Nucleotide-Regulated Tough and Rapidly Self-Recoverable

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Nucleotide-regulated tough and rapidly self-recoverable hydrogels for highly sensitive and durable pressure and strain sensors. Qin Zhang, Xin Liu, Xiuyan Ren, Fei Jia, Lijie Duan, and Guanghui Gao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02039 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Chemistry of Materials

Nucleotide-regulated tough and rapidly self-recoverable hydrogels for highly sensitive and durable pressure and strain sensors. Qin Zhang, Xin Liu, Xiuyan Ren, Fei Jia, Lijie Duan*, Guanghui Gao* Polymeric and Soft Materials Laboratory, School of Chemical Engineering, School of Chemistry and Life Science, and Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, China Corresponding authors: Lijie Duan, Guanghui Gao E-mail: [email protected]; [email protected]. ABSTRACT: Flexible and stretchable hydrogels have drawn much attention as wearable sensors, however, most hydrogel sensors usually suffer from poor mechanical toughness and self-recovery, causing great limitation on application for repeatedly sensing. Here, highly stretchable, tough, and anti-fatigue hydrogels are fabricated by incorporating nucleotide (adenosine monophosphate, AMP) into hydrophobic association polyacrylamide (PAAm) networks. AMP as a dynamic connected bridge significant regulate the mechanical performances of hydrogels, wherein the anionic phosphate groups from AMP form ionic bonds with cationic micelles, and the nucleobases of AMP bind with PAAm chains through hydrogen bonds. The noncovalent synergistic interactions in the hydrogel networks contribute to achieving fast selfrecoverable and anti-fatigue behaviors at room temperature without any external stimuli. More importantly, the nucleotide-regulated hydrogel presents durability and high sensitivity as pressure and strain sensors for the detection of various mechanical deformations. As a result, the hydrogels are successfully designed as wearable sensors for sensing various large and subtle human motions, including the bending of elbow, wrist, finger, and even the vibrations of the rib cage and larynx. It is envisioned that the nucleotide-regulated hydrogels would find broad applications in electric skins, medical monitoring, soft robotics, and flexible touch panels.

Flexible pressure and strain sensors have drawn much attention in electric skin1,2, human movement detection3–5, and health monitor6,7. They are capable to sense biological activities and provide feedback via the changes of outputting electric signals. With the development of advanced materials, many reliable sensors have been successfully fabricated, including carbon-based sensors8–11, nanowire based sensors12–14, and conductive polymer based sensors15,16. However, these sensors usually exhibit low working range and suffer from rapid deterioration of conductivity under repeatedly large deformation, causing limitation on their further application17,18. Therefore, an extremely stretchable sensor with anti-fatigue conductivity is of significance to fabricate a flexible wearable device with the increasing complexity and multi-functionality. Hydrogels, as a class of soft water-based materials, show great potential in wearable sensors due to their excellent biocompatibility, intrinsically high stretchability, and tunable conductivity19–21. The application of hydrogels in flexible sensors requires high mechanical toughness and excellent self-recovery to achieve large range strains sensing and long-term cycling stability. Up to now, numerous efforts have been devoted to design tough and fast self-recoverable hydrogels through integrating dynamic noncovalent bonds (e.g., ionic bonds, hydrogen bonds, hydrophobic association) into macromolecular network to effectively dissipate energy22–24. Despite the complex network structures and strong crosslinking interactions endowed the hydrogels with

high mechanical performances, these hydrogels tend to need a high recovery temperature and/or long resting time after loading. Therefore, it is desirable to design a simple and effective strategy to fabricate hydrogels with excellent mechanical properties, remarkable self-recoverable and antifatiguing behavior at ambient temperature. Nucleotides are the essential components of DNA and RNA, which have received considerable attention in numerous areas, such as encapsulation25,26, enzyme mimics27, and sensing28,29. Nucleotides consist of negatively charged phosphate-sugar backbone and nucleobase. Considering the multiple bonding sites of nucleotide molecules, it was speculated that introducing nucleotides to hydrogels could form hydrogen bonding, electrostatic interactions, π-π stacking, hydrophobic association and metal coordination, which imparts high mechanical strength and rapid recoverability to hydrogels via reversible noncovalent interactions. Herein, a facile and effective approach was designed to fabricate a highly tough and fast self-recoverable hydrogel by introducing a model nucleotide of adenosine monophosphate (AMP) into hydrophobic association polyacrylamide (PAAm) hydrogels. The dynamic noncovalent cross-linking network regulated by nucleotide endowed hydrogels with large elongation, high toughness, rapid self-recovery and remarkable fatigue resistance. More importantly, the nucleotide-regulated hydrogels were both

