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Article Cite This: Chem. Mater. 2018, 30, 4289−4297

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Stretchable, Conductive, and Self-Healing Hydrogel with Super Metal Adhesion Yimeng Wang,† Furong Huang,† Xibang Chen,† Xiao-Wei Wang,‡ Wen-Bin Zhang,‡ Jing Peng,† Jiuqiang Li,† and Maolin Zhai*,†

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†

Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation Chemistry Key Laboratory of Fundamental Science, Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Center for Soft Matter Science and Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Currently, adhesive and self-healable hydrogels have highlighted their potential in tissue adhesives, sealants, and implantable electronic devices. For electronic device adhesives, a combination of adhesive, conductive, and selfhealing properties is required. Here, we demonstrated a onepot synthesis method of a novel self-healing hydrogel (named GOxSPNB) with various ratios of graphene oxide to soluble starch and poly(sodium 4-vinyl-benzenesulfonate-co-N-(2(methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide) by a γ-radiation technique. The resultant hydrogel based on a totally physical cross-linking network exhibits fast automatic self-healing ability, nontoxicity, ionic conductivity about 10.5 mS dm−1, and super highly reusable metal adhesion with 60.5 MPa adhesive strength to the copper plates at room temperature, nearly one magnitude larger than other reversible adhesives that have been reported. Meanwhile, it also shows extreme adhesive strength to some organic substrates, such as porcine skin (130.70 kPa) which is the highest strength to date. This self-healable adhesive hydrogel, especially for metal substrates, has great potential in hydrogel glues for the design and fabrication of smart electronic device adhesives.

1. INTRODUCTION Self-healing hydrogels can heal or repair their original properties which improves the reliability and durability of the materials. Recently, many novel self-healable hydrogels were designed and can be mainly divided into two parts: the extrinsic and the intrinsic self-healing materials according to their working mechanisms.1,2 The intrinsic self-healing hydrogels which were based on reversible chemical bonds, including covalent and noncovalent bonds, have attracted increasing attention, especially the hydrogels with reversible noncovalent bonds such as hydrogen bonding,3 electrostatic interaction,4 host−guest interaction,5 metal coordination6,7 and so on, because of the fast recovery behavior without any external stimuli, e.g., pH, UV light, or temperature. However, the low mechanical strength of the self-healing hydrogels with reversible noncovalent bond’s low bond energy and weak interactions limited their usage. To overcome these shortcomings, new strategies to enhance the mechanical strength include preparing the gels with double-network (DN),8 topological sliding rings,9 macromolecular microsphere gel,10 or nanocomposites (NC).11 Among them, NC gels have been widely studied due to their remarkable mechanical properties and facile preparation. For example, Wang et al. © 2018 American Chemical Society

reported a transparent self-healable hydrogel based on water, clay, and organic components with a 0.5 MPa storage modulus (G′) and it can be molded into shape-persistent, free-standing objects.12 More recently, Xia et al. proposed a novel strategy to improve stretchability, fatigue resistance, and self-recoverable properties of hydrogels using SiO2-g-poly(butyl acrylate) core−shell inorganic−organic hybrid latex particles.13 As a result, the formulated hydrogels showed a tensile strength of 1.48 MPa, stretchability of 2511%, and toughness of 12.62 MJ m−3. Compared with other nanocomposites (e.g., gold and silver nanoparticles, carbon nanotubes, SiO2), the low cost, large surface area, and good hydrophilicity of graphene oxide (GO) make it an excellent nanoaddition.14 Moreover, GO has various oxygen functional groups, such as hydroxyl (−OH), carbonyl (CO), and carboxyl (−COOH) which can easily bond with polar bulk polymers, and therefore reinforce the mechanical strength of the hydrogels. No doubt, self-healing hydrogels have many practical applications, such as in ionic conductors,15 wearable devices,16 Received: March 26, 2018 Revised: May 21, 2018 Published: June 13, 2018 4289

DOI: 10.1021/acs.chemmater.8b01260 Chem. Mater. 2018, 30, 4289−4297

Article

Chemistry of Materials

Figure 1. (a) Synthesis route of N-(2-(methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide (MOBAB). (b) Schematic illustration of the preparation of GO3SPNB hydrogel.

