crystal product, yield 1.3g (50%). 1H NMR (400 MHz, DMSO-d6) δ ppm: 8.33 (2H, s, amide), 6.35-6.15 (2H, m, CH2=CH–), 6.15-6.00 (2H, m, CH2=CH–), 5.70-5.50 (2H, m, CH2=CH–), 3.43 (4H, t, -CH2-NH-, J=6.5Hz), 2.84 (4H, t, CH2NH-, J=6.8Hz). 13C NMR (100 MHz, DMSO-d6) δ 165.24, 132.07, 125.75, 38.46, 37.65. Preparation of gels: Ion30/AAm0/Water15 gel was prepared as follows: MAANa (30mM, 3.24g), DMC (30mM, 8.33g), were first dissolved in water (15 mL). Next, the DMSO solution of Cy (0.06mM, 15.6mg, dissolved in 50μL) and the KPS aqueous (0.3mM, 81mg, dissolved in 2mL) were added to the solution. Then, TMEDA (0.3 mM, 40.3 μL) was added to the previous solution and stirred for 30 s. The resulting solution was poured into molds and reacted at 35 °C for 2 h to obtain Ion30/AAm0/Water15 gels. Ion30/AAm0/Water10 gel was prepared as follows: MAANa (30mM, 3.24g), DMC (30mM, 8.33g), were first stirred with glycerol (5mL) to obtain viscous liquid with color of milky white. After that, water (10mL) was added and keep stirring until fully mixed. Ultrasonic vibration was used to remove bubbles to obtain a colorless, transparent solution. Next, the DMSO solution of Cy (0.06mM, 15.6mg, dissolved in 50μL) and the KPS aqueous (0.3mM, 81mg, dissolved in 500μL) were added to the solution. Then, TMEDA (0.3 mM, 40.3 μL) was added to the previous solution and stirred for 30 s. The resulting solution was poured into molds and reacted at 35 °C for 2 h to obtain Ion30/AAm0/Water10 gels. The preparation process was shown in Scheme 1. Ion30/AAm0/Water5 gel was prepared as the Ion30/AAm0/Water10 gel. The corresponding amount of glycerol was 10mL and water was 5mL. Ion30/AAm0/Water0 gel was prepared as Ion30/AAm0/Water15 gel and water was replaced by glycerol. The content of each component was shown in Table 1.


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Scheme 1. The preparation process of gels. Table 1. Recipes for the gels Samples

MAANa

DMC

H2O

Glycerol

Cy

KPS

TMEDA

Ion30/AAm0/Water15

30mM

30mM

15mL

0mL

0.06mM

0.3mM

0.3mM

Ion30/AAm0/Water10

30mM

30mM

10mL

5mL

0.06mM

0.3mM

0.3mM

Ion30/AAm0/Water5

30mM

30mM

5mL

10mL

0.06mM

0.3mM

0.3mM

Ion30/AAm0/Water0

30mM

30mM

0mL

15mL

0.06mM

0.3mM

0.3mM

Mechanical Tests: Tensile measurement was performed on a tensile tester (SHIMADZU, model AGS-X, 100N, Japan) at room temperature, with a constant velocity of 80 mm min−1. All gels were prepared as cylinders with length of 30 mm and diameter of 4.5 mm. For tensile loading−unloading cyclic tests, the gels samples were stretched to 1000% strain and then returned to the initial length with a constant velocity of 80 mm min−1. The cyclic tests were performed five times with no time interval between each cycle. Conductivity measurements: 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 voltage of the electrochemical workstation during test was set