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Figure 1. Schematic illustration of AMP-regulated hydrogels. pressure and strain sensitive, which exhibited high sensitivity and durability as pressure and strain sensors. Besides, a variety of human motions consisting of the bending of elbow, wrist, and finger, coughing, breathing, and even speaking could be precisely discriminated by this hydrogel sensor. It is envisioned that the nucleotide-regulated strategy would inspire the design and development of high mechanical performance and anti-fatigue soft materials. EXPERIMENTAL SECTION Materials Sodium hydroxide (NaOH, AR) was supplied by Fuchen Chemical Reagent Factory (Tianjin, China). Cetyltrimethylammonium bromide (CTMAB, AR) was obtained from Huishi Biochemical Reagent Co. (Shanghai, China). Sodium chloride (NaCl, 99.5%) was obtained from Beijing Chemical Factory (Beijing, China). Acrylamide (AAm, 99%), hexadecyl methacrylate (HMA, 95%), N,N,N’,N’tetramethylethylenediamine (TMEDA, ≥ 97%), 5’-adenosine monophosphate (AMP, ≥ 98%), and potassium persulfate (KPS, 99.5%) were purchased from Aladdin Reagent Co. (Shanghai, China). Deionized water was obtained from laboratory water maker at 25 °C (18.2 MΩ cm resistivity). Preparation of Hydrogels Firstly, 0.16 g NaOH was put in deionized water (100 ml) to get alkaline solution. Next, 0.32 g NaCl and 0.8 g CTMAB were dissolved in 10 ml NaOH solution by stirring at 40 °C

for 30 min. Thereafter, 0.0435 g HMA was added in the solution with stirring at 40 °C for 2h. Then, 0.3 g AMP and 2 g AAm were added into the solution until a transparent solution obtained by stirring at 70 °C. 35 L TMEDA and 0.02 g KPS were put into the above solution with stirring for 5 min when the solution was cooled to 25 °C. And then, the prepared solution was transferred to a mold consisting of two glass plates and one silica rubber spacer (100×100×2 mm3). Finally, the AMP-regulated hydrogels were obtained through the polymerization reaction for 12 h at 35°C. The resulted hydrogels were named as AMP-x, where x refers to the molar ratio of AMP/AAm. The hydrogels prepared by above method were denoted as AMP-3% hydrogels. Morphological Observation The micro-structure of the AMP-regulated hydrogels was visualized by using a scanning electron microscopy (SEM, JSM 6510, JEOL, Japan) with a 100 times of magnification factor. The hydrogel samples were firstly freeze-dried and exposed their inner structures by breaking in liquid nitrogen. And then, the platinum was coated on the fracture surface of specimens. Mechanical Measurement The tensile properties were tested by using the AGS-X tensile tester (SHIMADZU, Japan) at a 100 mm min-1 velocity at room temperature. The hydrogel samples were prepared as a dumbbell-shape (30 mm×4 mm×2 mm). Tensile cyclic loading-unloading tests were performed under the same


