Multiple Weak H-Bonds Lead to Highly Sensitive, Stretchable, Self

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Biological and Medical Applications of Materials and Interfaces

Multiple Weak H-Bonds Lead to Highly Sensitive, Stretchable, Self-Adhesive and Self-Healing Ionic Sensors Haiyan Qiao, Pengfei Qi, Xiaohui Zhang, Linan Wang, Yeqiang Tan, Zhaohui Luan, Yanzhi Xia, Yan-Hui Li, and Kunyan Sui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20380 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Applied Materials & Interfaces

Multiple Weak H-Bonds Lead to Highly Sensitive, Stretchable, Self-Adhesive and Self-Healing Ionic Sensors Haiyan Qiao,‡ Pengfei Qi,‡ Xiaohui Zhang, Linan Wang, Yeqiang Tan, Zhaohui Luan, Yanzhi Xia, Yanhui Li, and Kunyan Sui*

State Key Laboratory of Bio-fibers and Eco-textiles, College of Materials Science and Engineering, Shandong Collaborative Innovation Center of Marine Biobased Fibers and Ecological textiles, Institute of Marine Biobased Materials, Qingdao University, Qingdao 266071, China.

KEYWORDS: weak hydrogen bonds, sensitivity, self-adhesiveness, self-healing, ionic sensors

ABSTRACT: Herein, we demonstrate that a ternary ionic hydrogel sensor consisting of tannic acid (TA), sodium alginate (SA), and covalent cross-linked polyacrylamide (PAM) as skinmountable and wearable sensors. Based on the multiple weak H-bonds and synergistic effects between the three components, the as-prepared hybrid hydrogel exhibits ultra-stretchability with high elasticity, good self-healing, excellent conformability, and high self-adhesiveness to diverse substrates both in air and underwater. More importantly, the ternary hydrogel exhibits high strain sensitivity especially under subtle strains with a gauge factor of 2.0, which is close to the theoretical value of the ionic hydrogel sensors; an extremely large workable range of strain

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(0.05%-2100%); and a low operating voltage 0.07 V. Consequently, the sensor demonstrates superior sensing performance for real-time monitoring of the large and subtle human motions, including limb motions, swallowing, smiling and wrist pulse. Therefore, it is believed that the STP hydrogel has a great potential applications in health monitoring, smart wearable devices, and soft robots.

1. INTRODUCTION Stretchable and wearable electronics have grown explosively over the last decade because of their great potential applications in various areas, such as human-machine interfaces, healthcare monitoring and soft robots.1-4 As a wearable sensor, several minimum combinations are required: high sensitivity, high stretchability, good adhesiveness conformably to arbitrary surface, selfhealing, low power consumption, biocompatibility and so on.5-8 Tremendous efforts have been devoted to achieve these performances integrated into stretchable wearable electronics.9-11 Current existing stretchable electronics mostly comprise electrical conducting materials with insulating elastomeric substrates.12-15 The electronic conductors achieved high sensitivity at small strains, but failed to monitor large strains due to the percolation networks, resulting in a narrow workable strain range. In addition, poor conformability, poor biocompatibility and delamination of the electronic conductors pose great challenges in applying electrical conductors in specific applications, such as detecting various biological signals of human skin or soft tissue organs.16,17 Most recently, flexible and stretchable ionic hydrogel conductors have been developed as strain sensors, which enable the devices to achieve high transparency, stretchability and biocompatibility. 18-24 Moreover, self-adhesive and self-healable ionic hydrogels were designed in the last few years.25-32 However, ionic hydrogel strain sensors suffer from low


