Ultraflexible Self-Healing Guar Gum-Glycerol Hydrogel with Injectable

Aug 1, 2018 - Recently, flexible, injectable, and strain-sensitive hydrogels have attracted great research interest for application as electronic skin...
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Article Cite This: ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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Ultraflexible Self-Healing Guar Gum-Glycerol Hydrogel with Injectable, Antifreeze, and Strain-Sensitive Properties Xiaofeng Pan,† Qinhua Wang,† Dengwen Ning,† Lei Dai,‡ Kai Liu,*,† Yonghao Ni,†,§ Lihui Chen,† and Liulian Huang† †

ACS Biomater. Sci. Eng. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 08/16/18. For personal use only.

College of Material Engineering, Fujian Agriculture and Forestry University, No. 15 Shangxiadian Road, Cangshan District, Fuzhou City, Fujian Province 350002, China ‡ College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China § Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B5A3, Canada ABSTRACT: Recently, flexible, injectable, and strain-sensitive hydrogels have attracted great research interest for application as electronic skin and wearable strain sensors. The synergistic integration of high flexibility, rapid self-healing, and antifreezing properties makes injectable, strain-sensitive, and self-healing guar gum hydrogels still a great challenge. Here, inspired by the strong hydrogen bonding of glycerol and water, the chelation cross-linking between glycerol and borax, we constructed a compact three-dimensional dynamic cross-linked net formed of glycerol−water−borax. Under stress, dynamic interactions of glycerol−water−borax net act as sacrificial bond energy for effective dissipation, which enables the hydrogel to achieve high flexibility, stretchability, and injectability. More importantly,because of the presence of glycerol, the antifreeze and moisturizing properties of the gel are improved. The hydrogel also exhibited an ultrafast self-healing ability of only 15 s. In addition, the results show that the hydrogel has self-adhesive properties and strain sensitivity. The hydrogels have the potential to be used to make flexible, wearable, and 3D-printable electronic skin and strain-sensitive sensors. KEYWORDS: self-healing, hydrogel, injectability, antifreezing, strain-sensitive

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healing effect, flexibility, and stability of borax-cross-linked GG hydrogels at the same time by a facile method. Glycerol (1,2,3-propanetriol or glycerin) is a transparent, odorless, nontoxic liquid. Glycerol can reduce the freezing point of water, so the glycerol/water solution is usually used in nontoxic antifreeze.20,21 Lu et al. have used glycerol/water mixed system to prepare antifreeze hydrogel, and the hydrogel can maintain good elasticity and flexibility under a low temperature environment.22 More importantly, as a polyhydric alcohol, small-molecule glycerol can form a complex with cross-linker borax. After forming a complex with borax, chelated bonds were formed between glycerol and borax.23 K et al. have used glycerol as a plasticizer for borax cross-linked PVA film, increasing the film flexibility.24 The polyhydroxy structure of guar gum is similar to PVA. So, these bonds may improve the shortcomings of GG hydrogels. In this work, the multifunction guar gum−glycerol ionic hydrogel (GG-glycerol hydrogel) were prepared in the

INTRODUCTION In recent years, hydrogels with intelligent functions, such as self-healing,1 3D printing,2−4 functional response,5 light sensitivity,6 temperature sensitivity,7 strain sensitivity,8 and shape memory,9 have attracted much attentions in electronic skin and flexible wearable devices fields. A lot of natural polymers have been used to prepare smart hydrogels. For example, chitosan,10 agarose,11 sodium alginate,12 cellulose,13 and guar gum.14 Among these natural polymers, guar gum (GG) extracted from the guar seeds15,16 is an edible, low-cost, water-soluble polyhydroxy polysaccharide. It is also a suitable raw material for the preparation of smart wearable hydrogels. Ionic hydrogel can convert mechanical deformation of hydrogels to electrical signals, which is a good strategy for making intelligent strain-sensitive devices.17,18 It is a simple and common method to prepare GG ionic hydrogel by using borax solution as a cross-linker and conductivity fluid. However, borax-cross-linked GG hydrogels are very brittle, and have weak flexibility, slow self-healing effect, and poor lowtemperature tolerance.19 These shortcomings limit the wide range application of GG hydrogels in electronic sensor devices. Therefore, it would be greatly expected to improve the self© XXXX American Chemical Society

