Autonomous Chitosan-Based Self-Healing Hydrogel Formed through

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Article Cite This: ACS Appl. Polym. Mater. 2019, 1, 1769−1777

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Autonomous Chitosan-Based Self-Healing Hydrogel Formed through Noncovalent Interactions Zhong-Xing Zhang,*,† Sing Shy Liow,† Kun Xue,† Xikui Zhang,† Zibiao Li,† and Xian Jun Loh*,†,‡ †

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634, Singapore ‡ Department of Materials Science and Engineering, National University of Singapore, Singapore 117576, Singapore

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S Supporting Information *

ABSTRACT: A facile strategy was developed for the formation of an autonomous chitosan-based self-healing hydrogel. This hydrogel was fabricated using in situ free radical polymerization of acrylic acid (AA) and acrylamide (AM) in the presence of chitosan in dilute acetic acid aqueous solution under mild conditions. The in situ formed hydrogel is mainly composed of chitosan graft copolymers (CS-g-P(AM-r-AA)) and a small amount of nongrafted copolymers (P(AM-r-AA)), which interact with each other through a combination of multiple noncovalent interactions, including the interchain electrostatic complexation between −[AA]− segments and positively charged amino groups of chitosan, the H-bonding between −[AM]− segments, and the H-bonding between −[AM]− segments and the chitosan backbone. Owing to the cooperation of these noncovalent interactions and the reversible nature of the noncovalent network structure, the obtained hydrogel exhibits rapid network recovery, high stretchability, and efficient autonomous self-healing properties. The hydrogel can also dissolve completely in dilute acidic aqueous solution under mild conditions, visibly reflecting the unique network feature of this selfhealing hydrogel system. KEYWORDS: self-healing, hydrogel, chitosan, noncovalent interactions, in situ polymerization

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INTRODUCTION In nature, self-healing is a key protective feature of biological materials and systems, taking place from the single molecule level (e.g., DNA repair) to the macroscopic level (e.g., closure and healing of injured skin and blood vessels). The intrinsic self-healing processes are autonomously triggered upon the external damage, and eventually the initial functions are regenerated. As a result, living organisms can survive in the wild and prolong their lifetime. By contrast, most man-made materials are irreparably damaged by cracks at the macro- or microscale, and they eventually lose the structural integrity as well as mechanical and other properties with propagation of these cracks.1 Hydrogels are three-dimensional networks of hydrophilic polymer chains that are chemically and/or physically crosslinked.2−6 Hydrogels possess a lot of attractive inherent properties, such as structural similarities to biological tissue, rubbery consistency, low interfacial tension to human tissue, responsiveness to external stimuli, adaptivity, tunability, biocompatibility, and biodegradability, that make them successful for a wide range of biomedical applications in drug delivery, tissue engineering, wound healing, for personal health care applications, and so on.7−11 In recent years, an exciting trend has emerged involving the use of hydrogels in engineering materials for robotics, electronics, actuators, sensors, and so on.12−17 An impetus behind this innovation © 2019 American Chemical Society

is the repurposing of sustainable and innovative soft materials originally developed for biomedical applications.18−22 To fulfill the purpose, it becomes essential to enhance hydrogel’s durability by introducing autonomous self-healing ability.13 The self-healing of a hydrogel is mostly achieved by constitutional dynamic chemistry, where dynamic bonds contribute to the preparation of polymer networks with the ability to undergo reversible bond breaking−re-formation processes. To date, both reversible chemical bonds (including phenylboronate complexation, disulfide bonds, imine bonds, acylhydrazone bonds, reversible radical reaction, and Diels− Alder reactions) and noncovalent interactions (including hydrophobic interactions, host−guest interactions, hydrogen bonds, crystallization, polymer−nanocomposite interactions, and multiple weak intermolecular interactions) have been utilized to fabricate self-healing hydrogels.23 However, most of the self-healing hydrogel systems have been made based on synthetic polymers; either these systems require sophisticated synthesis to incorporate the desired reversible interactions, or the components and reagents inside the systems are not biocompatible, which limited the application of self-healing hydrogels in biomedical and personal/health care areas. In Received: April 5, 2019 Accepted: May 28, 2019 Published: May 28, 2019 1769