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sensitivities to monitor subtle strains and signal hysteresis owing to their inelastic/viscoelastic property.15,21,29-33 Nevertheless, it remains a grand challenge in achieving a combination of the above mentioned critical features for new generation of flexible sensors. It is noted that gauge factor (GF), a strain sensitivity factor of a sensor, is defined as GF= (ΔR/R0)/ε, where R0 is the resistance at 0% strain, ΔR is the relative resistance variation upon stretch, and ε is the applied strain.11,24,34 In the ideal case, which is the volume of the material do not change upon extern stretching, GF can be expressed by GF=ε+2 (Equation S1).24 Based on this, we can infer that the strain sensor could achieve high sensitivity with high GF if the sensor exhibits ideal elastomer properties. On the other hand, it is demonstrated that weak H-bonds in the materials can dynamically break and reassociate upon stretch, thus endowing the hydrogel with elastomer properties and self-healing property. 35-37 In light of this hypothesis, we have designed a ternary hybrid ionic conductor that integrate high sensitivity, high stretchability, excellent elasticity, self-healing and self-adhesiveness. The ionic hydrogel strain sensor is comprised of sodium alginate (SA), tannic acid (TA), and chemically cross-linked polyacrylamide (PAM) (denoted as STP) with the following rationale: i) Multiple weak dynamic networks formed between the three contribute to the hydrogel with high elasticity, high stretchability, low modulus, and selfhealing properties. ii) SA, which is a biodegradable and biocompatible natural polyelectrolyte, provides large amount conductive ions to ensure high sensitivity of the ionic hydrogel. 38-40 iii) TA, which is a natural available polyphenolic compound having numerous functional catechol and pyrogallol groups endows the good self-adhesiveness.41-43 This strategy has several advantages, including: i) facile one-pot method and low cost. ii) tunable properties in a wide range. iii) biocompatibility and nontoxicity. 38,41,44 The resulting ionic conductor displayed high sensitivity with a gauge factor of 2.0 close to the theoretical value of the hydrogel at small strains, extremely


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low operating voltage, a broad strain window, strong self-adhesiveness and self-healing properties. As a result, the sensor can achieve real-time monitoring the large limb motions and subtle muscle movements of human body with high sensitivity and good repeatability. 2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Sodium alginate (SA) was purchased from Qingdao Hyzlin Biology Development Co., Ltd. The molecular weight Mw = 3.15 ×105, Mw/Mn = 2.55, G:M =1:1. The SA powder was dried and kept in a desiccator to avoid moisture absorption prior to use. Tannic acid (TA), acrylamide (AM), ammonium persulfate (APS), N,N'-methylenebisacrylamide (MBAA) and N,N,N',N'-tetramethylethylenediamine (TEMED) are commercially available from Aladdin and used without additional treatment. 2.2. Preparation of the STP Hydrogels. All the hydrogels were synthesized by one-pot method. SA solution was prepared by dissolving SA powder in deionized water at room temperature under vigorous stirring for 4 h and then heated in 50 ℃ water bath for 0.5 h until the homogeneous solution was formed. TA solution (0-0.10 wt%) was then added into SA solution under vigorous stirring for 2 h. Subsequently, AM was added to the SA/TA precursor solution and stirred for 2 h, where the concentrations of SA and AM were fixed at 2 wt% and 20 wt%, respectively. APS, MBAA, and TEMED were then added to the above solution and stirring for 10 min in an ice bath. The precursor solution was injected into a plastic tube (diameter D = 5.0 mm) and a self-prepared glass mold (length × width × thickness = 170.0 mm × 100.0 mm × 2.0 mm), respectively. Then, the samples were placed at 50 ℃ for 3 h to afford the final hydrogel.


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2.3. Characterizations. Fourier transform infrared spectrum of SA, TA, SA/TA solution and STP hydrogel (CSA = 2 wt%, CTA = 0.10 wt %, CPAM =20%) over wavenumber range of 4000 cm-1 and 400 cm-1 were obtained at room temperature (Nicolet 5700, Thermo, Waltham, MA, USA). 2.4. Mechanical Tests. The tensile and compression tests were performed using a universal test machine (WDW-5T, China) equipped with a 250 N load cell for tension and 2000 N load cell for compression. The specimens with a diameter of 5 mm were subjected to tensile testing with the loading rate of 100 mm/min. The cylindrical samples with a diameter of 15 mm and a height of 12 mm were used for compression tests with the rate of 5 mm/min. The compressive strain was defined as h/h0, where h and h0 are the height under compression and the original height. The tensile loading-unloading test was performed for 10 times with the loading rate of 100 mm/min, and the compressive loading-unloading test was also performed for 10 times with the loading rate of 5 mm/min. The tensile strength and strain was obtained from the failure point, and the elastic modulus was determined by the average slope over 10-30% of elongation according to the stressstrain curve. The toughness was defined according to the area under a stress-strain curve by integration. The energy dissipation was calculated based on the area of the loading-unloading cycles. 2.5. Self-healing Tests. The microscopic morphology of the hydrogel with a scratch about 100 μm was observed using optical microscope. Rheological measurements were performed on a rotational rheometer (MCR 301, Anton Paar, Australia) with a gap of 1 mm and a parallel plate geometry of 25 mm in diameter. The samples were 25 mm in diameter and 1 mm in thickness. Rheology test was performed to study the collapse and reformation of the hydrogel network by setting alternant oscillation strains of 1% and 2000% at a fixed angular frequency of 10 rad/sec. The time for healing was fixed at 80 sec/cycle. The self-healing property was quantitatively