Received: June 7, 2018 Accepted: August 1, 2018 Published: August 1, 2018 A

DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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ACS Biomaterials Science & Engineering

was purchased from Aladdin Industrial Corporation (Shanghai, China). All reagents used in this work were of analytical grade and used without further purification. Preparation of GG Hydrogel. The GG hydrogel was prepared by dissolving 0.15 g of Guar Gum in 10 mL of deionized water, and then 4 wt % borax solutions was added into the guar gum solution until the gel was completely formed. Preparation of GG-glycerol Hydrogel. The preparation process of GG-glycerol hydrogel is shown in Figure 1. First, 2 mL of glycerol was dissolved in 18 mL of deionized water. Then, 0.3 g of GG powder was added in the glycerol/water solvent and stirring by magnetic stirrer. Finally, 4 wt % borax solutions were gradually dropped until the gel is completely formed. Solutions with different borax concentrations (0.5, 1, 2, and 4 wt %) were used to prepare different hydrogel samples.

glycerol/water solution system. First, the glycerol/water solvent was used as a dispersion medium for hydrogels, then borax was used as a cross-linking agent. The hydrogel gelation process is as follows: (1) The borax dissolves in water and readily decomposes into borate ions and boric acid, forming an alkalescent boric/borate solution (eqs 1 and 2). (2) The cross-

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CHARACTERIZATION

Water Absorption Measurement. The swelling studies were performed by immersing as-prepared wet GG hydrogel and GGglycerol hydrogel samples in a standard saline solution (0.9 wt % NaCl) at room temperature. Samples were taken at regular intervalsand the weight was measured after the surface moisture was wiped off with filter paper. The swelling index was calculated as follows: swelling index = (Ws − Wd)/Wd where Ws is the weight of swollen hydrogel and Wd is the initial weight of the wet hydrogel. All experiments were done in triplicate. Scanning Electron Microscopy. The GG hydrogel was freezedried and then coated with gold for field-emission scanning electron microscopy (FE-SEM) analysis. The GG-glycerol hydrogel cannot be completely freeze-dried due to the glycerol. Prior to the analysis, the sample was rinsed with deionized water to remove glycerol. The glycerol-free hydrogel is freeze-dried and then coated with gold for FE-SEM analysis. To observe the structure of the hydrogel crosssection, we exposed the cross-section by fracturing the composites in liquid nitrogen. All the samples were photographed by Nova Nano SEM 230(FEI S.R.O., Czech Republic) at an accelerating voltage of 5.0 kV. FTIR Spectroscopy. FTIR spectra of the GG-glycerol hydrogel were inspected with a Nicolet 380 FTIR spectrometer (Thermo Electron Instruments Co., Ltd., USA) in the frequency range of 4000−400 cm−1 with a total of 32 scans and resolution of 4 cm−1. The freeze-dried sample was mixed with KBr to form an ultrafine powder, and the sample was then pressed into thin slices. DSC Analysis. GG hydrogels and GG-glycerol hydrogels were investigated for their freezing temperature using a differential scanning calorimetry (DSC 214, Netsch). The cooling cycle was

linking process is divided into two steps, monoidal complexation (eq 3) followed by a cross-linking reaction to form the diol−borax complex (eq 4).25 (3) Glycerol and borax undergo a cross-linking reaction similar to the guar gum cross-linking process after the addition of glycerol (eqs 5 and 6), glycerol and guar gum may also be cross-linked by borax (eq 7).Guar gum and glycerol especially, with a wide range of sources, good biocompatibility, and complete degradation properties, have extensive advantages as smart sensor hydrogels of direct contact with the human skin. This facile and low-cost method for preparing flexible and self-healing natural hydrogels with antifreezing, injectable, and strain-sensitive properties will have a wide application in biomedicine, three-dimensional scaffolds, artificial skin, wearable sensor devices, and other fields.