to 0.1V. The conductivity was calculated from the formula σ=L/(S*R) where σ is the conductivity in S cm−1, L was the distance between each two electrodes, R was the resistance of the gels, and S was the cross-sectional area of the gels. All gels were prepared as cylinders with diameter of 4.5 mm. The distance between each two electrodes was set as 1.33cm. Anti-freeze test and measurement of conductivity at different temperature: The anti-freeze test was performed by placing the gels in a refrigerator (set at -20 °C) for 24 hours and observing whether they were frozen. Then took them with forceps to observe whether the gels remained soft. Further anti-freeze properties were tested by dynamic temperature-sweep measurements. The effect of temperature on conductivity was measured by placing the gels at room temperature (20 °C) as well as in refrigerators with 4 °C and -20 °C for 24 hours, respectively. Then maintain the temperature and measure their conductivity. The frequency of the current used to measure conductivity was 1 kHz. Rheological measurement: Rheology measurements were performed on a rheometer (Ares G2, TA). The test was carried out by dynamic temperature-sweep measurements with a range of 20 ° C to 20 ° C and the angular frequency was set as 10 rad/s. Self-healing test and measurement of self-healing efficiency at different temperatures: The selfhealing test was performed by cutting and healing the gels, then stretching the healed gels without retention time and comparing the stretched length. The self-healing time was 5s. The self-healing efficiency at different temperatures was measured by cutting and healing the gels at 20°C, 4°C and -20°C, respectively, and measure the conductivity before (σ0) and after(σ) healing under set temperatures. Self-healing efficiency was calculated by formula (σ/σ0)*100% and the frequency of the current used to measure conductivity was 1 kHz. Adhesive evaluation: The adhesive strength of gels on various material surfaces was measured by peeling measurements using a texture analyser (CT3-1000, USA) and 90° peeling mode.


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The samples were prepared with length of 80 mm, width of 20 mm, and thickness of 6 mm. As a stiff backing for gels, PET film was bonded onto gels with cyanoacrylate adhesive. The prepared samples were tested with the standard 90° peeling test with a constant peeling speed of 5 mm s−1 after 700 g of weight pressure for 30 min. The substrate materials included aluminium and PTFE. The interfacial toughness was determined by dividing the plateau force in the steady-state region of the peeling process by the width of the gel sheet. Demonstrations of conductive performance: The demonstrations of conductivity were performed by placing the gel in a circuit to replace a portion of the metal wire. In the demonstration of conductive performance at low temperature, Ion30/AAm0/Water10 and Ion0/AAm60/Water15 hydrogels were placed on two sets of copper sheets (20*30 mm), two pieces of copper sheets in each set, and the distance between the two pieces of sheets was 5mm. After the gels and the copper sheets were frozen together at -20 °C for 24 hours, wires and batteries (3.7 V lithium ion battery) were respectively connected and then observe the LED light. In the demonstration of 220V alternating current conductivity, Ion30/AAm0/Water10 gel was placed on two copper sheets (30*60 mm) with metal wires connected, and the distance between the copper sheets was 5mm. Connect a 220V 35W LED to the circuit and then connect the wires to 220V 50Hz AC power, continue to power on and observe the LED illumination. Demonstrations of motion sensing capabilities: The demonstration of motion sensing capability was divided into the following two parts: The first was to explore the resistance variations of Ion30/AAm0/Water10 gel against strain. A multimeter (OWON B35T, with Bluetooth and data logging function) for measuring resistance was attached to both ends of the Ion30/AAm0/Water10 gel and placed together on the tensile tester. The resistance variations rate was recorded on the multimeter during stretching, and the rate was tested in the range of 0-1000% strain. Next, the demonstration of limb motion sensing was carried out by attaching the multimeter to both ends of the gel and placing them on a finger. Connect the multimeter