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evaluated by a tensile-heal-tensile test. The hydrogel was stretched into two halves, and then the two halves contacted again and self-healed for different time. 2.6. Tensile Adhesion Tests. Tensile adhesion test was conducted to measure the adhesive ability of the ternary gel to the surface of various substrates with a bonding area of 25.0 mm×25.0 mm×1.0 mm by the universal testing machine. The adhesion strength was quantitatively calculated by the maximum load divided by the initial bonded area. The tensile adhesion strength was also further confirmed under water. Different substrates were chosen to verify the adhesiveness including polyethylene, glass, aluminium and porcine skin, where the porcine skin was used to mimic the adhesiveness of hydrogel to human skin tissues. Adhesion-strip cyclic tests were also conducted to evaluate the repeatability. 2.7. Electrical Tests. A strain sensor was assembled using the ternary STP hydrogel. The electrical signals of the hydrogel sensors were accounted by a digital source-meter (Keithley 2450). The relative change of the resistance is calculated by the equation: ∆R/R0 (%) = (R-R0)/R0 (%), where R0 and R are the resistance without and with applied strain, respectively. The gauge factor (GF) defined as GF = (∆R/R0)/ε, where ∆R/R0 is the relative change of resistance and ε is the applied strain. 2.8. Cytotoxicity assay: The cytotoxicity of STP hydrogel materials was tested using the MTT colorimetric method.45 The hydrogel was sterilized and then placed in a 1640 culture medium without FBS to prepare the extract by at 37 ℃ in a 5% CO2 atmosphere for 24 h. The L929 cells were resuscitated and suspended in the culture medium, seeded on 96-well plates (1000 cells/well) and incubated at 37 ℃ in a 5 % CO2 atmosphere for 24 h. The medium was then replaced by the extract, using the culture medium itself as a control. After incubating for 24, 48 and 72 h, the cells


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were treated with MTT of 20 μL/well (5 mg/mL in PBS filtered for sterilization) and incubated for 4 h at 37 ℃ in a 5% CO2 atmosphere. After that the MTT was removed, dimethyl sulfoxide (DMSO) of 200 μL/well was added, and shook for 10 min. The absorbance values were read in triplicate against a reagent blank at a test wavelength of 490 nm (Tecan GENios, Tecan Austria GmbH, Salzburg, Austria). Cell viability was calculated using the following equation: Cell viability (%) = Atest/Acontrol × 100%, where Atest and Acontrol were the absorbance values of the test and control groups, respectively. 3. RESULTS AND DISCUSSION 3.1. Design and Synthesis of the STP Hydrogel. Briefly, a precursor solution was prepared by mixing SA and TA at first. Though the solution became pink color due to the auto-oxidation of a small fraction of catechol/pyrogallol groups to quinone, 46-49 the catechol/pyrogallol groups were still dominant. Then the monomer (AM), cross-linker (MBAA), initiator (APS) and catalyst (TEMED) were added and polymerized at constant temperature of 50 °C for 3 h (Figure 1a). In the ternary ionic multi-bond network hydrogel (STP), the presence of SA dramatically increased the conductivity of the resulting hydrogel, making them as ideal ionic conductor. The multiple and reversible weak H-bonds formed between TA, SA and PAM make high energy dissipation for the hydrogel during stretching to ensure high extensibility, low stiffness and great elasticity (Figure 1b). Additionally, the PAM networks could significantly stand the large deformations during stretching and maintain the elasticity of the hydrogel. Moreover, the reconstruction of multiple Hbonds and the catechol groups of TA could be contributed to the hydrogel with good self-healing property and strong self-adhesive character. Thus the synergistic effects of SA, TA and PAM enable the hydrogel high sensitivity, high stretchability, excellent self-healing ability and selfadhesiveness. FTIR was used to verify the characteristics of the weak H-bonds. Two strong


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absorption bands at 3350 cm-1 and 3270 cm-1 were observed ascribed to the symmetrical stretching vibration of hydroxyl groups of SA and TA, respectively, while a left shift around 3427 cm -1 was observed for hydroxyl of SA/TA, indicating the formation of intra- or intermolecular weak Hbonds.43,50 The characteristic peaks of PAM shifted to lower wavelengths 3200 cm-1, showing that the radical chain transfer reactions has occurred in the STP hydrogel, and the weak bonds formed between TA and PAM were also demonstrated (Figure S1). The rheological tests also demonstrated the gel character and the elastic recovery property of the hydrogel, further suggesting the existence of weak and reversible H-bonds among the three components (Figure S2).

