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Rapid self-healing, stretchable, moldable, antioxidant and antibacterial tannic acid-cellulose nanofibril composite hydrogels

Wenjiao Ge, Shan Cao, Feng Shen, Yuyuan Wang, Junli Ren, Xiaohui Wang

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China

A B S T R A C T

Here, we designed a self-healing composite hydrogel with antioxidant and antibacterial activities by using cellulose nanofibrils (CNF) and tannic acid (TA) as functional additives. Excellent mechanical stability, moldability, stretchability and rapid self-healing ability without any external intervention were realized in one system due to the combined dynamic borate ester bonding between polyvinyl alcohol-borax (PB) and multi-hydrogen bonding between different components. The rheological measurements indicated the incorporation of CNF and TA to PB system substantially affected the viscoelasticity of hydrogels. The unique antioxidant and antibacterial properties were achieved due to the complexation of TA. These high performance multifunctional hydrogels opens a window for a broad application in the field of smart devices and surface engineering.

1. Introduction

Self-healing hydrogels are cross-linked polymer network structures capable of autonomously repairing from an event of damage. They offer substantial benefits to prolong the life span of material and restore and/ or retain original properties (Taylor & In Het Panhuis, 2016) and thus show great potential for application in a broad range of fields, from coatings and drug delivery vehicles to soft robotics. Self-healing hydrogels rely on the energy dissipation by the dynamic bonds, either intramolecular interactions (e.g. hydrogen bonding, ionic bonding, etc.) or dynamic covalent bonds (e.g., Schiff base, acylhydrazone, disulfide, borate ester, etc.). Numerous polymers are available for the fabrication of self-healing hydrogels, e.g., poly (ethylene glycol) (Li, Zhang, Fortin, Xia, & Zhao, 2015), poly (acrylic acid) (Shao et al., 2018), poly (vinyl alcohol) (PVA) (Lu et al., 2016), hyaluronic acid (Yu, Cao, Du, Wang, & Chen, 2015), chitosan (Qu, Zhao, Ma, & Guo, 2017), guar gum (Pan et al., 2018). Among these polymers, PVA has attracted widespread interests as a well-known water-soluble, highly crystalline, nontoxic and biocompatible polymer (Baron et al., 2019; Chunshom, Chuysinuan, Techasakul, & Ummartyotin, 2018). On the other hand, current demanding applications of self-healing hydrogels, especially in smart material field, need them to simultaneously satisfy several functions. In respond to the needs, a number of adhesive or conductive PVA hydrogels have been developed by introducing functional additives such as polydopamine (Liu et al., 2018), Ag dots (Pourreza & Ghomi, 2017), carbon nanotube (Cai et al., 2017), polypyrrole (Ding et al., 2018). However, the inclusion of functional additives tend to weaken the dynamic interactions that holding the hydrogel network, and thus result in damaged mechanical strength. Therefore, fabrication of multi-functional self-healing hydrogels with reliable mouldability, good mechanical stability and stretchability is still a challenge. In this work, highly stretchable, moldable, rapid self-healing hydrogels with good antioxidant and antibacterial properties were designed based on embedding CNF and TA into PVA and borax hydrogel networks. CNF with high strength and dimensional anisotropy natural sourcing (De France, Hoare, & Cranston, 2017) served as a renewable reinforcing agent. TA, a weakly acidic polyphenolic compound possessing excellent antioxidant, antibacterial, antimutagenic, antigenic activities and good biodegradability (Hong, 2016), was majorly used to endow the hydrogels bioactivities. The variations of the chemical structures and crystallinity before and after cross-linking were discussed, and the rheological behaviors, morphology, self-healing, antioxidant and antibacterial properties of the hydrogels were also evaluated.