DOI: 10.1021/acsapm.9b00317 ACS Appl. Polym. Mater. 2019, 1, 1769−1777

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ACS Applied Polymer Materials

Figure 1. Schematic cartoon showing the formation of chitosan-based physical hydrogel by in situ polymerization.

radical polymerization of acrylic acid (AA) and acrylamide (AM) in the presence of chitosan in dilute acetic acid aqueous solution under mild condition with ammonium persulfate (APS) as initiator and N,N,N′,N′-tetramethylethylenediamine (TEMED) as accelerator (Figure 1). By rational design of the gel-forming formulation and careful control of reaction conditions, the chemical cross-linking which usually occurred in normal free radical polymerization system due to radical coupling can be effectively suppressed. As a result, an autonomous chitosan-based self-healing hydrogel by only noncovalent interactions was obtained in a facile one-pot process. The hydrogel in this work showed rapid network recovery, high stretchability, and efficient autonomous selfhealing properties at high water content (up to 90 wt %) because of the noncovalent nature of the network structure. This hydrogel can also be totally dissolved in dilute acidic aqueous solution under mild conditions. This property will contribute to degradation and/or recycling of this soft material after use.

recent years, self-healing hydrogels based on natural polysaccharides (such as alginate, carrageenan, chitosan, dextran, hyaluronic acid, and xanthan) have aroused peoples’ interests due to the sustainability, biocompatibility, and biodegradability of these natural resources.24−32 Among them, the most noteworthy is the chitosan-based self-healing hydrogels. Many studies have been performed for the development of this novel soft material and application of them in a wide variety of areas like wound healing,24,33−35 cell encapsulation,36 drug delivery37−39 and oil/water separation,40 and so on. Chitosan (CS) is the second most ubiquitous natural polysaccharide after cellulose on earth with a broad range of applications. It is derived from chitin by N-deacetylation and is consequently a copolymer of N-acetylglucosamine and glucosamine. Structurally, chitosan contains a lot of amino groups on the backbone ready for derivation and modification. So far, most chitosan-based self-healing hydrogels are based on dynamic covalent Schiff-base formation.41 Other notable selfhealing mechanisms based on dynamic disulfide bonds42 and Diels−Alder click reactions43 have also been reported in the literature. By comparison, noncovalently bonded chitosanbased self-healing systems are seldom reported. Noncovalent interactions (such as electrostatic interactions and H-bonding) have been employed to fabricate chitosan-based hydrogels for many years.44,45 However, it is in very recent years that the design of a chitosan-based self-healing hydrogel through noncovalent interactions began to arouse peoples’ interests. Several examples were found in the literature including chitosan/poly(vinyl alcohol), modified chitosan/alginate, and chitosan/poly(acrylic acid) systems.46−48 These self-healing hydrogel systems are usually composed of two components. Upon mixing with chitosan in aqueous solution, another component interacted with chitosan macromolecules through hydrophobic interactions, electrostatic interactions, H-bonding, and so forth so as to induce the formation of 3D network. By introduction of graphite oxide or ions like Fe3+, dualnetwork physical chitosan-based self-healing hydrogels were obtained.49,50 Herein we present a different chitosan-based physical selfhealing hydrogel system. It was fabricated through in situ free