Figure 1. Schematic illustration of the high performance ternary hydrogel. (a) The synthesis of STP hydrogel. (b) Multiple weak H-bonds formed among SA, TA and PAM.


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3.2. Mechanical Proprieties of the STP Hydrogel. A series of tests were carried out to quantitatively examine the mechanical properties of the STP hydrogel as shown in Figure 2. It can be seen that the stretchability of the ternary hydrogel increased with an increase of TA content. The extremely stretchable hydrogel with failure strain of 4000% was obtained with 0.10 wt% TA, which was almost 4 times higher than the original SA/PAM hydrogel and outperform the most existing ionic hydrogels (Figure 2a,b). The high stretchability may be mainly attributed to the feature of multiple bonding in polyphenol. On the other hand, the maximum toughness of the ternary hydrogel with 0.10 wt% TA content was 1.1 MJ/m3, and the modulus decreased with the increasing of TA content (Figure 2c), which contributes to the good conformal to arbitrary surfaces. The elasticity and recovery properties of the STP hydrogel were demonstrated by successive loading-unloading cycles without any lapse time between the cycles as shown in Figure 2d. After a slight hysteresis in the first cycle, the stress-strain curves for the rest cycles were subsequently overlapped, showing a typical elastomer-like behavior.51,52 The recovery ratio of peak stress was achieved to 94% even after 10 cycles (Figure 2d inset). In addition, the STP hydrogel with cylinder shape could withstand a high compression strain of 90% without any fracture and fully recovered its original shape immediately after releasing the load (Figure 2e). Similarly, after first cycle, the recovery ratio still remained above 90% of peak stress after 10 cycle compressions, suggesting the outstanding compression elasticity (Figure 2e inset). These results demonstrate the excellent elasticity and fast recovery property of the hydrogel, which is attributed to the high energy dissipation via rapid break and association of weak H-bonds in the STP ionic hydrogel upon large deformations.


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Figure 2. Characterization of the mechanical properties of STP hydrogels with different concentration of TA (CTA) at fixed concentrations of SA (2 wt%), AM (20 wt%), MBAA (0.06 wt%, mass percent concentration to AM). (a) Typical photograph of the tensile test showing superhigh stretchability. (b) The stress-strain curves of STP hydrogels as a function of CTA. (c) Elastic modulus and toughness calculated from stress-strain curves of STP hydrogels as a function of CTA. (d) The relaxation cycles to the hydrogel (CSA= 2 wt%，CTA= 0.10 wt%，CPAM= 20 wt%) under a tension of 1000% and (e) a compression of 90%. The inset are the recovery ratio of peak stress for tension cycle and compression cycle. 3.3. Self-Adhesive Properties of the STP Hydrogel. Due to the low modulus and pyrogallol/catechol groups, it is expected that the obtained STP hydrogels deserve a good conformability and excellent self-adhesive abilities. As shown in Figure 3a and Figure S6, the


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hydrogel could tightly adhered to arbitrary shape surfaces and diverse materials, including silver, plastic, glass, skin, leaf, stone, carton and biological tissue, indicating the hydrogel has a substantial broad application to various substrates. Besides, a STP hydrogel can firmly adhere two weights together and support a heavy load in air and underwater, respectively, which was over 830 times and 330 times more than its own weight, suggesting the high adhesive strength (Figure 3b). We studied the effect of concentration of TA on the adhesion property of the hydrogels. The results showed that the adhesion strength of the STP hydrogel was increased with the TA concentration (Figure S8), indicating that TA plays a critical role in improving the adhesion of the hydrogels. The adhesion strength of the hydrogel (CSA= 2 wt%，CTA= 0.10 wt%，CPAM= 20 wt%) to representative substrates was characterized by a tensile adhesion test as shown in Figure 3c and Figure S9. In air, the adhesion strength to representative substrates was in a range of 15-46 kPa, outperforming the most of mussel-based adhesive hydrogels reported up to now (Table S1 for comparison). According to the previous reports,53-57 the catechol groups have a versatile chemistry that permits strong adhesion to inorganic surfaces via forming strong and reversible coordination interactions, while quinone groups were able to bind to organic surfaces via forming covalent bond. Therefore, due to the presence of a large amount catechol groups in the obtained STP hydrogel, the adhesion strength to inorganic surfaces (e.g., aluminium) was much higher than organic surfaces (e.g., porcine skin). Furthermore, the STP hydrogel also exhibited strong adhesive ability underwater. The adhesion strength under water was in a range of 10-20 kPa (Movie S1). Though the obtained value is about half comparing to that in air, it is strong enough to be adhered to various substrates and has promising applications under water, such as underwater sensor or microphone.58-60