2. Experimental section

2.1. Materials

Cellulose nanofiber oxidized by TEMPO-mediated oxidation (CNF,1 wt%) was obtained from Tianjin Woodelfbio Cellulose Co., Ltd. Poly

(vinyl alcohol) (PVA, 98.099.0% alcoholysis) and borax (sodium tetraborate decahydrate, 99.5% purity) were supplied by Sinopharm Chemical Reagent Co., Ltd. TA, DPPH (2,2-Diphenyl-1-picrylhydrazyl,97.0% purity), and 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid ammonium salt) (ABTS, 98% purity) were purchased from Aladdin-reagent Inc. All other reagents were of analytical grade without further purification.2.2. Hydrogelation PVA powder (5.0 wt%) was added to CNF suspension (1.0 wt%) under vigorous stirring at 90 °C until the homogeneous solution formed. And then, borax aqueous solution (0.6 wt%) with various amount of TA was added into the above solution in a ratio of 1:5 (v/v). After stirring, homogeneous hydrogels were formed. The mass ratio of TA to PVA were 5/1, 10/1, 15/1 and 20/1 w/w, and the corresponding hydrogels were labeled as PBCT1, PBCT2, PBCT3 and PBCT4, respectively. The PB, PBC and PBT hydrogels were prepared with similar methods without CNF and TA, TA, and CNF, respectively.

2.3. General characterizations The functional groups on PBCT hydrogels were characterized by using fourier transform infrared (FT-IR) spectroscopy (VERTEX 70, Germany) equipped with a universal attenuated-total-reflection (ATR) probe. X-ray diffraction (XRD) patterns were recorded on a D8 ADVANCE X-ray diffractometer (Germany) using Cu Kα1 radiation in the 2θ range from 5° to 60°. The morphology of the hydrogels was investigated using a scanning electron microscope (SEM, EVO 18, Germany) at an accelerating voltage of 5 kV. Prior to observation, all the freeze-dried hydrogel samples were cut to expose to their inner structures and sprayed with a thin gold layer.

2.4. Rheological test

Dynamic rheological behaviors of the PBCT hydrogels were analyzed at 25 °C using a rheometer (TA ARES G2, USA). Samples were prepared in the shape of a disk with a diameter of 15mm and a

thickness of 6 mm. The frequency sweep was carried out to measure storage modulus (G) and loss modulus (G) as a function of angular frequency (ω) over the range of 0.1100 rad/s at 1% strain amplitude. The loss tangent (tan δ), complex modulus (G*) and complex viscosity (η*) were calculated based on the value of Gand G(tan δ=G/ GG*= G'2 + G2 , η*=G*/ω). The self-healing property was investigated by straining the hydrogels under an alternatively changing amplitude of oscillatory strain. To determine the critical strain region, the dynamic strain sweep from 0.01 to 1000% at ω=10 rad/s was investigated. Then amplitude oscillatory strains were switched from small strain (200 s for each interval) to large strain (200 s for each interval), and two cycles were carried out. 

2.5. Self-healing performance The hydrogel was cut into two parts, and then the fractured two parts were brought into contact with each other to observe the selfhealing phenomena. To estimate ex-situ self-healing behavior, four blocks of freshly prepared PBCT hydrogels stained with methyl orange, amaranth and thymol blue were put together without any external force. All the situations of the hydrogels were photographed.

2.6. Antioxidant assay

Antioxidant activity was measured using two different in vitro assays (i.e., DPPH, ABTS assay). For DPPH assay, a fresh DPPH/ethanol (40 mM) solution was used for the measurements. Afterwards, 50 mg of hydrogels was immersed in 3 mL of DPPH solution and kept in the dark at room temperature for 30 min. The absorbance change at 517 nm was measured using a UVvis spectrophotometer (SHIMADZU UV-1800, Japan). DPPH radical scavenging activity was calculated as the following formula: DPPH scavenging activity=(AblankAsample)/ Ablank×100%, where Ablank is the absorbance of DPPH solution, Asample is the absorbance of samples mixed with DPPH solution. Each sample was carried out in triplicate. ABTS assay was carried out through a previously reported method with slight modification. The ABTS radical cation (ABTS%+) was produced by reacting 7.4mM ABTS stock solution with 2.6mM potassium persulfate (K2S2O8) and allowing the mixture to stand in the dark at room temperature for 24 h. Before usage, the ABTS%+ solution was adjusted with ultrapure water to an absorbance of 0.70 ± 0.02 at 734 nm. Then, a determined amount of hydrogel samples was mixed with 3 mL ABTS%+ solution and incubated in the dark for 20 min. The absorbance of the resulting solution at 734 nm was monitored by

