Ultrastretchable and Stable Strain Sensors Based on Antifreezing and

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Ultrastretchable and Stable Strain Sensors Based on Anti-freezing and Self-healing Ionic Organohydrogels for Human Motion Monitoring Jin Wu, Zixuan Wu, Xing Lu, Songjia Han, Bo-Ru Yang, Xuchun Gui, Kai Tao, Jianmin Miao, and Chuan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20267 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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ACS Applied Materials & Interfaces

Ultrastretchable and Stable Strain Sensors Based on Anti-freezing and Self-healing Ionic Organohydrogels for Human Motion Monitoring Jin Wu*,1, Zixuan Wu1, Xing Lu1, Songjia Han1, Bo-Ru Yang1, Xuchun Gui1, Kai Tao*,2, Jianmin Miao3, and Chuan Liu*,1

1State

Key Laboratory of Optoelectronic Materials and Technologies and the Guangdong

Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China 2The

Ministry of Education Key Laboratory of Micro and Nano Systems for Aerospace,

Northwestern Polytechnical University, Xi'an, 710072, China 3School

of Mechanical and Aerospace Engineering, Nanyang Technological University,

Singapore 639798, Singapore

KEYWORDS: stretchable strain sensor, anti-freezing organohydrogels, self-healing, antidrying, human motion detection

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ABSTRACT Ionic hydrogels, a class of intrinsically stretchable and conductive materials, are widely used in soft electronics. However, the easy freezing and drying of water-based hydrogels significantly limit their long-term stability. Here, a facile solvent-replacement strategy is developed to fabricate ethylene glycol (Eg)/glycerol (Gl)-water binary anti-freezing and antidrying organohydrogels for ultrastretchable and sensitive strain sensing within a wide temperature range. Due to the ready formation of strong hydrogen bonds between Eg/Gl and water molecules, the organohydrogels gain exceptional freezing and drying tolerance with retained deformability, conductivity and self-healing ability even stay at extreme temperature for a long time. Thus, the fabricated strain sensor displays a gauge factor of 6, which is much higher than previously reported values for hydrogel-based strain sensors. Furthermore, the strain sensor exhibits a relatively wide strain range (0.5-950%) even at -18 °C. Various human motions with different strain levels are monitored by the strain sensor with good stability and repeatability from -18 to 25 °C. The organohydrogels maintained the strain sensing capability when exposed to ambient air for nine months. This work provides new insight into the fabrication of stable, ultrastretchable and ultrasensitive strain sensors using chemically modified organohydrogel for emerging wearable electronics.


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1. INTRODUCTION Electronic devices with stretchable, self-healing and transparent characteristics are highly demanded in the next-generation electronics, such as electronic skin (e-skin), touch panels, electrodes, wearable devices and soft robots.1-4 Soft robots can deal with uncertain tasks in various environments by integrating multiple sensing functionalities.1 Strain sensing is an important function of soft robots for their skin-like interaction with the real world. Furthermore, such stretchable strain sensors can compliantly conform to nonplanar and complex surfaces for monitoring of human/machine activities, personal health and surroundings.5-7 Traditional nanomaterials, such as low-dimensional carbon materials, metal nanowires and silicon nanoribbons, have been engineered on elastic substrates to fabricate stretchable strain sensors.2,

8-12

However, the structure engineering strategy imposes

complicated procedures to fabricate the sensors with limited stretchability (usually less than 200%), self-healing ability and device density.1, 8, 13 In contrast, the intrinsically stretchable materials provide a direct solution with simple fabrication process, low cost, high mechanical robustness and high density of devices.1, 14 In the past decade, soft materials with self-healing ability like human skin have drawn tremendous attention in modern flexible electronics for prolonged service life under harsh mechanical conditions.1, 15 In addition to e-skin, the concept of ionic skins has been proposed recently with increasing applications of ionic conductors such as ionic hydrogels and ionic liquid in human activity and health monitoring.16-18 Analogous to human skin, ionic skins also transduce signals using ions.16 Recently, ionic conductive hydrogels have been intensively studied for their applications in various ionotronic devices, including flexible sensors, artificial muscles, skins and axons, owing to their intrinsically high stretchability, ionic conductivity, self-healing ability and tunable electrical and mechanical properties.4,