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Moreover, the cyclic tensile adhesion tests showed a good repeatability and durability of the STP hydrogel (Figure 3d). It still survived about 85% of the original adhesion strength after five cycles, superior to the most existing adhesive hydrogels (Table S1 for comparison). Moreover, it is worth noted that the STP hydrogel could adhere conformably to the complex surfaces of our skin with moderate adhesive strength, which could be easily peeled off for more than 30 cycles without any pain for the human body (Figure S10), indicating the safety, biocompatibility and tenderness of this ionic hydrogel. Therefore, the STP ionic hydrogel can be used as a skinmountable sensor for obtaining precise signals of motion without the adhesives, which often significantly reduce and interfere the sensitivity of the sensors.

Figure 3. Self-adhesive properties of the STP hydrogels (CSA= 2 wt%，CTA= 0.10 wt%，CPAM= 20 wt%). (a) The hydrogel firmly adhered to various materials, including silver, plastics, silica glass, porcine skin, leaf, stone, carton and organ. (b) The hydrogel (0.6 g) adhered between the


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two weights and supported the maximum load of 500 g in air and 200 g underwater, respectively. (c) The adhesive strength of the hydrogels to different substrates in air and under water tested by tensile adhesion tests. (d) Repeatable adhesion behaviors of the hydrogel to different substrates tested by cyclic tensile adhesion tests. 3.4. Self-Healing Properties of the STP Hydrogel. As expected, the resulting STP hydrogel exhibited good self-healing properties owing to the multiple weak H-bonds. As demonstrated in Figure 4a, a cylindrical shape of STP hydrogel was cut into two halves and then put together to allowing healing under room temperature without any external stimuli. After 2 h, the two cut healed together completely and could be further stretched, indicating good self-healing property of the STP hydrogel. It was also proved from the microscope view via the optical microscope images and the rheological experiments. The microscopic morphology of the STP hydrogel with a 100 µm scratch was monitored by optical microscopy over time as shown in Figure 4b. The scratch became thinner and eventually disappeared after 80 minutes that demonstrated the well self-healing property of the obtained hydrogel. The rheological strain sweep was conducted to study the collapse and reformation of hydrogel network. Figure 4c showed the changes of storage moduli (G′) and loss moduli (G″) of the STP hydrogel with the alternate stains at a fixed time of 80 s. At the oscillation strain of 1%, G′ > G″, suggesting the gel-like character of hydrogel with integrated network. Then switching the strain to 2000%, both G′ and G″ were decreased substantially and inverted to G′ < G″, which indicated the gel was transformed into a sol state due to the disruption of hydrogel networks. While switching the strain back to 1%, G′ and G″ was recovered to the initial values immediately and inverted to G′ > G″ again. The fast gel-sol and solgel transitions were ascribed to the dynamically break and reassociate of H-bonds in the system, which demonstrated the excellent self-healing property of the STP hydrogel. Additionally, the


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time-dependent of the mechanical properties recovery of the hydrogel was studied by the tensile test (Figure 4d,e). The results showed that the mechanical properties of the STP hydrogel were completely regain with recovery ratio above 90% after healing 80 min autonomously, resulted from the reconstruction of multiple reversible non-covalent weak H-bonds build by catechol groups of TA with SA and PAM.28,30,61

Figure 4. The self-healing properties of STP hydrogels (CSA = 2 wt%, CTA = 0.10 wt%, CPAM = 20 wt%). (a) A tear-heal test was conducted to confirm the self-healing of the hydrogel, the healing time was about 2h. (b) Optical microscopy images of the healing process where a 100 μm wide


