Eco-Friendly, Self-Healing Hydrogels for Adhesive and Elastic Strain

7 days ago - Intelligent skinlike materials have recently attracted tremendous research interests for employing in electronic skin, soft robotics, and...
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Eco-Friendly, Self-Healing Hydrogels for Adhesive and Elastic Strain Sensors, Circuit Repairing, and Flexible Electronic Devices Zhuo Zhang,† Zhiliang Gao,† Yitong Wang,† Luxuan Guo,† Chaohui Yin,‡ Xiaolai Zhang,† Jingcheng Hao,*,† Guimin Zhang,§ and Lusheng Chen∥ †

Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Ministry of Education, and School of Physics, Shandong University, Jinan 250100, P. R. China § State Key Laboratory of Generic Manufacture Technology of Chinese Traditional Medicine & National Engineering and Technology Research Center of Chirality Pharmaceuticals, Lunan Pharmaceutical Group Co. Ltd., Linyi 276000, P. R. China ∥ College of Chemistry, Chemical Engineering and Material Science, Shandong Normal University, Jinan 250014, P. R. China

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S Supporting Information *

ABSTRACT: Intelligent skinlike materials have recently attracted tremendous research interests for employing in electronic skin, soft robotics, and wearable devices. Because the traditional soft matters are restricted in unsatisfactory mechanical performances or short-term usage, these materials are adverse to practical applications. Here, intriguing conductive hydrogel materials with multifunctionality (MFHs) are fabricated by using poly(acrylic acid) (PAA), dopamine-functionalized hyaluronic acid (DHA), and Fe3+ as ionic cross-linker. The mussel-inspired networks with delicate combination of physical and chemical cross-linking possess synergistic features of inherent viscoelasticity, high stretchability (800%), and durable self-adhesiveness to various substrates. Owing to the abundant hydrogen bonds and multiple metal coordination interactions between Fe3+, catechol, and carboxylic groups, the matrix reveals repeatable thermoplasticity and autonomous self-healing property both mechanically and electrically (98% recovery in 2 s). When served as strain sensors, the MFHs can distinctly perceive complex body motions from tiny physiological signal (breathing) to large movements (knee bending) as human motion detecting devices. Moreover, the MFHs were explored as ideal material for circuit repairing, programming, and switches constructing because of their excellent properties. Consequently, these eco-friendly hydrogel ionotronic devices can be promising candidates for next-generation intelligent wearable devices and human−machine interfaces.

1. INTRODUCTION Modern flexible electronics have received a myriad of research attention for their extensive applications in the fields of human−machine interfaces1−3 and energy storage devices.4−7 Among them, flexible wearable strain sensors, such as electronic skin, mimicking human skin for daily healthcare and self-diagnosis, can repeatedly monitor quantifiable electrical signals upon various external deformation. These devices are highly desired due to their simple transduction mechanism, low power consumption, and compatibility with large-area processing techniques.8−11 Currently, there are three mature approaches to improve their sensitivity (gauge factors): (i) inducing microcracks in strain sensors,12,13 (ii) designing three-dimensional (3D) conductive networks,14,15 and (iii) endowing strain sensors with rough microstructure.16,17 Nevertheless, focusing on the building blocks, the reported materials, e.g., conductive polymers, carbon-based materials, and organic photovoltaics, are almost hard and lacking of integrated multifunctionality, such as stretchability, selfhealability, self-adhesiveness, and recyclability, which give rise © XXXX American Chemical Society

to the profound challenges to achieve eco-friendly strain sensors with high performance stability and comfortable human experience.3,18−20 In 2004, Suo et al. proposed a novel concept of “ionic skin” based on ionic transduction through hydrogels or ionic gels, which provides great avenues to simply design intelligent skinlike strain sensors with transparency, biocompatibility, and high stretchability in contrast to the traditional and sophisticate “electronic skin”.21 As a consequence, the soft hydrogels as promising candidates for flexible wearable devices have achieved significant progress in the subsequent development. A representative research conducted by Wu et al. first incorporated self-healing properties into a bioinspired mineral hydrogel by metal chelation as compliant ionic skin, which could repeatedly repair structural damage and recover to their functional states, thereby enhancing their durability and Received: November 19, 2018 Revised: March 5, 2019

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DOI: 10.1021/acs.macromol.8b02466 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules prolong the service life.22 The intelligent strain sensors significantly decrease the repair costs and improve economical profits, meeting the escalating requirements of eco-friendly wearable strain sensors. However, the strain sensors cannot be attached directly onto the human body, which need additional adhesive tape or binding straps, leading to tedious operation processes during long-term practical application. To ameliorate these defects, developing compliant, self-adhesive, and selfhealing hydrogel materials by blending polydopamine with PVA hydrogels for epidermal strain sensors emerged as alternative choices, which show high sensitivity at both ultralow and high strain.23 Despite the successful incorporation of self-adhesiveness, the toughness and stretchability of these hydrogel materials are so weak that they are vulnerable to rupture after debonding. Overall, only few attempts were made to integrate multifunctional features such as mechanical strength, healing property, adhesiveness, and renewability into one hydrogel system, which is highly desirable for exploitation of next-generation eco-friendly strain sensors and creation deep integration between the natural and the artificial. To this end, we fabricated multifunctional conductive hydrogel materials (MFHs) by facile one-pot free radical polymerization with intriguing features, including inherent viscoelasticity, comprehensive mechanical performances, rapid self-healing capability, durable self-adhesiveness, eco-friendly reusability, and strain-dependent conductivity. In this hybrid gel system, acrylic acid (AA) monomers were polymerized in the presence of mussel-inspired dopamine-functionalized hyaluronic acid (DHA) polymer and ionic cross-linker, Fe3+, to establish an interpenetrating network coexisting multiple noncovalent interactions and covalent bonds, endowing the MFHs with high stretchability and toughness under deformation due to the efficient energy dissipation. The rich inter- and intrachain hydrogen bonds inside the matrix render the thermoplastic performance to the MFHs, resulting in ecofriendly reusable applications. The excellent and reversible selfadhesiveness to various surfaces including organic and inorganic substrates was provided by the abundant catechol groups and quinone groups on the DHA. Owing to the multiple noncovalent interactions, the MFHs exhibited fast and durable self-healing properties, 97% mechanical recovery in 30 s and electrical recovery with 98% in 2 s, which is conducive to enhance the stability and prolong the lifetime of hydrogel devices. Significantly, the densely porous networks and dispersive Fe3+ enable the MFHs to display stable and repeatable signal variations upon mechanical deformations for utilizing as flexible strain sensors to accurately perceive realtime complex human motions. Furthermore, the MFHs can also be applied for circuit repairing, programming, and switches constructing in a flexible electronic field.

dissolved in 50 mL of phosphate-buffered saline (PBS) solution at pH = 5.5. EDC (960 mg) and NHS (576 mg) were slowly added to the DHA PBS solution at a ratio of 1:4:4 (DHA:EDC:NHS) and then reacted for 2 h to activate the carboxylic group of DHA. Subsequently, DA (948 mg) was added to mixed solution with vigorous stirring at a 4:1 molar ratio to DHA and reacted at room temperature for 9 h. The pH of the resulting solution was maintained to 5.5 by using 1 mol L−1 HCl solution during the reaction. To remove unreacted DA and purify the final product, the solution was dialyzed (MWCO = 8000 Da) exhaustively against a pH = 5.0 HCl solution for 3 days and then dialyzed against deionized water for 2 days. The resulting mixture was lyophilized and stored at −20 °C before use. 1H NMR analysis was performed to confirm the structure of the DHA conjugates. The content of catechol was determined by measuring absorbance at 280 nm using a UV−vis spectrometer, and quantitative measurement was achieved with a DA standard. 2.3. Fabrication of Multifunctional Hydrogels (MFHs). The MFHs were prepared by in situ polymerization of AA into DHA polymer networks with the Fe3+ as ionic cross-linker. Typically, 1 mL of DHA (2 wt %), 500 μL of AA, 0.2% MBAA (mol %, MBAA:AA), and 1% Fe(NO3)3 (mol %, Fe(NO3)3:AA) were mixed homogeneously with vigorous stirring to form precursor solution. After degassing for 10 min, 800 μL of 5% APS (wt %, APS:AA) aqueous solution was added to initiate the polymerization. Additionally, the degree of chemical cross-linking was changed by varying the concentrations of MBAA (cMBAA) from 0 to 2%. The physical crosslinker content was enhanced by increasing the concentrations of Fe(NO3)3 (cFe) from 0 to 3%. For comparison, single cross-linked PAA/Fe3+ hydrogels and PAA/DHA hydrogels were obtained through the same process as that of the MFHs without the addition of DHA or Fe3+. 2.4. Measurements and Characterization. 1H NMR spectra were recorded on a Bruker Avance 400 spectrometer (Bruker, Germany). UV−vis spectra were obtained using a U-4100 UV−vis spectrometer with a scanning speed of 600 nm/min. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded from a VERTEX-70/70v FT-IR spectrometer (Bruker Optics, Germany). The cross-sectional images of freeze-dried hydrogels with the gold coating were observed on a scanning electron microscopy (SEM, Zeiss G300). Wide-angle X-ray diffraction (WAXD) experiments were measured on a Rigaku D/Max 2200PC diffractometer equipped with a graphite monochromator and Cu Kα radiation (λ = 0.15418 nm) in the range of 10° < θ < 60°. Rheological measurements were performed on a Haake RheoStress 6000 rheometer with a cone−plate sensor system (C35/1° Ti L07116; diameter: 35 mm; core angle: 1°). The rheological tests can be conducted at varied temperature with the help of a cyclic water bath. The compressive tests at 5 mm/min and tensile experiments at 50 mm/min were conducted by Texture Analyzers (Food Technology Corporation). The adhesive strength of the hydrogels was determined by lap shear testing performed on a commercial tensile tester at 50 mm/min (Tensile Tester AG-2000A, Shimadzu). 2.5. Rheological Measurements. Initially, the linear viscoelastic region of the hydrogels was defined by conducting amplitude sweep tests at a fixed frequency of 1 Hz. Besides, the mechanical strength of the hydrogels with different cross-linking densities as well as the thermoplasticity at different temperature (25, 40, 60, and 75 °C) were investigated through the amplitude sweep tests. With the frequency ranging from 0.01 to 10 Hz, frequency sweep tests were performed under an appropriate stress selected from the linear viscoelastic region. The self-healing properties of the MFHs were estimated first by the steady-shear measurements under the CR (control rate) mode.25 The shear rate was increased from 1 to 1000 s−1 and then decreased from 1000 to 1 s−1. Moreover, the alternate step strain sweep was performed at a fixed oscillatory frequency of 1 Hz. Oscillatory shear stress (γ) was switched from small stress (γ = 1 Pa) to subsequent large stress (γ = 8000 Pa, which is larger than upper limit value of linear viscoelastic regions) with 150 s for every stress interval. Except as indicated otherwise, the tests were performed at 25 °C.

2. EXPERIMENTAL SECTION 2.1. Materials. Acrylic acid (AA, >99%), ammonium persulfate (APS, >98%), 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC, >98%), and dopamine hydrochloride (DA, >98%) were purchased from Aladdin Reagent. N,N′-Methylenebis(acrylamide) (MBAA, >98%) and N-hydroxysuccinimide (NHS, >98%) were purchased from J&K Scientific Ltd. Sodium hyaluronic acid (SHA, Mw = 370 kDa) was obtained from Freda Biochem Co., Ltd. (Shandong, China) and used directly. Ferric nitrate (Fe(NO3)3· 9H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Synthesis of DHA Conjugates. To prepare the DHA, DA was chemically conjugated to the carboxyl group of SHA using EDC and NHS as previously described.24 Briefly, DHA (500 mg) was B

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Figure 1. Schematic of the synthetic process for the MFHs and the corresponding interactions inside the multifunctional networks. 2.6. Lap Shear Strength Testing. To estimate the adhesion of the MFHs, we examined their adhesive strength at various surfaces (including glass, Teflon film, and porcine skin) by lap shear strength tests.26 The substrates without contaminants were cut into 4 × 2.5 cm2 strips and employed as adhered materials. The preweighed hydrogels (100 mg) were sandwiched among the same prepared substrate surfaces with an overlapping area of 2.5 × 2.5 cm2. The joint area was compressed under 100 g of weight for 10 min to ensure the hydrogels entirely adhered to the overlapped area uniformly. For comparison, samples using PAA/Fe3+ hydrogels as adhesives were prepared by the same way and tested. After measuring the contact area of substrate using digital caliper, the adhesion test was immediately processed with a strain rate of 5 mm/min until the joint split. The adhesion strength was determined by the maximum load divided by the known overlapping area of the adhesive joint. 2.7. Electrochemical Tests. To evaluate the conductivity of the PADA hydrogels, electrochemical impedance spectra were measured on CHI760D electrochemical workstation at frequencies ranging from 0.01 Hz to 100 KHz with a 0.005 V voltage amplitude utilizing ITO glasses as cell electrodes and Teflon as a spacer. Data were analyzed using Z-view software to obtain the Nyquist plots. 2.8. Capacitance based Strain Sensor. A capacitive strain sensor was fabricated to detect the electrical changes of the PADA hydrogel and monitor large-scale human motions. A stretchable dielectric layer (Very High Bond (VHB) 4905, 3M) was sandwiched by two hydrogel layers which were linked to two metallic electrodes. An additional VHB was applied to wrap the whole capacitive strain sensor, resulting in protection and evaporation control. Capacitance measurements were taken using the impedance analyzer (HewlettPackard 4192A LF) at an AC voltage of 1 V and a sweeping frequency of 1 kHz.

one-pot in situ free radical polymerization of AA in the existence of Fe(NO3)3 (noncovalent cross-linker), MBAA (covalent cross-linker), and DHA as the adhesive compound. It is worth noting that the concentration of AA and DHA was fixed at 25 and 1 wt %, respectively. Covalent bonds and noncovalent interactions are two main types of interactions in the networks, which give the multifunctionality to the hydrogels. On the one hand, the chemically cross-linked PAA chains and DHA polymer sustain a permanent dense skeleton, which can efficiently dissipate energy after external load application. On the other hand, the metal coordination of Fe3+ ions with carboxylic groups of PAA as well as DHA and catechol with Fe3+ maintain a reversibly loose network, endowing the MFHs with self-healing property. Furthermore, the existence of catechol, hydroxyl, and carbonyl groups in PAA and DHA chains forms extensive inter- and intrachain hydrogen bonding, leading to the thermoplasticity of the MFHs. The synergistic effect of chemical cross-linking and noncovalent interactions facilitates smooth stress-transfer, contributing to the high stretchability, elasticity, and toughness of the MFHs. Owing to the unoxidized catechol groups of DHA, the MFHs own the self-adhesiveness to various substrates. The interpenetrate networks were permeated with free ferric irons, which imparts the ionic conductivity to the MFHs. Evidently, these features of the MFHs could be affected by the diverse compositions, which would be demonstrated detailedly in the following discussion. To exhibit and confirm the chemistry behind the MFHs, as illustrated in Figure S3, the metal−ligand coordination can be substantiated from the FT-IR spectrum. From the PAA/DHA hydrogel curve, a single absorption peak at 1710 cm−1 was attributed to the symmetrical stretching vibration of CO of −COOH. While in the spectrum of the MFHs with addition of Fe3+, a noticeable change has been detected in a new peak at 1595 cm−1 corresponding to the typical peak of stretching asymmetric vibration of the −COO−. It is testified that the carboxylic groups of hydrogel molecules convert into ionized carboxyl group after chelating with Fe3+, generating the split of the single peak into two peaks.25 As the SEM micrographs revealed (Figure S4), both the PAA/Fe3+ complexes and the MFHs exhibited 3D interconnected networks with uniform macroporous structures. Compared with the PAA/Fe3+ hydrogel, the MFHs possess larger pore size ranging from 8

3. RESULTS AND DISCUSSION 3.1. Design Strategy and Characterization of Multifunctional Hydrogels (MFHs). The MFHs synthesis process and mechanism are illustrated in Figure 1. First, DHA conjugates were synthesized by modifying the SHA with DA by the EDC/NHS coupling reaction (Scheme S1 in the Supporting Information). The successful modification of DA was verified by 1H NMR (Figure S1), as indicated by the appearance of catechol proton peaks between 6.6 and 6.8 ppm and the new peaks belonging to the −CH2− group close to the catechol ring at around 2.8 ppm. In accordance with the UV adsorption at 280 nm (Figure S2), the substitution of DA on the SHA backbone was calculated to be 33.4 ± 0.3%. Subsequently, the as-prepared MFHs were fabricated via C

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Figure 2. (A) G′ and G″ as a function of the stress for the MFHs (cMBAA = 0.1 mol %, cFe3+ = 1 mol %). (B) Oscillatory shear test result for the MFHs (cMBAA = 0.2 mol %, cFe3+ = 1 mol %). Compressive test of the MFHs with (C) different cMBAA, the cFe3+ was fixed at 1 mol %, and (D) with different cFe3+, the cMBAA was fixed at 0.1 mol %. (E) Photograph of the samples which could stretch to 800%. Optical images of PAA/DHA (F), PAA/Fe3+ (G), and the MFHs (H) at 90% compressive strain.

to 23 μm, which is beneficial for their high stretchability and large deformation. The MFHs with different polymer constituent displayed the similar porous structure, illustrating the high reproducibility of this preparation method. Rheology tests were performed to evaluate the viscoelasticity of the hydrogels, where all samples were permitted to develop a fully cross-linked network after 3 days standing at 25 °C. Amplitude sweep tests were conducted to estimate the viscoelasticity of MFHs initially (Figure 2A), which show the curves for storage modulus (G′) and loss modulus (G″) as a function of stress. During the stress from 1 to 8300 Pa, G′ and G″ remained constant, defined as the linear viscoelastic region where the hydrogels exhibited solidlike behavior (G′ > G″) and the steady networks remained unbroken under externally large deformations. When the stress reached 8960 Pa, the intersection of the G′ curve and the G″ curve can be monitored, certifying that the reaching of a solid−liquid

transition point and the hydrogel networks begin to be damaged. With the stress increased continually, G′ sharply decreased and became lower than G″, indicating the solidlike networks thoroughly collapsed into a quasi-liquid state.27 As the frequency sweep test revealed (Figure 2B), the G′ was conspicuously greater than the G″ over the entire frequency range, demonstrating that the MFHs possessed elastic dominant networks. 3.2. Stretchability and Toughness of MFHs. The MFHs with comprehensive mechanical performance reveal highly stretchable and mechanically compliant under strain, which is promising for the fabrication of flexible electronic devices. Given that the variation of composition could result in different cross-linking ratios in networks, a variety of the MFHs with diverse cMBAA and cFe were prepared to be evaluated the toughness and stretchability. As shown in compressive stress− strain curves (Figure 2C), the MFHs (cFe = 1 wt %) containing D

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Figure 3. (A) A bulk of the MFHs can be cut into two pieces and then healed under ambient conditions. (B) G′ and |η*| values of the MFHs (cMBAA = 0.1 and cFe3+ = 1 mol %) when alternate stress switched from small strain (γ = 1.0 Pa) to large strain (γ = 9000 Pa) with a loading period of 150 s. (C) Steady-shear test results of the MFHs (cMBAA = 0.1 and cFe3+ = 1 mol %). (D) Healing efficiency of the original and the self-healed hydrogels with different cFe3+. (E) Digital photographs of the electrically healing process. The LED could be lit again once the two cut hydrogel blocks were contacted together. (F) Real-time current tests during the self-healing process (cMBAA = 0.1 and cFe3+ = 1 mol %).

different cMBAA were compressed under a raising strain. As expected, the compression strength and stiffness increased apparently with growing cMBAA from 0.08 to 0.2%, which should ascribe to an increasing covalent cross-linking ratio and more dense networks. It was observed that the hydrogels cannot be fabricated without MBAA. Accordingly, the cumulatively stability of hydrogel has a positive correlation with the concentration of MBAA (Figure S5). Subsequently, the compression strength and stiffness increased with the increases of Fe3+ contents from 0 to 0.5 mol % at a fixed cMBAA (0.1 wt %) (Figure 2D). Nevertheless, following by a further increment in Fe3+, the compression strength and stiffness displayed a decreasing trend. The impacts of Fe3+ on stretchability were further evaluated by tensile tests. As depicted in Figure 2E, the MFHs (cMBAA = 0.1 mol %, cFe3+ = 1 mol %) possess high stretchability with 800% elongation at break. Furthermore, Figure S6 shows that the extensibility was improved remarkably by introducing appropriate Fe3+ (0−0.5 mol %), while the hydrogels became much weaker with a continued increase of cFe (1−2 mol %). It is speculated that an appropriate amount of ferric ions could enhance the crosslinking density and expedite the polymerization of PAA, contributing to an improvement of mechanical performance. However, the excess iron loading (exceeding 1 mol %) may restrain the radical polymerization, remaining the networks with non-cross-linked Fe3+ which harm the mechanical performance of the MFHs. Qualitatively, a 90% strain was exerted on the MFHs to visibly display the toughness, compared to its pure parent hydrogels (Figures 2F−H). It is apparently that both PAA/DHA hydrogels and PAA/Fe3+ hydrogels exhibited observably damages and plastic deformations at 90% strain compression (Figures 2F,G), in contrast to the nearly intact structure of the MFHs (Figure 2H). Consequently, after introducing DHA and Fe3+, the additional multiple noncovalent interactions and hydrogen bonding could dramatically reinforce the matrix, which play a vital role in mechanical strength of the MFHs. 3.3. Self-Healing Behavior of the MFHs. Conductive hydrogels incorporated self-healing properties are highly anticipated, since they can maintain both mechanical and

electrical performances even under unfavorable damage. These hydrogels could be further utilized as flexible electronics with long-term service life span. In the MFHs system, the abundant reversible bonds in the networks imparted autonomously healable properties both mechanically and electrically within transient healing durations under ambient conditions (≈62% humidity at 25 °C). As shown in Figure 3A, the mechanical self-healing behavior of the MFHs was examined first. Asprepared hydrogel (cMBAA = 0.1 and cFe3+ = 1 mol %) was cut apart by a blade, and then the fresh cuts were merged together into an entity under mild force to facilitate the automatically healing process. After 10 s, the rehealed hydrogel can sustain repeated stretch without crack, indicating that they have a reliable self-healing property. Additionally, the continuous step stress sweep28 and steady shear tests25 were conducted to further verify the recovery property of the MFHs. As revealed in Figure 3B, when the oscillatory shear stress suddenly changed from 1 to 9000 Pa for 150 s, the G′ and the complex viscosity (|η*|) decreased considerably, representing the collapse of networks under large stress which exceeds the critical point (Figure 2A). Once the stress declined to the original value (1 pa), both G′ and |η*| recovered to primary values promptly, indicating a visible conversion of hydrogel from the quasi-liquid state to gel-like state. The collapse and reversion process could be reproduced many times, implying the intrinsic and rapid self-healability of the MFHs. As seen from Figure 3C, the instantaneously apparent viscosity and thixotropic behavior were also detected within a wide range of shear rates from 1 to 1000 s−1. It is obvious that the apparent viscosity declines gradually with the increasing of shear rate, exhibiting a typical shear-thinning behavior. As the shear rate drops to initial state, the viscosity could almost recovered, demonstrating that the MFHs could nearly restore its original state.29 To elucidate the underlying mechanism of the selfhealing process, the effect of cFe3+ was further investigated by compressive tests (Figure S7). One can see that the compression strength of the self-healed hydrogels (in 30 s) manifested little spoilage after exhaustive destruction, while the hydrogels without Fe3+ were evidently destroyed. To quantify the self-healing performances of the MFHs, the healing E

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function of temperature over a range from 25 to 75 °C. Obviously, the values of G′ and τ declined gradually with the raised temperature, testifying that the hydrogels got softened owns to the damage of hydrogen bonding, which is accordance with the macroscopic behavior (Figure S8B). Accordingly, this thermoplastic hydrogel can be a promising candidate for shapeable and reusable electronics. 3.5. Durable Self-Adhesiveness of the MFHs. The strong self-adhesiveness to various surfaces, including organic and inorganic surfaces, was another distinctive and advantageous feature of the MFHs, ascribing to the enough catechol groups of DHA chains imitating the mussel adhesion mechanism. As demonstrated in Figure 5A, two pieces of glass slides were glued together by the MFHs, and each side could sustain a heavy load of 200 g, revealing excellent adhesion to glass substrate. Furthermore, two metal sheets (zinc sheet) were bonded together by the hydrogels, which could bear a 200 g weight (Figure 5B). Besides inorganic substrate, the MFHs could also tightly adhere to the organic surfaces, such as the plastic surface (Figure 5C). The adhesion strength of the MFHs to the representative surfaces, such as porcine skin, zinc sheet, and Teflon, was quantified by lapshear experiments (Figure S9A). The PAA/Fe3+ hydrogel without adding DHA was selected as the control. As clarified in Figure 5D, compared with the control group, the MFHs exhibited significant improvement in adhesive strength, which was apparently influenced by different tested surfaces. Specifically, the highest adhesion strengths were 33.2 kPa for metal, 12.6 kPa for porcine skin, and 11.1 kPa for Teflon. More importantly, the MFHs possessed repeatable and durable adhesive capability, attested to by conducting adhesion peel-off tests on three different typical substrates over three cycles (Figure 5E). In the first cycle, the hydrogel was attached to the same substrate, then it was peeled off by a tensile load, and ultimately the same hydrogel was readhered before the next cycle. It was found that the adhesion peel-off process had a negligible impact on the adhesive strength of the MFHs, and adhesion strengths were kept at about 32.3 kPa for metal, 11.4 kPa for porcine skin, and 10.7 kPa for Teflon after three cycles. The high and durable adhesion to various surfaces could be explained by mussel chemistry. The unoxidized catechol groups firmly anchored on the inorganic surfaces mainly by the reversible coordination interactions, whereas the oxidized o-quinone provides attachment to organic surfaces via forming irreversible covalent cross-linking (Figure S9B).32 Additionally, the self-adhesiveness strength of the catechol groups was stronger than that of the quinone groups. Hence, the MFHs could attach to not only the inorganic substrates, such as glass and metal, but also the organic substrates, such as porcine skin and Teflon. The high self-adhesiveness makes the MFHs promising candidates for epidermal electronics, or implantable sensors, which desire intimate contact between the devices and the curved or dynamic surfaces to accommodate the complex motions. 3.6. Electrochemical Performance and Circuit Repairing Simulation. The existence of free ions in networks, such as Fe3+ ions, contributed to the ionic conductivity of the MFHs. Moreover, the porous structure is prerequisite for strain-dependent conductivity since it serves as the pathway to control the diffusion of ions, even if external pressure exerts on the soft matrix, which is conducive to obtain tunable electromechanical property and fabricate strain-sensitive wearable devices. To display the conductivity directly, the as-

efficiency is defined as S′/S, where S′ is the compression strength of the self-healed hydrogels at 90% strain and S is that of original hydrogels. The healing efficiency enhanced dramatically with the addition of Fe3+, which could be further improved by increasing the cFe3+ (Figure 3D). All the results confirmed that Fe3+ has crucial influence on the self-healing process. As clarified in Figure 1, the prominent self-healing performance of the MFHs is ascribed to the diversely reversible bonds in networks. When two bulk gels were brought together, dynamic metal coordination interactions between Fe3+, catechol, and carboxylic groups prompted ferric ions to move freely, leading to the repairing process. In addition, the polymer chains could intertwine to form abundant hydrogen bonding due to the existence of large amounts of hydroxyl, catechol, and carbonyl groups on polymers. In conclusion, the dynamic bonds could achieve to energy dissipation effectively under accidental destruction, imparting spontaneously healing capacity to the MFHs. Moreover, the MFHs also presented excellent restoration of the electrical properties after the repeated self-recovery. As shown in Figure 3E, the MFHs could light a green LED in a complete circuit, displaying adequate ionic conductivity. When the hydrogel was thoroughly bifurcated, the bulb was extinguished due to the open circuit. Once the two pieces were brought together, the bulb was lit again without a distinctly dim behavior. Correspondingly, the real-time current test (Figure 3F) further reveals a 98% recovery approximately just within 1−2 s in the electrical self-healing process, implying a total regeneration of the 3D conductive matrix for the selfhealed hydrogels. 3.4. Thermoplastic Performance of the MFHs. A vast amount inter- and intrachain hydrogen bonding, distributed in soft matrix served as dynamic bridges, could response to different temperatures and consequently endow the MFHs with thermoplasticity (Figure 1).30,31 Undergoing heating above 50 °C, the as-prepared hydrogels became softened due to the breakage of hydrogen bonds, which could be transformed shapes casually but still in a solidlike gel state (Figure 4). As the temperature decreasing, the MFHs retained their desired shape with the reconstruction of hydrogen bonding, accommodating further electrical applications feasibly. Besides, the mechanical strength of MFHs under different temperature was inspected. Figure S8A illustrates the variation for storage moduli (G′) and yield stress (τ) of typical hydrogel samples (cMBAA = 0.1 and cFe3+ = 2 mol %) as a

Figure 4. Thermoplastic performance of the MFHs. Images show that the hydrogels can be manipulated into various shapes under heating and be fixed after cooling. F

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Figure 5. Self-adhesive MFHs could firmly adhered to various surfaces, such as glass (A), metal (B), and plastic (C). (D) Adhesive strength of the hydrogels to different substrates tested by lap-shear experiments. (E) Repeatable adhesive capability of the MFHs to different substrates.

Figure 6. (A) Images for strain-dependent conductivity performance of the MFHs. Electrochemical impedance plot of the hydrogels with different cFe3+ (B) and the corresponding equivalent electrical circuit (C).

prepared MFHs (cMBAA = 0.1 and cFe3+ = 1 mol %) acted as electric wires which were linked to a circuit with a LED bulb (3.7 V), and the bulb was lit once the circuit is connected visibly (Figure 6A). Imposing stretch on the hydrogel, the LED dimmed remarkably because of the coinstantaneous enhancement of pathway length and shrinkage of cross-sectional area and ultimately the restraint of ion transport. After the force was withdrawn, the luminance of bulb obviously recovered to its initial state, representing the strain-sensing performance of the MFHs with unbroken ions pathway. Subsequently, electrochemical impedance spectroscopy was conducted to quantitatively assess conductive properties, and the results were exhibited as Nyquist plots. As can be seen in Figure 6B, after fitting to an equivalent electrical circuit (Figure 6C), the Nyquist plots of hydrogels uniformly consisted of a semicircle and a line, which were mainly controlled by charge transfer and mass transfer, respectively. A semicircle in the high-frequency region was observed, the starting point of which was defined as

the electrolyte resistance (Rs), and the diameter of the semicircle mainly indicated the charge-transfer resistance (Rp).33 The corresponding impedance parameters of the MFHs with different Fe3+ content are presented in Table S1. In contrast to the hydrogel without Fe3+ (the inset in Figure 6B), the hydrogels with Fe3+ showed markedly smaller Rs and Rp. Additionally, the Rp decreased remarkably with the increase of Fe3+ content, suggesting that metal ions had dominated impact on the conductivity of the MFHs. Owing to the excellent self-healing ability and reshapability, the conductive MFHs can serve as ideal material for circuit repairing and programming. As shown in Figure 7A, the series circuit, parallel circuit, and series-parallel circuit could be constructed successfully by linking LEDs using self-repairing MFHs wires without any adhesives. Besides designing 2D circuit, the self-adhesive MFHs wires could be fixed upon 3D rhombus object easily with on-demand programmed patterns, indicating their possible application as eco-friendly and humanG

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Figure 7. (A) The series circuit (a), parallel circuit (b), and series-parallel circuit (c) connected by MFHs wires. (B) Schematic illustration of the switch and circuit. (C) Photographs of MFHs based single-pole double-throw (SPDT) switch connected to the on−off programmed circuit.

Figure 8. (A) Schematic design and mechanism for the capacitive strain sensors. Plots of capacitance change of the hydrogel sensors as a function of time for the applied strain (B) and twist angle (C).

constant, S is the effective area of the conductive layer, and d is the distance between the two electrodes), capacitive sensors can be used to measure the applied force via detecting variation of S and d, which can be impacted due to the change of the area or the distance between the two conductive layers.35 As shown in Figure 8A, a simple capacitance-based strain sensor was constructed by incorporating two MFHs films with a dielectric layer (VHB), followed by connecting with a digital analyzer through electrical wires to record corresponding signals. The sensing response is defined as S (%) = (C − C0)/C0 × 100% = ΔC/C0 × 100%, where C0 and C are the capacitance without and with applied strain, respectively. Initially, the capacitive strain sensor presented incremental responses to the strain ranging from 100 to 200% due to an increase in capacitance Figure 8B. The stable capacitance variations under 180° twist angle applied on the

friendly for long-lasting electronic devices (Figure S10). Additionally, the MFHs could also be utilized to assemble flexible electronic devices, such as switches (Figure 7B). A single-pole double-throw switch made by MFHs was connected to the on−off circuit, in which the LEDs can be switched to light by altering the connection type between MFHs wires and MFHs switch (Figure 7C). 3.7. Strain Sensors for Monitoring Human Body Motion. Considering that the MFHs integrated many desirable features, we investigated their promising application as strain sensors for perceiving real-time bodily motions. In general, capacitance sensors exhibit competitive advantages of high strain sensitivity without temperature disturbance and precise modification of the device by analysis of the simple governing equation.34 As described by C = εS/4πkd (C is the capacitance, ε is the dielectric constant, k is the electrostatic H

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Figure 9. Relative capacitance changes versus time for the bending of finger (A), opisthenar (B), wrist (C), swallowing (D), and bending of knee (E). The detection of tiny movements caused by the blood pressure in the artery of the chelidon (F) and different breathing modes (G).

cuff hinders the blood flow. Based on this physiological process, the sensors fixed on the wrist revealed distinct realtime capacitance signals as the blood pressure rose or fell, which could be utilized as a proof-of-concept blood-pressure sensor (Figure 9F). Furthermore, the MFHs sensor was assembled onto the surface skin of chest tightly to monitor physiological breathing (Figure 9G). The curves illustrated different breathing modes, such as slow breathing, deep breathing, and breathing hold, which could detect the breathing condition before and after exercise. All the results further certified that the hydrogel strain sensors developed in this study possess excellent electrical stability and repeatability, making it an ideal candidate for human health monitoring as wearable devices.

MFHs were observed even after several cycles, indicating their long working life and excellent stability (Figure 8C). To investigate the real applications, the capacitance-based strain sensors were directly attached to the index finger, wrist joint, opisthenar, knee, and throat without any extra adhesive tapes to monitor normal body motions. When the finger bended to different angles at 30°, 60°, and 90°, an increase of relative capacitance change (from ∼111% to ∼174%) was monitored in Figure 9A. The relative capacitance change can be maintained at a certain angle without obvious loss of the change in the capacitance peak. Figures 9B,C demonstrate the relative capacitance changes for strain sensor-attached opisthenar and wrist with different bending conditions, respectively. The distinguishing and repeatable electrical signals testified the sensing stability of the strain sensors. When the device is adhered to the throat, it presented reproducible and distinct signal patterns during swallowing, indicating that the MFHs-based strain sensors could accurately monitor the tiny movements, as shown in Figure 9D. Figure 9E presents the effectively monitoring of knee joint bending with different bending angles (60°, 90°, and 120°). It is apparently that the real-time capacitance change of the sensor enhanced as the bending angle increased with good repeatability and stability, reflecting the potential application in walking state (speed and angle) recording and physiotherapy (ambulation training for stroke patients recovering). To explore the practical application of the MFHs as wearable sensors, the slight physiological changes such as breathing and blood pressure were further detected. When the aerometer was attached onto the left arm, the blood pressure in the artery of the chelidon would increase with the inflatable

4. CONCLUSIONS In summary, this study developed a new class of conductive MFHs based on PAA, DHA, and Fe3+ via facile one-pot free radical polymerization. The subtle combination of physical and chemical cross-linking was employed to meet the high stretchability and toughness due to the efficient energy dissipation. Inspired by the mussel chemistry, DHA chains with catechol groups and quinone groups were introduced into hydrogel substrates, imparting the MFHs with strong and durable self-adhesiveness to various surfaces, including organic and inorganic substrates. Besides, the thermoplastic performance provided by abundant inter- and intrachain hydrogen bonds, ensured that the moldable MFHs alter their shape casually for convenient and recyclable employment. More importantly, after loading Fe3+ as ionic cross-linker, the system with multiple reversible bonds exhibited efficient and durable I

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(7) Chen, D.; Wang, D.; Yang, Y.; Huang, Q.; Zhu, S.; Zheng, Z. Self-Healing Materials for Next-Generation Energy Harvesting and Storage Devices. Adv. Energy Mater. 2017, 7, 1700890. (8) Jin, H.; Abu-Raya, Y. S.; Haick, H. Advanced Materials for Health Monitoring with Skin-Based Wearable Devices. Adv. Healthcare Mater. 2017, 6, 1700024. (9) Wang, X.; Dong, L.; Zhang, H.; Yu, R.; Pan, C.; Wang, Z. L. Recent Progress in Electronic Skin. Adv. Sci. 2015, 2, 1500169. (10) Wang, T.; Zhang, Y.; Liu, Q.; Cheng, W.; Wang, X.; Pan, L.; Xu, B.; Xu, H. A Self-Healable, Highly Stretchable, and Solution Processable Conductive Polymer Composite for Ultrasensitive Strain and Pressure Sensing. Adv. Funct. Mater. 2018, 28, 1705551. (11) Wang, S.; Lin, L.; Wang, Z. L. Triboelectric nanogenerators as self-powered active sensors. Nano Energy 2015, 11, 436−462. (12) Chen, S.; Wei, Y.; Wei, S.; Lin, Y.; Liu, L. Ultrasensitive Cracking-Assisted Strain Sensors Based on Silver Nanowires/ Graphene Hybrid Particles. ACS Appl. Mater. Interfaces 2016, 8, 25563−25570. (13) Kang, D.; Pikhitsa, P. V.; Choi, Y. W.; Lee, C.; Shin, S. S.; Piao, L.; Park, B.; Suh, K. Y.; Kim, T. I.; Choi, M. Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system. Nature 2014, 516, 222−226. (14) Cao, J.; Lu, C.; Zhuang, J.; Liu, M.; Zhang, X.; Yu, Y.; Tao, Q. Multiple Hydrogen Bonding Enables the Self-Healing of Sensors for Human-Machine Interactions. Angew. Chem. 2017, 129, 8921−8926. (15) Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H. M. Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition. Nat. Mater. 2011, 10, 424−428. (16) Wei, Y.; Chen, S.; Lin, Y.; Yang, Z.; Liu, L. Cu-Ag core-shell nanowires for electronic skin with a petal molded microstructure. J. Mater. Chem. C 2015, 3, 9594−9602. (17) Pang, C.; Koo, J. H.; Nguyen, A.; Caves, J. M.; Kim, M. G.; Chortos, A.; Kim, K.; Wang, P. J.; Tok, J. B.; Bao, Z. Highly SkinConformal Microhairy Sensor for Pulse Signal Amplification. Adv. Mater. 2015, 27, 634−640. (18) Tee, B. C. K.; Wang, C.; Allen, R.; Bao, Z. An electrically and mechanically self-healing composite with pressure- and flexionsensitive properties for electronic skin applications. Nat. Nanotechnol. 2012, 7, 825−832. (19) Guo, C.; Yu, Y.; Liu, J. Rapidly patterning conductive components on skin substrates as physiological testing devices via liquid metal spraying and pre-designed mask. J. Mater. Chem. B 2014, 2, 5739−5745. (20) Liu, L.; Luo, S.; Qing, Y.; Yan, N.; Wu, Y.; Xie, X.; Hu, F. A Temperature-Controlled, Conductive PANI@CNFs/MEO2MA/ PEGMA Hydrogel for Flexible Temperature Sensors. Macromol. Rapid Commun. 2018, 39, 1700836. (21) Sun, J.; Keplinger, Y. C.; Whitesides, G. M.; Suo, Z. Ionic Skin. Adv. Mater. 2014, 26, 7608−7614. (22) Wang, Z. Q.; Sun, S.; Zhu, W.; Wu, P. A Bioinspired Mineral Hydrogel as a Self-Healable, Mechanically Adaptable Ionic Skin for Highly Sensitive Pressure Sensing. Adv. Mater. 2017, 29, 1700321. (23) Liu, S.; Zheng, R.; Chen, S.; Wu, Y.; Liu, H.; Wang, P.; Deng, Z.; Liu, L. A compliant, self-adhesive and self-healing wearable hydrogel as epidermal strain sensor. J. Mater. Chem. C 2018, 6, 4183− 4190. (24) Park, H. J.; Jin, Y.; Shin, J.; Yang, K.; Lee, C.; Yang, H. S.; Cho, S. W. Catechol-Functionalized Hyaluronic Acid Hydrogels Enhance Angiogenesis and Osteogenesis of Human Adipose-Derived Stem Cells in Critical Tissue Defects. Biomacromolecules 2016, 17, 1939− 1948. (25) Gao, Z.; Sui, J.; Xie, X.; Li, X.; Song, S.; Zhang, H.; Hu, Y.; Hong, Y.; Wang, X.; Cui, J.; Hao, J. Metal-Organic Gels of Simple Chemicals and Their High Efficacy in Removing Arsenic(V) in Water. AIChE J. 2018, 64, 3719−3727. (26) Chung, H.; Grubbs, R. H. Rapidly Cross-Linkable DOPA Containing Terpolymer Adhesives and PEG-Based Cross-Linkers for Biomedical Applications. Macromolecules 2012, 45, 9666−9673.

self-healing property within 2 s without needing any external stimuli in both mechanical and electrical aspects, which is a precondition for stable working and long-life service. Considering above desirable multifunctionality, the MFHs were assembled into a capacitive strain sensors, which could distinctly perceive complex body motions from tiny physiological signal (breathing) to large movements (knee bending) as human motion detecting devices. Moreover, the MFHs can also be applied for circuit repairing, programming and switches constructing in flexible electronic field. Overall, the musselinspired MFHs pave a new way to explore versatile applications in eco-friendly soft ionotronics, human motion monitoring devices, and artificial intelligence field.

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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/acs.macromol.8b02466. Synthesis and characterizations of SHA; FT-IR spectrum, SEM images and thermoplastic characterizations of the hydrogels; the mechanical performances and selfhealing efficiency of MFHs with different cMBAA and cFe3+; image of lap shear strength test setup; the corresponding parameters of electrochemical impedance tests (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-531-88366074. ORCID

Jingcheng Hao: 0000-0002-9760-9677 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (Grant No. 21420102006) and the Natural Science Foundation of Shandong Province (ZR2018ZA0547).

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REFERENCES

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