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EXPERIMENTAL SECTION

Materials. Chitosan (deacetylation value 75.0−85.0%) and N,N,N′,N′-tetramethylethylenediamine (TEMED, >98%) were purchased from Tokyo Chemical Industry Co., Ltd. (TCI). Ammonium persulfate (APS, >98%), acrylic acid (AA, 99%), and acrylamide (AM, >98%) were purchased from Sigma-Aldrich. All other reagents were used as received without further purification. Fabrication of Chitosan-Based Self-Healing Hydrogel. In a typical procedure, a chitosan solution (2.5 wt %) was made by dissolving a known amount of chitosan powder in dilute acetic acid aqueous solution (1.0 wt %) with vigorous stirring at room temperature for 2 h. To the chitosan solution (8.0 g), determined amounts of AA and AM were added. The mixture was stirred and purged with nitrogen for 20 min at room temperature. Subsequently, desired amounts of TEMED and APS were added to the above mixture. The mixture was stirred and purged with nitrogen for another 10 min at room temperature. Then the mixture was sealed and placed in water bath at 30 °C for 24 h without shaking. The obtained hydrogel was rinsed with DI water, and then the water droplets were removed by filter paper. After that, the hydrogel was kept at 4 °C in a sealed Petri dish in which droplets of DI water were added to ensure a humid environment. 1770

DOI: 10.1021/acsapm.9b00317 ACS Appl. Polym. Mater. 2019, 1, 1769−1777

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ACS Applied Polymer Materials In this work, a series of hydrogels with different properties were synthesized by varying the reaction temperature, the initial amount of AA and AM, and the concentrations of initiator. The details of these hydrogel formulations as well as some important experimental results are listed in Table S1 (see the Supporting Information). According to different test requirement, the hydrogel samples were fabricated into a cylindrical shape (diameter of 10 mm and length of 50 mm) or a rectangular strip (length of 35 mm, width of 25 mm, and thickness of 2 mm). General Characterization Methods. Proton nuclear magnetic resonance (1H NMR) spectra were recorded at room temperature on a JEOL ECA 500 MHz NMR spectrometer operating at 500 MHz. Chemical shifts were reported in parts per million (ppm) on the δ scale and were referenced to residual protonated solvent peaks: DMSO-d6 spectra were referenced to (CHD2)(CD3)SO at δH 2.50, CDCl3 spectra were referenced to CHCl3 at δH 7.26, and D2O spectra were referenced to HDO at δH 4.70. Fourier transform infrared (FTIR) spectra of samples in KBr pellet were recorded on a PerkinElmer FTIR 2000 spectrometer in the region of 4000−400 cm−1; 64 scans were signal-averaged with a resolution of 2 cm−1 at room temperature. Before measurement, the hydrogel samples were washed in deionized water and ethanol three times to remove any reactant residues from the sample and then dried in a vacuum oven at 50 °C for 2 days. Size exclusion chromatography (SEC) of the selfhealing hydrogel was performed with a Shimadzu SCL-10A and LC10AT system equipped with a Sephadex G-100 column (size: 2.5 × 32 cm2), a Shimadzu RID-10A refractive index (RI) detector, and a Shimadzu SPD-10Avp UV−vis detector. Acetic acid (HOAc)− sodium acetate (NaOAc) buffer (0.3 M HOAc/0.2 M NaOAc) was used as the eluent with a flow rate of 0.5 mL/min. The freeze-dried hydrogel was dissolved in the same buffer (2.0 mg/mL) for testing. Molecular weight analysis was calculated against dextran standards. Scanning electron microscopy (SEM) was performed on a JEOL FESEM JSM6700F operating at an acceleration voltage of 5 kV, and all of the freeze-dried hydrogel samples were fractured and coated with gold in a vacuum before observation. Monomer Conversions. Monomer conversions for in situ polymerization were calculated by 1H NMR spectroscopy based on the determination of residual acrylamide in final hydrogel using decanoic acid as an internal standard. After the in situ polymerization, a measured amount of hydrogel (usually 1.0 g) was soaked in ethanol (1.5 mL) for 12 h (shrinking) and then allowed to hydrate in distilled water (1.5 mL) for 12 h (swelling). The final solution outside the hydrogel in each step was collected. This shrinking and swelling process was repeated two times. After this purification, the hydrogel sample was recycled and dried in a vacuum oven at 50 °C for 2 days for solid content study. The collected solutions were combined, and the mixture was saturated with sodium chloride until the added sodium chloride did not readily dissolve. Subsequently, a measured amount of decanoic acid (usually 10 mg) in ethyl acetate (3 mL) was added as internal standard. The organic phase was separated, and the mixture was further extracted with fresh ethyl acetate (2 × 3 mL). The organic extracts were combined and dried over anhydrous Na2SO4; a 3 mL aliquot was filtered, evaporated at room temperature, and redissolved in deuterated solvent (DMSO-d6). The integral ratio of the residual acrylamide/standard was calculated from the 1H NMR spectrum by comparing the integrations of the three vinyl protons of acrylamide (5.50−6.25 ppm) to the integration of the signal in the region 0.78−0.92 ppm where three methyl protons of standard molecule resonate. The residual amount of acrylamide monomer in hydrogel and monomer conversion were then calculated from a calibration curve (see Figure S1, R2 = 0.999). Self-Healing Efficiency. Self-healing efficiency of hydrogel samples was determined based on uniaxial elongation measurement according to reported methods in the literature with some necessary modifications.51,52 For this test, all hydrogel samples were fabricated into a cylindrical shape with a diameter of 10 mm and length of 50 mm. The hydrogel sample was cut into two halves from the middle, and then the two halves were brought into contact and left undisturbed at room temperature for 1 day.

The uniaxial elongation measurements were performed on an Instron 5567 universal testing machine (Norwood, MA) with a crosshead speed of 20 mm/min at room temperature. The percentage elongation at break of the original hydrogel sample (λ) and the healed hydrogel sample (λ′) was measured. The healing efficiency (f) is defined as f = λ′/λ. The percentage elongation at break λ = (L − Lo)/ Lo × 100%, where L is the length of the hydrogel sample at break and Lo is the original length of the hydrogel sample. For reproducibility, at least three samples were measured for each hydrogel formulation, and the results were averaged. The healing efficiency, f, takes values between 0 and 1. It is obvious that the closer the value of f is to 1, the better the self-healing capability of the hydrogel is. By comparison, if the value of f is closer to zero, it indicates that the hydrogel sample has lower self-healing ability. In this work, all hydrogel samples were screened by using this method. Dynamic Oscillatory Rheological Experiments. The dynamic oscillatory rheological experiments were performed using an AR2000 stress-controlled rheometer (TA Instruments). All measurements were performed with a plate geometry (20 mm diameter) equipped with a Peltier-based temperature control. Prior to testing, a little water was applied to the outer edges of the hydrogel sample to prevent dehydration, and a self-made solvent trap was also employed to minimize the evaporation of water from hydrogel samples during testing. All the measurements were conducted at 25 ± 0.1 °C. Strain amplitude sweeps were usually performed first to determine the linear viscoelasticity region (LVR). All rheological measurements were performed in triplicate to make sure the data are reliable. The quantitative recovery test of hydrogel samples was performed on this machine by using a two-step procedure. Briefly, a hydrogel sample was subjected to increasing strain amplitude (e.g., 0.01 to 300%) at constant low frequency. After that, the recovery of storage modulus (G′) and loss modulus (G″) were monitored with time at constant low strain amplitude and frequency. To evaluate the repeated recovery ability, the hydrogel sample was alternatively subjected to small strain amplitude (e.g., 1%) and large strain amplitude (e.g., 300%) at constant low frequency, and the recovery of G′ and G″ was monitored with time. Tensile Tests. Tensile tests were performed on an Instron 5567 universal testing machine (Norwood, MA) at room temperature. For this test, all hydrogel specimens had a length of 35 mm, a width of 25 mm, and a thickness of 2 mm. The gauge length between the two clamps was ∼10 mm. The loading rate was 20 mm/min. For evaluation of self-healing ability, the hydrogel specimen was cut into two halves from the middle, and then the two halves were brought into contact and left undisturbed at room temperature for 1 day prior to test. The tensile stress−strain curves were then recorded on this computer-controlled universal testing machine.