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scratch on the hydrogel changed over time. (c) Changes of G′ and G″ with time at alternant oscillation strains of 1% and 2000% and at an angular frequency of 10 rad/sec for the hydrogel. The time for healing was fixed at 80 sec/cycle. (d) The tensile stress-strain curves and (e) the mechanical recovery of strain, stress, and toughness of virgin hydrogel and the cut hydrogel with different healing times. 3.5. High Performances of the STP Hydrogel Strain Sensors. Owing to the large amounts of ions released from SA and the low elastic modulus of the hydrogel, it can be expected that the hydrogel could be used as ionic strain sensor with high sensitivity and stability. The electromechanical responses of the STP ionic hydrogel to tensile strain have been characterized. The strain sensor could monitor an extremely low strain down to 0.05% at a voltage of 0.5 V (Figure 5a), which is the lowest value of the existing ionic hydrogels and comparable to that of those best electronic strain sensors (Table S1 for comparison). And, the sensitivity of GF at subtle strain is about 2.0 (0.05% < ε < 100%), which is close to the theoretical value of strain conductors and more higher than most reported ionic sensors (Figure S11). It is resulted from the elastomerlike properties of the STP hydrogel owing to the multiple weak H-bonds. In addition, our hydrogel sensor could also detect the maximum strain of 2100% with high GF up to 9.0 (Figure 5b,c), demonstrating the large workable range of strains (0.05%-2100%) and high sensitivity of the STP hydrogel. Meanwhile, the electromechanical loading-unloading curves were stable over 800 cycles at a small strain of 100% and 10 cycles at a large stain of 1000% (Figure 5d,e) , indicating the long lifetime of the STP hydrogel sensor. Additionally, effective current could be generated in the hydrogel with an extremely low voltage down to 0.07 V for strains in the range from 100% to 600% (Figure 5f). Such low applied voltage is contributed to operate such sensors with portable energy devices, thus facilitating their


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utilization in lightweight wearable devices, as well as eliminate electrochemical reaction effectively during the function of ion conductors.3 The hydrogel also exhibits a rapid and autonomous electrical self-healing property demonstrated by connecting the LED bulb to the circuits (Figure S12, Movie S2).

Figure 5. The electromechanical properties of the STP hydrogel sensor (CSA = 2 wt%, CTA = 0.10 wt%, CPAM = 20 wt%). (a, b) Time-dependent relative resistance changes of the sensors at a small strains (< 5%) and a large strain (2100%) under the applied voltage of 0.5 V, respectively. The insect pictures are the luminance of a LED with the increase of tensile. (c) The gauge factor variations of the STP hydrogel with strain. (d, e) Stability of relative resistance changes of the sensors for 800 cycles at 100% strain and for 10 cycles at 1000% strain under the same applied voltage of 0.5 V. (f) Time-dependent relative resistance changes of the sensors under strains in the range (100% to 600%) at a low voltage of 0.07 V. The ternary hydrogel strain sensors are expected to have an excellent performance for humanmachine interaction and healthcare monitoring considering its ultrasensitive property. Also, thanks


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to the low modulus and excellent adhesiveness, the STP hydrogel could conformably and tightly adhere to the dynamically moving parts of the human body to directly detect the signals of the strains without the interference of the tapes. The typical human motions were detected as show in Figure 6. The relative resistance rapidly increased along with the bending angles from 0°to 120° and displayed a step-by-step curves (Figure 6a, Movie S3). Besides, the real-time resistance changes of the strain sensor were both kept consistent during walking with legs holding and elbow bending (Figure 6b,c). It shows that the hydrogel sensor could monitor and distinguish the motions of the finger, knee and elbow in a fast response time about 0.4 s (Figure S13) and a good repeatability, which was attributed to its features of sensitivity, electrical stability and good elasticity.

Figure 6. Demonstration of the STP hydrogel (CSA = 2 wt%, CTA = 0.10 wt%, CPAM = 20 wt%) as wearable sensors for real-time monitoring various human motions. (a) The time-dependent relative resistance changes vs. time of bending the index finger at different bending angles (0º, 30º, 45º, 90ºand 120°). (b, c) The time-dependent relative resistance changes vs. time of the bending and


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releasing of knee and elbow, respectively. (d, e) The time-dependent relative resistance changes vs. time when sensor attached on throat and corners of the mouth to sense small movements, such as swallowing and smiling. (f) The subtle muscle movements of wrist pulse of the same person at ease (red lines) and after exercise (blue lines). The inset is magnification of a single period of the pulse-waveform. Subtle motions were also accurately to be monitored, including swallowing, smiling and wrist pulse. Both of the relative resistance changes is below 5% when swallowing and smiling (Figure 6d,e). In addition, the breathing modes including regular breathing and rapid deep breathing after exercise, could be clearly distinguished based on real-time resistance changes (Figure 6f). The amplitude and frequency were evidently increased after exercise with a blood pulse of 92 beats per minute contrast to the regular blood pulse of 72 beats per minute. The Figure 6f inset is the magnification of a single period of the pulse-waveform. It indicates that our flexible strain sensor could precisely detect and distinguish the tiny signals such as recognizing the systolic pressure (P1) and diastolic pressure (P2).62 Although previous ionic conductors reported the achievable for monitoring the wrist pulse, the accuracy is low and with poor repeatability due to the low sensitivity under subtle strains.22,23,63 Thus our STP hydrogel sensor with extremely high sensitivity, high stability and biocompatibility is more suitable for applying in medical fields. It could be used in real-time detection of abnormal respiration to help for the early diagnosis of diseases such as asthma, hearth failure, embolism, and so on.64-66 3.6. Biocompatibility of the STP Hydrogel. As a skin-like hydrogel sensor, biocompatibility is also necessary. The in vitro cytotoxicity studies was performed by culturing L929 cells in the hydrogel extract for 24, 48, and 72 h. As shown in Figure 7a, the morphology of the L929 cells cultures for 72h displays a healthy growth state. The relative growth rate value of the STP hydrogel


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was higher than 100% and the cell viability values were comparable to the control, indicating that the STP hydrogel was non-cytotoxicity and had a good biocompatibility (Figure 7b). Therefore, the STP hydrogel was a human-friendly material, which might be further implied in biomedical and tissue engineering.

Figure 7. (a) Morphology of the L929 cells cultured for 3 days in hydrogel extract. (b) The results of the cytotoxicity tests in L929 cells treated with the STP hydrogel extract. 4. CONCLUSIONS In summary, we successfully developed a ternary hybrid ionic conductor as skin-mountable and wearable strain sensor through the associated interactions of the weak H-bonds and synergistic effects between TA, SA and PAM. This hydrogel integrated the properties of stretchablity, low modulus, good recovery, self-adhesive, self-healing, sensitivity and biocompatibility into one system. The healing time was within 2 h and the mechanical recovery was up to 96%. The skinlike strain sensor exhibited a broad strain window (0.05%-2100%) with a high GF of 2.0 and 9.0


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at subtle (< 100%) and large (2100%) strains, respectively. And the small strains below 600% can be detected at a very low voltage of only 0.07 V, which sufficiently saved energy and simplified the test conditions. It is demonstrated that the hydrogel could monitor large limb motions of human body (finger, knee, elbow), and subtle muscle movements (smiling, chewing, wrist pulse), showing potential applications in skin-like electronics, human-machine interaction, and artificial prosthetics. Thus we believe that this study provides a new strategy for design of skin-like multifunctional ionic hydrogels as flexible and wearable sensing devices. ASSOCIATED CONTENT Supporting Information Equation of the theoretical value of the strain sensor under small strain (Equation S1); Characterizations of the STP hydrogel (Figure S1,2); comparison of mechanical properties of STP hydrogel prepared with different SA, AM and MBAA content, respectively (Figure S3-5); photographs for adhesion between different adherends and hydrogels (Figure S6); the schematics of tensile-adhesion testing and tensile adhesion test under different conditions (Figure S7-10); GF comparison of some typical reported sensors (Figure S11); the electrical recovery of the selfhealing hydrogel (Figure S12); the response time of the hydrogel sensor (Figure S13); comparisons of some strain sensors (Table S1). (PDF) Movie S1 for the adhesion process underwater of hydrogels to diverse substrates. (AVI) Movie S2 for the healing process for ternary hydrogels as self-healable electronic interconnects with LEDs. (AVI)


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Movie S3 for the relative resistance changes versus time for monitoring the finger bending in real time. (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡Haiyan Qiao and Pengfei Qi contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (51573080, 51873094), the Key Research and Development Project of Shandong Province (2016GGX102005), Technology Development Project of Shinan District of Qingdao (2018-4-007-ZH) and Program for Taishan Scholar of Shandong Province.


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ToC Figure:


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Multiple Weak H-Bonds Lead to Highly Sensitive, Stretchable, Self

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