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EXPERIMENTAL SECTION

Materials. Guar gum (over 99.99% purity) was obtained from Chengdu Aike Reagent Co., Ltd. (Chengdu, China) Borax (sodium tetraborate decahydrate, over 99.5% purity, Na2B4O7·10H2O, Mw = 381.37 g/mol) and glycerol (1,2,3-propanetriol, over 99.99% purity)

Figure 1. Schematic illustration of the preparation of the GG-glycerol hydrogel. B

DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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ACS Biomaterials Science & Engineering performed from 30 °C to −70 °C at a rate of 5 °C/min under argon protection. Rheological Analysis. Rheological behaviors of GG hydrogel and GG-glycerol hydrogel were analyzed with a stress-controlled rheometer Rotational Rheometer MARS III Haake (Thermo Scientific, Germany) by using a parallel plate geometry with a diameter of 35 mm. Samples were subjected to a strain sweep test in which they were deformed at different shear strain (the modulus G was independent of the applied strain). To ensure that each measurement is measured in the linear viscoelastic region, we selected 5% of the deformation in the test. Oscillatory frequency sweeps were performed from 1 to 10 Hz at 25 °C under each hydrogels condition. For time sweep tests, the GG-glycerol hydrogel and fresh self-healed GG-glycerol hydrogel, the storage modulus G′ and loss modulus G″ of the original and the healed GG-glycerol hydrogel were measured at a strain of 5% and frequency of 1.0 Hz, respectively. Microscopic Observation of Self-Healing Process. The selfhealing process of the GG-glycerol hydrogel was observed by an Optical Microscope (magnified 40 times, Axio-Imager LSM-800, Germany). After the sample was sliced, a scratch was manually cut, and then the crack of the sample was continuously observed under the same microscope. Adhesion Tests. The GG-glycerol hydrogel was adhered to the surface of the copper sheet with an adhesion area of 20 mm × 20 mm. The samples were pulled at a crosshead speed of 10 mm/min at room temperature using a digital tensile machine (KJ-1065B, Kejian instrument Co. Ltd., Dongguan, China) until the samples were separated completely. The adhesion strength was calculated according to the maximum tension divided by the adhesion area. Electrical Tests. The resistance measurements were obtained using an LCR meter (TH2832, Changzhou, China) at an AC voltage of 1 V and a sweeping frequency of 1 kHz. The LCR meter was used to detect simultaneous resistance changes under various stimuli on the GG-glycerol hydrogel sensor.

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Figure 2. (a) Effect of different borax concentration (wt %) on gel formation. (b) Swelling properties of wet GG-glycerol hydrogel and GG hydrogel in standard saline solution (0.9% NaCl) at room temperature.

RESULTS AND DISCUSSION Wet Hydrogel Cross-Linking and Swelling Analysis. The glycerol content in the hydrogels has a great influence on the gel formation. The GG-glycerol hydrogel can be formed in an ideal state only when the glycerol content is 10% (v/v). Therefore, the glycerol content of 10% (v/v) in the hydrogel was chosen for further study. To further explore the effect of borax concentration on gelation, we cross-linked GG-glycerol hydrogels with different concentrations of borax, and the results are shown in Figure 2. As the concentration of borax increases, the degree of cross-linking of the GG-glycerol hydrogels gradually increases. When the concentration of borax is 4 wt %, gel can be completely forms. Generally, the concentration of borax should be very low when it is used as a cross-linker for the hydrogels. For example, Dai et al.19 successfully prepared GG hydrogels containing Ag nanoparticles with 1 wt % of borax as a cross-linker. However, the concentration of borax needed for GG-glycerol hydrogel is much higher (4 wt %) than that of the GG hydrogel. This may be due to the glycerol in the GG-glycerol system consuming most of the borax. The swelling properties of the wet hydrogels were studied, and the results are shown in Figure 2b, the GG hydrogel first absorbs water and gradually loses weight. After about 16 h, the hydrogel dissolved and lost its gel characteristics. However, GG-glycerol hydrogel absorbed a lot of water rapidly within 10 h, and the final swelling ratio is about 25%. The possible reason is that borax cross-linked hydrogel is not firm enough and the guar gum is water-soluble,26 leading to a decrease in the hydrogel weight until the gel status is completely lost. Compared to GG hydrogel, the water adsorption ability of