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RESULTS AND DISCUSSION Formation of Self-Healing Hydrogels. The in situ free radical polymerization produced random synthetic copolymer chains (P(AM-r-AA)), and most of them were grafted onto chitosan backbone forming chitosan graft copolymer (CS-gP(AM-r-AA)) (see Scheme S1). The latter was the major component of the resultant hydrogel. The graft copolymer chains interacted with each other by means of multiple Hbonding interactions (synthetic polymer chain−synthetic polymer chain, synthetic polymer chain−chitosan backbone, and chitosan backbone−chitosan backbone) and electrostatic interactions between the negatively charged −[AA]− units on synthetic chains and positively charged amino groups on the chitosan backbone. As a result, a hydrogel based on multiple noncovalently cross-linked networks was formed. The reversible nature of these noncovalent interactions together with the unique structural feature rendered he hydrogel designed and synthesized here possessing rapid network recovery, high stretchability, and efficient autonomous self1771

DOI: 10.1021/acsapm.9b00317 ACS Appl. Polym. Mater. 2019, 1, 1769−1777

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ACS Applied Polymer Materials healing properties. Recently, Jiang et al. reported a hydrogel system by using chitosan, AA, and AM, and the obtained hydrogel showed toughness, self-healing, and metal ion adsorption properties.53 However, our hydrogel system exhibits a great difference from that in not only appearance (our hydrogel is transparent) but also network structure and properties due to very different hydrogel composition and fabrication conditions we adopted. To investigate the role of each component on the hydrogel formation, oscillatory time sweep experiments were performed to monitor the variation of G′ and G″ of a representative hydrogel-forming formulation containing 2.25 wt % chitosan, 0.056 mol L−1 AA, 0.893 mol L−1 AM, 0.00296 mol L−1 APS, and 0.0167 mol L−1 TEMED at 30 °C (Table S1, entry 13). Three control samples (control 1, no AA; control 2, no chitosan; and control 3, no AM) were test under the same conditions as that of representative hydrogel-forming solution. For each sample, the oscillatory time sweep experiment was performed at a low frequency of 10 rad/s and a small strain of 1.0% to ensure that the experimental conditions did not interfere with the gelation process at 30 °C (Figure 2). The inserted photos in Figure 2 show the appearance of each sample solution after in situ polymerization.