and tissue repair.17,18 Among them, adhesive hydrogels, which have potential applications in medical adhesives, industrial assembly, manipulation systems, and smart-printing systems, have gained unprecedented attention in the past few years.19 For biological applications, Li et al. reported a design of tough adhesives, which combined chemical and physical processes at the interface and in the bulk of the adhesive to achieve high adhesion energy on various wet and dynamic surfaces.18 In a word, achieving high adhesion requires two features: the strong bonds with the substrates and strong interactions inside the hydrogels. However, an ideal hydrogel adhesive should possess not only high adhesive strength, but also plenty of features for specific applications, such as self-healing, ionic conductivity, and biocompatibility for implantable electronic devices. To the best of our knowledge, no such kind of hydrogel adhesive has been reported to show extremely high adhesive strength so far. Herein, we report a novel NC hydrogel (named GO3SPNB) with fast automatic self-healing ability, good biocompatibility, high ionic conductivity of about 10.5 mS cm−1, and outstanding reusable metal adhesion with 60.5 MPa adhesive strength to the copper plates at room temperature, in which GO, soluble starch, and poly(sodium 4-vinyl-benzenesulfonateco-N-(2-(methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide) (P(NaSS-co-MOBAB)) synthesized by a onepot γ-radiation polymerization are stable through physical cross-linking. Our design is based on the following strategies. First, GO sheets served as interlamellar bridges by hydrogen bonds will endow the hydrogel with good mechanical strength and layered morphology. Second, highly adhesive components such as soluble starch and acrylate derivatives may provide the hydrogel with excellent adhesive behavior. Third, multiphysical cross-linking in the hydrogel, such as electrostatic interaction and hydrogen bonds, benefits its fast intrinsic self-healing ability. Moreover, γ-radiation is a simpler, cleaner, and more effective technique which can be carried out at room temperature without initiators and catalysts. And γ-ray is able to sterilize and disinfect the hydrogel system. Furthermore, the free NaBr introduced into the hydrogel may enhance its ionic

conductivity. We believe that this design principle may open the door to construct the next generation of harmless conductive adhesive materials, especially for implantable electronic devices.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium 4-vinyl-benzenesulfonate (NaSS) was purchased from Nanjing Asian Chemical Co., Ltd. Graphite oxide was bought from the Sixth Element Materials Technology Co., Ltd. (Changzhou, China). 2-(Dimethylamino)ethyl methacrylate (DMAEMA) was ACROS reagent purchased from Aldrich. 1-Bromobutane with purity of >98% is Lancaster reagent of Alfa Aesar. Soluble starch (A.R., Mn = 5832 g/mol, PDI = 9.05) was obtained from Beijing Chemical Works. HeLa cells were obtained from the American Type Culture Collection (Manassas, VA, U.S.A.). Cell culture medium (DMEM high glucose, Gibico) and fetal bovine serum (FBS) (Qualified, New Zealand origin) were obtained from Invitrogen. 3(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were purchased from AMRESCO (Solon, U.S.A.). All other chemicals were of reagent grade and were used without any purification. 2.2. Preparation of GO Suspension. The solid graphite oxide was dispersed in distilled water (1, 3 mg/mL) under magnetic stirring for 30 min and exfoliated under sonication (Y99−2 DN ultrasonicator, China) for 2 h. The fully exfoliated GO suspension was obtained and characterized by FTIR, Raman, XRD, XPS, SEM, and TEM. 2.3. Synthesis of N-(2-(Methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium Bromide (MOBAB). N-(2(Methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide (MOBAB) was prepared and purified according to a procedure published earlier by Xu et al.20 (Figure 1a). DMAEMA and 1bromobutane with a mole fraction of 1:1.3 were put in a roundbottom flask. The reaction was carried out at 50 °C for 6 h under the protection of N2, using acetone as solvent. Then the excess solvents were removed by vacuum distillation at 35 °C. Then the product was purified and precipitated by petroleum ether several times. After being dried in vacuum at 40 °C, the final product was obtained as a white powder, giving a 90% yield. 2.3.1. 1H NMR (Figure S1) (D2O, 500 MHz, ppm). δ (ppm) 0.91− 0.88 (t, J = 7.5 Hz, 3H, NCH2CH2CH2CH3), 1.33−1.30 (m, 2H, NCH2CH2CH2CH3), 1.75−1.69 (m, 2H, NCH2CH2CH2CH3), 1.89 (s, 3H, CH2C(CH3)-CO2), 3.11 (s, 6H, N(CH3)2), 3.37−3.33 (t, J = 4290