UVvis assay. ABTS%+ scavenging activity was evaluated based on the following formula: ABTS%+ scavenging activity=(AblankAsample)/ Ablank×100%, where Ablank and Asample were the absorbance of ABTS%+ solution before and after mixing with samples.

2.7. Antibacterial assay

The antibacterial activity of the hydrogels was assessed against Staphylococcus aureus (S. aureus) ATCC 6538 by using punch well agar diffusion method. In brief, the petri dishes containing bacteria cultured agar plates (105 CFU/mL) were punched with 6 mm-diameter wells. Subsequently, 200 mg of the PB-based hydrogels were filled into these pre-punched wells and incubated at 37 °C for 24 h. 100 μL of PBS served as the control. Each test was performed three replicates and the diameter of the zone of inhibition around each well was measured and recorded.

3. Results and discussion

3.1. Fabrication of PBCT hydrogels

In this study, we designed a kind of self-healing PBCT hydrogels with high water content, good adhesiveness and excellent antioxidant and antibacterial activities. The schematic representation of the hydrogels is shown in Fig. 1a. PVA could be well dissolved in the CNF/ water dispersion, afterwards TA and borax were simply blended in to form PBCT hybrid hydrogels. In this system, CNF with several micrometers in length (Fig. S1a) was applied as reinforcement nanofiller, while TA was chosen for its abundant phenolic hydroxyl groups and superior properties. Dynamic chemical bonds and hydrogen bonds among PVA, borax, CNF and TA formed a physically-chemically crosslinked double network, leading to the self-healing property of PBCT hydrogel. In this hydrogel, the monoborate ions from borax could react with cis-diol in PVA, CNF and TA to form dynamic borate ester complex (Huang et al., 2017), leading to strong interactions among these components (Fig. 1b). Meanwhile, chain entanglements, inter- and intramolecular hydrogen bonding between the different compents also contributed to the formation of 3D network within the PBCT hydrogels. Interestingly, the obtained hydrogels with high water content (Fig. S1b) could be readily molded and remolded into various shapes (Fig. 1c), suggesting their high ductility and flexibility. Moreover, the PBCT hydrogels exhibited good adhesion to the finger epidermis even during the stretching process. Their adhesive strength to porcine skin was also quantified by a tensile adhesion test with the highest strength up to