For representative hydrogel-forming solution (Figure 2a), at the beginning, G′′ was larger than G′, which was expected because the sample was in a liquid state and viscous properties dominated. Both moduli increased fast with time during the first 1 h due to the starting of polymerization and grafting of the synthetic polymer chains onto chitosan backbone. According to literature, the grafting happened at the amino and hydroxyl sites along the chitosan backbone (see Scheme S1). The increasing rate of G′ was higher than that of G′′, which led to a G′ and G′′ crossover eventually. This phenomenon was related to the formation of multiple noncovalent networks as the growth of chitosan graft copolymer and the self-assembly of these macromolecules. Thus, the elastic properties eventually became dominant. The gelation time, i.e., the time required to achieve the crossover, was 2.4 h according to this test. After 2.4 h both G′ and G′′ continued to increase. However, the former increased much faster than the latter, implying the noncovalent networks became stronger with the in situ polymerization. Control 1 (no AA, Figure 2b) showed an obvious increase in G′ and G′′ during the first 0.6 h which was very similar to that of hydrogel-forming formulation (Figure 2a). However, after that, though both moduli showed further increase, G′′ was always larger than G′ throughout the entire reaction period (4 h), showing the reaction mixture was in the solution state and there was no stable polymer network formed. Another observation was that the values of both G′ and G′′ were much lower than those of the hydrogel-forming formulation (Figure 2a). According to these results, it was clear that AA should play a key role in the formation of stable polymer network in this system. AA introduced the electrostatic interactions between synthetic polymer chains and chitosan backbones, which not only ensured the stability but also enhanced the mechanical strength of the networks in this system. Control 2 (no chitosan, Figure 2c) showed only a little increase in G′ and G′′ with reaction time. In addition, the G′′ values were always higher than G′ throughout the entire reaction period (4 h) showing there was also no hydrogel formed in the circumstance of no chitosan. It was proposed that the role of chitosan is to provide cores for synthetic polymer chains to graft so as to form big graft copolymer macromolecules with many synthetic side chains able to interact with neighboring macromolecules via multiple Hbonding and so on. For control 3 (no AM, Figure 2d), both G′ and G′′ did not show noticeable increase with reaction time. In addition, the G′ and G′′ became very closer to each other, but no crossover throughout the entire reaction period (4 h), showing the starting reaction solution became a paste-like mixture. From these findings, it was concluded that the role of AM was to form synthetic side chains of chitosan graft copolymer. The side chains should be sufficiently long and flexible, so as to make the chitosan graft copolymer macromolecules efficiently interact with each other for stable network formation. On the other hand, the existence of PAM would determine the mobility of the networks in this system. The long and flexible PAM side chains would make the functional groups accessible to each other across the cut and rejoined interfaces of the hydrogel for an efficient re-formation of noncovalent interactions.54 Through this test, it can be seen that each component is indispensable to the hydrogel formation. The fundamental

Figure 2. Oscillatory time sweep measurements of (a) representative hydrogel formulation, (b) control 1 (no AA), (c) control 2 (no CS), and (d) control 3 (no AM) showing the variations of G′ (filled symbols) and G″ (open symbols) as functions of reaction time at constant angular frequency ω of 10 rad/s and strain of 1.0% at 30 °C. The inserted photos show the appearance of each sample solution after in situ polymerization. 1772

DOI: 10.1021/acsapm.9b00317 ACS Appl. Polym. Mater. 2019, 1, 1769−1777

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ACS Applied Polymer Materials

Figure 3. Comparison of the rheological properties of resultant hydrogels prepared by formulations with AA (normal samples, entries 9−14 in Table S1) and without AA (as controls) with the variation of initial APS concentrations. (a) Changes of G′ and G′′ with APS. (b) Changes of tan δ with APS. Measurement conditions: 1% of strain, 10 rad/s, 25 °C. Each point represents the mean value ± SD. The X-axis represents the ratio of APS concentration to the maximum APS concentration of 0.0148 mol L−1.