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thickness = 75 × 25 × 1 mm3) to each other using 0.2 g GO3SPNB hydrogel.22 The overlapped adherends were compressed under a 300 g weight for 10 min. The tested sample geometry had an overlap length of 20 mm and a thickness of 0.35 ± 0.05 mm. After a drying step of 48 h at room temperature (expect for porcine skin, through a 24 h drying step, which was kept wet during the tensile test by continuously spraying of water), the glued samples were tested with a speed of 6 mm min−1 until the two plates were fully separated from each other. To investigate the adhesive healing capability of the hydrogels, the samples were tested using the above test procedure, and then were put back together to recover the same bond area by adding 100 μL distilled water on the surface of the gels uniformly. All adhesion tests were carried out at room temperature. Measurements of each sample were repeated at least five times and were averaged for a given sample in order to reduce the experimental error. 2.9. Cytotoxicity Test. The in vitro cytotoxicity test (MTT assay) was performed by using the HeLa cell line, and the corresponding results are shown in Figure S7. Cell viability was compared to control cells that had been incubated in a culture dish without hydrogels. Hela cells were seeded in a 96-well at a density of 1 × 104 cells per well in high glucose Dulbecco’s modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum and 1% penicillin− streptomycin and incubated for 24 h (37 °C, 5% CO2). After 24 h, cells were washed with fresh medium and treated with hydrogels. Prior to this, 30 mg hydrogels of each kind were washed thrice everyday with 1000 μL of diluent PBS for 3 days. After 24 h of incubation at 37 °C, the hydrogels and medium were removed. One mL fresh medium (DMEM) and 10 μL of MTT solution were added to each well, and the incubation continued for further 4 h. The medium containing unreacted MTT was removed, and formed insoluble formazan crystals were dissolved in 200 μL of dimethyl sulfoxide (DMSO). The absorbance of formed formazan product was measured at 540 nm using an EnSpire multimode plate reader (PerkinElmer, U.S.A.). The relative cell viability in percentage was calculated according to eq 2:

9 Hz, 2H, NCH2CH2CH2CH3), 3.72−3.70 (t, 2H, NCH2CH2O), 4.57 (s, 2H, NCH2CH2O), 5.73 (s, 1H, alkeneH), 6.11 (s, 1H, alkeneH). 13C NMR (Figure S2 of the Supporting Information, SI) (D2O, 500 MHz, ppm): δ 168.51, 135.21, 127.70, 65.26, 62.18, 58.44, 51.26, 23.97, 19.08, 17.27, 12.79. 2.4. Synthesis of Hydrogels. The synthesis route of the hydrogels is shown in Figure 1b. 100 mg soluble starch, 1.03 g (5.0 mmol) of NaSS, and 1.47 g (5.0 mmol) of MOBAB were dissolved in 5.0 mL certain content of GO suspension at 80 °C. Then the above viscous liquid was transferred to a glass tube of 10 mm diameter. Finally, the tube was sealed and irradiated to form the hydrogel with 5, 10, 15, 20, and 30 kGy dose at a dose rate of 80 Gy min−1 at room temperature. For comparison, the composites without GO or soluble starch were prepared under the same conditions. 2.5. Characterization of Hydrogels. Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spotlight200, U.S.A.), Micro Raman imaging Spectroscopy (Thermo Fisher DXRxi, U.S.A.), and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra, U.K.) were used to characterize the chemical properties of the resultant gels. X-ray Power Diffractometer (XRD, PANalytical X’Pert Pro, NED) was used to analyze the different crystal distribution strength of the sample, and the sample was operated at 40 kV and 40 mA. Scans were made from 5° to 85°(2θ) at the step of 5° min−1. The morphology of both GO and freeze-dried hydrogel was observed using scanning electron microscopy (SEM, Hitachi S-4800, Japan) and transmission electron microscopy (TEM, Tecnai F30, NED). The electrochemical property was recorded using an electrochemical instrument (Autolab PGSTAT302N, CH). According to the procedure published earlier by Hou et al.,21 the ionic conductivity of the hydrogel was measured at 25 °C through an alternating current (AC) impedance method (Figure S5a) over the frequency range from 100 kHz to 1 Hz. The sample was 25 mm (width) × 12 mm (length) × 0.65 mm (thickness), and sandwiched by copper sheets using 0.2 g GO3SPNB hydrogel. The conductivities of samples were listed in Table S1. The ionic conductivity of the hydrogel was calculated according to eq 1: σ=