GG-glycerol hydrogel is improved, and it has good resistance to dissolution in water. This may be attributed to the tight dynamic cross-linking and hydrogen bond between glycerol− water−borax, limiting the out flow of water within the hydrogel. However, the GG-glycerol hydrogel still degraded in water after about 36 h, this was possibly because most of the glycerol was displaced from the hydrogel in a long time, causing the borax−glycerol−water system in the hydrogel to be destroyed.27 FTIR Analysis. The FTIR spectra of GG and GG-glycerol hydrogel are shown in Figure 3. A characteristic broad absorption band at 3430 cm−1 can be found in GG-glycerol hydrogel, which is the O−H stretching vibration of the GG polymer. The sharp peak at 1230 cm−1 belongs to the asymmetry extension of B−O−C, which supports the crosslinking of GG by borax. The absorption band of GG associated with O−H bending at 1643 cm−1 moves to 1637 cm−1 of GGglycerol hydrogel, indicating intermolecular/intramolecular hydrogen bond vibrations.28 Hydrogel Mechanical and Injectable Properties. Hydrogels with poor stretching ability are difficult to withstand mechanical deformation and then stably move deformation into an electrical signal. This defect is fatal to the preparation of sensors.29 The GG hydrogel is fragile and hard to stretch, but the ultrasoft GG-glycerol hydrogel with good flexibility can not only be extruded into a film but also stretched and twisted dramatically. The simple dynamic reversible cross-linking of borax is not a steady structure. Lu et al.25 have considered C

DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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folded fish-net network can be stretched, and the aggregated guar gum polymer chains move into interlocking lines. When the hydrogel is pulled into a thin film, the dynamic hydrogen bond and cross-linked structures in the collapsed grid rapidly break and quickly combine to maintain the shape of the enlarged network, and the guar gum chains slip and disperse from the net. This elastic space network may allow the hydrogel to maintain a large deformation, and the fracture and rearrangement of the cross-linked net effectively dissipates energy, rendering the hydrogel a good flexibility.30 Rapid preparation and injectable properties of hydrogels at room temperature are critical for preparing cell-print scaffolds for 3D-printed wearable electronic device.31 Because GG hydrogels have poor flexibility and a slow self-healing effect,32 it is difficult to deform and inject after preparation. This is also a common problem in most hydrogels. However, the GGglycerol hydrogel exhibited an excellent injectable property. As shown in Figure 4b, the GG-glycerol hydrogel can be smoothly extruded through a needle onto a plate. The injectable property can be attributed to an equilibrium of hydrogels structure: (1) Enough shear force will damage the hydrogel. In the viscosity (η) analysis of the GG-glycerol hydrogel, its viscosity decreased with the increasing frequency, indicating that the hydrogel has typical shear thinning properties; (2) Once the shear force removes, it can quickly recover.33 Various shapes injected by syringe can be obtained with GG-glycerol hydrogel. Morphology Observation. The cross-section morphologies of freeze-dried hydrogels are shown in Figure 5. The GGglycerol hydrogel is prone to form the homogeneous structure

Figure 3. (a) FTIR spectra of GG. (b) FTIR spectra of GG-glycerol hydrogel.

adding MFC to form an interpenetrating network to improve the mechanical properties of borax cross-linked hydrogel, but the effect of polymers reinforcement is not significant enough. However, the nanostructured glycerol may provide a strong bond effect, resulting in an obviously improved flexibility of the hydrogel. As shown in Figure 4a, glycerol can form a strong hydrogen bond with water and a reversible chelation crosslinking with borax, giving rise to a bridge connecting water and borax molecules, resulting in a fishnetlike structure composed of small molecules water, glycerol, and borax. The guar gum molecules are trapped in the net by dynamic borax crosslinking and hydrogen bond. When the gel is stretched, the

Figure 4. (a) Mechanism model of GG-glycerol hydrogel extension and stretching. (b) Syringe injection of different shapes and variation of viscosity in variable frequency scanning of GG-glycerol hydrogel. D

DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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small molecule glycerol on the hydrogel, the same volume of glycerol/water solution and deionized water were frozen at −16 °C for 4 h. The result is shown in Figure 7a, it was found

Figure 5. (a) Cross-section FE-SEM pictures of GG hydrogel. (b) Cross-section FE-SEM pictures of GG-glycerol hydrogel.