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Figure 2. Mechanical property exhibition of the AMP-regulated hydrogels (AMP-3%): (a) stretching exhibition, (b) antipuncturing exhibition, (c) compression exhibition, (d) lifting a weight of 350 g without any damage. condition. To avoid water volatilization, silicone oil was coated on the hydrogel samples before the experiment. The compression properties were measured by the CT31000 texture analyzer (Brookfield, U.S.A) with loading velocity of 5 mm s-1 at room temperature. The samples were prepared into cylinder (20 mm diameter15 mm thickness). The maximum compressive strain was 80%. The successive compression cycling tests were performed under the same condition without any testing time between each compressive cycle. Rheological Measurement The storage modulus (G’) and loss modulus (G”) of hydrogels were measured by a Physical MCR 302 rheometer (Anton Paar) with a parallel-plate geometry (25 mm diameter) at 25 °C. The hydrogel were prepared into a cylinder (25 mm diameter, 2 mm thickness). The strain sweep was conducted under a fixed frequency () of 10 rad/s in the strain range from 0.01 to 1000%. In addition, the frequency sweep test was carried out in the frequency range from 0.1 to 100 rad/s under a constant 0.1% strain (). Swelling Behavior The hydrogels were swollen in deionized water until reaching swelling equilibrium at 25 °C with replacing the deionized water every 12 h. The excess water on the surface of the samples was wiped with a filter paper before being weighted. The swelling ratio of the hydrogels was calculated according the equation: Swelling Ratio= (Ws-Wi)/Wi where, Ws is the weight of the swollen samples and Wi is weight of the original samples. Electrical Measurement The electrical resistance of the hydrogel samples was tested by the electrochemical station (Princeton Applied

Research Model 2273). The change of relative resistance for hydrogels is defined as follows: ∆R/R0=(R-R0)/R0×100% where, R0 and R respectively denotes the original resistance at the strain of 0% and the real-time resistance under certain strain. RESULTS AND DISCUSSION Nucleotides, as the basic structural unit and building block for DNA and RNA, comprise a nitrogen-containing base joined to a pentose and a phosphate group. Here, an AMPregulated hydrogel with high stretchability, toughness, and fatigue resistance was prepared by the micelle copolymerization of AAm with hydrophobic monomer HMA in the alkaline solution consisting of cationic surfactant CTMAB, NaCl, and AMP (Figure 1). The alkaline environment ensured AMP to be completely dissolved in aqueous solution via the deprotonation of phosphate groups. In this system, AMP served as a dynamic connected bridge, which not only formed ionic bonds with cationic hydrophobic association micelles, but also interacted with PAAm chains via hydrogen bonds. The ionic bonds, hydrogen bonds, and hydrophobic association in the hydrogel could effectively dissipate energy under deformation, resulting in excellent mechanical properties. These reversible noncovalent bonds were crucial for selfrecovery, allowing for automatically and rapidly restoring mechanical properties at ambient environment. In addition, the presence of NaCl endowed the hydrogel with excellent conductivity, making it an ideal ionic conductor for flexible sensors. It is noteworthy that the chloride ions can also interact with ammonium ions by electrostatic interactions. However, the chloride ions did not contribute to the mechanical properties of hydrogels because no new crosslinking points were formed. Ultimately, the AMPregulated hydrogel with NaCl exhibited high mechanical behaviors, rapid self-recovery and conductive performance, which displayed high sensitivity and durability as a strain