and the smartphone via Bluetooth. When the finger bended, the resistance changed, and the corresponding data would be recorded on the smartphone. Demonstration of network signal transmission performance: Cut the network cable and connected a piece of copper sheet (15*20 mm) to each end of the wires. Fix them in a one-toone correspondence according to the color of the wires. The distance between two copper sheets in each set was 3mm. Then the Ion30/AAm0/Water10 gels were placed on each set of copper sheets to connect the wires, one end of the cable was connected to a computer and the other was to a router. Use the program "ping.exe" to check the connection between the computer and the router and open several web pages to check the network transmission performance. Results and discussions Mechanical properties These gels were prepared by the introduction of ionic polymers via electrostatic interaction and chemical crosslinking. The gels exhibited excellent tensile properties in Figure 1. Among them, Ion30/AAm0/Water10 could withstand an elongation of nearly 8000% (Figure 1a). This was mainly caused by the constant destruction and reformation of the electrostatic interaction in the gel during the stretching process, which dissipated a large amount of energy. It is worth noting that the fracture stress gradually increased from 6 kPa to 56 kPa and the elongation at break decreased from 8000% to 2000%, as the glycerol content increased in the gel. This is due to the fact that glycerol could form a large number of hydrogen bonds and the increasing crosslinking could enhance the fracture stress. However, the significantly increased crosslink density of hydrogen bonds limited the movement of molecular chains, inhibiting the elongation. Moreover, the gels exhibited excellent recovery properties. When the samples (Ion30/AAm0/Water10 or Ion30/AAm0/Water5) were stretched for 5 cycling times with a


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1000% deformation, the maximum stress did not change significantly in Figure 1b and 1c. The recovery performance was achieved by the addition of Cy as a chemical crosslinker, maintaining the stability of network structure during stretching without irreversible deformation. However, the maximum stress of Ion30/AAm0/Water0 was significantly reduced after continuous 5 stretching cycles, which was caused due to excessive glycerol in the gel (Figure 1d). According to the literature 68, we further analyzed the changes in the cyclic stress-strain. During the cyclic stretching process, most of the energy dissipation and residual strain occured in the first cycle. Ion30/AAm0/Water10 showed limited softening, and the change in the stressstrain curve in the subsequent cycles was negligible (Figure 1b). This was due to that the electrostatic crosslinking of the polyelectrolyte was broken during stretching in the first cycle, resulting in softening. Since the strength of the electrostatic interaction was weak, the softening was not obvious. Moreover, the electrostatic interaction was almost destroyed and recrosslinked after first stretching cycle and did not exhibit a gradual destruction characteristic. Thus, the stress-strain curve of the Ion30/AAm0/Water10 gel reached a steady state from the second cycle. Although the stress–stretch curves still exhibited hysteresis, but the hysteresis loops were much narrower than those in the beginning cycles. As the glycerol content increased, hydrogen bond crosslinking introduced by glycerol molecules became significant, which enhanced the mechanical properties of the gel. Since the strength of hydrogen bond crosslinking was strong, the softening became remarkable during cyclic stretching, and became remarkable as the glycerol content increased (Figure 1c and 1d). Further, hydrogen bond crosslinking was gradually resolved in multiple cycles of stretching, resulting in further softening and accumulated residual strain. Thus, Ion30/AAm0/Water0 exhibited a gradual softening during multiple cycles of stretching.



Figure 1. The (a) tensile tests and loading−unloading cyclic tests of (b) Ion30/AAm0/Water10, (c) Ion30/AAm0/Water5 and (d) Ion30/AAm0/Water0 gels. Ion30/AAm0/Water15 failed to perform these tests due to its fluidity. Conducive property The gels exhibited conductive properties based on ionic polymer networks, which released a large number of free-moving ions by ionization of water, thereby achieving electrical conductivity. The network of Ion30/AAm0/Water15 with completely free glycerol, was completely ionized and released the maximum concentration of free-moving ions. The strongest conductivity reached about 0.035 S/cm (Figure 2a). As the glycerol content increased, the conductivity showed a significant decrease and its downward trend was fitted to the curve


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in Figure 2b. This demonstrated that the addition of glycerol inhibited the ionization of polymer, resulting in a decrease of free ions in the gel. A similar phenomenon could be observed by reducing the concentration of ionic units in the gel (Figure S1). This property could be used to obtain a series of gels with different conductivity properties by controlling the glycerol content for different application environment. Another notable phenomenon was observed that the conductivity of the gels was not affected by the frequency of current, which was different from the phenomenon reported in the literature.25 This indicated that the conductive gels had the potential to be used as signal transmission materials.