with smaller pores and tighter networks. The strategy of improving the mechanical properties of hydrogel by adding nanomaterials to reduce the pore size has been proven.34 When the nanostructure glycerol is added, the internal space of the hydrogel shrinks and the pores dimensions size decreases, this will lead to a lean in the density of the polymer network.35 This tight 3D network may have changed the weak mechanical properties of the GG hydrogel. When resisting stress concentrations, the uniform distribution of pores distributes the stress more evenly throughout the hydrogel network, and small pores and an increased number of pores can be used as a barrier against crack propagation.36 This is a direct reason for excellent ductility and flexibility of GG-glycerol hydrogels. Rheological Analysis. It can be seen from Figure 6 that the storage modulus (G′) of both GG hydrogel and GG-

Figure 7. (a) Deionized water and water glycerol mixture solution (volume ratio 9:1) for freezing at −16 °C for 4 h. (b) Before and after GG hydrogel and GG-glycerol hydrogel for freezing at −16 °C for 1 h. (c) DSC thermograms of GG-glycerol hydrogel and GG hydrogel from 30 °C to −70 °C. The cooling rate was 5 °C/min.

that pure water has been completely frozen. However, some small ice crystals were only formed in glycerol/water solution, and these ice crystals can be dissolved after shaking. To verify the antifreeze performance of GG-glycerol hydrogel, we froze GG hydrogel and GG-glycerol hydrogel at −16 °C for 1 h to obtain two new hydrogels as shown in Figures 7b. The GG hydrogel had completely frozen and was very hard. The GGglycerol hydrogel remains soft and elastic, indicating that the glycerol has antifreeze effect on GG hydrogels. The results of DSC analysis are shown in Figure 7c. For GG hydrogels, a sharp peak was observed at −13.8 °C, which can be attributed to the freezing of water in the hydrogel. Interestingly, for the GG-glycerol hydrogel, a peak at −18.7 °C was found and the peak became smaller. The possible reasons for the antifreeze property of glycerol are (1) Glycerol can form strong hydrogen bonds with water molecules and disrupt the formation of ice crystal lattices at subzero temperatures.22 (2) The water−borax−glycerol complex improves the stability of polymers hydrogels systems in lowtemperature environments.39 Self-Healing Property Analysis. The self-healing hydrogel can quickly restore its network structure and multifunction after damage. For the manufacture of wearable electronic devices and electronic skin, it is essential to achieve high safety, reliability, and durability.40 To prove the rapidly self-repair property of the GG-glycerol hydrogels, the macroscopic selfhealing tests using inspection of direct visual and optical microscopy methods were carried out and the results were

Figure 6. Storage modulus G′ and loss modulus G″ of GG-glycerol hydrogel and GG hydrogel versus frequency.

glycerol hydrogel is obviously greater than their loss modulus (G″) at high frequencies, indicating the presence of classical cross-linked hydrogel network.37 In all frequency range, the G′ of GG hydrogels is larger than GG-glycerol hydrogel. It is known that the lower the storage modulus generally indicates the lower the stiffness. So, the GG-glycerol hydrogel is easy to deform, and has better flexibility, which is also consistent with the FE-SEM analysis and the excellent ductility performance of GG-glycerol hydrogel. Antifreeze Property and DSC Analysis. The elasticity of hydrogels is inevitably affected by temperature, hydrogels lose their elasticity owing to the freezing of solvent (i.e., water) at subzero temperatures.38 To investigate the antifreeze effect of E

DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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Figure 8. (a) Demonstration of self-healing ability for GG-glycerol hydrogel at room temperature. (b) Original and self-healed GG-glycerol hydrogel in time sweep test (frequency, 1.0 Hz; strain, 5%). (c) Optical microscopy to observe the self-healing process of hydrogel crack.

glycerol, which can fill up the gap between the polymer chain and the cross-linking agent, providing more hydrogen bonds and borax cross-linking sites, facilitating recombination of borax-diol and hydrogen bonding at the contact surface.25 Ultrafast and fully restored internal structure of the hydrogel can be achieved by a multibond interaction, surpassing most self-healing effects of hydrogels, which shows potential for the manufacture of self-healing smart devices. Self-Adhesive and Strain-Sensitive Properties. It is troublesome that most wearable devices made from hydrogels require other tools to fix positions, and the prepared selfadhesive hydrogels can perfectly solve this problem. Most hydrogels are hard and have poor ductility. A common strategy is to use dopamine as a binder to give the hydrogel selfadhesive properties,43 We have a facile method to prepare GGglycerol hydrogel films that have the self -adhesive ability. It can be seen from Figure 9a that the GG-glycerol hydrogel film exhibited adhesion and mechanical compliance for human skin. Skin of human arm was pasted on transparent hydrogel film, after the rotation of 90°, it is still tightly fit and does not fall. According to the tensile adhesion test results, the adhesion of