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Figure 3. (a) The tensile curves and (b) the corresponding elastic modulus and toughness of the hydrogels with different AMP contents. (c) The tensile curves and the corresponding first derivative line of the AMP-3% hydrogels. (d) The compressive curves of the hydrogels with different AMP contents and (e) the corresponding stress. (f) 60 times successive compressive curves of AMP-3% hydrogels. and pressure sensor. The AMP-regulated hydrogels exhibited extraordinary stretchability and excellent toughness. The AMP-3% hydrogels can be easily stretched to 20 times its original length without rupture (Figure 2a). The Figure 2b illustrated that the hydrogel was tough enough to resist to the puncture with a surgical knife blade. Moreover, the hydrogel also demonstrated high compressive toughness, which could withstand large compressive strain of 80% and rapidly recover to its initial shape after unloading (Figure 2c). In addition, the hydrogel could also hold up a load of 350 g without any damage in Figure 2d. Figure 3a illustrated the tensile curves of the AMPregulated hydrogels with diverse AMP contents. The pure hydrophobic association hydrogels (AMP-0%) showed low fracture strength of 90 kPa. In contrast, a significant increase of fracture stress was observed after introducing AMP into the hydrogels. The fracture strength enhanced to 620 kPa at the strain of 2160% when the content of AMP increased to 3%. What’s more, both toughness and elastic modulus of the hydrogels improved dramatically as the contents of AMP increased. For example, the toughness could reach up to 3.63 MJ/m3, which was over 3 times that of the pure hydrophobic association hydrogels (Figure 3b). Impressively, the AMP-regulated hydrogels displayed unique strain stiffening behavior, which is similar with the mechanical response of many biological gels30,31. As shown in Figure 3c, from the yield point A, the slope of the tensile curve of AMP-3% hydrogels increased as the strain increased until point B. The hydrogels became much stiffer with the increase of strain to counteract large deformation and maintain their structural integrity during the process. This phenomenon could be attributed to the rearrangement of dynamic cross-linking structure during elongation32. When the hydrogels were loaded with a small stress, the PAAm chains were gradually oriented along with the stress direction. After reaching the yield point, the coiled PAAm chains began to elongate and slide, accompanied by the fracture of ionic bonds, hydrogen bonds, and hydrophobic

association. When the large strain applied, the hydrogel suffered from disruption and collapse of cross-linking networks. During the process, large amounts of energy would be dissipated, therefore increasing the resistance against crack propagation, and causing the enhancement of stiffness with the increase of the applied strain. The compression property of the hydrogels was also measured. Figure 3d and 3e displayed that the compression strength of the hydrogels enhanced with the increase of AMP contents. Moreover, 60 times successive compression tests were performed on the AMP-3% hydrogels to investigate the compressive fatigue resistance of hydrogels. From Figure 3f, the loading-unloading curves with evident hysteresis were nearly overlapped, implying the excellent compression recovery and anti-fatigue property of the AMP-regulated hydrogels. Besides, cyclic tensile tests were performed to investigate the mechanism of energy dissipation for the AMP-regulated hydrogels. Successive cyclic tensile curves of AMP-3% hydrogels without resting time between each cycle were shown in Figure 4a. An evident hysteresis loop could be observed in the first tensile cycle, suggesting that large amounts of energy was dissipated effectively by the fracture and reconstruction of ionic bonds, hydrogen bonds, and hydrophobic association. It was found that the hysteresis loops from the second to fifth cycles were nearly overlapped and less than that of the first cycle, implying that the hydrogel suffered from structural changes during first tensile cycle and remained similar network structure in the following cycles. As shown in Figure 4b, the hydrogels exhibited nearly unchanged dissipated energy for the second to five cycles, indicating the remarkable fatigue resistance of the hydrogels. In addition, the hydrogels exhibited fast selfrecovery ability. Figure 4c and 4d showed five times cyclic tensile tests on the same hydrogel with 10 min resting time at room temperature. The dissipated energy in first cycle was 24.58 kJ/m3, and the second cycle after 10 min resting time exhibited dissipated energy of 19.34 kJ/m3, the toughness recovery achieved 79% within 10 min. The excellent selfrecovery was attributed to the physical cross-linking


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Figure 4. (a) Five times cyclic tensile curves of AMP-3% hydrogels at 600% strain without any resting time and (b) the corresponding dissipated energy. (c) Five times cyclic tensile curves of AMP-3% hydrogels with 10 min resting time and (d) the corresponding dissipated energy. (e) Continuous cyclic tensile curves of different AMP-3% hydrogels at different max and (f) the corresponding dissipated energy. and rapidly recoverable network of hydrogels regulated by dynamically connected bridge structure. Compared with the previous works20,22,23,24,35, the strategy regulated by the small molecule of AMP enabled hydrogels with a simple, flexible and easy-to-recovery network architecture and a fast selfrecoverable process without any external stimuli. Furthermore, continuous cyclic loading-unloading tests were performed on hydrogel at different maximum strains (max). As shown in Figure 4e and 4f, the hydrogels exhibited low hysteresis in every cycle, suggesting that the hydrogels possessed excellent resilience. Besides, the hysteresis loops increased with increasing max, and the dissipated energy increased from 5.08 kJ/m3 at max = 200% to 108.80 kJ/m3 at max = 1600%. Oscillatory rheology tests were performed to study the viscoelastic behavior of the hydrogels at 25 °C. Firstly, the strain sweep for AMP-0% and AMP-3% hydrogels in the strain range of 0.01-1000% at  =10 rad/s was investigated. In Figure 5a, both hydrogels exhibited an obvious straindependent viscoelastic response. G’ was consistently greater than G” at linear viscoelasticity region (  0.01-1%), which is a character of viscoelastic materials. Figure 5b showed the frequency dependence of G’ and G” for the hydrogels. During the entire frequency range, G’ of both hydrogels was higher than G”, indicating elastic solid like behavior of the hydrogels33. Moreover, G’ of AMP-3% hydrogels was higher than that of AMP-0% hydrogels, suggesting that the additional cross-linking supplied by AMP did exist in the hydrogel networks34. The frequency dependence of loss factor (tan δ = G”/G’) for the hydrogels was illustrated in Figure 5c. The AMP-3% hydrogels displayed much lower value of tan δ than AMP-0% hydrogels. Because the integration of AMP could ameliorate the network structures, leading to very few imperfection of cross-linking networks and excellent elasticity35,36. Additionally, G” and tan δ of AMP-3% hydrogels decreased with increasing the frequency, and more significant decrease was occurred in the low frequency region. This suggested that the hydrogels underwent more relaxation of the polymer networks37.

The swelling performance of the AMP-regulated hydrogels was studied. The hydrogels were swollen in distilled water at room temperature until mass equilibrium. The AMP-0% hydrogels were collapsed after 36 h, while the AMP-3% hydrogels even kept integrity after 1450 h (Figure 6a and 6b). Because the pure hydrophobic association hydrogels without AMP possessed low cross-linking density, leading to unstable network structure. The corresponding swelling curves were illustrated in Figure 6b. The AMP-3% hydrogels demonstrated high swelling ratio, which achieved swelling equilibrium after 1450 h. Figure 6c and 6d demonstrated SEM images of the hydrogels. The AMP-0% hydrogels exhibited large porous structure. After introducing AMP, the pore size of the hydrogel became narrow, suggesting that AMP could effectively regulate the network structure of hydrogels and generate strong interactions. The AMP-regulated hydrogels exhibited excellent ionic conductivity due to the abundance of Na+ and Cl- in the hydrogel system. Figure 7a showed a complete circuit consisting of a LED bulb and a piece of AMP-regulated hydrogel. It was seen that the bulb became much brighter when pressure applied. The ionic transport distance in the hydrogel network decreased under compressive stress, leading to the improvement of ionic migration efficiency38,39. The result demonstrated that the hydrogels possessed pressure sensitivity. Based on the pressure sensitivity of the AMP-regulated hydrogels, a smart wearable wristband was successfully fabricated for the real-time pressure detection. As presented in Figure 7b, when the wearable wristband was worn on human wrist, it could monitor subtle changes of pressure under human finger touch, and the magnitude of the relative resistance changes increased with the increase of applied pressure. Furthermore, the pressure sensors made from the AMP-regulated hydrogels displayed high sensitivity. In Figure 7c, the sensors could response to different pressures when subjected to repeatedly compressive loading and unloading at a fixed compressive velocity of 60 mm/min, and the electrical signals were highly replicative. Moreover, the hydrogel pressure sensors exhibited frequency-



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Figure 5. (a) Strain dependence of G’ and G” for the hydrogels at =10 rad/s. (b) Frequency dependence of G’ and G” for the hydrogels at  =0.1%. (c) Frequency dependence of Tan δ of the hydrogels.

Figure 6. (a) The photographs of swollen hydrogels after swelling 36 h. (b) Swelling curves of the hydrogels. The SEM images of the AMP-0% hydrogels (c) and AMP-3% hydrogels (d). dependence when applied a pressure of 25.0 kPa (Figure 7d). Besides, Figure 7e presented the curve of the relative resistance changes for the pressure sensors under 150 cyclic compressive loading-unloading, and the relative resistance changes were nearly unchanged, revealing the high cycling stability. The AMP-regulated hydrogel also exhibited superb sensitivity as a strain sensor. Gauge factor (GF) was the ratio between relative resistance variation and applied strain, which is a common parameter to assess the sensitivity of the hydrogel sensors. GF = (∆𝑅/𝑅0)/ = [(𝑅 ― 𝑅0)/𝑅0)/], where R0 and R respectively denotes the original resistance at the strain of 0% and the real-time resistance under certain strain. Figure 8a was the dependence curve of the relative resistance variations with applied strain from 0% to 1000%, which fits to the polynomial equation y=A2 + B + C ( y and  respectively denotes the variation of relative resistance and the applied strain). It was found that the fitting curve was in agreement with experimental data. Gauge factor under the strain range of 0-100% was evaluated. As shown in Figure 8b, gauge factor was 0.74 in the strain range of 0-10%. With the applied strain increased, gauge factor reached 1.77 (10 <  < 60%) and 2.57 (60 <  < 100%), which were higher than those of the previously reported hydrogel strain sensors40–42,

indicating that the hydrogel sensors demonstrated high strain sensitivity. Figure 8c illustrated that the hydrogel strain sensors displayed frequency dependence, which was greatly important for the precise detection. When the hydrogel sensor was exposed to various strains at a fixed strain speed of 200 mm/min, the relative resistant changes gradually increased with the increasing strain from 50 to 200% (Figure 8d). In addition, no obvious shift of the electric signal baseline was observed under repeated deformation even at the large strain of 200% in Figure 8e, confirming the high conductive stability of the hydrogel sensors. To investigate the anti-fatigue conductivity, 100 cycling tensile loadingunloading tests were performed on the hydrogel sensor under 50% strain (Figure 8f), and no obvious degradation of electric signals was seen during the cycling process. The high strain sensitivity and durability are significant for the longterm usage of the hydrogel sensors. The excellent mechanical properties, high strain sensitivity, and conductive stability enabled AMP-regulated hydrogels as ideal candidates for a wearable strain sensor. Thus, the hydrogels directly attached to human skins for monitoring the diverse human motions as wearable strain sensors. Figure 9a showed the relative resistance variations of the hydrogel strain sensors for the different finger bending angles. It was


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Figure 7. (a) A complete circuit consisted of a LED bulb and a piece of AMP-3% hydrogel. (b) A wearable wristband for subtle pressure detection based on the AMP-regulated hydrogels. (c) Relative resistance changes of the pressure sensors under various pressures (20.0 kPa and 30.0 kPa) at a constant compressive speed of 60mm/min. (d) Relative resistance changes of the hydrogel pressure sensors at different frequencies under 25.0 kPa pressure. (e) Relative resistance changes of the pressure sensors under 7.7 kPa pressure during 150 cyclic compressive loading-unloading.

Figure 8. (a) The relative resistance variations of AMP-regulated hydrogels as a function of tensile strain. (b) Gauge factor under the strain range of 0-100%. (c) The relative resistance changes under cyclic tensile unloading-unloading at different frequencies with a strain of 50%. (d) The relative resistance changes under different cycling tensile strains at a fixed tensile speed of 200 mm/min. (e) No degradation of electric signals under repeated large strain. (f) Anti-fatigue conductivity of the hydrogel under a strain of 50% for 100 tensile loading-unloading cycles. seen that as the bending angles increased from 45° to 90°, the relative resistance changes of the sensors increased. Moreover, the resistance values were consistent when the finger was held at a certain angle. As shown in Figure 9b and 9c, the hydrogel sensors could also distinguish the bending of wrist and elbow, and the bending of elbow displayed a larger relative resistance change than the bending of wrist due to the larger deformation. Meanwhile, the relative resistance changes were highly stable and repeated, which indicated that the wearable strain sensors possessed antifatigue conductivity and high strain sensitivity. Apart from the ability for monitoring large-scale human motions, the AMP-regulated hydrogel strain sensors could

also perceive subtle deformation. As shown in Figure 9d, the strain sensor could sense micro-muscle movement when the fist was in a relaxed or clenched state, and a regular and reproducible relative resistance change was observed under repeated motions. Moreover, the hydrogel sensors can sensitively detect subtle physiology signals, including the vibrations of the rib cage and larynx. The hydrogel strain sensors could accurately discriminate different breathing modes (regular breath and deep breath). It was obvious in Figure 9e, the vibration amplitude of regular breath was lower than the deep breath. In addition, when fixed on the larynx directly, the hydrogel strain sensors could monitor the changes in electric signals when the tester coughed (Figure 9f). The curves of relative resistance changes were quite



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Figure 9. Exhibition of the AMP-regulated hydrogel as wearable strain sensors. (a) The relative resistance variations of the hydrogel strain sensors upon diverse finger bending angles (45° and 90°). The responsiveness of the hydrogel sensors to different human motions: (b) wrist and (c) elbow. (d) The curves of relative resistance variations when making a fist. (e) The responsiveness of the hydrogel sensors to different breathing modes. (f) The curves of relative resistance variations when coughing. The curves of relative resistance variations when monitoring different voice signals: (g) “strength”; (h) “hydrogel”; (i) “where are you from”. regular and stable under multiple coughed process. Furthermore, the hydrogel strain sensor also possessed the ability for voice recognition. As shown in Figure 9g-h, the different phrases could be distinguished by attaching a piece of hydrogel on the larynx. The pronunciation of “strength” exhibited a characteristic peak, where the word of “hydrogel” caused three peaks. Moreover, four characteristic signals were observed when saying “where are you from”. These characteristic peaks presented highly reproducibility when repeated the same phrases. Besides, the intensity of the tone was also be reflected on the characteristic peaks. The hydrogels exhibited potential application in vocal recovery and phonatory rehabilitation as sensors. CONCLUSION In summary, we designed a simple and general method to construct a novel nucleotide-regulated hydrogel by introducing AMP into hydrophobic association PAAm networks. AMP as the dynamic connected bridge endowed hydrogels with high mechanical properties by bonding to both the cationic hydrophobic association micelles and to the PAAm chains. The hydrogel exhibited large elongation (2160%) and high toughness (3.63 MJ/m3). The synergistic noncovalent interactions in the hydrogel networks accounted for fast self-recoverable behavior and fatigue resistance under room temperature without any external stimuli. Furthermore, the hydrogels presented ultra-high pressure and strain sensitivity and durability, which could be designed as wearable sensors for pressure and strain detection. The hydrogel sensors could accurately monitor and distinguish both large-range human movements and subtle physiological signals, such as the motions of various human joints, fisting, coughing, breathing, and speaking. It

was envisioned that the AMP-regulated strategy would open a new path for fabricating flexible hydrogel sensors with reliable mechanical properties and durability. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by grants from National Natural Science Foundation of China (Nos. 51873024 and 51703012), Science and Technology Department of Jilin Province (No. 20180101207JC), Education Department of Jilin Province (No. JJKH20191296KJ and JJKH20191306KJ), and Jilin Provincial Development and Reform Commission (No. 2019C052-2). REFERENCE (1) Qiao, Y.; Wang, Y.; Tian, H.; Li, M.; Jian, J.; Wei, Y.; Tian, Y.; Wang, D.-Y.; Pang, Y.; Geng, X.; et al. Multilayer Graphene Epidermal Electronic Skin. ACS Nano 2018, 12, 8839–8846. (2) Gerratt, A. P.; Michaud, H. O.; Lacour, S. P. Elastomeric Electronic Skin for Prosthetic Tactile Sensation. Adv. Funct. Mater. 2015, 25, 2287–2295. (3) Cai, G.; Wang, J.; Qian, K.; Chen, J.; Li, S.; Lee, P. S. Extremely Stretchable Strain Sensors Based on Conductive


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TABLE OF CONTENT FIGURE


Nucleotide-Regulated Tough and Rapidly Self-Recoverable

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