Figure 2. (a) The conductivity at alternating currents with different frequencies of Ion30/AAm0/Water15,

Ion30/AAm0/Water10,

Ion30/AAm0/Water5

and

Ion30/AAm0/Water0 gels. (b) The conductivity with different glycerol contents at 1kHz. Anti-freezing property It was noticed that these gels would not be frozen in the environment of below zero degree. It was found from Figure 3a that the gel remained soft and stretchable even if it was stored at -20 °C for 24 h. To understand the anti-freezing principle of the gels, a series of glycerol-free gels with different concentrations of ionic monomer were investigated. As the concentration of ionic monomer was reduced, the gels were gradually frozen (Figure S2a). This indicated that



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the high concentration of ionic monomer in the gels could obstruct the freezing process. In addition, a series of gels without any ion units were also prepared and their anti-freezing properties were investigated by adjusting the content of glycerol. The results showed that the addition of glycerol caused the gels difficult to be frozen based on high ion concentration and the addition of glycerol in Figure S2b. In surprise, the gels exhibited the almost same intensity of electrical conductivity at different temperature in Figure 3b. The effect of temperature on the electrical conductivity could be compensated by an appropriate formula, indicating the ability to be used in the environment of below zero degree.

Figure

3.

(a)

The

flexibility

and

stretchability


of

Ion30/AAm0/Water15,

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Ion30/AAm0/Water10, Ion30/AAm0/Water5 and Ion30/AAm0/Water0 gels after freezing for 24h at -20°C. (b) The conductivity of Ion30/AAm0/Water10 gel at different temperatures. (c) Dynamic temperature-sweep measurements of Ion30/AAm0/Water15, Ion30/AAm0/Water10, Ion30/AAm0/Water5 and Ion30/AAm0/Water0 gels. To further study the anti-freezing performance of these gels, dynamic temperature-sweep measurements were performed (Figure 3c). The modulus of all the samples did not change significantly in the range of -20 °C to 20 °C, indicating that these samples were not frozen in the temperature range, showing significant anti-freezing performance. The test results showed that the storage modulus (G') and loss modulus (G'') of all gels gradually decreased with the increased temperature. This indicated that the molecular chains of all gels were relaxed with increased temperature. In addition, as the glycerol content increased, G' increased, indicating that glycerol could increase the crosslink density of the gel network. This was because that the glycerol molecule could easily form hydrogen bonds, resulting in an increased crosslinking density. The FT-IR measurement supported this result (Figure S3). It was noteworthy that the Ion30/AAm0/Water0 sample exhibited a phase transition region at temperatures below -7.7 °C. This was a zone that exhibited a transition from elastic stage to glass stage as the temperature decreased. This was also showed a relaxation of chain segments between entanglements. Despite this, Ion30/AAm0/Water0 was still soft in this state and still exhibited certain antifreezing performance. Self-healing property Due to the existence of electrostatic interaction in the gels, in general, gels could exhibit selfhealing properties. The experimental results supported this statement. As shown in Figure 4a, when the gels were cut, the contact surface was aligned at the fracture and the gels were stretched immediately. The results showed that these gels exhibited self-healing properties with



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varying degrees. The self-healing ability showed a trend of first increasing and then decreasing (Figure

4a).