4.8 K Pa (Fig. S2). 3.2. FTIR and XRD analysis In order to reveal the interactions among the PVA, borate ion, CNF and TA, the ATR-FTIR spectra of PBCT hydrogels and their compositions are shown in Fig. 2a. For neat PVA, the prominent peaks showed OeH stretching at 3300 cm1, CeH alkyl stretching at around 29402905 cm1, and CeO stretching of secondary alcohols at 1086 cm1 (Kumar, Negi, Bhardwaj, & Choudhary, 2012). The symmetric CeC stretching vibration was observed at 1142 cm1, which is the characteristic of semi-crystalline PVA (Kumar et al., 2017). In the spectra of TA, spectral peaks at 1705 cm1 due to C]O stretching vibration, 14421609 cm1 due to aromatic stretches, 1311 cm1 (outof- plane vibration of OeH bonds), and other clear characteristic peaks at 10191185 cm1 (substituted benzene rings vibrations), and 754 cm1 (in-plane vibration of OeH bonds) in the fingerprint region were observed (Hu et al., 2018). After crosslinking, several distinct peaks characteristic of borax and borate can be observed in the spectra of PB hydrogel. The peaks at 1426 and 1338 cm1 related to the asymmetric stretching vibration of BeOeC bonds, and the peak at 834 cm1 derived from BeO stretching vibration from residual borate (Koysuren, Karaman, & Dinc, 2012; Kumar et al., 2017). The peak at 664 cm1 was responsible for the bending of BO-B linkages within borate network (Spoljaric, Salminen, Luong, & Seppälä, 2014). These strong evidences confirm the crosslinking formation and the presence of borate networks. Moreover, the spectrum of CNF shows typical strong hydroxyl band for eOH stretching vibration at 3335 cm1, eCH2 bending at 1424 cm1, and CeO stretching vibration at 1031 cm(Nie et al., 2018; Xu, Krietemeyer, Boddu, Liu, & Liu, 2018). The spectrum of PBC with CNF is similar to that of PB hydrogel. Specifically, the peak around 1058 cm1 was ascribed to CeO stretching vibration of CNF backbone, which can be identified in PBC hydrogel sample. With the participation of TA, the spectrum of PBCT shows variations in intensity and shifting of peak from 3335 to 3365 cm1, which may be due to eOH stretching vibration of TA and the intermolecular interaction among PVA, CNF, TA and borate via hydrogen bonds. Meanwhile, strong signals at 1713 and 758 cm1 resulted from the characteristic of TA, suggesting the successful incorporation of TA. The XRD patterns of PBC and PBCT hydrogels are shown in Fig. 2b. The pure PVA exhibited three typical diffraction peaks at 2θ=19.6°, 2θ=22.9°, and 2θ=40.8°, corresponding to the (101), (200), and (103) planes of PVA crystallites (Peng, Xiao, Tang, & Zhou, 2014). With the incorporation of borax, only a blunt amorphous peak centered at 2θ=20.0° can be observed. The disappearance of the typical peaks of PVA crystallites suggests that strong interaction between borate and the eOH groups of PVA leads to the destruction of well-organized PVA structure (Han et al., 2017). By further addition of CNF, PBC exhibited diffraction peaks at 2θ=22.6°, indicating the existence of the CNF component in the hydrogel. Furthermore, the presence of TA in PBCThydrogels had no effect on the crystalline behavior but slightly strengthen the intensity of diffraction peaks comparing with PBC. Hence, the crosslinking between PVA, borax, CNF and TA exerts strong effects on their structures and crystalline behaviors.

3.3. Morphology analysis Fig. 2CH gives the fractured surfaces of freeze-dried studied materials by SEM analysis. Obvious differences in the pore size, and roughness of pore walls of the different samples were observed. Compared with PB hydrogel, the PBC hydrogel exhibited a denser network with smaller pore sizes and higher surface area. The featured nanofibrils were observed in the CNF-contained hydrogel for their intermolecular interactions with PVA and borax species. Moreover, the CNF composites did not exhibit visible aggregates under 1000 magnification, indicating an effective dispersion of CNF throughout the PB matrix (Naseri, Deepa, Mathew, Oksman, & Girandon, 2016). TA can bound to the PB matrix and form an entangled network on the pore walls. Increasing the amount of TA in the composite hydrogel leads to large area film-like regions and larger pores, which might explain the higher moisture uptake for this system (Spoljaric et al., 2014). Consequently, the incorporation of CNF and TA showed noticeable influence on the PB matrix and played a critical role in achieving desired performance of hydrogel. These 3D interconnected porous structures are beneficial to the mechanical and responsive performances of the hydrogels.

