Mussel-Inspired Cellulose Nanocomposite Tough Hydrogels with

Article Cite This: Chem. Mater. 2018, 30, 3110−3121

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Mussel-Inspired Cellulose Nanocomposite Tough Hydrogels with Synergistic Self-Healing, Adhesive, and Strain-Sensitive Properties Changyou Shao,† Meng Wang,† Lei Meng,† Huanliang Chang,† Bo Wang,† Feng Xu,† Jun Yang,*,† and Pengbo Wan‡ †

Chem. Mater. 2018.30:3110-3121. Downloaded from pubs.acs.org by MIDWESTERN UNIV on 01/24/19. For personal use only.

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, No 35, Tsinghua East Road, Haidian, Beijing 100083, China ‡ Center of Advanced Elastomer Materials, State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: The remarkable progress in efforts to prepare conductive self-healing hydrogels mimicking human skin’s functions has been witnessed in recent years. However, it remains a great challenge to develop an integrated conductive gel combining excellent self-healing and mechanical properties, which is derived from their inherent compromise between the dynamic cross-links for healing and steady cross-links for mechanical strength. In this work, we design a tough, self-healing, and self-adhesive ionic gel by constructing synergistic multiple coordination bonds among tannic acid-coated cellulose nanocrystals (TA@CNCs), poly(acrylic acid) chains, and metal ions in a covalent polymer network. The incorporated TA@CNC acts as a dynamic connected bridge in the hierarchically porous network mediated by multiple coordination bonds, endowing the ionic gels the superior mechanical performance. Reversible nature of dynamic coordination interactions leads to excellent recovery property as well as reliable mechanical and electrical self-healing property without any assistance of external stimuli. Intriguingly, the ionic gels display durable and repeatable adhesiveness ascribed to the presence of catechol groups from the incorporated tannic acid, which can be adhered directly on human skin without inflammatory response and residual. Additionally, the ionic gels with a great strain sensitivity can be employed as flexible strain sensors to monitor and distinguish both large motions (e.g., joints bending) and subtle motions (e.g., pulse and breath), which enable us to analyze the data on the user interface of smart phone via programmable wireless transmission. This work provides a new prospect for the design of the biocompatible cellulose-based hydrogels with stretchable, self-adhesive, self-healing, and strain-sensitive properties for potential applications in wearable electronic sensors and healthcare monitoring.

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INTRODUCTION The advent of smart wearable devices has risen exponentially to be next-generation electronics.1−3 As an important subfield of wearable devices, wearable and flexible strain sensors for transducing mechanical deformation into electrical signals have been prosperously developed in electronic skins,4 human− machine interaction,5−7 human motion detection,8 personalized health monitoring,9−11 and so forth. Self-healing wearable devices are able to restore their structure and functionality from damage and thereby possess increased safety, reliability, and durability, which may facilitate the optimization of integrated wearable devices for other functions.12 The development of self-healing wearable devices relies considerably on self-healing and flexible soft materials applicable to biological environments. As a typical soft material with structural resemblance to biological tissue, self-healing conductive gels have been increasingly investigated due to their good viscoelasticity and biocompatibility, which are ideal to be a candidate for applications in wearable and flexible strain sensors.13−15 © 2018 American Chemical Society

Emerging research into self-healing conductive gels mimicking natural healing feature involves release of healing agents, dynamic covalent bonds (chemical cross-links), and noncovalent interactions (physical cross-links).16 Of particular interest are noncovalent interactions considering their autonomous self-healing features without the external stimuli (e.g., pH,17 UV light,18 or temperature19), which are generally utilized, separately or in combination, such as hydrogen bonds,20 hydrophobic interaction,21 host−guest interaction,22 and metal coordination bonds.23−25 Excellent mechanical properties of self-healing conductive gels are the prerequisite for wearable strain sensors to achieve high and stable performance under repeated deformation state. However, self-healing conductive gels typically exhibit poor mechanical properties because of their inherent compromise Received: March 19, 2018 Revised: April 17, 2018 Published: April 17, 2018 3110

DOI: 10.1021/acs.chemmater.8b01172 Chem. Mater. 2018, 30, 3110−3121

Article

Chemistry of Materials

Figure 1. (a) Conversion of CNC (1.02 wt %) to TA@CNC (1.2 wt %) under alkaline conditions. (b) Chemical structure of TA. (c) AFM images of CNC and TA@CNC. (d) FTIR spectra of CNCs, TA and TA@CNC.

these adhesive PDA gels typically struggle to be imparted outstanding mechanical properties without sacrificing other desired properties. Moreover, the high costs of dopamine and the characteristically dark color of PDA hydrogels may be impediments for the practical applications. Tannic acid (TA), as a low-cost and biocompatible plant polyphenol, has been used as precursors for the formation of multifunctional coatings.36−38 Because of their analogous structure to dopamine, these tannic acid coatings deposit under similar conditions with PDA, yet are colorless and significantly less expensive than dopamine, which are recognized as a better alternative to PDA for the design of adhesive hydrogels with biocompatibility, nontoxicity, and nonirritant to human skin.39−42 Given that metal-phenolic networks (MPNs) composed of polyphenols and metal ions allow for the versatile engineering of functional surfaces and particles, the surface coatings made from the coordination-driven interfacial assembly of MPNs are a promising strategy toward facile engineering of functional materials, which is of interest in areas ranging from electronics and catalysis to biomedicine.43−46 Herein, inspired by mussel-inspired adhesive mechanism, we designed a tough, self-healing, and self-adhesive ionic gel with high strain sensitive features, introducing a promising candidate for wearable sensors. The ionic gel was prepared by constructing synergistic interfacial dynamic coordination bonds among tannic acid-coated CNCs (TA@CNCs), poly(acrylic acid) chains, and metal ions in a covalent polymer network. The TA-coated CNC acting as a dynamic connected bridge endows the ionic gels with hierarchically porous network mediated by multiple reversible coordination bonds, leading to the significant mechanical reinforcement of ionic gels. The existence of dynamic multiple coordination bonds contributes to the fast self-recovery as well as reliable mechanical and electrical self-healing properties of ionic gels. Besides, the ionic gels display excellent and repeatable adhesiveness to various substrates including human skin tissue, ascribing to the presence of catechol groups from the incorporated TA. Notably, the ionic gels exhibited high strain sensitivity as a

between the dynamic cross-links for healing and steady crosslinks for mechanical strength.26,27 Therefore, it is extremely necessary, but generally challenging, to create an integrated conductive gel with both excellent self-healing and mechanical performance. In the past decade, several approaches have been proposed to address this need, frequently including incorporating nanoscale fillers into the polymer gel networks to efficiently dissipate energy via the rupture of reversible sacrificial bonds.28,29 These nanofillers interact with the surrounding polymer phase to tune the interfacial interactions and thereby simultaneously improve the toughness and self-healing capability of the conductive gel. In addition to nanoclay, graphene oxide, and carbon nanotubes, increasing interest has focused on the use of the rod-shaped cellulose nanocrystals (CNCs) with native crystalline structure, which is ascribed to their plant-based biocompatibility, excellent mechanical properties, and modifiability.30−32 These nanoparticles composed of cellulose backbone chains intrinsically assembled into the unique hierarchical structures allow for high level of interactions between adjacent nanoparticles and the polymer phase, contributing to the simultaneous improvements of selfhealing and mechanical performance. However, the wearable strain sensors assembled by selfhealing conductive gels need additional adhesive tapes and straps to avoid the interfacial delamination between the sensors and the contacted substrates, leading to complicated and timeconsuming operation process during long-term practical application. The self-adhesive behavior can partially ameliorate this concern by attaching directly onto the skin or other tissues of the human body, promoting the stability in the signal detection and transport of wearable strain sensors under repeated deformation state. Thus, a substantial need exists for the self-adhesive conductive gels with excellent self-healing and mechanical properties for applications in wearable strain sensors. Several mussel-inspired adhesive polydopamine (PDA) gels have been proposed to address this need, which attracted the tremendous attention as epidermal sensors and implantable bioelectronics.33−35 While elegant and promising, 3111


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Figure 2. (a) Schematic illustration of ionic gel synthetic process that includes the in situ polymerization to form nanocomposite hydrogels and then immersion in Al3+ solution to produce ionic coordination. (b) SEM images of cross-section fracture morphology of ionic hydrogel with interconnected porous structures and some pullout filamentous fibrils at the crack tip region. (c) UV−vis spectra of PAA, PAA-TA@CNC, PAA-Al3+, TA@CNC-Al3+, and PAA-TA@CNC-Al3+. (d) Possible coordination modes among TA@CNC, PAA, and Al3+: (I) metal-phenolic coordination between TA@CNCs; (II) metal-carboxylate coordination between PAA chains; (III) hybrid bridging between TA@CNCs and PAA chains.

wearable strain sensor, which could precisely monitor and distinguish both large motions and subtle motions, enabling us to analyze the data on the user interface of smart phone by programmable wireless transmission. Moreover, the ionic gels demonstrate no obvious allergic reactions and residual to the human skin, providing a new prospect for biocompatible and wearable sensors based on the design of cellulose reinforced conductive gels with stretchable, self-adhesive, and self-healing properties.

colloidal stability yet but with a slightly yellow discoloration. The tannic acid primer has a PDA-like chemical structure containing catechol groups (Figure 1b), which form the higher molecular weight species due to the in situ oxidative polymerization of plant polyphenol under alkaline conditions, leading to the decrease in solubility. Presumably, the decreased solubility of tannic acid and its inherent affinity through the intermolecular interactions toward cellulose leads to deposition onto the surface of CNC particles.47 The AFM images (Figure 1c) recorded the evolution from well-dispersed and isolated CNC particles to tannic acid-coated CNC (TA@CNC), exhibiting a remarkably thickened wall after the assembly coating process that can be further confirmed by the dimension distribution (Figure S1). FTIR was used to corroborate the successful surface modification (Figure 1d), where the spectrum of TA@CNC appeared some notable stretching peaks in contrast to unmodified CNCs including a peak at 813 cm−1 from distortion vibrations of CC in the benzene rings and peaks at 1531 and 1612 cm−1 from stretching vibrations of the C−C aromatic groups.48 A two-step method was applied to synthesize TA@CNC ionic gels as illustrated in Figure 2a. Initially, all reactants of acrylic acid (AA, monomer), ammonium persulfate (APS, initiator), N,N′-methylenebis(acrylamide) (MBA, chemical

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RESULTS AND DISCUSSION Design and Synthesis of Tannic Acid-Coated Cellulose Nanocrystal (TA@CNC) Ionic Gel. In this study, we proposed a self-adhesive, self-healing, and tough ionic gel with the incorporation of TA@CNC as dynamic motif. Extraction of CNCs was achieved by the hydrolysis process of sulfuric acid, which resulted in sulfate half ester groups on the CNC surfaces. The electrostatic repulsion among these groups imparted colloidal dispersion and stability without precipitation or flocculation to the resulting CNCs aqueous solution, which was a prerequisite for fabricating the subsequent nanocomposites with homogeneously dispersed CNC networks. As shown in Figure 1a, mixing CNCs and tannic acid (Figure 1b) led to coated CNCs (TA@CNC), which remained in 3112


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Figure 3. Mechanical properties of the TA@CNC ionic gels. (a) Stress−strain curves for ionic gels with different TA@CNC contents. (b) Optical images of [email protected] ionic gels showing high flexibility from a free-standing state to a stretched state up to 2900% elongation. (c) Tensile properties for [email protected] at different strain rates. (d) Toughness of ionic gels with different TA@CNC contents. (e) Continuous cyclic tensile loading−unloading curves at 1200% strain without resting time between each cycle. (f) Cyclic tensile loading−unloading curves under different strains (400%, 800%, 1200%, 1600%). (g) Rapid recovery of the ionic gel to its original shape after undergoing a series of compressive deformations of 85% and subsequently releasing force. (h) Typical successive loading−unloading compression tests of [email protected] for seven cycles at 85% strain without resting intervals (the inset image shows the corresponding recovery ratio for each cycle).

the formation of coordination bonds ascribed to the presence of Al3+.49 UV spectroscopy shows that the absorbance of TA@ CNC-Al3+ and PAA-Al3+ are 1.14 and 0.86 at 458 nm, respectively, corroborating that the coordination bonds exist not only between TA@CNCs (I, metal-phenolic coordination), but also between PAA chains (II, metal-carboxylate coordination), while the PAA-TA@CNC-Al3+ displays the higher absorbance of 1.67, inferring the presence of the coordination bonds between TA@CNCs and PAA chains (III, hybrid bridging). Mechanical Properties. We conducted a series of tensile and compressive tests to investigate the mechanical properties of obtained ionic gels. Figure 3a shows the stress−strain curves of ionic gels with different TA@CNC contents and the detailed parameters are summarized in Table S2, which indicate that the mechanical properties of ionic gels are significantly improved and can be tailored by controlling the TA@CNC contents. Notably, [email protected] ionic gel exhibits the highest elasticity and enable to withstanding high degrees of stretching to 2900% without fracture (Figure 3b). The mechanical properties of ionic gels were found to be strain-rate dependent, where the

cross-linker), and TA@CNC were added into a single water pot for the preparation of covalently cross-linked composite gels via free radical polymerization. Subsequently, the obtained composite gels were soaked in 0.1 mol/L Al3+ aqueous solution to form ionically cross-linked domains that utilized the binding affinity of ionic coordination interactions in the covalently cross-linked network. SEM images displayed the morphology of the obtained ionic gel with hierarchically porous structure in Figure 2b, where some pullout TA@CNC fibril bundles can be observed in the interpenetrating channel from neighboring polymer matrix. Moreover, one can note that these thick fibril bundles range from 1 to 5 μm, which is larger than the length of individual TA@CNC, revealing that the bridged TA@CNC may be strongly binding by reversible association of coordination bonds. UV−vis spectra were used to examine the existence of coordination bonds (Figure 2c), and the hypotheses of possible coordination modes were proposed (Figure 2d). PAA and TA@CNC solution show a negligible absorbance in the range of 400−600 nm, whereas PAA-Al3+, TA@CNC-Al3+, and PAA-TA@CNC-Al3+ solution display the evident absorption with a shoulder peak at 458 nm, implying 3113


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Figure 4. Self-healing properties of the TA@CNC ionic gels at 25 °C. (a) Ionic gel is cut into two pieces and recombined. Then the fractured gel can self-healed automatically into a complete gel and sustain stretching without failure after 30 min healing. (b) Typical stress−strain curves of the original and self-healed [email protected] ionic gels with different healing time. (c) Healing efficiency of ionic gels as a function of TA@CNC content after 30 min healing. (d) G′ and G″ of [email protected] ionic gel in the alternate step strain test (strain = 1 and 100%) at a fixed time interval of 200 s.

higher strain rate contributes to the higher fracture stress in Figure 3c. While the strain rate approaches 160 mm/min, the fracture stress levels off and exhibits the less rate-dependent response, implying the limitation of the ability of coordination bonds to dissipate energy. With increase in strain rate, the fracture strain increases initially and then decrease from 120 mm/min because the strain rate is too high for physical crosslinks to break and reform and thereby relax the high stresses at the crack tip, enabling the complex dissociation of coordination bonds tend to be negligible with a lower fracture strain.50 The enhancement of TA@CNC on toughness is shown in Figure 3d, in which the optimized toughness (5.60 MJ/m3) of ionic gels is 43-fold higher than pristine PAA gels. In fact, Young’s modulus exhibited a similar trend profit by the stiffening effect of TA@CNC (Figure S2). The ionic gel was also extremely notch-insensitive (Figure S3), indicating that the mechanical reinforcement of TA@CNC may be derived from resistance against crack propagation in blunting and energy dissipation. Considering that the network in ionic gels incorporated with physical cross-links via multiple ionic coordination interactions, one can expect excellent self-recovery ability on the obtained ionic gel. In Figure 3e, the large hysteresis loop can be observed in the first loading−unloading cycle, implying the effective energy dissipation. However, no substantial decrease in maximum strain is noted at the following cycles (Figure S4), suggesting excellent self-recovery ability by energy dissipation. Then we applied the recovery ratio and the residual strain ratio to further evaluate recovery efficiency (Figure S5), and found that the recovery ratio could exceed as high as 92.5% with 12% residual strain, confirming good resilience and fatigue resistance of ionic gels.

The hysteresis behavior could be determined to reflect the energy dissipation and evaluate the toughness of ionic gels. Figure 3f reveals that the ionic gels could effectively dissipate energy with pronounced hysteresis loops. From the quantified results of energy dissipation efficiency in Figure S6, we note that the ionic gels could dissipate more energy with an increase in strain but display a decrease tendency in energy dissipation ratios. This phenomenon can be explained by the synergistic effects between covalent cross-links and multiple coordination interactions. With small strains applied, dynamic TA@CNC motifs mediated transient networks associated by reversible coordination bonds tend to preferentially break to dissipate energy efficiently, and thereby survive the covalent network. Although the strain is gradually increased and large deformation is applied, covalent bonds serving as permanent cross-links start to rupture irreversibly along with the synergy of the weaker but dynamic coordination bonds to maintain the integrity of primary structure, resulting in the lower energy dissipation efficiency with increased strains during the cyclic tests. Figure 3g presents outstanding compressive toughness and recovery property of the ionic gel, which is so tough enough to withstand a series of compressive deformations of 85% and even fully recover after removing the pressure. The fatigue resistance property of ionic gel is investigated by successive cyclic compression tests at 85% strain. According to Figure 3h, the stress−strain curves with the evident hysteresis almost coincide with first cycle, and the recovery ratio could exceed as high as 98.5% after second cycle and still more than 96% after seven cycles from the insets, implying extraordinary compression recovery and fatigue resistance of ionic gels. Self-Healing Properties. To demonstrate the self-healing behaviors of TA@CNC ionic gels, we initially carried out a 3114


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Figure 5. Adhesive properties of the TA@CNC ionic gels. (a) Ionic gel can adhere to fingers without the assistance of additional adhesive tapes and withstand the recoverable stretch of 200%. (b) Exhibition of stripping lag and no residual behavior in the process of peeling from the human skin. (c) Adhesion strength of ionic gels with different substrates tested by tensile adhesion tests. (d) Repeatable adhesion behavior of the adhesive ionic gels to different substrates tested by cyclic tensile adhesion tests.

links. Additionally, Figure 4c demonstrated that the increase of TA@CNC content was able to improve effectively the selfhealing efficiency of the ionic gels with 30 min healing, highlighting the crucial role of dynamic TA@CNC motifs for the reversible rearrangement in the self-heling process of ionic gel. Notably, the rigid but dynamic TA@CNC motifs associated by coordination bonds enable the prevention of crack propagation across the interface and mediate multiple coordination bonding transient network, which enhances network stability with allowed superior mechanical property and dissipates energy during deformation with imparted good recovery, respectively. The microscopic self-healing behavior of the ionic gel was proved by rheological experiments to study the breakup and reformation of the ionic gel network. The alternate step strain test (strain = 1% and 100%) of the [email protected] ionic gels at a fixed time interval of 200 s was carried out as shown in Figure 4d. When the hydrogel was subjected to a small amplitude oscillatory shear (strain = 1%), elastic modulus is greater than loss modulus and both moduli do not change with time, indicating the formation of self-standing gel with intact network under small oscillatory strain. Afterward, the hydrogel was subjected to a large amplitude oscillatory shear (strain = 100%), the elastic moduli (G′) and loss moduli (G″) values were inverted immediately and accompanied by a drastic decrease, which indicated that the gel was converted into the sol state ascribed to the disruption of gel network. By switching the strain from a large strain of 100% to a small strain of 1% at a fixed frequency (1.0 Hz), the gel-like character (G′ > G″) was recovered instantaneously without any significant decrease in

macroscopic self-healing test by direct visual inspection. As illustrated in Figure 4a, the rectangle-shaped specimens were cut into halves using blades and incisions were highlighted by the stain of methylene blue for better visual inspection. Then the fresh cut surfaces of two separate pieces were immediately recombined without applied stress and were placed in a sealed vessel to minimize the effect of water evaporation at 25 °C. After healing for the prescribed contact time, the incision of ionic gel can self-heal automatically and disappeared almost completely without any other additional stimuli or healing agents. Moreover, the self-healed ionic gel was strong enough to sustain stable self-supporting and stretching without failure at the interface, indicating that the ionic gel possessed efficient self-healing ability as well as outstanding mechanical performance. Moreover, we further investigated the tensile properties of self-healed gels with different healing time to evaluate the selfhealing abilities of ionic gels. Figure 4b indicates that the fracture stress of healed ionic gel displayed significant enhancement with increasing healing time. This time-dependent self-healing behavior involves a two-stage process distinguished by the increase amplitude of healing efficiency (HE) that is defined as the tensile strength ratio between the healed gel and original gel at break (Figure S7). In the earlier stage, the fracture stress of healed gel significantly increased with increase in the healing time, and HE rapidly increased to about 88% within 20 min. Subsequently, the increase of HE slowed down in the later stage ascribed to the self-healing process gradually reached equilibrium state with HE up to 92% after 30 min healing, implying excellent self-healing performance of ionic gels based on the reversible coordination cross3115


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Figure 6. Strain sensitive TA@CNC ionic gels assembled into wearable strain sensor to monitor various human motions in real time. (a) Complete circuit composed of a LED bulb and our ionic gel, showing the LED response to applied strains on the gel sample under the loading−unloading cycles. (b) Relative resistance change of ionic gel sensor as a function of the applied strain, and the inset image shows the effects of various applied strain on the gauge factor. (c) Stability of the ionic gel strain sensor by repeatedly applying a strain of 55% for more than 1000 cycles. (d) Relative resistance change of the ionic gel strain sensor adhered onto the forefinger to monitor its bending with different angles. The insets show the ionic gel strain sensor adhered onto the finger bending at different angles. (e) Standard shooting action with the demand for perpendicularity among wrist, elbow, and shoulder. The corresponding relative resistance changes were demonstrated for (f) opisthenar, (g) elbow, and (h) shoulder joint bending in the shooting practice, respectively. The insets show the strain sensor adhered onto the wrist, elbow, and shoulder.

nation between excellent self-adhesiveness and rapid selfrecovery of the TA@CNC ionic gels (Video S1). Besides, the ionic gel could adhere to human skin and was able to be easily removed without any irritation, damage, or pain, thereby the macroscopic peeling test on human skin was displayed to further elucidate the adhesive performance of ionic gels. As illustrated in Figure 5b, it can be observed that a crack initiates at the gel−skin interface and the distinct stripping lag appeared in the subsequent propagation, thereby the adhesive ionic gels could not be immediately peeled off from the substrate during the process of peeling test and no residual adhesive behavior of hydrogels after detachment from the human skin. In contrast, there was almost no stripping lag process in the pristine PAA gels so that the crack can easily propagate along the interface without kinking or significantly deformation (Figure S9), corroborating that the adhesion properties of PAA gels was significantly enhanced due to the incorporation of TA-coated CNCs. We quantified the adhesion strength of the ionic gels to the typical surfaces by a tensile adhesion test and the results were depicted in Figure 5c. The ionic gels on the surface of aluminum exhibit the highest adhesion strength among the substrates, which should be attributed to the synergy of metal complexation and hydrogen

each repeatable cycle of the recovery. Therefore, the rapid sol− gel transition with the complete recovery of the gel network after disruption corroborated the excellent self-healing capability of the ionic gel, and the self-healing mechanism was the reconstruction of reversible ionic coordination complexation in the gel network system. It is worth noting that the self-healing behavior of this ionic gel occurs autonomously without any external intervention, which is ideal to extend its practical applications in comparison to the healable gels that depend on external treatment. Self-Adhesiveness Properties. Given the catechol groups of oxidized polyphenols mimicking the mussel adhesion mechanism, the TA@CNC ionic gels exhibited the unique self-adhesiveness to a wide range of surfaces. As demonstrated in Figure S8a, the ionic gels firmly adhered to the merged glass slides without additional adhesives and enable to support a heavy load of 350 g. The ionic gels also displayed strong adhesion to other surfaces such as polytetrafluoroethylene (PTFE), rubbers, wood, and carnelian (Figure S8b−e). Interestingly, Figure 5a indicates that the adhesiveness of the ionic gel is strong enough to adhere to fingers and withstand the recoverable stretch of 200% without the assistance of additional adhesive tapes, corroborating the excellent combi3116


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Figure 7. (a) Real-time resistance change signal of the ionic gel strain sensor adhered onto the radial artery of the wrist to monitor the pulse under relaxation and exercise conditions. (b) Detection of tiny movement caused by breathing. The inset shows the gel sensor adhered on the chest. (c) Real-time bodily motion monitoring system using a gel strain sensor and smart phone.

sensitivity of resistance for the TA@CNC ionic gel can be further evaluated by the gauge factor (S), which is defined as the ratio of relative resistance change (ΔR/R0 = (R − R0)/R0) to the applied strain (ε), S = (ΔR/R0)/ε = [(R − R0)/R0]/ε, where R0 and R are the original resistance at 0% strain and the resistance under stretching, respectively. Figure 6b reveals that the relative resistance change increased linearly at first with a gauge factor of 0.23 (0% < Δε < 40%) increased to 0.63 (40% < Δε < 65%) and then exhibited an exponential dependency upon applied strain with a gauge factor up to 4.9 (65% < Δε < 75%). The inset image depicted that the gauge factor of the ionic gel was 5.5 at the strain of 100% and increased to 7.8 as the strain increased to 2000%, which is significantly improved compared to the most previously reported piezoresistive electronic strain sensors52 and capacitive soft strain sensors,53 indicating that our ionic gels possess high strain sensitivity with ultrastretchability. As shown in Figure 6c, the relative resistance change of the ionic gel strain sensors remained stable after more than 1000 cycles of 55% strain except a slight fluctuation. The insets revealed their excellent reproducibility of the relative resistance change of ionic gel, corroborating a remarkable electrical stability. In addition, we investigate the electrical self-healing property of the ionic gel connected in series with a blue LED bulb in the circuit. As shown in Figure S11 and Video S3, the LED bulb was successfully lighted by a 1.5 V power supplier in the electric circuit connected the ionic gel, then LED bulb was extinguished in the open circuit when the ionic gel was severed with a razor blade. After putting the two separate parts together immediately and self-healing via the reformation of dynamic coordination cross-linking among the contacted interfaces without any external stimuli, the circuit was restored and the LED bulb was lighted again, implying the rapidly and effectively electrical self-healing property of ionic gel. The relative resistance change of the healed gel recovered to 97.1% of its original state within 3 s by the real-time measurements (Figure

bonding between ionic gel and aluminum. Moreover, the hydrophobic interaction and hydrogen bonding are probably the two primary interactions in the adhesion of ionic gels for glasses and PTFE, whereas the adhesion of ionic gels on hogskin and rubber should be related to hydrogen bonding. Additionally, the effective adhesion based on reversible physical interactions displayed repeatability and durability. The cyclic peel-off tests were conducted that adhering the adhesive ionic gel onto the surface of substrate and then peeling it off by a tensile load, subsequently readhering the same ionic gel before the following cycle, and the results of adhesion strength were recorded in Figure 5d. It was noted that the ionic gels exhibited repeatable and durable adhesion with a slight decrease in adhesion strength after three cycles yet surviving more than 70% of the original adhesion strength, which were quite different from the previous adhesive hydrogels that are made by using strong oxidative agents and could not be reactivated after the complete curing.51 Importantly, the reversible physical interactions impart the biocompatible and repeatable adhesion to the TA@CNC ionic gels without the sacrifice of other desirable properties (mechanical performance and self-healing property), which were attractive in practical application as wearable strain sensors. Ionic Gel-Based Strain Sensor. We designed a complete circuit connecting a light-emitting diode (LED) bulb with the ionic gel sample to examine its strain sensitivity under the loading−unloading cycles. As illustrated in Figure 6a, the LED darkened with increasing strain and then became brighter immediately as the stress was released, signifying the fast resistance response to the applied strain on the ionic gel (Figure S10). Moreover, it was noted that the ionic gel displayed high strain sensitivity in a high amplitude deformation of 0−200% according to its fast response ability where the variation of the LED brightness could keep pace with the applied strains even at the increasing frequency of the tensile loading−unloading cycle (Video S2). The strain 3117


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then converted into digital signals by analog-to-digital (A/D) convertor. Subsequently, the digital signals were transmitted wirelessly to the smart phone through the Bluetooth links, enabling us to analyze the data on the graphical interfaces of smart phone. These results indicate that our mechanical robust, self-healable, and self-adhesive ionic gel is an ideal candidate to assemble the stretchable and wearable strain sensors with the remarkable sensitivity for human activity monitoring and personal healthcare diagnosis.

S12), indicating that the ionic gels possess an extraordinary electrical restoration performance. Owing to their excellent self-adhesiveness, high strain sensitivity, remarkable electrical stability, and fast self-healing capability of the ionic gels possess tremendous potential applications in wearable devices. Thus, we assembled a wearable strain sensor from the ionic gel directly attached on the human skin to detect the joints bending and stretching of diverse human motions. Figure 6d illustrates the detection of the bending of the forefinger with a stepwise increase (decrease) in angles. The strain sensor was stretched when the finger bent to different angles at 0°, 30°, 60°, and 90°, resulting in the increased resistance. The resistance value of the strain sensor was constant when the forefinger maintained at a certain angle due to the excellent electrical stability, thereby the real-time resistance change could be precisely monitored in response to the finger bending with different angles. Importantly, it is noted that the relative resistance change of the strain sensor increased to different levels with increasing the bending degree of the forefinger and immediately returned to their original levels when the forefinger was stepwise straightened, indicating the highly sensitive sensing performance of the ionic gel strain sensor. We demonstrated the applications of the ionic gel strain sensors for detection of both large motions and subtle motions in the basketball training, which expect that this soft sensor can provide the accurate analysis to correct the shooting posture and diagnose the personal healthcare. Figure 6e displays the standard shooting action with the demand for perpendicularity among opisthenar, elbow, and shoulder. The amplitude of corresponding signals was discernible for different motions of joints bending since different joints bending led to various elongations of the sensor. Therefore, the ionic gel strain sensor is able to both detect and distinguish human joints bending without the assistance of additional adhesives. As shown in Figure 6f, bending of the opisthenar could be precisely tracked by monitoring the relative change of the resistance in the repetitive shooting exercise. Similarly, the motions of elbow and shoulder joints bending could also be recorded and distinguished by real-time detection of the ionic gel strain sensors based on the distinctly differentiable patterns of response curves in Figure 6g and h. Moreover, the ionic gel strain sensors are sensitive enough to enable to accurately detect and promptly recognize the subtle motions including pulse and breath, which is beneficial for realtime monitoring of personal healthcare during the sports training. Pulse is a crucial physiological signal for the systolic and diastolic blood pressure as well as the heart rate, which can be detected by the ionic gel strain sensors attached to the radial artery. Figure 7a presents the real-time resistance change signal of the sensor under relaxation and exercise conditions. It clearly displays the enhanced amplitude and irregular pulse shapes after exercise with frequency increasing to 118 beats min−1 in contrast to regular pulse shapes in relaxation with frequency of 75 beats min−1, implying the high strain sensitivity and ultrafast response of the ionic gel strain sensor. Figure 7b shows the physiological breathing signal of volunteer in relaxation, which explicitly manifests the discriminable respiration rate and depth determined by the amplitude of the peaks. As a proof of concept, a real-time human motion monitoring system was designed as depicted in Figure 7c. Current ionic gel strain sensor was used for human motion detection, where the resistance changes of the ionic gel sensors were captured and

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CONCLUSION In summary, a combination of covalent cross-link and multiple coordination cross-links was employed to prepare the TAcoated CNC (TA@CNC) reinforced ionic gels with excellent mechanical integrity, self-healing, and self-adhesive properties. Inspired by the adhesion mechanism of mussels, tannic acid containing catechol groups is anchored onto the surface of CNCs by surface deposition, leading to the TA-coated CNC (TA@CNC). The TA@CNC acting as a dynamic connected bridge imparts the ionic gels with hierarchically porous network mediated by multiple reversible coordination bonds, contributing to the remarkable mechanical properties involving ultrastretchability (fracture strain of 2952%), high compression performance (95% strain without fracture), and toughness (5.60 MJ/m3). Dynamic features of multiple coordination bonds lead to the excellent recoverability as well as reliable mechanical and electrical self-healing properties of ionic gels through reversibly break and reform. Notably, the rigid but dynamic TA@CNC motifs enable to prevent crack propagation across the interface and mediate transient network associated by multiple coordination bonds, highlighting that the TA@ CNC plays a vital role to weave mechanically robust and dynamic networks. Intriguingly, the chemical nature of TA endows these ionic gels high and repeatable self-adhesive capability to diverse substrate without inflammatory response and residual. Additionally, the ionic gels exhibit high strain sensitivity as a wearable strain sensor, which could precisely monitor and distinguish both large motions and subtle motions, enabling us to analyze the data on the user interface of smart phone by programmable wireless transmission. This study provides a positive exploration to fabricate cellulose-based conductive hydrogels with stretchable, self-adhesive, selfhealing, and strain-sensitive properties for their potential applications in wearable electronic sensors, healthcare monitoring, and even the soft intelligent robots.

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MATERIALS AND METHODS

Materials. The never-dried bleached softwood Kraft pulp obtained from DongHua Pulp Factory was applied to prepare cellulose nanocrystal (CNC) suspensions. Tannic acid (TA), acrylic acid (AA), ammonium persulfate (APS), N,N′-methylenebis(acrylamide) (MBA), and aluminum chloride hexahydrate (AlCl3·6H2O) were purchased from Beijing Lanyi Co. Ltd., China. Tris(hydroxymethyl)aminomethane (Tris) was purchased from Sigma-Aldrich. All other chemicals purchased were analytical grade unless otherwise noted and were used as received. Extraction of Cellulose Nanocrystals. Cellulose nanocrystals (CNCs) were prepared by the well-established sulfuric acid hydrolysis method as reported in our previous work.54 In brief, 4 g of pulp was added into 200 mL of 55 wt % sulfuric acid at 50 °C for 90 min under mechanical stirring (350 rpm), where amorphous regions were preferentially hydrolyzed. The suspension was diluted with deionized water and concentrated by centrifugation, followed by dialysis against water until neutrality. Then high-pressure homogenization was 3118


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Adhesion Tests. Tensile adhesion testing was performed to measure the adhesive strength of TA@CNC ionic gels on the various substrate surfaces (Figure S13). The hydrogels were adhered to the surface of the specimens with a bonded area of 20 mm × 20 mm. The samples were pulled at a crosshead speed of 10 mm/min until the occurrence of separation, using a universal Zwell/Roell mechanical tester under the ambient conditions. It is noted that the adhesion test was immediately conducted once the hydrogel was attached on the substrate surface without curing time, and the adhesion strength was calculated by the maximum load divided by the initial bonded area. The substrates that we choose for investigation were aluminum (Al), polytetrafluoroethylene (PTFE), glasses, hogskin, and rubber. Electrical Tests. The real-time electrical signals of the strain sensors based on the resistance changes of the hydrogels in a different state were recorded by CGS-8 Intelligent Sensing Analysis System. The relative change of the resistance was calculated by Ohm’s law (R = U/I) on the basis of the application of a constant voltage to the strain sensors induces changes in the electrical current under different strains.

performed to increase dispersity and stability of the suspension, and finally the homogenized CNC suspension was sonicated for 10 min (300 W) and cryopreserved (4 °C) before usage. Preparation of Tannic Acid Coated Cellulose Nanocrystals (TA@CNC). The TA@CNC suspension was prepared via a one-pot water-based process. In detail, Tris buffer solution (1 M) was dropwise added into the 150 g CNC suspension (∼1.02 wt %, containing about 1.53 g CNCs) until pH was adjusted to 8.0. Subsequently, 0.51 g of tannic acid was introduced and the resultant suspension was magnetically stirred for 6 h at room temperature, obtaining the final TA@CNC suspension with concentration of 1.2 wt % in this work. Hydrogels Preparation. A two-step strategy was applied to synthesize TA@CNC ionic gels. Initially, 6 g of AA monomer, 50 mg of APS initiator, and 20 mg of MBA chemical cross-linker were dissolved in 24 mL of deionized water. Then TA@CNC aqueous suspension was added and stirred vigorously for 10 min, followed by sonication (300 W) in an ice−water bath for 10 min to form a uniform mixture. After degassing by bubbling nitrogen for 5 min, the mixture was poured into PTFE molds under nitrogen atmosphere at 50 °C for 2 h to achieve the covalently cross-linked composite gels. Afterward, the obtained composite gels were immersed in 0.1 mol/L AlCl3 aqueous solutions at room temperature for 2 h to form ionic crosslinked gels. At last, the Al3+-loaded ionic gels were soaked in deionized water for 24 h to remove superfluous cations. The sample codes for ionic gels (TA@CNC-x) were defined by the weight ratio of the TA@ CNC against AA (constant at 6 g). For example, 1.0% TA@CNC ionic gel referred to a gel loaded with 60 mg of TA@CNC is designated as [email protected], and the detail composition of TA@CNC ionic gel are shown in Table S1. Spectroscopic Analyses. UV−vis absorption spectra were recorded using a Cary 5000 spectrophotometer. Fourier transform infrared (FTIR) spectroscopy of the samples was recorded using PerkinElmer FTIR Spectra 2000 at room temperature. The test specimens were prepared using the KBr disc method and analyzed over the range of 400−4000 cm−1. Morphological Observation. For atomic force microscope (AFM, Bruker Multimode 8), the topographic images were captured using tapping mode in fluid, which was driven by photothermal excitation. For cross-section observation by scanning electron microscopy (SEM, Hitachi 3500S), the fractured hydrogel samples were freeze-dried for 24 h, followed by putter-coating with platinum for 30 s observation at an accelerating voltage of 6 kV. Mechanical Tests. All mechanical tests were performed at room temperature using a universal mechanical tester (Zwell/Roell) equipped with a 200 N load cell. A layer of silicone oil was coated on the hydrogel surface during the testing and storage time to minimize water evaporation. The uniaxial tensile test was performed on the rectangular-shape specimens (10 mm in width, 6 mm in depth, and 35 mm in length). The initial distance between two clamps was 15 mm, and the constant stretching rate was 60−180 mm/min. The unconfined compression tests of the cylindrical sample (30 mm height and 20 mm diameter) were conducted at a crosshead speed of 10 mm/ min. Self-recovery properties were investigated by typical tensile and compressive loading−unloading tests, the details of mechanical tests of ionic gels were shown in the Supporting Information. Self-Healing Experiments. The rectangular-shape specimens (10 mm in width, 6 mm in depth, and 35 mm in length) were cut into halves using blades, and then the two separate halves were brought into contact immediately without applied stress and were stored in a sealed vessel to minimize the water evaporation. During the healing process, no other stress or outside stimulate was applied on the interface. After self-healing, the uniaxial tensile test was measured again to calculate the healing efficiency (HE) (defined as the tensile strength ratio between the healed gel and original gel). Besides, the microscopic self-healing behavior of hydrogels was proved with the alternate step strain test (strain = 1% and 100%) at a constant frequency of 1.0 Hz to investigate the rupture and rearrangement in the cross-linked network of hydrogel. The continuous step strains were transformed with 200 s for every strain interval as literature reported.55

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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.chemmater.8b01172. Compositions of ionic gels; mechanical properties summary of ionic gels; details of mechanical tests of ionic gels; dimension distribution histograms of CNCs and TA@CNC; Young’s modulus and toughness of ionic gels as a function of TA@CNC content; critical strain and fracture energy for notched ionic gels as a function of TA@CNC content; stress−time tensile curves during cyclic loading−unloading process at 1200% strain; recovery ratio and residual strain ratio of ionic gels; dissipated energy and corresponding energy dissipation ratio of ionic gels at different strains; time-dependent self-healing behaviors of [email protected] ionic gels; resistance change in electrical self-healing process of conductive gel; images for excellent adhesiveness of ionic gel with various substrates; images of pristine PAA gels without stripping lag process (PDF) Video of adhesiveness of ionic gel (AVI) Video of ionic gel displaying high strain sensitivity (AVI) Video of electrical self-healing property of ionic gel (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-10-62337223. ORCID

Changyou Shao: 0000-0003-2464-7792 Feng Xu: 0000-0003-2184-1872 Jun Yang: 0000-0002-1633-4465 Pengbo Wan: 0000-0001-8178-4262 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21674013, 21404011). 3119


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Mussel-Inspired Cellulose Nanocomposite Tough Hydrogels with

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