Ion30/AAm0/Water10

had

the

best

self-healing

property

and

Ion30/AAm0/Water0 exhibited no self-healing property. The reason for this phenomenon could be attributed to the electrostatic interaction in the gels. An increase of the glycerol content inhibited the ionization of polyelectrolyte, resulting in a decrease of self-healing properties. The similar phenomenon was observed in Figure S4, when the ion unit content in the gel decreased. A small amount of glycerol did not have a significant effect on inhibition of ionization, and it was possible to construct crosslinking via hydrogen bonds. This indicated that the self-healing ability of the gels were determined by electrostatic interaction and hydrogen bonding. The addition of a small amount of glycerol could increase the hydrogen bond density, which was advantageous for improving the self-healing property. However, excessive glycerol inhibited the ionization of polyelectrolyte and significantly reduced selfhealing performance. Besides, the effect of temperature on self-healing efficiency had also been measured for Ion30/AAm0/Water10 at 20 °C, 4 °C and -20 °C, respectively. The selfhealing efficiency were measured by the change in conductivity after immediately contacting with the broken gels (Figure 4b). It was found that the higher the temperature, the higher the self-healing efficiency (Figure 4c). The self-healing efficiency could reach 96% at 20 °C and 94% at 4 °C. The self-healing efficiency at -20 °C was the lowest (85%). The decrease of temperature resulted in slow movement of molecular chains. Despite this, the gel still exhibited efficient self-healing properties. This allowed the gel devices with long service life and could easily recover the performance after being destroyed by external forces.


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Figure 4. (a) The self-healing properties of Ion30/AAm0/Water15, Ion30/AAm0/Water10, Ion30/AAm0/Water5 and Ion30/AAm0/Water0 gels at room temperature. (b) Photograph of self-healing efficiency test. (c) The self-healing efficiency of Ion30/AAm0/Water10 gel at different temperatures.



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Adhesive property These gels exhibited excellent adhesive behavior to various materials interfaces in Figure 5. Peeling force tests were measured at the surface of aluminum and polytetrafluoroethylene (PTFE) in Figure 5(a,b). The adhesive force exhibited a tendency of firstly increasing and then decreasing as the content of glycerol increased. Ion30/AAm0/Water5 exhibited the highest adhesive strength. To further determined the role of glycerol in adhesion properties, additional peeling force tests of the gel without glycerol were measured and the adhesive strength would exhibit a significant decrease in Figure S5. This indicated that the adhesion process was determined

by

electrostatic

interaction

and

hydrogen

bonding.

However,

in

Ion30/AAm0/Water0 gel, excessive hydrogen bonds lead to too high cohesive force and cause a significant decrease for adhesion properties. Moreover, the Ion30/AAm0/Water10 gel exhibited suitable adhesion properties and was capable of adhering tightly to a variety of interfaces in Figure 5c. In particular, its adhesion to PTFE allowed it to withstand a weight of 2.5 kg (Figure 5d). This excellent adhesion property facilitated the compounding of a gel with a variety of materials to prepare complex devices. In addition, it was also able to adhere closely to the skin and would not fall off due to movement, indicating its potential application for using in wearable devices (Figure 5e).


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Figure 5. (a) The peeling test of on Ion30/AAm0/Water10, Ion30/AAm0/Water5 and Ion30/AAm0/Water0 gels at aluminum surface and (b) PTFE surface. (c) Adhesive exhibition of

Ion30/AAm0/Water10

gel

adhering

to

diverse

materials

and

(d)

skin.

(e)

Ion30/AAm0/Water10 gel adhering and lifting 2.5 kg of weight at PTFE surfaces. Ion30/AAm0/Water15 failed to perform all of these tests due to its fluidity. Motion detection and signal transmission The gel (Ion30/AAm0/Water10) was used to explore the application as an electric conductor due to excellent conductive properties. Since the gel had anti-freezing properties, it could be used as a conductive gel material working in low temperature environments. Here, the conductivity of the gel in a cold environment was demonstrated. The Ion30/AAm0/Water10



gel still possessed the significant electrical conductivity at -20 °C and could illuminate the LED (Figure 6a). However, the acrylamide-based hydrogel Ion0/AAm60/Water15 had no conductivity and cannot illuminate the LED. (Figure 6b) This indicated that this gel had the ability to work at low temperatures, but conventional hydrogels cannot. Further, the ability of the gel to transfer high voltage currents had also been demonstrated. As shown in Figure 6c, the gel could be used to light up an LED lamp stably under 220V AC power (Movie S1), indicated that the gel could work stably under high voltage alternating current environment.

Figure

6.

Demonstration of conductivity of (a) Ion30/AAm0/Water10 and (b)

Ion0/AAm60/Water15. (c) Lighting up an LED light (220V, 50Hz, 35W) by using Ion30/AAm0/Water10 gel


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Subsequently, the relationship between resistance and elongation was determined. The resistance of the gel could be increased with stretching, exhibiting a smooth curve (Figure 7a), which allowed it to be used in fields such as motion sensing. Considering that the gel could be stretched multiple times under 1000% strain without obvious mechanical change. This gel enabled motion sensing and worked well up to 1000% strain. Thereafter, the sensing properties of the hydrogel on limb movement were measured in Figure 7b. As the finger moves, the electrical resistance of the gel undergone a significant change, exhibiting sensing performance. This change process could also be recognized and recorded by a smart phone in Figure 7(c,d), indicating that the gel could be applied as a motion sensor in wearable devices.

Figure 7. (a) The resistance variations of Ion30/AAm0/Water10 gel against strain. (b) The recorded resistance variations of the gel in response to the finger bending. (c) The principle of motion monitoring by wireless connection to send the sensing signals to the smartphone. d) The sensing performance of smartphone for detecting the finger bending via remote monitoring. The conductivity of the gel was stable at different current frequencies, indicating that it could be a good material for signal transmission. In order to verify its signal transmission capability,



the gel was used to replace a part of metal cables to transmit network signal in Figure 8. The results showed that the gels showed network transmission performance which was similar with the traditional network cables. The wired network was identified and worked steadily. Multiple web pages could be opened quickly, and the network latency was kept at low level (Movie S2). This indicated that the gel could be used as signal transmission carriers instead of conventional metal wires.

Figure 8. Photographs of transmitting wired network signals with the gel (the network latency data were tested by the program of ping.exe) Some studies have achieved the motion sensing by preparing gels or composites using glycerol or polyelectrolytes 69,70. Simultaneously, these studies demonstrated the ability to work at high temperatures and enabled temperature detection. In contrast, this work demonstrated good electrical properties in low temperature environments, which developed the soft materials for electronics at low temperatures. Besides, this conductive gel exhibited outstanding ultrastretchability, self-healing and adhesive properties. Its good motion sensing and signal transmission properties was suitable for many intelligent applications in extremely harsh


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environments. Conclusion In summary, conductive gels with ultra-stretchability, anti-freezing, self-healing and adhesive properties was successfully prepared by using water-glycerol mixed solvent. The gels exhibited excellent tensile properties and fatigue resistance. These gels exhibited excellent electrical conductivity and their conductivity could be controlled by adjusting the glycerol content. Importantly, the gels could maintain a flexible and stretchable state as well as good conductivity at low temperatures of -20 °C. These gels also had efficient self-healing properties and the healing efficiency was up to 96%. The excellent performance of adhesion properties allowed it to adhere tightly to a variety of materials including skin. It could be allowed to work stably in AC current up to the voltage of 220V without being destroyed. Moreover, it could also be used as motion sensing devices and signal transmission materials. As a result, the high stretchable, low temperature resistant, self-healing, high-adhesive conductive gel could be widely utilized in fields of artificial intelligence systems and wearable devices.

Supporting Information. Supporting Information is available free of charge on the ACS Publication website: Preparation of samples of control groups, conductivity and peeling test of hydrogels with different polyelectrolyte content, anti-freezing and self-healing properties of the control groups, FT-IR of polyelectrolyte gels, 1H and 13C NMR spectra of Cy and movies demonstrating conductive performance (Movie S1) and signal transmission performance (Movie S2). ACKNOWLEDGMENT 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 Education Department of Jilin Province (Nos. JJKH20191296KJ and JJKH20191306KJ).

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Conductive Organohydrogels with Ultrastretchability, Antifreezing, Self

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