3.4. Rheological properties

To further investigate the viscoelasticity and ductility led by the incorporation of CNF and TA into the PB hydrogels, various dynamic oscillation rheology measurements were employed to gain insight into the properties of the hydrogels. Typically, gels formed by permanent covalent bonds display frequency-independent moduli, whereas frequency- dependent moduli was observed for gels cross-linked with temporary, reversible bonds (Roberts, Hanson, Massey, Karren, & Kiser, 2007). The Gand Gof the prepared hydrogels in the ω range of 0.1100 rad/s are illustrated in Fig. 3ac. There was an apparent change in both Gand Gas a function of increasing ω, regardless of the hydrogel composition, which is in accordance with previous observations of other PB based hydrogels. In the low frequency region where tan δ > 1, a liquid property was signified because the time scale probed is longer than the lifetime of the reversible crosslinks, thus it is allowable for the network to restructure (Deng et al., 2012). As ω proceeded, Ggradually approached Geven well past G, whereas G″ reached its peak and then started to decrease. At higher ω, there is insufficient time allowed for the labile crosslinks to dissociate (Roberts et al., 2007). Thus, the difference between Gand Gis getting larger (tan δ < 1) and gels exhibit elastic-like behavior. The crossover between Gand Gwas identified as the indication of gelation. In this work, we are particularly interested in the following characteristic parameters to quantify the rheological functions of the PBCT hydrogels: the relaxation time (trelax) (Fig. 3d) and the plateau value of the storage modulus obtained at high angular frequencies (Gmax) (Fig. 3e). The relaxation time trelax can be determined by trelax=2π/ωc, where ωc is the crossover ω at which tan δ=1 (Piest, Zhang, Trinidad, & Engbersen, 2011). Generally, trelax reflects the average lifetime of the crosslink. Hydrogels with longer relaxation times show more elastic behavior and have a better shape stability (Piest et al., 2011). As shown in Fig. 3f, neat PB hydrogel possessed a small trelax value of 5.71 s. As expected, the trelax of PBC hydrogel was almost 5 times longer than that of PB hydrogel due to the incorporation of 1% CNF, revealing the subtle effect of CNF. With further addition of TA into the PBC matrix, the relaxation processes of the hydrogels (PBCT1, 3 and 4) were also slowed down with increased trelax values. They could be more stable in shape compared to PB hydrogel. The increase of trelax can be explained by an increase in polymer entanglements, a higher crosslink density, or a combination of these (Ding et al., 2018; Ivanov, Larsson, Galaev, & Mattiasson, 2004). Remarkable difference was also observed in Gmax values of all the hydrogel samples. Compared with the pure PB hydrogel, an increase of Gmax was generated in PBC because of the entanglement and crosslink performance of CNF in the hydrogel matrix. With the participation of TA, the Gmax values declined with the increase of TA content, suggesting a disassociate effect of TA to the PBC hydrogel network.

In addition, according to the plots of G* and η* as a function of ωhydrogels with the incorporation of CNF and TA exhibited the similar trend with the results of Gmax. It is further confirm that CNF acts as reinforcing filler and crosslink role in the 3D networks of the PB hydrogels. While existence of TA would cause consumption of borate, thus leading to reduced borate to react with PVA and decrease in viscoelasticity properties. In this view, the presence of CNF and TA brought about pluralism and otherness for the PB matrix. Their viscoelasticity properties can be easily tailored by adjusting different amount of TA. 3.5. Shape stability and tensile property Although ductility and elongation were facilitated via crosslinking with borax, the PB and PBC hydrogels still displayed flow characteristic after inversion for 0.5 h, indicating weak material stability (Fig. 4a). Fortunately, this limitation can be overcomed with the addition of CNF. As result, PBC and PBCT can be inversed for over 24 h without any changes in shape, indicating enhanced shape stability. The mechanical behavior of the prepared hydrogels was tested by manual stretching and the results are shown in Fig. 4b. The PB hydrogel broke easily in the process of stretching. With the addition of CNF, the tensile strength exhibited an increasing trend and the elongation at break of PBC hydrogel was higher than 800%. These results highlight the significance of adding CNF into the PB matrix systems. CNF as a reinforcement phase resulted in a remarkable improvement in tensile strength and stability. The participation of TA made PBC hydrogel softer and became more accessible to stretch without fracture. The stretchability of PBCT4 even can reach 2000%. 3.6. Self-healing property Cut/heal test was conducted to investigate the self-healing characteristics of the PBCT hydroges. As shown, a piece of hydrogel was cut into two parts and then brought into contact at ambient environment without any hesitation or any external intervention (Fig. 5a). As expected, the fractured two parts could adhere to each other and be hold up to support their own weight. The self-healing mechanism is explained by the dynamic borate ester bonds between the hydroxyl of PVA (or CNF, TA) and borate. When the hydrogel was cut into parts, many reversible borate ester bonds were cleaved at the cut interfaces. These non-associated groups were exposed and had a strong tendency to link together. Thus, reconstruction of dynamic bonds was facilitated across the cut interfaces once the parts were brought in contact (Wang,Tao, & Pan, 2016). Moreover, four single fresh hydrogel blocks stained