l RS

(1)

relative cell viability (%) = (A sample − Ablank )/(Acontrol − Ablank ) × 100%

(2)

where l is the thickness of the gel, and S is the contact area between the hydrogel and copper plates. R is the bulk resistance which was obtained from the complex impedance diagram. 2.6. Self-Healing Properties of Hydrogels. The hydrogel was cut into two pieces, and then the fractured surfaces of the two pieces were simply brought into contact with each other without any further operation to observe the self-healing phenomenon immediately. 2.7. Rheological Tests. Rheological measurements were carried out with a stress-controlled rheometer (TA Instruments, AR-G2) equipped with a steel-coated parallel-plate geometry (8 mm diameter) at 25 °C. The gap distance was fixed at 0.8 mm. The storage modulus (G′) and loss modulus (G″) were recorded. Stress sweep measurement at fixed frequency (6.28 rad s−1) provides information about the mechanical strength of the gel sample. Frequency sweep was conducted from 6.28 to 628 rad s−1 at 0.1% strain amplitude, which was used to examine the viscous or elastic response of the tested gel to rapid or slow movements (at high or low frequencies). The rheology analysis of hydrogels was carried out to monitor qualitatively the selfhealing process. Recovery tests were performed by straining each hydrogel to failure under increasing strain from 1% to 500% (250%, 1000%) and then using time sweep test to monitor the recovery of G′ and G″ as a function of time. 2.8. Adhesion Tests. Lab shear strength tests were performed using an Instron 3365 material test system with metal substrates (aluminum, copper, zinc, brass, and iron plates) and organic substrates (porcine skin, polytetrafluoroethylene (PTFE), and poly(methyl methacrylate) (PMMA) plates) adherends at room temperature. Porcine skin obtained from a local slaughter house was shaved and washed by deionized water before using, which was used for simulating human skin. Test specimens were prepared according to M. A. Zadeh’s work by gluing two identical plates (length × width ×

Measurements of each sample were repeated at least five times and were averaged for a given sample in order to reduce the experimental error.

3. RESULTS AND DISSCUSSION 3.1. Synthesis and Characterization of GOxSPNB Hydrogels. The synthesis routes of cation monomer N-(2(methacryloyloxy)ethyl)-N,N-dimethylbutan-1-aminium bromide (MOBAB) and the GOxSPNB hydrogel, where x defines the content of GO (mg/mL), are shown in Figure 1. A more detailed description of the synthesis routes is provided in the Experimental Section, and the hydrogels with different GO contents as well as using various absorbed doses were prepared through one-pot method. All reactants of soluble starch, NaSS, and MOBAB were first added to a GO suspension at 80 °C, followed by a γ-radiation-induced-polymerization process for preparing the hydrogel. The GO3SPNB hydrogel was prepared at 15 kGy absorbed dose unless otherwise stated. As shown in Figure S3, GO3PNB without soluble starch and GO0SPNB without GO are liquid-like which could not form hydrogels. All the additions are stable at studied absorbed doses in our experiments, according to our previous works.23−25 Different from other chemically linked hydrogels, here we presented a fully physically linked self-healing hydrogel. The reversible ionic bonds between sulfo groups of NaSS and quaternary ammonium of MOBAB as well as the hydrogen bonds among 4291


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Figure 2. (a) FTIR spectra of the NaSS, MOBAB, GO, soluble starch and GO3SPNB gel. (b) Raman spectra of the GO3SPNB gel and GO. (c) XRD patterns of GO3SPNB gel and GO sheets. (d) XPS spectra of GO3SPNB gel and GO. Deconvoluted C 1s XPS spectra of (e) GO and (f) GO3SPNB gel.

interlayer distance of 7.6 Å.29 The gel containing GO showed an amorphous halo with a maximum at 2θ of 20.3° without the diffraction peak of GO and the characteristic XRD peaks of NaBr. These XRD results suggested that the GO sheets were uniformly dispersed and intercalated within the polymer matrix owing to the hydrogen bonding between GO and the P(NaSSco-MOBAB) polymer.27,30 As shown in Figure 2d−f, the GO3SPNB gel showed a N 1s peak at 398.1 eV, arising from quaternary ammonium group in the MOBAB. The C 1s XPS peaks of the GO and hydrogel were deconvoluted; for GO, there were three typical peaks at 284.8, 286.9, and 288.4 eV, ascribed to unoxidized graphite carbon (C−C/CC), C− OH/C−O−C and OC−OH groups, respectively.31 The hydrogel revealed peaks centered at 284.6, 286.1, and 288.9 eV that were respectively assigned to CC/C−C, C−O−/C−N/ C−S, and CO groups. To investigate the microstructure and morphology of the GO sheets and the freeze-dried gel, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out as shown in Figure 3. Figure 3a,b showed that the GO sheets exhibited lamellar morphology. Due to the hydrogen bonding between GO sheets and P(NaSS-co-MOBAB) polymer, the freeze-dried gel also showed a layered structure under 500 magnifications (Figure 3c). And its internal polymer chains are entangled with each other to form a micro three-dimensional network structure (Figure 3d). The layered and porous structure of the hydrogel may play an important role in its mechanical properties.31 3.2. Rheological Properties and Self-Healing Properties of GOxSPNB Hydrogels. To a certain extent, the rheological properties of gel-like soft materials could determine their practical usages, and therefore it is significant to study the rheological properties of our hydrogels.32 For these studies, storage modulus (G′) was used as a measure of hydrogel strength, and tan δ, which is the ratio of the loss modulus (G′′)

GO, soluble starch, and the P(NaSS-co-MOBAB) are expected to contribute to the fast autonomous self-healing property. Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy were used to identify the chemical structure of the resultant gel. Herein, the FTIR spectra of NaSS, MOBAB monomer, freeze-dried GO, soluble starch, and the GO3SPNB dried gel are shown in Figure 2a. The characteristic peak of the stretching vibration of −OH group shows a displacement at 3430 from 3200 cm−1, and the stretching vibration of C−O is moved to 1032 from 1039 cm−1. The lower wavenumber shifting of the characteristic peaks of key polar groups indicates the formation of hydrogen bonds among GO, soluble starch, and P(NaSS-co-MOBAB) polymers.26 And the FTIR results suggest that the GO-containing cross-linking network is obtained through opening the CC double bonds in the monomers, while the main functional groups remained. As shown in Figure 2b, the Raman spectrum of the GO3SPNB gel showed a slight peak shift compared with the GO. The G peak at 1590 cm−1 was blue-shifted to 1598 cm−1 while the D peak at 1350 cm−1 was red-shifted to 1346 cm−1, going from GO to the gel. And the D/G intensity ratio of the gel was about 0.97 which is slightly higher than that of GO (0.96). These phenomena can be ascribed to the hydrogen bonding between the GO sheets and the P(NaSS-co-MOBAB) polymer, which may induce a small quantity of new defects during the radiation polymerization, as reported previously.27 Both FTIR and Raman spectrum of GO3SPNB gel confirm the existence of hydrogen bonding and electrostatic forces, in other words, there existed physical cross-linking among GO, soluble starch and P(NaSS-co-MOBAB) polymer.28 X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS) were tested to further confirm the synthesis of GO3SPNB gel to analyze its chemical and physical structure. We can see in Figure 2c, GO itself exhibited the typical XRD peak at 2θ of 11.7°, corresponding to the 4292


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over the storage modulus (tan δ = G′′/G′), was used as a measure of hydrogel elasticity. We investigated the effects of GO content and absorbed dose on hydrogel formation. As shown in Figure 4a, b, the G′ increased with increasing GO content and absorbed dose. The sample without GO failed to form a hydrogel in the studied dose range, which indicated the GO was indispensable as nanofiller in our system.30 The GO sheets serve as interlamellar bridges by hydrogen bonds, which guide the formation of the layered hydrogel and enhance the mechanical strength of the hydrogels.12 Furthermore, a denser gel network and high degree of gelation, which may enhance the mechanical strength of the hydrogels, were obtained by increasing the absorbed dose.33 Considering the mechanical strength and irradiation time of the resultant hydrogels, we chose GO3SPNB hydrogel prepared at 15 kGy absorbed dose (G′ = 2100 Pa) for further experiments. The frequency dependence of the storage and loss oscillatory shear moduli (G′ and G′′, respectively) confirmed hydrogel-like behavior as G′ is dominant across the whole range of frequencies (Figure S4a). And the sample showed

Figure 3. (a) The SEM and (b) TEM images of the freeze-dried GO suspension. (c, d) The SEM images of the freeze-dried GO3SPNB hydrogel.

Figure 4. (a) Oscillatory rheological properties (ω = 6.28 rad s−1, γ = 2.5%, 25 °C) of GOxSPNB hydrogels with various GO contents prepared at 15 kGy. (b) Oscillatory rheological properties (ω = 6.28 rad s−1, γ = 2.5%, 25 °C) of GO3SPNB hydrogels prepared at various absorbed doses. (c) Photographs portraying the self-reparability of a GO3SPNB hydrogel at room temperature: the original GO3SPNB hydrogel → the separated pieces → immediately self-healed GO3SPNB hydrogel → stretched GO3SPNB hydrogel. (d) Fully stretched GO3SPNB membrane → toughness test of the GO3SPNB membrane by 1 mL centrifuge tube’s pointed end. (e) Strain-dependent (ω = 6.28 rad s−1, 25 °C) oscillatory shear rheology of GO3SPNB hydrogel. (f) Step-strain measurements of GO3SPNB hydrogel over three cycles (ω = 6.28 rad s−1, 25 °C). 4293


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was deduced from Nyquist plots (Figure S5b) and its original ionic conductivity could reach 10.5 mS dm−1 . More interestingly, its ionic conductivity can be recovered after cut-healing cycles as shown in Figure S5b and Table S1. Compared with its original counterpart, the ionic conductivity of healed hydrogel only slightly decreased. After 10 cut-healing cycles, GO3SPNB hydrogel still presented more than 80% of its original ionic conductivity, which implied that our hydrogel restored ionic conduction after self-healing. Due to its selfhealable, reshapable, conductive, and adhesive ability, this hydrogel can be used as an emergency circuit repair material. 3.4. Adhesion Behavior of the GO3SPNB Hydrogel. Polyacrylate is widely used as adhesives because of its strong hydrogen bonding which has been already industrially produced by DuPont and Bayer. Meanwhile, starch adhesive is made by starch or starch derivatives due to its gelatinization feature, which has been used in the adhesive industry for many years.36,37 Fahihnejad et al. reported that the adhesive properties are determined by the surface density of H-bonding groups and the bulk viscoelasticity of the polymer determines its viscous forces.38 With similar structure and multiple hydrogen-bonding groups, GO3SPNB hydrogel showed satisfactory adhesion force even on a glass surface. The wet hydrogel tightly adhered on the glass and could support a 300 g weight (Figure 5c). To vividly display the adhesive behavior of GO3SPNB hydrogel, the images of hydrogels adhering to various solid materials were illustrated in Figure 6a. The

dramatic decreases in viscosity under shear, which is a typical behavior of a physical cross-linking hydrogel system.30,34 As shown in Figure 4c, when we cut the hydrogel into two separate pieces and then bring them into contact, they automatically rejoin immediately in the ambient environment without any external stimuli. Meanwhile, the resultant hydrogel can be stretched into a membrane which can tolerate a certain strain (Figure 4d, Movies S1 and S2). To further assess the self-healing behavior of the hydrogel, we conducted the rheological recovery tests. As shown in Figure 4e, strain amplitude sweeps of the samples demonstrated a typical elastic response of hydrogels. The G′ value of the GO3SPNB hydrogel rapidly decreases above the critical strain region (γ = 160%), indicating a collapse of the gel state to a quasi-liquid state.35 Besides, this hydrogel exhibited very fast recovery of its mechanical properties without any stimuli after a large strain breakdown, known as thixotropy (Figure 4f and S4b). When the oscillatory shear strain was increased from 1% to 250% and maintained for 50 s, G′′ was a little higher than G′; however, they immediately recovered their original values once the strain went back to 1% (Figure 4f). Similarly, when we later applied the larger strains (500% and 1000%) followed by small strain (1%), G′ also quickly restored the initial value. Moreover, as shown in Figure S4b, the storage modulus G′ (2038 Pa) of the recovered hydrogel nearly recovered its initial value (2042 Pa) within 80 s after stopping the large strain (500%). The fastrecovery capability feature of GO3SPNB hydrogel may result largely from the multiphysical cross-linking network structure, that is to say, the electrostatic interaction between the cation and anion polymer chains, the hydrogen bonding among soluble starch, GO sheets and P(NaSS-co-MOBAB) polymer chains in GO3SPNB hydrogel all benefit the self-healing ability. The recovery process in GO3SPNB hydrogel does not involve reassembly of chemical bonds and therefore is quick. 3.3. Conductive Behavior of the GO3SPNB Hydrogel. XPS (Figure 2c) confirmed that there was free NaBr inside the hydrogel system which may provide certain ionic conductivity. To some extent, this hydrogel can be designed to be an emergency circuit repair material. A battery-powered circuit was built to visually demonstrate the electrical healing (Figure 5a, Movie S3). As shown in Figure 5b and Movie S4, parallel circuit and series circuit were achieved, respectively, by connecting two LEDs using the resultant hydrogels. Furthermore, the ionic conductivity of the GO3SPNB hydrogel

Figure 6. Adhesive exhibition of GO3SPNB hydrogel adhering to (a) various solid materials, including PTFE (polytetrafluoroethylene), PP (polypropylene), glasses, NR (natural rubber), copper, and iron; (b) various biological tissues including heart, liver, kidney, bone, skin, and muscle.

samples exhibited an effective adhesive behavior on the various organic and inorganic substrates, including polytetrafluoroethylene (PTFE), polypropylene (PP), glasses, natural rubber (NR), copper, and iron. To test GO3SPNB hydrogel’s potential for the practical applications of human portable equipment and tissue adhesive, the adhesion behavior of hydrogel with various biological tissues was explored and shown in Figure 6b. Each adhesive hydrogel can easily adhere to the chicken’s biological tissues including heart, liver, kidney, bone, skin, and muscle. Moreover, the as-prepared device could hold up 350 g weights at room temperature (Figure 7a, b). To evaluate the potential usage of our gel as a self-healing adhesive matter, lap shear testing was performed by using various metal substrates and organic substrates. Figure 7c demonstrates the strong adhesive strength of GO3SPNB gel to various metal substrates. Especially, the adhesive strength to copper plates was observed to be super high (60.5 MPa), which is the highest value compared with published papers.22,39,40 The super high adhesive strength may be ascribed to the possible adhesion

Figure 5. (a) Conductive behavior and conductive function selfhealing behavior of the GO3SPNB hydrogel at room temperature: original hydrogel→ broken hydrogel→ self-healed hydrogel. (b) Parallel circuit and series circuit connected by GO3SPNB hydrogel. (c) Photograph showing the adhesion properties of GO3SPNB hydrogels on glass slides. 4294