7, 19-22

Especially, double networked (DN) hydrogels composed of a rigid network and a ductile matrix display enhanced mechanical robustness, strength and functionalities compared with 3 ACS Paragon Plus Environment


corresponding single networked counterpart.23 However, conventional water-based hydrogels inevitably freeze below subzero temperatures and thus become rigid, fragile and nonconductive.24-26 Therefore, the applications of hydrogel-based devices at low temperatures are inhibited. Even at room or higher temperatures, hydrogels inevitably dry out due to the water evaporation. The freezing and drying induced losses of flexibility, stretchability, conductance and other properties are two intrinsic problems of water-based hydrogels, which significantly impair their stability, durability and application scope of fabricated devices. Recently, some hydrogel-based strain and pressure sensors are reported with excellent stretchability.8, 16, 23, 27-30 However, the problems of easy freezing and drying still need to be resolved before the long-term reliable application can be realized.7, 31-32 For example, Sun and co-workers utilized a polyacrylamide (PAM) hydrogel to fabricate an ionic skin with strain and pressure sensitivities based on the capacitance change of sandwich structures upon deformation.16 Lee and co-workers employed polyvinyl alcohol (PVA) based self-healing hydrogels to fabricate extremely stretchable strain sensors with a gauge factor (GF) of 1.51.8 However, both the aforementioned hydrogel sensors need to be encapsulated in order to prevent the water evaporation. Even so, partial water evaporation was still inevitable during the long-term application.8 Furthermore, it is difficult for the strain sensors to work at subzero temperature due to the easy freezing of water in the hydrogels. In addition, previously reported hydrogel based strain sensors suffer from low sensitivity (GF ≤ 2.3).8 A superior strategy to the current encapsulation method is the development of intrinsically anti-freezing and anti-drying hydrogels. Recently, some endeavors have been made to enhance the freezing and drying tolerances of hydrogels by modifying either the solvent or ionic compounds.24-26, 31-34

However, the anti-freezing agents are largely introduced in reactors in the polymerization

step in previous works, which requires specific synthetic conditions to meet the demand of polymerization for different networks. Thus, a more universal and feasible route needs to be


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developed to fabricate anti-freezing and water-retention organohydrogels for the emerging applications, such as wearable strain sensor. Ethylene glycol (Eg) and Gl are well-known effective anti-freezing agents and humectants widely used in industry.24 They inhibit the freezing of water by forming strong hydrogen bonds with water molecules and meanwhile disrupting the hydrogen bonds between water molecules.24, 26 Notably, by controlling the Eg concentration in the water-Eg binary solution, the freezing point of solution can be decreased to as low as -40 °C, which is much lower than those of pure Eg and water.26, 35 Herein, anti-freezing and anti-drying PAM/carrageenan DN organohydrogel with Eg/Gl-water binary solvent are produced from corresponding DN hydrogel using a convenient solvent-replacement strategy for high-performance strain sensing application. The solvent-replacement strategy provides a convenient route to tune the concentration of Eg/Gl in the binary solution. The anti-freezing organohydrogels maintain their stretchability (up to 950% strain), conductance, self-healing ability and strain-sensitivity even at such low temperature as -18 °C. Thus, the strain sensors based on organohydrogels can monitor various human motions with high sensitivity (GF=6), wide strain range (0.5%400%), wide temperature range and excellent stability. 2. EXPERIMENTAL SECTION 2.1. Materials Eg and Gl were purchased from Aladdin. All other chemicals, including carrageenan, acrylamide (AM), KCl, N,N-methylenebisacrylamide (MBA) and ammonium persulfate (AP), were purchased from Sigma-Aldrich. 2.2. Synthesis of DN Hydrogels and Organohydrogels The PAM/carrageenan hydrogel was synthesized using a facile one-pot polymerization method. Specifically, the chemicals including 7.5 g AM, 1.5 g carrageenan, 0.005 g MBA and 0.09 g KCl were added into 41.0 g deionized water and magnetically stirred at 95 °C for five hours. After the system was cooled to 75 °C, 0.0375 g AP was added into the mixture and 5 ACS Paragon Plus Environment


stirred for 1 min. Subsequently, the solution was poured into a glass petri dish, stored at 6 °C for one hour, and heated at 95 °C for another hour. The hydrogel was formed after the system was cooled to room temperature, and therefore could be removed from the glass dish. A facile solvent-replacement approach was employed to fabricate the organohydrogels from the PAM/carrageenan hydrogel. Specifically, the as-synthesized hydrogel samples were soaked in 100 wt% Eg, 100 wt% Gl, 20 wt% Eg + 80 wt% H2O and 20 wt% Gl + 80 wt% H2O for one hour for the generation of corresponding organohydrogels. To make a comparison, the as-synthesized hydrogel sample was also soaked in 100 wt% H2O for the same time to generate corresponding water-based hydrogel. After the samples were removed from the solutions, the extra solvent on the surface of organohydrogels was removed with a filter paper. 2.3. Sensor Characterization and Test The organohydrogels were deployed as channel material to fabricate strain sensors using the two-electrode configuration. Conductive metallic wires were attached to the two ends of sensors and served as the electrodes. The resistance/conductance of sensors was monitored using a Keithley analyzer (Model 2400) after applying a fixed voltage of 3 V on the sensors. To apply desired strains to the sensor for the electromechanical test, the two ends of the samples were fixed on a home-built motorized stretching stage, which was controlled by a Zolix SC300-3A motion controller and corresponding software. For the low-temperature strain sensing, the electromechanical properties of the organohydrogels sensor were studied immediately when it was removed from the -18 °C environment. Similar setups were employed for human motions monitoring, thermal sensitivity and humidity sensitivity studies. The differential scanning calorimetry (DSC) spectra of samples were obtained on a DSC-204 F1 (Netzsch, Germany). 3. RESULTS AND DISCUSSION