with different colors of dyes were put together and autonomously merged into an integral block within 10 s (Fig. 5b). The resultant hydrogel could be stretched into a membrane, indicating fast ex-situ selfhealing performance of PBCT hydrogel (Chen, Hao, Hao, Guo, & Jiang, 2018). The reason for the complete merging is due to a sufficient amount of free hydroxyl groups and borate on the surface of the hydrogels. They tend to establish borate ester bonds and hydrogen bonds at the contacted interface. Rheological recovery test was further employed to assess the selfhealing behavior of the hydrogel. From the result of strain sweep of the PBCT3 hydrogel (Fig. 5c), Gcurve intersected Gcurve at a strain of 30.71% determining the critical point. By further increasing the strain, the Gvalue decreased and was lower than G, revealing a collapse of the gel state to a quasi-liquid state (Shang et al., 2017). Afterwards, continuous step strain measurements at a constant ω of 10 rad/s were performed to determine the strain-induced damage and recovery behavior of the PBCT3 hydrogel (Fig. 5d). A high strain of 100% beyond the critical strain was applied to disrupt the hydrogel network. As displayed, the Gvalue of the hydrogel dropped dramatically along with an inversion of Gexceeding G. Following this, a low strain of 1% was applied to inspect the recovery of the hydrogel structure. The Gvalue immediately restored to its initial value. The destruction and recovery properties of the hydrogel were alternatively repeated two cycles indicating the rapid self-healing ability of the hydrogel. 3.7. In vitro antioxidant activity The in vitro antioxidant activities of the prepared PBCT hydrogels were evaluated by measuring their ability to scavenge DPPH radicals and ABTS%+ radicals. Fig. 6a shows the absorption curves of DPPH radical solution before and after exposure to hydrogel samples. DPPH radicals gave a strong absorption at 517 nm in the visible spectroscopy for the existence of odd electron. Nevertheless, the absorption reduced when the electron pairs with a hydrogen atom from the antioxidant. The reduced DPPH was created, and the color turned from purple to yellow (Wei et al., 2019). The degree of decolorization is directly proportional with the number of electrons scavenged. As shown in Fig. 6c, the PB and PBC samples did not show any DPPH radical scavenging activity. After the addition of TA, about 8.4% of the DPPH radicals were scavenged by PBCT1, highlighting an antioxidant response. The free radicals scavenging ability of TA is based on the phenolic hydroxyl groups, which are strong free radical terminators that can reduce free DPPH radicals to yellow-colored diphenyl-picrylhydrazine (Li et al., 2019). With increasing amount of TA, a more remarkable DPPH scavenging activity could be obtained. In the hydrogel of PBCT4, the antioxidant activity increased up to 91.7%. ABTS assay is based on the antioxidant ability to react with ABTS%+ radical cation generated in the assay system. Fig. 6b shows characteristic absorption spectra of the ABTS%+ and its reduction product ABTS. The ABTS%+ was blue-green in color and had absorption maxima at wavelength of 734 nm. After addition of antioxidants, the pre-formed blue-green ABTS%+ was reduced to ABTS and converted back to its colorless neutral form (Roberta Rea, Proteggentea, Pannalaa, Yanga, & Rice-Evansa, 1999). Afterwards, the reaction of ABTS%+ with antioxidants can be determined by the decrease in the absorbance at 734 nm. The scavenging values of the samples against ABTS%+ were shown in Fig. 6d. The hydrogel matrix prepared without TA showed poor ABTS%+ scavenging activity. On the other hand, the hydrogels with TA showed a good antioxidant behavior, which was attributed to electron transfer or hydrogen atom donation from TA segments to ABTS%+ free radicals. Increasing amount of TA resulted in a higher ABTS%+ scavenging activity. PBCT4 showed a maximum radical scavenging efficiency of 99%. This trend was similar to the scavenging activity observed with the DPPH radicals. These results demonstrate that the PBCT hydrogels have excellent antioxidant capacity originating from TA. 3.8. Antibacterial activity To investigate the antibacterial properties of the PBCT hydrogels, Gram-positive bacterium (S. aureus) was used as model bacteria in our experiment. In Fig. 7a, no obvious inhibition zone was observed around the control disc loaded with PBS. In contrast, it was noticeable that the diameter of inhibition zone of the hydrogels against S. aureus increasedwith increasing amount of TA. Thus, the antibacterial properties can be tailored by adjusting the contents of TA. In addition, the PB and PBC hydrogels also exhibited antibacterial activity for S. aureus, which may be due to the existence of the negative borate (Tavakoli & Tang, 2017). These results indicate the PBCT hydrogels had antibacterial activity against S. aureus, which was probably led by TA.

4. Conclusions

In summary, we adopted a facile approach to fabricate a kind of multifunctional PBCT composite hydrogels based on CNF, TA and PB matrix. By using a combination of CNF and TA as nanofillers and functional additives, the hydrogels performed multi-functions including stretchable, mechanically stable, antioxidant and antibacterial properties. Due to the reversible borate ester bonds and hydrogen bonds, these hydrogels exhibited moldability and self-healing property within 10 s without any external stimuli. The combination of rapid self-healing ability, antioxidant and antibacterial activities, along with an easy process of preparation, make these materials ideal candidates for the design of smart self-healing devices and dynamic surface coatings. Acknowledgement This work was supported by the National Natural Science Foundation of China (51673072). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2019.115147. References Baron, R. I., Bercea, M., Avadanei, M., Lisa, G., Biliuta, G., & Coseri, S. (2019). Green route for the fabrication of self-healable hydrogels based on tricarboxy cellulose and poly(vinyl alcohol). International Journal of Biological Macromolecules, 123, 744751. Cai, G., Wang, J., Qian, K., Chen, J., Li, S., & Lee, P. S. (2017). Extremely stretchable strain sensors based on conductive self-healing dynamic cross-links hydrogels for human-motion detection. Advanced Science, 4(2), 1600190. Chen, W. P., Hao, D. Z., Hao, W. J., Guo, X. L., & Jiang, L. (2018). Hydrogel with ultrafast self-healing property both in air and underwater. ACS Applied Materials & Interfaces, 10(1), 12581265. Chunshom, N., Chuysinuan, P., Techasakul, S., & Ummartyotin, S. (2018). Dried-state bacterial cellulose (Acetobacter xylinum) and polyvinyl-alcohol-based hydrogel: An approach to a personal care material. Journal of Science: Advanced Materials and Devices, 3(3), 296302. De France, K. J., Hoare, T., & Cranston, E. D. (2017). Review of hydrogels and aerogels containing nanocellulose. Chemistry of Materials, 29(11), 46094631. Deng, G., Li, F., Yu, H., Liu, F., Liu, C., Sun, W., et al. (2012). Dynamic hydrogels with an environmental adaptive self-healing ability and dual responsive solgel transitions. ACS Macro Letters, 1(2), 275279.

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