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bility by electrostatic interaction and hydrogen bonds as the dynamic bonds. This resultant GO3SPNB hydrogel tolerates extreme stretch which can transform from bulk hydrogel into transparent membrane hydrogel, and features an ionic conductivity of 10.5 mS dm−1. And the hydrogel can completely heal itself immediately without any external stimuli at room temperature. More importantly, with highly adhesive components such as soluble starch and acrylate derivatives inside, the gel exhibits super adhesion to metal substrates and organic substrates. For example, the adhesion strength to the copper plates was about 60.5 MPa which is almost one magnitude larger than reported hydrogel adhesives. This may be ascribed to the possible adhesion interactions between the gel and substrates as well as the electrostatic interaction and hydrogen bonding inside the gel. Moreover, the self-healing adhesive hydrogel can be repeatedly used five times on the aluminum plates without obvious strength loss. We used porcine skin as biological substrates to simulate human dermis and the adhesive strength reached 130.70 kPa which is the highest level compared with the published literatures. Meanwhile, the cell viability of human cervical cancer (HeLa) cells remained at 100% in the GO3SPNB hydrogel. We believe that our new design strategy, the simple one-pot radiation polymerization method, and the self-healing adhesive hydrogel system, could be generally applied to develop a new generation of harmless conductive adhesive materials.

Figure 7. (a) Schematic representation of lap shear test geometry. (b) Photograph showing 350 g weights putting on the glass plates by adhesion with the dried gel. (c) Adhesive strength of the dried gel to different metal substrates. (d) Stress−strain plots of the healable gel tested using lap shear test for five times on Al plates. (The inset picture shows the surface of the tested sample after five cycles.)

interactions between the gel and substrates including hydrogen bonding, hydrophobic interaction, and metal-complexation as well as the electrostatic interaction and hydrogen bonding inside the gel.41 Furthermore, Figure S6 shows the adhesion strength to various organic substrates, including porcine skin, PTFE, and PMMA plates. We chose porcine skin as tissue substrate, because it can closely resemble human skin and is robust enough to ensure that adhesive failure finally occurs at the interface. The adhesion strength of GO3SPNB hydrogel to porcine skin reached 130.70 kPa, which was almost one magnitude larger than reported hydrogel adhesives.42−45 An ideal self-healing adhesive should not only show great adhesive strength, but also restore its adhesion properties in the case of damage. To investigate the recovery performance of the hydrogel, five cycles of wetting treatment-lap shear tests were performed on the Al substrates as shown in Figure 7d. The lap-shear strength was in the range of 47.35 ± 2.49 MPa for all the test cycles. The slight decrease of the strength may be due to the mass loss of the dried gel during the delamination. And the inside digital photo showed that there was no obvious interface damage on the Al substrates. The above results, together with the conductive features of the GO3SPNB hydrogel, suggested that our hydrogel may have potential as a useful self-healing adhesive in various industrial fields, especially electronic binders. 3.5. Cytotoxicity Tests of the GO3SPNB Hydrogels. We further evaluated the biocompatibility of the GO3SPNB hydrogels prepared at various absorbed doses by MTT assay using HeLa cells (Figure S7). After 24 h of incubation with the hydrogels, the viability of the HeLa cell line was typically 100%, as compared to the control (medium without hydrogel). This low cytotoxicity along with the superior adhesive behavior to porcine skin (Figure S6) shows that our hydrogel has great potential for use in biomedical applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01260. 1 H NMR spectrum, 13C NMR spectrum of MOBAB, photograph presentations of all samples, rheological characterization of hydrogels, electrical healing study of GO3SPNB hydrogel, adhesive strength of GO3SPNB to various organic substrates, and cell viability data of GO3PNB hydrogel (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) Movie S4 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.Z.). ORCID

Wen-Bin Zhang: 0000-0002-8746-0792 Maolin Zhai: 0000-0002-6038-3512 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Science Challenge Project (No. TZ2018004) and National Natural Science Foundation of China (Project No. 11575009, 11375019, 21474003, and 91427304) is acknowledged for supporting this research.

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4. CONCLUSIONS We designed and prepared a stretchable, conductive, biocompatible hydrogel with autonomous self-healing capa-

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