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A modified one-pot method was exploited to synthesize the water-based PAM/carrageenan DN hydrogel. Different from previously reported photoinduced polymerization of PAM,20 here the polymerization of PAM is initiated by heating. AP and MBA were utilized as the thermal initiator and cross-linker respectively for the polymerization of PAM. The PAM and carrageenan were covalently and ionically cross-linked, respectively, generating the PAM/carrageenan DN hydrogel. Figure 1a illustrates the fabrication of organohydrogels from the corresponding DN hydrogel using a facile solvent-displacement process. Specifically, the synthesized water-based DN hydrogel was soaked in Eg/Gl solution directly for the replacement of water molecules with outside ubiquitous Eg/Gl molecules. Driven by the concentration difference, the molecule exchange proceeded rapidly until an equilibrium state was reached. As such, the original water-based hydrogel was changed to the organohydrogel with Eg/Gl-water binary solvent, which displayed boosted anti-freezing and anti-drying performances. To investigate the influence of solvents on the anti-freezing behavior of products, the same batch of hydrogels was immersed in five different solutions, including 100 wt% H2O, 100 wt% Eg, 100 wt% Gl, 20 wt% Eg + 80 wt% H2O and 20 wt% Gl + 80 wt% H2O, for the same time, producing corresponding samples marked as 1, 2, 3, 4 and 5#, respectively (Figure b-h). When these samples were placed at -18 °C for one hour, the waterbased hydrogel (1#) froze completely and thus could not withstand a stretching strain, while the other four unfrozen organohydrogel samples (2-5#) maintained their excellent stretchability (≥900% strain) due to the unfreezing state (Figure 1c-f). The freezing state of samples can also be verified by their color change after solvent replacement. Specifically, after one-hour cooling treatment, the original transparent hydrogel (1#) became white and opaque, indicating the frozen state. By contrast, the organohydrogels (2-5#) maintained the high transparency, suggesting the unfrozen state.



Figure 1. a) Schematic illustrating the synthesis of anti-freezing DN organohydrogels from DN hydrogel via a facile solvent-replacement strategy. The DN hydrogel froze at -18 °C, while the anti-freezing Eg- and Gl-DN organohydrogels maintained their stretchability at such low temperature as -18 °C. b) Photographs of five different samples obtained by soaking the same batch of hydrogels in five different solutions for one hour and placing them at -18 °C for one hour. The sample 1 froze and thus could not be stretched. c), d), e) and f) Photographs of samples 2, 3, 4 and 5# stretched to 950%, 950%, 900% and 950% strains, respectively. g) Digital image of the five samples obtained by placing them at -18 °C for six hours. h) Photograph shows the sample 3# in (g) can be stretched to 900% strain. The relatively anti-freezing capability of the organohydrogels was further investigated by storing the one-hour cooled organohydrogels (2-5#) at -18 °C for prolonged five hours (Figure 1g-h and Figure S1-2, Supporting Information). It was found that the samples soaked in pure Eg and Gl (2-3#) avoided freezing and thus could still be stretched to as large as 500% 8 ACS Paragon Plus Environment

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and 900% strains, respectively. By contrast, the samples soaked in low-concentration Eg and Gl solutions (4-5#) have already frozen and could not bear such deformations. This demonstrates that the concentration of Eg/Gl in binary solvent plays an important role in determining the anti-freezing property of generated organohydrogels. The lowest freezing point of Eg/Gl-waterbinary solution can be obtained when the weight percentages of Eg and Gl are around 70% and 66%, respectively.24-25, 35 Apparently, immersion of the water-based hydrogel in the solution with such low concentration as 20 wt% Eg/Gl + 80 wt% H2O cannot enable the concentration of Eg/Gl in organohydrogels to reach 70%/66%. By contrast, the exchange of water in the hydrogel with 100 wt% Eg/Gl can bring the concentration of Eg/Gl in the organohydrogels close to 70%/66%. Thus, the samples 2-3# exhibit better anti-freezing ability than the samples 4-5#. A series of photographs in Figure S2 (Supporting Information) show the samples 2-3# can still be stretched to different strains even after storage at -18 °C for six hours, demonstrating the excellent subzero tolerance. In fact, the samples 2-3# can maintain their flexibility even when staying at -18 °C for further prolonged time, such as 24 hours (Figure S3, Supporting Information). Although -18 °C is the lowest temperature provided by our current experimental setup, the organohydrogels may be able to avoid freezing at lower temperatures. Thus, the solvent replacement strategy provides a convenient platform to tune the freezing tolerance of organohydrogels by programming the concentration of anti-freezing agents. The remarkably improved anti-freezing property of produced organohydrogels also indicates that Eg/Gl has been successfully modified in the hydrogel via the solvent-replacement process. Furthermore, we have systematically investigated the anti-freezing properties of the hydrogels/organohydrogels by measuring the DSC spectra of samples, which were obtained by immersing the hydrogel in Eg/Gl aqueous solution with different Eg/Gl concentrations (Figure S4, Supporting Information). A sharp peak around -16.4 °C was observed on the DSC spectra of water-based Eg/Gl-DN hydrogel, corresponding to the freezing point of the Eg/Gl9 ACS Paragon Plus Environment


DN hydrogel.25 The lower freezing point of the Eg/Gl-DN hydrogel compared with that of pure water is mainly attributed to the existence of ionic compound (KCl).31 With the introduction of Eg/Gl, the peak positions shifted rapidly to the negative direction. For the organohydrogels obtained by placing the hydrogel in 20 wt% Eg and Gl, the peaks positions were located at -36.6 and -26.7 °C, respectively. Interestingly, for the organohydrogels prepared by immersing the hydrogel in 75 wt% Eg/Gl or above, no obvious peak was observed even when the temperature was decreased as low as -130 °C, which was also the lowest scanning temperature provided by our DSC system. It indicates the extraordinary antifreezing ability of the organohydrogels. After extracting the freezing points of the Eg/Gl-DN organohydrogels from the DSC curves, the phase diagrams of organohydrogels were constructed to reveal the relationship between the anti-freezing property and the ratio of Eg/Gl to water (Figure S5, Supporting Information). The enhanced anti-drying ability of organohydrogels is attributed to the key role of Eg/Gl in disrupting the formation of ice crystal lattices at subzero temperatures by forming strong hydrogen bonds with H2O molecules.24 The formation of hydrogen bonds between H2O molecules is also disrupted by Eg/Gl molecules.26 Water can exist in hydrogel in three different states: “free water”, “intermediate water” and “nonrotational bound water”.36 The free water freezes at normal freezing points and exchanges rapidly with Eg/Gl in the solventreplacement process. The intermediate water is loosely bound with DN polymer chains or other chemicals in the hydrogels, and thus freezes at lower temperatures than normal freezing points. The nonrotational bound water is also known as unfrozen bound water, which is bound to polymers or solvent with more than one hydrogen bond.36 The free water can be replaced by the anti-freezing Eg/Gl in the solvent-replacement process. Furthermore, some free water becomes intermediate or unfrozen bound water due to the interaction with Eg/Gl via forming hydrogen bonds. Therefore, the freezing point of organohydrogels is greatly lowered due to the significantly reduced content of free water. 10 ACS Paragon Plus Environment

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The excellent stretchability of organohydrogels is attributed to the noncovalent interactions such as dynamically dissociated and re-associated hydrogen bonds and double helices of carrageenan, which can function as sacrificial bonds to effectively dissipate energy before the breaking of covalent PAM chains during mechanical deformations.37 Some permanent deformation was observed after unloading of the large strain up to 950% or 900% strain, which may be attributed to the breakage of some polymeric chains at such large strains (Figure S6, Supporting Information). Notably, the solvent displacement also significantly increases the mechanical strength of organohydrogels. As shown in Figure S7 (Supporting Information), the elastic moduli of the DN hydrogel, Eg and Gl organohydrogels were calculated to be 247, 387 and 417 kPa, respectively, from the stress-strain curves over 0-75% strain. The enhanced mechanical robustness of organohydrogels in comparison with the DN hydrogel is resulted from the formation of hydrogen bonds between polymer chains and Eg/Gl, which may also assist in the bridge of polymer chains.24 A pronounced hysteresis loop was observed on the loading and unloading curves, reflecting the effective energy dissipation (Figure S7b, Supporting Information).38 In addition to the mechanical deformability, the organohydrogels maintain their selfhealing ability and conductivity after storage at subzero temperature or exposure to ambient air for a long time. The movement of K+ and Cl- ions both in water and Eg/Gl solvents contributes to the conductivity of the organohydrogels.7,

39

The electrical conductance and

self-healing properties of Eg/Gl organohydrogels at different states were investigated by connecting them in a circuit in series with a LED indicator (Figure 2). After the Eg/Gl organohydrogels were stored at -18 °C for six hours and then removed, they could still act as conductors to lighten the LED indicator (Figure 2a-f). When the Eg/Gl organohydrogels were bifurcated, reconnected at the broken surface, heated at 85 °C for 30 min, and then cooled to room temperature, the Eg/Gl organohydrogels were readily self-healed and therefore could be employed as conductors to lighten the LED indicator again. Notably, the self-healed Eg- and 11 ACS Paragon Plus Environment


Gl-DN organohydrogels could still withstand the strains as large as 150% and 170%, respectively, without losing their conductance and structural integrity, indicating the occurrence of self-repairing for both the mechanical and electrical characteristics. The selfhealing efficiency was further explored by measuring the conductance change of the organohydrogel before, during, and after self-healing (Figure S8, Supporting Information). The conductance decreased to nearly zero after cutting. However, it recovered to 81.8% initial level after self-healing at room temperature. Furthermore, the conductance was completely recovered after a healing-cooling cycle, indicating the high efficiency of the self-healing process. Importantly, the Eg-DN organohydrogel can still be exploited as a conductor to lighten the LED indicator in a circuit after exposure to ambient air for as long as nine months, revealing the excellent water retention capability (Figure 2g). After the organohydrogel was cut to two parts, the LED indicator extinguished. However, the LED indicator was lightened again immediately when the two bifurcated parts were brought to contact with each other at room temperature, indicating the self-healing of electrical property (Figure 2i). The conductivity of hydrogel changed very little after nine months passed. In Figure 2g-i, the brightness of LED is weaker when compared to Figure 2a-c, which is attributed to both the different voltages applied on the LED indicators and different kinds of LED indicators used in the self-healing tests. The efficient self-repairing is owing to the dynamic dissociation and reassociation of both hydrogen bonds and ions mediated double helical structures of carrageenan.26, 38 Because hydrogen bonds can dynamically dissociate and re-form at room temperature, the self-healing of electrical conductance can occur at room temperature.8 However, the self-healing of the mechanical property is mainly attributed to the dissociation of double helices at high temperature, followed by the re-association of double helices at low temperature. Thus, it requires a heating and cooling cycle to repair the broken surfaces. To evaluate the anti-drying capability of the organohydrogels, we further explored the anti-drying capability by measuring the weight loss of the water-based DN hydrogel and the Gl-DN 12 ACS Paragon Plus Environment

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organohydrogel under different temperature over time (Figure S9, Supporting Information). There was some weight loss observed for both the DN hydrogel and the Gl-DN organohydrogel with prolonged exposure time at high temperature (50-70 °C). The weight loss increased with temperature. However, the weight loss of the DN hydrogel was several times higher than that of the Gl-DN organohydrogel, indicating the significantly enhanced anti-drying capability by replacing water with Gl in the hydrogel.

Figure 2. The thawy organohydrogels after storing at -18 °C for six hours were connected in a circuit in series with an LED indicator to investigate their conductivity and self-healing ability. a-c) Photographs of the Eg-DN organohydrogel at different states: before cutting (a), bifurcated and then self-healed (b), and stretched at 156% strain after self-healing (c). d-f) Photographs of the Gl-DN organohydrogel: before cutting (d), self-healed (e), and stretched at 170% strain (f). g-i) Photographs of the Eg-DN organohydrogel (blue dye marked) exposed to ambient air for nine months at different states: before cutting (g), completely bifurcated (h), and after self-healing (i). 13 ACS Paragon Plus Environment


The

remarkable

anti-freezing,

anti-drying

and

self-healing

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attributes

of

the

organohydrogels (samples 2 and 3#) render them attractive for stable strain sensing within a broad temperature range. After the Eg-DN organohydrogel was stored at -18 °C for six hours, its strain-sensitivity maintained and thus could be employed for strain sensing at subzero temperature (e.g., -18 °C) (Figure 3). The electromechanical property of the Eg-DN organohydrogel sensor is evaluated by monitoring its relative resistance change ΔR/R0 (%) during deformation, where ΔR is the resistance variation relative to its original resistance R0. Figure 3a shows a typical relative resistance variation versus strain curves when different strains were applied. The resistance increased monotonically from 11.8 to 169.9 KΩ with increased strain up to 400%, which is the maximal strain provided by our home-built stretching stage (Figure S10, Supporting Information). The plot of response versus strain displayed two linear regions with different slopes, corresponding to the GF of 1.9 in the strain range of 0-250%, and the GF as large as 6 in the strain range of 250-400%, respectively. The GF exhibited by our strain sensor is 2.6 times higher than that of previously reported strain sensors based on other hydrogels.8,

34, 40

For example, Lee’s group developed single wall

carbon nanotube (SWCNT)/hydrogel based strain sensor with the highest GF of 1.51.8 Niu and co-workers reported organogel based strain sensor with the GF of 2.3.34 Although strain sensors based on inorganic nanomaterials such as graphene, CNT, silicon nanoribbon and metal nanostructures display high GF, their stretchability and self-healing ability are limited.6, 10, 41-47

In addition to the competitive stretchability and sensitivity, our organohydrogel strain

sensors provide additional advantages of self-healing capability and high transparency (Figure S2c, Supporting Information), an indication of very competitive overall sensing performance in comparison with previously reported stretchable strain sensors (Table S1, Supporting Information).8,

11, 16, 18, 24, 26, 34, 40-42, 48

The strain sensing performance of the Eg-DN

organohydrogel was compared before and after self-healing (Figure S11, Supporting Information). The self-healed Eg-DN organohydrogel still displayed clear response to 14 ACS Paragon Plus Environment

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temperature variation. Furthermore, no appreciable change in GF was found after cutting and self-healing, further verifying the high healing efficiency. Particularly, our strain sensor can experimentally detect extremely small strains e.g., 0.5%2%, which are generally too small to be detected by the hydrogel-based strain sensors reported in the literatures (Figure 3b).34 Furthermore, our sensor displayed a high GF of 2.2 in the small strain range of 0.5-2%, which is even higher than that of the conventional metal strain gauge (2.0) (Figure S12, Supporting Information).2 In addition to small strain, the sensor also showed high sensitivity and good repeatability in the detection of moderate and high strains (Figure 3c-d). The high sensitivities at both large and small strains coupled with the extremely wide detection range of strain enable the strain sensor to detect almost the full range of human activities.2 It's worth noting that the sensor generated stable output signal during the loading and unloading of 40% strain for as many as 310 cycles (Figure 3d-g). Only small response variations were observed in the repeated sensing process, demonstrating the excellent stability, repeatability and durability. The stable and reproducible response was also observed in the detection of small strain of 20% in repeated 65 cycles (Figure S13, Supporting Information). Furthermore, this sensor displayed the relative resistance change of 317.7% to the large strain of 300% with a standard deviation (SD) as small as 4.4% in repeated 11 cycles, which confirmed the good stability of this kind of strain sensor (Figure 3c and Figure S14, Supporting Information). Generally, the sensor displayed a very small hysteresis in the detection of such large strain as 300% (Figure S15, Supporting Information). However, a minor hysteresis was observed in the small strain range (50-100%), which may be attributed to the viscoelastic nature of polymers and the interaction between K+ and carrageenan.49 The strain-sensitivity of the organohydrogel is mainly attributed to the dimension change induced resistance variation at certain strain range (0-250% in Figure 3a).50 The resistance of a rectangular organohydrogel slice can be described by this function: R = ρ×L/S, in which R, ρ, L and S are the resistance, resistivity, length and cross-sectional area of the organohydrogel 15 ACS Paragon Plus Environment


slice, respectively.50 Hence, the resistance of the strain sensor is proportional to the length and inversely proportional to the cross-sectional area. In addition to the geometrical effect, the piezoresistive effect should also be considered at large strains (250-900% in Figure 3a).8, 49 At very large strains, the breakage of some polymeric chains leads to the occurrence of permanent deformation and the change in ionic concentration, which also contributes to the resistance variation. This can explain the occurrence of two-linear ranges in the response versus strain curve in Figure 3a. The resistance of the sensor is around 10 KΩ at 0% strain, which is higher than that of the sensors based on carbon materials and metal nanowires but lower than that of the sensors made of ionic liquid.12, 18 The organohydrogel slice displayed clearly increased length and decreased cross-section area upon stretching, which led to an increased resistance (Figure S2, Supporting Information). This organohydrogel strain sensor provides distinct merits of low temperature tolerance, ultrasensitivity, wide detection range, good stability and flexibility, which are advantageous for practical applications, especially for low-temperature human motion monitoring, as discussed below.


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Figure 3. Electromechanical properties of the Eg-DN organohydrogel sensor at -18 °C. a) Plot of the relative resistance variation (ΔR/R0) versus strain at -18 °C. Inset: photograph of the organohydrogel at 400% strain. b) and c) Real-time responses of the organohydrogel to repeated small (0.5-2%) and large strains (300%), respectively. d) Response of the organohydrogel to repeated loading and unloading of 40% strain for 310 cycles, showing the good stability of this strain sensor. e), f) and g) Selected two sensing cycles in (d) around 14, 276 and 1436 s, respectively. The anti-freezing and flexible strain sensor can be conveniently attached on human body to monitor various human motions from small scale (e.g., finger typing) to large scale (e.g., knee joint bending) at -18 °C (Figure 4). Figure 4a shows the response to the repeated bending of a forefinger with the frequency of 1/3 HZ. The resistance increased rapidly and 17 ACS Paragon Plus Environment


sharply upon forefinger bending, which is attributed to the stretching of the organohydrogel together with the finger skin. The signal recovered rapidly and completely upon the relaxation of the finger. The sensors displayed stable and reproducible response in the cycling test with a low noise level. The response/recovery time (t90) of the sensors was defined as the time taken to achieve 90% of the equilibrium point.51-52 In this way, the response and recovery time was calculated to be 140 and 100 ms, respectively, from the dynamic response curve within one cycle, indicating the fast response and recovery speeds (Figure 4b). Due to the sufficient sensitivity, this sensor can detect the tiny finger movement of typing on a keyboard (Figure 4c). Each press on the keyboard produced a clear response peak. The minor inconsistence in the response signals in different cycles is attributed to the slight trembling of the finger when typing different words. Additionally, the sensor is capable of detecting large-scale motion such as repeated bending of the knee joint with pretty stable and repeatable response (Figure 4d). The large-scale motion produced much higher signal level than the small-scale motion, indicating the capability of the sensor to distinguish the human motions with different scales.

Figure 4. Human motions monitoring using the organohydrogel strain sensor stored at -18 °C for six hours. a) Real-time response of the sensor to the bending of a forefinger. b) Analyses of the response and recovery time t90 when the forefinger bending was monitored. c) 18 ACS Paragon Plus Environment

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Response signal to the typing with the forefinger on a keyboard. d) Response of the wearable sensor to repeated squats. Insets in (a), (c) and (d) showed the sensors were directly attached on forefinger and knee for monitoring of corresponding motions. Notably, the strain sensors maintain their sensing functionalities, conductivity and mechanical deformability when exposed to ambient air (25 °C, 70% relative humidity (RH)) for as long as nine months, demonstrating the excellent anti-drying capability (Figure 5). The moisture holding capability of the organohydrogel is attributed to the hygroscopic property of Eg, which has a high boiling point of 198 °C at 1 atm and low volatility.34,

53

Eg/Gl can

readily form hydrogen bonds with water molecules, which prevents the water evaporation. Due to remarkable anti-drying capability of the Eg-DN organohydrogel, the strain sensor did not show appreciable degradation in the sensitivity after nine months passed, as shown in the comparative study (Figure S16, Supporting Information). The Eg-DN organohydrogel strain sensor placed in ambient air for nine months can still be stretched to 400% strain and twisted 360° without structural damage (Figure 5a-d). After nine months passed, the strain sensor can still detect multiscale strains with high sensitivity and good reproducibility at room temperature, demonstrating the long-term stability (Figure 5e-f). Specifically, a high GF of 4.5 was displayed by the sensor in the strain range of 200-400%. The response increased monotonically with increased strain from 0% to 260%. Furthermore, very small response variations were observed in the cycling test of detecting different strains, indicating the good stability (Figure 5f). As the stretchability, stability and strain-sensitivity of the sensor were retained after nine months, it can be utilized to monitor various human motions, such as handshaking, bending of elbow joint, finger and wrist, making a fist, etc. (Figure 5g-j and Figure S17, Supporting Information). As shown in Figure 5g, the strain sensor provides noticeable and repeatable response when the wearer shook hands with other people. The response increased monotonically with increased bending angles in the detection of the bending of elbow joint and finger, demonstrating the capability of the strain sensor to 19 ACS Paragon Plus Environment


distinguish different motion range of joints (Figure 5h-j). The signal produced by the flexion of large joint is higher than that generated by the movement of small joint, suggesting the ability of the sensor to discriminate the movement of different joints. Finally, as the temperature and humidity of human skin varied a lot to the environment, the dependences of the conductance and sensitivity of the hydrogel strain sensors on the environmental temperature and humidity are investigated.54 As shown in Figure S18 (Supporting Information), the sensor also exhibits noticeable responses to both temperature and humidity variations, although the responses are small when compared with the signal level generated by large strains. Furthermore, the Eg-DN organohydrogel strain sensors displayed well-defined response curves at different humidity and temperature with a small sensitivity variation (Figure S19, Supporting Information). The slightly decreased sensitivity with reduced environmental humidity and elevated temperature may be attributed to the dehydration induced concentration changes of ions and polymer chains in the organohydrogel.21 In practical strain sensing applications, to eliminate the influence of humidity, the organohydrogel can be encapsulated by a water-proof film or subjected to surface hydrophobic treatment to inhibit the adsorption and desorption of gaseous water molecules on the organohydrogel.55 To preclude the influence of temperature, a straininsensitive temperature sensor can be integrated to calibrate the strain sensor.50 Otherwise, it indicates the possible applications of the organohydrogel as wearable thermistor and humidity sensor. In addition to humidity and temperature, the organohydrogel sensor also exhibited small response to pressure (Figure S20, Supporting Information). No appreciable resistance change was observed when a 100 g object was placed on the organohydrogel. When the weight of object increased to 200 g, a small relative resistance change of 5.2% was observed, indicating the selectivity of the strain sensor.


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Figure 5. Room-temperature strain sensing characteristics of the anti-drying Eg-DN organohydrogel after exposure to ambient air for nine months. a-d) Photographs of the Eg-DN organohydrogel at different mechanical deformations, including relaxed state (a), 100% strain (b), 180% strain (c) and 360° rotation (d). e) Plot of relative resistance variation versus strain. A GF value of 4.5 was extracted from the linear fitting within 200-400% strains. f) Dynamic responses to a wide range of strains, including 45%, 85%, 150% and 260% strains, with five sensing cycles for each strain. g) Response signals of this sensor to repeated handshaking. h) and i) Real-time responses of the sensor to the bending of the elbow joint and the middle finger, respectively, with different flexion angles. j) Plots of the response versus bending angle of the elbow joint and finger. Insets in (g), (h) and (i): photographs of the motions of handshaking, elbow and middle finger, respectively. 4. CONCLUSIONS In summary, a facile solvent-replacement strategy is developed to fabricate anti-freezing and anti-drying PAM/carrageenan organohydrogels for high-performance strain sensing. Because 21 ACS Paragon Plus Environment


of the ready formation of strong hydrogen bonds between water and Eg/Gl molecules, the organohydrogels can maintain their deformability, self-healing ability and conductivity at subzero temperature or exposed to ambient air for a long time. These features endow the organohydrogel-based strain sensors with distinct advantages, including high sensitivity, ultrastretchability, self-healing ability, transparency, broad working temperature range and wide strain detection range. Specifically, this strain sensor can withstand 950% strain even at -18 °C with a GF of 6, which is 2.6 times higher than that of state-of-the-art hydrogel-based strain sensors. Notably, this sensor can detect multiscale strains from the small strain of 0.5% to the large strain of 400%. Due to the high sensitivity and flexibility, this strain sensor can be conformably attached on human body to detect various human motions with exceptional stability within a broad temperature range. Both tiny motion of finger typing and large movement of knee joint can be monitored with good reliability. Although only one kind of hydrogel is studied here, this general, simple and effective solvent-replacement strategy may be applicable to other hydrogels as well for the improvement of their properties. Given the above attributes, the organohydrogels based strain sensors hold great promise for a wide range of applications in soft robots, wearable electronics, e-skin, health monitoring, etc.10, 56

ASSOCIATED CONTENT Supporting Information Photographs showing the anti-freezing Eg-DN and Gl-DN organohydrogel placed at -18 °C for six hours can still be stretched to different strains; investigation of the transparency of the organohydrogel, photographs showing the excellent anti-freezing capability of the Eg- and Gl-DN organohydrogels; DSC spectra and phase diagrams of the Eg/Gl-DN organohydrogels with different Eg/Gl content; the mechanical reversibility of the hydrogel after a large strain is applied; typical tensile stress-strain relationship of the hydrogel and organohydrogels; plot of the relative conductance change of the organohydrogel before cutting, after cutting, initial 22 ACS Paragon Plus Environment

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healing and complete self-healing; plot of the weight losses of the hydrogel and organohydrogel versus time under different temperature; resistance variation of the Eg-DN organohydrogel sensor versus strain; comparison of the overall performances of state-of-theart stretchable strain sensors; comparison of the sensing performance of the Eg-DN organohydrogel before and after self-healing; relative resistance variation versus strain in the detection of small strains at -18 °C; response of the sensor to repeated loading and unloading of 20% strain for 65 cycles; the quantitative response of the Eg-DN sensor versus experimental cycle in the detection of 300% strain repeatedly; hysteresis of the Eg-DN organohydrogel-based strain sensor; comparison of the sensing performance of the Eg-DN organohydrogel before and after nine months; the wearable Eg-DN organohydrogel sensor was attached on human body for human motion detection at room temperature; dependences of the relative conductance change and sensing performance of the organohydrogel strain sensor on temperature and relative humidity; dynamic response of the organohydrogel to the loading and unloading of a 200 g glass slide on the organohydrogel. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX.

AUTHOR INFORMATION Corresponding Authors *Emails: [email protected] or [email protected] or [email protected]

Notes No competing financial interests have been declared.

Acknowledgements



J.W. acknowledges financial support from the National Natural Science Foundation of China, (61801525), the Guangdong Natural Science Funds Grant (2018A030313400) and the Guangzhou Science and Technology Project.

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TABLE OF CONTENT FIGURE


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Ultrastretchable and Stable Strain Sensors Based on Antifreezing and

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