feeding amount of AA could introduce more bonding sites between the synthetic polymer chains and chitosan backbones via the electrostatic interactions, contributing to both selfhealing ability and mechanical properties. To investigate more deeply the effect of AA on this self-healing hydrogel system, a series of hydrogels were prepared at 30 °C with varied initial AA/AM ratios in the range from 0 to 0.188 (see entries 3 and 5−8 in Table S1). For each formulation, the total monomer concentration was kept around 0.95 mol L−1 to ensure the prepared hydrogels by different formulations had similar solid content for comparison. It was found that the hydrogel’s selfhealing ability and rheological properties varied significantly with AA/AM ratio (Figure S3). At an AA/AM of 0 (i.e., without AA), the resultant sample showed a healing efficiency f near 50%. The sample was a very weak hydrogel with G′ around 60 Pa and tan δ over 0.5. There could be chemical cross-linking occurring partially in this case. With the increase of AA/AM to ca. 0.06, the healing efficiency showed a drop of 10% (from ca. 50% to 40%). However, the G′ of the sample significantly increased from ca. 60 to ca. 2100 Pa. This observation clearly revealed the effect of AA on the improvement of hydrogel’s mechanical property. When the AA/AM further increased to ca. 0.09, the sample’s healing efficiency decreased another 3%, while the G′ further increased ca. 300 Pa. Noteworthily, upon further increase of AA/AM from ca. 0.09 to ca. 0.13, the sample’s self-healing efficiency quickly dropped down to almost 0 (Figure S3a), while the sample’s G′ did not change noticeably (Figure S3b). The possible reason is that strong electrostatic interactions under this case compromised the flexibility and reversibility of the polymer network. When the AA/Am reached 0.19, the sample’s healing efficiency was still 0. However, the G′ showed a noticeable drop. On the contrary, both the G′′ and tan δ increased a little. The possible reason is that in this case too much negatively charged −[AA]− unit was incorporated into synthetic polymer chains, which caused electrostatic repulsion between synthetic polymer chains. As a result, the strength of the hydrogel sample decreased. Another interesting observation is that the transparency of the final hydrogel varied with the feeding amount of AA. At lower AA concentration ( G′ (i.e., tan δ > 1.0), while the normal samples (with AA) should always have G′′ < G′ (i.e., tan δ < 1.0). By observing and comparing the variations of G′, G′′, and tan δ of both normal hydrogel samples and control hydrogel samples, we can infer the possible network structure in the resultant hydrogel and the occurrence of the chemical cross-linking during the in situ polymerization (Figure 3). It can be seen that the control sample’s tan δ was always higher than that of normal sample in this study. However, under higher APS concentrations, the control samples also formed hydrogels with tan δ < 1.0, indicating there was chemical crosslinking happened in the polymerization system in the case of higher APS concentrations. With reducing APS, both normal sample’s and control sample’s tan δ increased. However, the normal sample’s tan δ was always 12%, the G′ value decreased rapidly and became closer to the G′′ value with the increasing strain. This indicated the partial collapse of the hydrogel network under large deformation. After strain was stopped, the hydrogel was found to recover its G′ and G′′ almost instantaneously (note: the time resolution of this measurement is ca. 6 s). Also, the recovered G′ and G′′ values were almost the same as those within LVR. This test showed that the network can be recovered rapidly and completely in this hydrogel system after collapse. This self-healing ability is attributed to reversibility of the noncovalently cross-linked network. The repeated recovery property of this hydrogel was investigated by largely straining the hydrogel (strain 300%) for 200 s and afterword monitoring the recovery at low strain (1%) for 150 s at frequency 10 rad/s (25 °C). It was found that the hydrogel system in this work can withstand several disruption−recovery cycles (at least three) without changing its mechanical properties (Figure 5b). The robustness of this hydrogel system is attributed not only to the noncovalent cross-linking nature of the network but also the synergistic cooperation of multiple noncovalent interactions in one system. This ability could allow this hydrogel system to find application in places where continuous motion and deformation will occur. Tensile tests were also performed to investigate the hydrogel’s self-healing ability. For comparison, the original hydrogel and self-healed hydrogel were tested under the same conditions. It was found that the hydrogel’s strain at break and stress at break can be recovered up to 82% and 78%,

respectively, after self-healing (Figure S6 and S7), which is consistent with the elongation experimental result (f ca. 88%). Purification, Regeneration, and Dissolution of Representative Self-Healing Hydrogel. The representative hydrogel prepared under the conditions in this work still contains a little amount of residual monomer (typically, monomer conversion ca. 94.6%, residual AM content ca. 0.35 wt %). If required, the hydrogel can be purified by successively soaking with ethanol−H2O (1/1, v/v), pure H2O, and sodium acetate buffer (0.1 M, pH 4.5) at room temperature (Figure S8). The method was safe and effective. After treatment, the residual AM content can be greatly reduced, allowing this hydrogel to pass the FDA regulation in high chance. The shape and appearance of the hydrogel can be recovered after the purification and regeneration process. Importantly, the hydrogel can still keep its self-healing and mechanical properties. This hydrogel system can also be totally dissolved in a dilute acetic acid or hydrochloric acid aqueous solution under mild conditions (Table S3). This is another evidence that this hydrogel system is only physically cross-linked in nature. Such ability will contribute to the fast degradation of the hydrogel system and make recycling after usage easier. Bulk Structure and Macromolecular Structure of Representative Self-Healing Hydrogel. The bulk structure of the chitosan-based self-healing hydrogel was investigated by using a JEOL FESEM. Before examination, the hydrogel samples were freeze-dried. After that, the dried samples were cut to expose their inner structure, and then the cross section was observed. SEM images revealed highly porous microstructure of dried hydrogel samples (Figure 6, a-1 to a-3). These pores were interconnected, and the inner wall surface of the pores was smooth. The pore size was around 20−40 μm in diameter. The freeze-dried hydrogel sample was further 1775

DOI: 10.1021/acsapm.9b00317 ACS Appl. Polym. Mater. 2019, 1, 1769−1777

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ACS Applied Polymer Materials Notes

examined by NMR spectroscopy. For this test, the freeze-dried hydrogel sample was cut into small pieces and then stirred in D2O containing DCl (0.10 M) at 40 °C until a clear solution was formed. The 1H NMR experiment was then performed at 25 °C. For the representative hydrogel prepared in this work, the signals at δ 3.0−4.0 ppm and δ 1.25−2.5 ppm were attributed to the protons of chitosan and synthesized polymer chain, respectively (Figure 6b, upper). There were resonance signals at δ 5.5−6.25 ppm which were due to residual monomers. The resonance signals at δ 2.90 and 3.56 ppm were due to TEMED (the accelerator for initiator APS). The purified and freeze-dried hydrogel sample was examined by NMR in the same way. As expected, the impurities (residual monomers and TEMED) had been removed completely after purification, while the resonance peaks due to the protons of chitosan and synthesized polymer chain did not show noticeable change.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by IAF-PP (HMBS Domain) Grant H17/01/a0/013 (OrBID): OculaR BIomaterials and Device.

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CONCLUSION In summary, we have successfully developed a facile method to design and fabricate a chitosan-based self-healing hydrogel by using the in situ polymerization technique. The on-site formation of chitosan graft copolymer, synergistic multiple noncovalent interactions, and only noncovalently cross-linked network are main features for this hydrogel system. The fresh hydrogel possessed high water content (up to 90%) and showed rapid network recovery (within 6 s), high self-healing efficiency (up to 88%), and high stretchability (elongation ratio at break up to 625%). The hydrogel can withstand the disruption−recovery cycle for several times. To date, chitosanbased hydrogels with high water content and outstanding selfhealing ability are seldom reported. Such a self-healing hydrogel system, based on synthetic−natural polymers, represents a new kind of adaptive, biomimetic soft material which may have the potential to link living and artificial systems and allow the development of biologically inspired soft devices in ways that are not possible with other synthetic or natural soft materials. In the next step, it is worth exploring the integration of this self-healing hydrogel systems with other components to gain multiple functionalities such as stimuli responsiveness, sensing, and conductivity for hitherto unimagined applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00317.

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REFERENCES

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Calibration curve for monomer conversion measurement; polymerization mechanism (Scheme S1); characterization results (Table S1); effect of the polymerization temperature, acrylic acid, and APS on the properties of hydrogel; characterization of representative hydrogel samples (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(Z.-X.Z.) E-mail: [email protected]. *(X.J.L.) E-mail: [email protected]. ORCID

Zibiao Li: 0000-0002-0591-5328 Xian Jun Loh: 0000-0001-8118-6502 1776

DOI: 10.1021/acsapm.9b00317 ACS Appl. Polym. Mater. 2019, 1, 1769−1777

Article

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DOI: 10.1021/acsapm.9b00317 ACS Appl. Polym. Mater. 2019, 1, 1769−1777