shown in Figure 8a. It was found that once two freshly prepared GG-glycerol hydrogels were brought into contact with each other, they immediately adhered and self-healed to form a single hydrogel without any irritation or external force. During the process, we can observe the diffusion of the red hydrogel at the contact interface, resulting in the blurred interface. It can be seen from Figure 8b that the storage modulus (G′) and loss modulus (G″) of the healed GGglycerol hydrogel are like those of the original hydrogel, indicating the full recovery of the internal network structure of the hydrogel.37 The self-repairing process of the hydrogel with cracks were observed using an optics microscope and the results are shown in the Figure 8c. It was found that the scratches in the hydrogel are blurred in 15 s, indicating an ultrafast self-healing process with cracks. The self-recovery rate of borax-cross-linked hydrogels is generally more than a few minutes.41,42 This self-healing effect is not fast enough for real-time monitoring of sensing devices. By introducing glycerol into the hydrogel system, the selfhealing ability of the GG-glycerol hydrogel is greatly improved. This may be attributed to the addition of small molecules of F

DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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sional scaffolds, artificial electronic skin, and 3D-printed wearable sensor devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lei Dai: 0000-0002-9011-2246 Kai Liu: 0000-0001-7833-2839 Yonghao Ni: 0000-0001-6107-6672 Liulian Huang: 0000-0003-3158-593X

Figure 9. (a) Self-adhesion of the transparent GG-glycerol hydrogel film. (b) Strain- sensitive detection model. (c) Relative resistance changes of the GG-glycerol hydrogel sensor. The insets show the hydrogel strain sensor adhered onto the finger straightening and bending.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the national key research and development program of China (Grant 2017YFB0307900).

the hydrogel to the copper sheet is about 2.5KPa. This property may be the reason that many exposed hydroxyl groups on the surface can be in good contact with the skin to generate hydrogen bonds. In addition, flexible GG-glycerol hydrogel has great strain-sensitive property. To investigate the strain sensitivity of the GG-glycerol hydrogel, we constructed a resistance detection circuit to monitor the change in resistance of the finger from stretching to bending (Figure 9b), and the results are shown in Figure 9c. When the finger starts to bend from the initial state, the resistance gradually increases. The resistance decreases when the finger returns to straightening. Meanwhile, the GG-glycerol hydrogel exhibits good repeatability. This is caused by the unique three-dimensional structure of ion-conducting GG-glycerol hydrogel: (1) The ions produced by borax dissolution provide conductivity of the hydrogel. (2) The three-dimensional network structure of the hydrogel provides a mobile channel for ions movement. When the finger is bended, the polymer network of the hydrogel will be stretched, resulting in the increase in the resistance of the hydrogel.34 This mechanical change is converted into an electrical signal, which is then received by the LCR meter, and the mechanical deformation of the hydrogel is continuously monitored. These results demonstrate that smart GG-glycerol hydrogels are promising for smart hydrogel applications such as self-adhesive electronic skin and wearable strain-sensitive devices.

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CONCLUSIONS In summary, a glycerol−water−borax system to form the flexible, self-healing, injectable, antifreeze, and strain-sensitive smart GG-glycerol hydrogel was developed. Our findings demonstrate that glycerol dual bonds functions result in a small pores network resisting crack propagation and quick reconstruction of energy dissipation, which cause the hydrogels to acquire high flexibility, ductility, self-adhesive, and good injectable property. More importantly, the glycerol with three hydroxyl groups provides a lot of cross-linking and hydrogen bond sites in the gap of polymers, enabling ultrafast self-healing capability and restoration of hydrogel internal structures, extending their service life. Furthermore, the glycerol inhibits the formation of ice crystals inside the polymers network, which increases the antifreeze effect of hydrogel. Finally, the small porous structure provides multilayer ions transport channels that can achieve significant stain-sensitivity. The results provide the possibility for constructing flexible, selfhealing, antifreezing, injectable, and strain-sensitive hydrogels as self-adhesive hydrogel dressings, injectable three-dimenG

DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsbiomaterials.8b00657 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX