Synthesis of strong and highly stretchable, electrically conductive hydrogel with multiple stimuli responsive shape memory behavior

Journal Pre-proof Synthesis of strong and highly stretchable, electrically conductive hydrogel with multiple stimuli responsive shape memory behavior Xiao Zhang, Junqi Cai, Wenqiang Liu, Weifeng Liu, Xueqing Qiu PII:

S0032-3861(19)31151-6

DOI:

https://doi.org/10.1016/j.polymer.2019.122147

Reference:

JPOL 122147

To appear in:

Polymer

Received Date: 13 November 2019 Revised Date:

28 December 2019

Accepted Date: 31 December 2019

Please cite this article as: Zhang X, Cai J, Liu W, Liu W, Qiu X, Synthesis of strong and highly stretchable, electrically conductive hydrogel with multiple stimuli responsive shape memory behavior, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2019.122147. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Author contributions Xiao Zhang: Conceptualization, Methodology, Software, Investigation, Writing - Original Draft. Junqi Cai: Validation, Formal analysis, Visualization. Wenqiang Liu: Writing - Review & Editing. Weifeng Liu: Validation, Formal analysis, Visualization, Writing Review & Editing, Supervision. Xueqing Qiu: Resources, Writing - Review & Editing, Supervision.

Strong and highly stretchable, electrically conductive hydrogel with multiple stimuli responsive shape memory behavior was synthesized.

Synthesis of strong and highly stretchable, electrically conductive hydrogel with multiple stimuli responsive shape memory behavior Xiao Zhang†, Junqi Cai†, Wenqiang Liu†, Weifeng Liu*†, Xueqing Qiu*†‡ † School of Chemistry and Chemical Engineering, Guangdong Engineering Research Center for Green Fine Chemicals, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China *Corresponding authors E-mail: [email protected] (W. Liu); [email protected] (X. Qiu). Tel.: +86 020-87114722.

Abstract A highly tough and conductive hydrogel with good shape memory behavior was facilely prepared via constructing the catechol-Fe3+ interactions in the poly(vinyl alcohol)

(PVA)

hydrogel

matrix.

The

hydrophobic

5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) was introduced to provide the catechol ligands for Fe3+. The fabricated TTSBI-2@Fe3+-12 nanocomposite hydrogel performed great toughness (9.23 MJ/m3), large tensile strength (3.25 MPa) and high extensibility (752%). The distinguished mechanical performance of the composite hydrogel was contributed by the synergy of nanophase separation structure formed by TTSBI in PVA matrix, strong hydrogen bonding interaction between PVA and TTSBI, and metal coordination interaction of catechol-Fe3+. The introduced Fe3+ also imparted good conductivity to the hydrogel. Moreover, the mechanical and conductive properties of the composite hydrogel could be flexibly regulated by the pH value. The conductive hydrogel showed excellent sensitivity to stretching, bending, twist, and compression. In addition, the hydrogel exhibited multiple-stimuli responsive shape memory behaviors. This work offers a

hierarchical self-assembly strategy to fabricate functional hydrogel with tailored mechanical, conductive properties and shape memory behavior for a series of promising applications such as flexible wearable electronics and intelligent actuators.

1. Introduction Hydrogel is a type of three-dimensional network structure that swells but does not dissolve in water [1-5]. Owning to its advantages of softness, flexibility, water-rich and good biocompatibility [6], hydrogel displayed many potential applications in the fields of biomedical engineering [7, 8], tissue engineering [9-11], flexible electronics [12], etc. Conductive hydrogels have aroused wide attention in recent years due to their promising applications for wearable sensors [13-16], super capacitors [17-21], medical diagnosis [22-24], etc. For example, recently, a highly stretchable supercapacitor assembled from polypyrrole-incorporated gold nanoparticle/carbon nanotube (CNT)/poly(acrylamide) (GCP@PPy) hydrogel was developed by Yu, et al [25], which performed excellent supercapacitor performance under complex mechanical deformations. An enzyme-like nanofibrous guanosine-molecular hydrogel from the self-assembly of guanosine was reported for printed flexible biosensors by Pei, et al [26], which provided a promising method for developing artificial enzymes and next-generation flexible bioelectronics. Recently, hydrogels with shape memory function has been expected to display the great potentials in soft actuators, intelligent robots, etc [27-31]. Shape memory hydrogel could transform from a temporary shape to its performant shape in response to external stimulus, such as thermal, electric, magnetic, light or chemical, etc [32-35]. In recent years, the non-covalent interactions including dynamic hydrogen bonds [36], dynamic ionic bonds [37], host-guest interactions [38], and metal-ligand binding interactions [39], hydrophobic association [40, 41], have been implemented in the construction of shape memory hydrogels. For instance, Wang, et al [42] reported a stretchable and transformable hydrogel-based energy harvester as well as a

self-powered mechanosensation sensor with unique programmable features, which could be triggered by thermal stimulation, demonstrating its potential application in wearable medical devices. Wu, et al [43] developed a photon upconversion lithography (PUCL) method using near-infrared light to adjust the size of patterns, envisaging regulate inflammation and vascularization of biomaterials. Although shape memory hydrogels have been commonly reported, the shape memory performance was usually single responsive and the reported shape memory hydrogels are generally weak in mechanical properties and non-conductive, which limits their vigorous development in the field of conductive devices [36-40]. Producing hydrogel with both good mechanical properties and electrical conductivities is a continuing challenge. Numerous efforts have been made to enhance the mechanical properties of hydrogel by constructing the microphase separation structure [44], multiple energy sacrificial networks [45-47], electrostatic interactions [48], hydrogen-bonding interactions [49], hydrophobic self-assembly [50], etc. Zhang [51] prepared a leeches-inspired hydrogel-elastomer with the elastic matrix formed by hydrophobic self-assembly. The composite material behaved as a hydrogel after adsorbing water and acted as an elastomer after dehydration. The compression strength of the hydrogel reached up to 1.6 MPa at 90% strain. High strength hydrogel based on hydrophobic interactions was also reported by Guo, et al [52]. The prepared hydrogel behaved good tensile strength (1.6 MPa) and high extensibility (900%), and even multiple shape-memory behaviors. 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) with centrally symmetric catechol structure displays many special properties, such as pH responsibility, strong metal ion complexing ability, and hydrophobicity [53]. Thus, introducing TTSBI into hydrogel through hydrophobic self-assembly could provide more potential possibilities for functional hydrogel. In this work, we developed a tough nanocomposite hydrogel via hierarchical self-assembly approach using TTSBI as a reinforcing agent. Uniform TTSBI nanoparticles (TTSBI-NPs) with hydrophilic catechol groups outward and hydrophobic groups inward in the PVA hydrogel matrix were achieved by hydrophobic self-assembly during the solvent exchange process.

After soaking the PVA/TTSBI nanocomposite hydrogel in FeCl3 solution, the catechol groups could further chelate with Fe3+ to generate the catechol-Fe3+ interactions, forming a stronger crosslinking network structure in the hydrogel and thus a strong and tough nanocomposite PVA hydrogel was obtained. The ferric ion also endued the composite hydrogel with good electrical sensory properties. In addition, the composite hydrogel performed multiple shape memory behavior under the stimuli of solvent, temperature, and ferric ion. The fabricated PVA nanocomposite hydrogel was expected to show good potentials in soft actuators, intelligent robots, flexible sensors and so on.

2. Experimental Section 2.1 Materials Poly(vinyl alcohol) (PVA) (hydrolyzed > 99.9 %; Mw = 130 kDa) was provided by Sigma.

5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) was

supplied by San Chemical Technology (Shanghai) Co., Ltd. Dimethyl sulfoxide (DMSO), ferric chloride (FeCl3), sodium hydroxide (NaOH) were purchased from Guanghua Chemical Reagent Co., Ltd in analytical grade. The hydrochloric acid solution was supplied by Guangdong Guangshi Agent Technology Co., Ltd with the concentration of 36~38 wt%. Aqueous solutions of pH = 5, 8, or 12 were prepared by adding a certain amount of HCl or NaOH into water. 2.2 Sample synthesis The PVA/TTSBI mixture with 10 wt% solid content was prepared firstly by adding a certain amount of PVA and TTSBI into DMSO aqueous solution, which was mixed by DMSO and H2O in a volume ratio of 4:1. The TTSBI in the total solid content of PVA mixture was added in 1 wt%, 2 wt%, 5 wt%, 10 wt%, 20 wt%, and marked as TTSBI-1, TTSBI-2, TTSBI-5, TTSBI-10, TTSBI-20, respectively. As shown in Fig. 1a, the PVA/TTSBI mixture was stirred for 2 h at 140 ºC, and was then poured into dumbbell

or cylindrical Teflon molds shortly. Finally, the molds with solutions were placed at a -18 ºC freezer for 12 h to form gels. The resulting gels were put in flowing water to extract out DMSO for 48 h. Ultraviolet spectroscopy (UV) was adopted to detect the water after solvent exchange to ensure there was no residue of DMSO in hydrogel. For comparison, PVA hydrogel without TTSBI was prepared by the same method. The TTSBI-2@Fe3+ ionic conductive hydrogels including TTSBI-2@Fe3+-5, TTSBI-2@Fe3+-8 and TTSBI-2@Fe3+-12 were prepared by first soaking TTSBI-2 in FeCl3 solution (0.05 M) for 4 h and then immersion in the solution at various pH (5, 8, 12) for 1 h, respectively (Fig. 1a). 2.3 Characterization 2.3.1 Attenuated total reflection Fourier infrared spectroscopy (ATR-FTIR) The attenuated total reflection Fourier infrared spectra (ATR-FTIR) of hydrogels were determined on a FTIR spectrometer (Thermo Nicolet, US) with the scanning range from 3750 to 550 cm-1. The hydrogels were lyophilized and flattened with a tablet machine before test. 2.3.2 Ultraviolet-visible spectroscopy (UV-vis) The transmittance of TTSBI-2 was determined by a UV-vis spectrophotometer (Shimadzu, Japan). The hydrogel sample was cut into a rectangular shape (6 cm × 4 cm) with thickness of 2 mm before test. 2.3.3 Scanning electron microscopy (SEM) SEM images of the TTSBI-NPs were characterized by a field-emission scanning electron microscopy (FE-SEM) (HITACHI SU-8220, Japan) operated at 10 kV. The TTSBI-NPs were prepared by dropping TTSBI/DMSO solution into the water slowly using a peristaltic pump with a speed of 0.4 rmp, then the mixture was dripped onto a thin silicon substrate and evaporated at ambient temperature for 8 h to obtain the sample for test. The cross-section morphologies of hydrogels were also achieved by field-emission scanning electron microscopy (FE-SEM) (HITACHI SU-8220, Japan). The hydrogel to be tested was freeze-dried and quenched with liquid nitrogen. All

samples were coated with gold before SEM test. 2.3.4 Mechanical properties tests Samples used for tensile test were made into dumbbells. The tensile tests of the hydrogel were performed on a universal testing machine (Dongguan Kejian Measurement Instrument Co., Ltd.) at 25 oC. Each sample was measured at a loading rate of 100 mm/min with an initial gauge length of 10 mm. Samples used for compression test were made into cylinders with a diameter of 25 mm and a thickness of approximately 10 mm. The uniaxial compression measurement of hydrogel was performed with an MST universal testing machine (MTS Systems Co. Ltd, China). The silicon oil was coated on the hydrogel samples to avoid water evaporation during the test course. 2.3.5 Conductive performance tests The rectangular hydrogel with thickness of 2 mm was prepared for the measurement of electrical conductivity (σ, S/m) according to the equation: σ = L/RA, where L (m) is the length between the two clips, R (Ω) is the electrical resistance and A (m2) is the cross-sectional area of the hydrogel. Electrical resistance of the hydrogel was measured by a digital display multimeter (VC890C+, Shenzhen Victor Co., Ltd., China) at the working voltage of 9 V. Each measurement was repeated 5 times and averaged. The relative resistance change of hydrogel was calculated as: ∆R = R-R0, where R and R0 represent the real-time resistance and the initial resistance, respectively.

3. Result and Discussion 3.1 Preparation of the composite hydrogels

Fig. 1. a) The synthesis procedure of the PVA composite hydrogel via freezing-thawing and solvent exchange method. b) Schematic illustration of the composite hydrogel.

As shown in Fig. 2a, when the TTSBI/DMSO solution was dropped into the water slowly, the hydrophobic TTSBI was able to self-assemble easily in the poor solvent of water to form nanospheres (~ 50 nm). After introducing TTSBI into the PVA matrix, uniform

PVA/TTSBI nanocomposite hydrogel

was

obtained

via a facile

freezing-thawing and solvent exchange strategy (Fig. 1a). The PVA nanocomposite hydrogel with a small loading content of TTSBI (TTSBI-1 and TTSBI-2) was transparent (Fig. S1 & S2). However, the TTSBI precipitated to form opaque hydrogel when the loading of TTSBI reached 5 wt% (TTSBI-5), and obvious TTSBI crystals were observed with the addition of TTSBI above 10 wt% (TTSBI-10). The macroscopic phase separation occurred at excessive loading of hydrophobic TTSBI. Further immersing PVA/TTSBI composite hydrogel into FeCl3 solution gained the samples of TTSBI@Fe3+ in light yellow. The color of the hydrogel became darker with the increase of pH value (Fig. 1a). This was because the iron ions could form more

coordination

numbers

at

higher

pH.

The

stoichiometry

of

catechol-Fe3+complexes (mono-, bis-, or tris-) could be controlled by the pH value via the deprotonation of the catechol hydroxyls [54]. Fig. 1b showed the schematic illustration of the composite hydrogel. During the solvent exchange process, the poor solvent of water drove the hydrophobic groups of TTSBI to agglomerate, forming uniform nanoparticles in the PVA hydrogel with hydrophilic catechol groups concentrating on the surface of the nanoparticles. The abundant catechol structure could generate strong hydrogen bonds with the PVA chains, which was verified by the ATR-FTIR spectra (Fig. S4). The blueshifts in ATR-FTIR spectra of hydroxyl group from 3300 cm-1 for pure PVA to 3316 cm-1 for TTSBI-10 fully demonstrated the hydrogen bonding interactions between the hydroxyl groups in PVA and catechol groups in TTSBI. Furthermore, the catechol groups stretching outside in TTSBI could chelate with iron ions, leading to the formation of catechol-Fe3+ interactions, which was beneficial to construct a stronger physical crosslinking network. As illustrated in Fig. 1b, the coordination between Fe3+ and catechol ligands was correlated to the pH value. The coordination states of catechol-Fe3+ complex exhibited mono- at the weakly acidic condition (pH = 5), bis-complex (pH = 8) and tris-complex (pH = 12) would form sequentially as the pH value increased [55, 56], leading to the formation of more intensive crosslinking network under alkaline conditions [54]. As shown in Figure S3, TTSBI-2@Fe3+-12 was placed in water with sulfosalicylic acid indicator. The water did not change color

after 12 h until a slight discoloration occurred after 24 h, suggesting the chelated ferric ion was restrained in the intensive hydrogel network with only a little leakage. The dispersion phase morphology of TTSBI in the composite hydrogel of TTSBI-2 was verified by the TEM image shown in Fig. 2c. The hydrophobic TTSBI dissolved in the benign solvent of DMSO, and self-assembled gradually in the PVA matrix to form nanospheres (~ 50 nm) during the solvent exchange process. No nanoparticles were found in the pure PVA hydrogel (Fig. 2b). Compared with the pure PVA hydrogel (Fig. 2d), the network structure of TTSBI-2 was much denser and more small pores of less than 1 µm was found in TTSBI-2 (Fig. 2e). Microfibrils were observed in TTSBI-2 (Fig. S4), which was mainly caused by the strong intermolecular interactions between PVA chains and catechol groups of TTSBI [57]. Fig. 2f showed the dense pore structure of TTSBI-2@Fe3+-12, which was much thicker than TTSBI-2.

Fig. 2. SEM images of the composite hydrogels. a) SEM image of TTSBI NPs separated from poor solvent of water. TEM images of the cross-sections of b) pure PVA hydrogel and c) TTSBI-2; SEM images of the cross-sections of d) pure PVA hydrogel, e) TTSBI-2 and f) TTSBI-2@Fe3+-12.

3.2 Mechanical properties of the composite hydrogel

Fig. 3. Mechanical properties of the composite hydrogels. a) Tensile stress-strain curves and b) toughness comparison of pure PVA and PVA nanocomposite hydrogel. Photograph demonstration of TTSBI-2@Fe3+-12 before and after c) extension and d) compression. e) Continuous loading-unloading compressive curves of TTSBI-2@Fe3+-12 for 5 cycles at 60% strain without residence time. f) Successive compressive loading-unloading curves of TTSBI-2@Fe3+-12 under varied strains (10% ~ 80%).

The tensile and compressive measurements were performed to evaluate the mechanical properties of the PVA nanocomposite hydrogel (Fig. 3 & Fig. S6), and the characteristic results were summarized in Tables S1. The tensile strength, modulus and toughness of PVA/TTSBI nanocomposite hydrogel were significantly improved compared with the neat PVA hydrogel. As shown in Fig. S6a, the tensile strength increased to 1.23 MPa from 0.90 MPa (neat PVA), and the elongation at break increased from 634% (neat PVA) to 774% with only 1 wt% addition of TTSBI (TTSBI-1). The tensile strength gradually increased but the extensibility decreased with the TTSBI loading content. The maximum tensile strength of 1.45 MPa was achieved in the sample of TTSBI-along with the Young’s modulus of 1.6 MPa, which was 61% and 60% increase compared with pure PVA hydrogel (Table S1), respectively. Further increasing the TTSBI content to 10 wt% (TTSBI-10) led a slight decline in the tensile strength and ductility, attributing to the macroscopic phase separation of

TTSBI particles in the PVA matrix (Fig. S1). The PVA/TTSBI nanocomposites behaved much stronger compressive strength and toughness than the PVA hydrogel (Fig. S6b). The improved mechanical performance was attributed to the formation of microphase separation structure (Fig. 2c) and much denser physical crosslinking network (Fig. 2e). After soaking TTSBI-2 into FeCl3 solution, the tensile strength of TTSBI-2@Fe3+ was significantly improved, while the extensibility was almost unchanged. As shown in Fig. 3a, the mechanical performance of TTSBI-2@Fe3+-5, TTSBI-2@Fe3+-8, TTSBI-2@Fe3+-12

increased

successively with

the pH value.

When

the

TTSBI-2@Fe3+ hydrogel was placed in alkaline condition (pH = 8 or 12), the tensile strength increased obviously. As shown in Fig. 3b, the tensile toughness of TTSBI-2@Fe3+-12 reached 9.23 MJ/m3, which was 3.7 times of pure PVA hydrogel (2.51 MJ/m3). The tris-coordinated catechol-Fe3+ sacrificial bonds constructed much denser physical cross-linking network at pH = 12, endowing the TTSBI-2@Fe3+-12 hydrogel high strength and toughness. Photograph demonstration of TTSBI-2@Fe3+-12 before and after extension and compression presented the excellent extensibility and resilience (Fig. 3c & 3d). The energy dissipation of TTSBI-2@Fe3+-12 was investigated by the hysteresis tensile measurement at a fixed strain of 300% (Fig. S6d). An evidently larger hysteresis loss than pure PVA hydrogel (Fig. S6c) was observed in the first tensile cycle of TTSBI-2@Fe3+-12, accompanied with a residual strain of 87%, which was slightly larger than that of PVA hydrogel (55%). The hysteresis loss of TTSBI-2@Fe3+-12 increased with the successive tensile strain along with an apparently increased residual strain (Fig. S6f). But for the pure PVA hydrogel, the increase of residual strain was a little smaller (Fig. S6e). All these results suggested that more microstructure changes occurred in TTSBI-2@Fe3+-12 than in the PVA hydrogel during the stretching process, due to the stronger physical cross-linking network [58]. The hysteresis compression deformation at 60% strain (Fig. 3e) and under varied strains (10% ~ 80%) (Fig. 3f) demonstrated that the TTSBI-2@Fe3+-12 hydrogel exhibited excellent resilient performance under compression compared with neat PVA

hydrogel (Fig. S6g). 3.3 Electrical properties of the composite hydrogel

Fig. 4. Electrical sensor properties of the composite hydrogels. a) The relative resistance change (△R/R0) of TTSBI-2@Fe3+-12 versus the tensile strain from 0 to 400% and images of the LED responding to applied strains. b) The reversible relative resistance changes-strain curves of tensile. c) The resistance change response of TTSBI-2@Fe3+-12 versus repeated loading and unloading of 100% tensile strain for 100 cycles. d) The relative resistance change of TTSBI-2@Fe3+-12 of varied finger bending angles (a: 0 °, b: 30 °, c: 60 °, d: 90 °). e) The conductive hydrogel (TTSBI-2@Fe3+-12) [a] before, [b] after being cut, [c] healed and [d] twisted as conductor to light a LED. f) The relative resistance change versus temperature variations for 20 cycles between 5 and 60 °C.

As shown in Table S2, the conductivity of the composite hydrogel improved after adsorbing iron ion, and its conductivity could be manipulated by pH. The conductivity of TTSBI-2@Fe3+ reached 0.261 S/m after soaking in FeCl3 solution for 4 h, which was much higher than TTSBI-2 (0.013 S/m) and neat PVA hydrogel (0.009 S/m). Further increasing the pH of TTSBI-2@Fe3+, the conductivity of TTSBI-2@Fe3+-5 (0.124 S/m), TTSBI-2@Fe3+-8 (0.084 S/m) and TTSBI-2@Fe3+-12 (0.055 S/m) decreased successively (Table S2). This was induced by the increased coordination

state at higher pH value, which increased the crosslinking density of the hydrogel network and thus greatly limited the movement of ions [59-62]. To examine the potential applications in flexible electronics, the electrical sensory properties of TTSBI-2@Fe3+-12 were evaluated. The relative resistance change ratio of the TTSBI-2@Fe3+-12 hydrogel increased gradually with the tensile strain from 0 to 400%, and the resistance change was more sensitive at large strain (Fig. 4a). The light-emitting diode (LED) indicator extinguished once the tensile strain exceeded 300%. Fig. 4b depicted the variation of relative resistance change within single cycle of the tensile loading-unloading process, suggesting that the relative resistance change of TTSBI-2@Fe3+-12 increased almost linearly within the tensile strain range from 0 to 100%. The stable electrical signal was exported during the continuous cycling tests of 100% tensile strain, demonstrating good sensitivity and durability of this conductive hydrogel as a strain sensor (Fig. 4c). When the TTSBI-2@Fe3+-12 hydrogel was stuck onto the finger or wrist, it implemented the transmission and collection of motion signals with the deformation of limbs. The motion exhibition of the index finger was shown in Fig. 4d. TTSBI-2@Fe3+-12 hydrogel could respond to the finger motion rapidly and repeatedly when the bending angle of finger changed from 0° to 90°, and the resistance increased in a stepwise manner from 0 to 11.8% (Fig. 4d). The resistance response for wrist bending was also manifest and repeatable (Fig. S7a). As shown in Fig. S7b, the conductive hydrogel performed high sensitivity in the detection of twist in different degrees. When the hydrogel was twisted 360° and 720°, the △R/R0 increased stepwise to 34.5% and 81.5%, respectively. The TTSBI-2@Fe3+-12 hydrogel was also electrically sensitive to compression. As shown in Fig. S7c, stable and obvious real-time electrical signal was output upon writing on a simple sandwich-structure device consisting of a piece of hydrogel in the middle and Teflon films on the top and bottom, suggesting the potential application in touch sensors. The TTSBI-2@Fe3+-12 hydrogel could also recover its original conductivity after being cut into two sections, which was visually presented in Fig. 4e. The LED indicator ([a]) quenched once the hydrogel was cut into two parts ([b]), but lighted

immediately when the two parts were touched with each other ([c]). When the hydrogel after healing was twisted, the brightness of LED was slightly reduced ([d]). The TTSBI-2@Fe3+-12 hydrogel also showed good response to temperature variations. The resistance of the ionic conductive hydrogel decreased with the temperature increasing from 20 °C to 60 °C, and the resistance recovered during the cooling process (Fig. S7d). Fig. 4f showed the relative resistance change with the temperature variations of the TTSBI-2@Fe3+-12 hydrogel for 20 cycles at 5 ~ 60 °C, implying good repeatability and stability for temperature sensing. Such an ionic conductive hydrogel with controllable mechanical and conductive performance showed a favorable prospect in flexible electronic device, soft intelligent robot, and electronic artificial skin, etc [63]. 3.4 Shape memory behavior of the composite hydrogel

Fig. 5. Multiple stimulus responsive shape-memory effect of the TTSBI-2 hydrogel. a) solvent stimulated shape memory; b) temperature stimulated shape memory; c) force anisotropy caused shape change after Fe3+ complexation in some special part of TTSBI-2, scale bar: 10 mm; d)

resistance change of TTSBI-2 during the shape deformation process (left); shape deformation process of hydrogel.

In recent years, shape memory hydrogel formed by hydrophobic association has been reported [32, 42, 43]. However, conductive hydrogel with shape memory function was rarely reported. The shape memory performance of TTSBI-2 under various stimulations was demonstrated in Fig. 5. The dumbbell-shaped hydrogel could switch between a spiral and a rectangle by changing the solvent (Fig.5a). The hydrogel was first placed in water and deformed into a spiral shape as the temporary shape, which was fixed by the physical crosslinking network formed by the hydrophobic self-assembly of TTSBI in water. When it was moved into DMSO, the temporary spiral shape restored to its original flat dumbbell shape quickly within 5 s, due to the dissociation of TTSBI in DMSO and the destruction of the physical crosslinking network formed by TTSBI self-assembly. Different from TTSBI as cross-linking sites, the crystalline region of the PVA molecular chains could also act as a temporary shape lock/switch upon heating/cooling process (Fig. 5b). The hydrogel was first placed in water at 55 °C, then deformed to a temporary spiral shape and placed in 25 °C for 600 s to fix the temporary shape. The hydrogel recovered to its permanent flat shape immediately when it was heated up to 55 °C. Interestingly, the hydrogel was able to switch from 2D to 3D by smearing FeCl3 solution (0.05 M) on one surface of the hydrogel in a flower shape, as shown in Fig. 5c. This was because the regional catechol-Fe3+ interactions generated a regionally denser cross-linking network and higher mechanical strength, resulting in uneven internal stress distribution, which caused the shape deformation. Once the sample was placed into DMSO, the flower recovered from 3D to 2D immediately. Interestingly, during the shape deformation process by smearing FeCl3 solution, the conductivity of the hydrogel varied as demonstrated in Figure 5d. This was because the content of ferric ion in hydrogel affected its shape memory properties. Meanwhile, the free ferric ion content also contributed to the conductivity of the hydrogel. During smearing FeCl3 solution, the free ferric ion content in the hydrogel increased, leading to the

increase in the conductivity (Figure 5d left). After immersion the hydrogel in alkaline solution, the free ferric ion was locked in the intensive hydrogel network, leading to the drop in the conductivity. Thus, the conductivity of the hydrogel changed during the shape deformation process, suggesting that the shape memory performance of hydrogel could be electrically detected. As hydrophobic TTSBI NPs acted as switches during deformation process providing numerous cross-linking sites, more complicated actuation could be designed based on the synergy of the hydrophobic association and metal complexation, endowing the conductive hydrogel with diverse features under multiple stimulations. This multi-functional performance also allowed the electrical signal detection during the shape deformation process, which might find great potentials in soft robots, and intelligent sensors etc.

4. Conclusions In summary, a robust ionic conductive hydrogel was prepared by a facile hydrophobic self-assembly strategy. The hydrophobic TTSBI was first incorporated to make the PVA composite hydrogel, followed by constructing the catechol-Fe3+ intermolecular interactions. Owing to the nanophase separation structure formed by TTSBI during the solvent exchange process and the hydrogen bonding interactions between TTSBI and PVA, the mechanical performance of PVA/TTSBI nanocomposite hydrogel was improved. Further constructing the catechol-Fe3+ interactions in the composite hydrogel led to a stronger physical crosslinking network, generating conductive hydrogel with high tensile strength (3.25 MPa) and excellent toughness (9.23 MJ/m3). Introducing the Fe3+ endowed the hydrogel with good conductivity and excellent electrical sensitivity, which could output stable electrical signal of resistance change in response to the stretch, compression, bending and twist of limb movements, and even to a slight deformation caused by handwriting. Moreover, the hydrogel also exhibited excellent temperature sensitivity. In addition, the conductive hydrogel behaved good shape memory performance under multiple stimulation conditions including temperature, solvent and Fe3+ concentration. This work provides a facile

route by hydrophobic self-assembly for conductive hydrogel with tailored mechanical and shape memory performance. The conductive hydrogel with multiple stimulus shape memory performance may find great potentials in soft robots and intelligent sensors etc.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgments The authors gratefully thank the National Natural Science Foundation of China (21706082), the Science and Technology Program of Guangzhou (201707020025, 201804010140), Guangdong Province Science Foundation (2017B090903003, 2018B030311052, 2019A1515012154) for the financial supports.

Conflict of interest

The authors declare no conflict of interest.

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Fig. 1. a) The synthesis procedure of the PVA composite hydrogel via freezing-thawing and solvent exchange method. b) Schematic illustration of the composite hydrogel.

Fig. 2. SEM images of the composite hydrogels. a) SEM image of TTSBI NPs separated from poor solvent of water. TEM images of the cross-sections of b) pure PVA hydrogel and c) TTSBI-2; SEM images of the cross-sections of d) pure PVA hydrogel, e) TTSBI-2 and f) TTSBI-2@Fe3+-12.

Fig. 3. Mechanical properties of the composite hydrogels. a) Tensile stress-strain curves and b) toughness comparison of pure PVA and PVA nanocomposite hydrogel. Photograph demonstration of TTSBI-2@Fe3+-12 before and after c) extension and d) compression. e) Continuous loadingunloading compressive curves of TTSBI-2@Fe3+-12 for 5 cycles at 60% strain without residence time. f) Successive compressive loading-unloading curves of TTSBI-2@Fe3+-12 under varied strains (10% ~ 80%).

Fig. 4. Electrical sensor properties of the composite hydrogels. a) The relative resistance change (△R/R0) of TTSBI-2@Fe3+-12 versus the tensile strain from 0 to 400% and images of the LED

responding to applied strains. b) The reversible relative resistance changes-strain curves of tensile. c) The resistance change response of TTSBI-2@Fe3+-12 versus repeated loading and unloading of 100% tensile strain for 100 cycles. d) The relative resistance change of TTSBI-2@Fe3+-12 of varied finger bending angles (a: 0 °, b: 30 °, c: 60 °, d: 90 °). e) The conductive hydrogel (TTSBI2@Fe3+-12) [a] before, [b] after being cut, [c] healed and [d] twisted as conductor to light a LED. f) The relative resistance change versus temperature variations for 20 cycles between 5 and 60 °C.

Fig. 5. Multiple stimulus responsive shape-memory effect of the TTSBI-2 hydrogel. a) solvent stimulated shape memory; b) temperature stimulated shape memory; c) force anisotropy caused shape change after Fe3+ complexation in some special part of TTSBI-2, scale bar: 10 mm; d) resistance change of TTSBI-2 during the shape deformation process (left); shape deformation process of hydrogel.

●A highly tough and conductive hydrogel was facilely prepared via freeze-thawing and solvent exchange method. ● Mechanical and conductive properties of the composite hydrogels could be regulated by the pH value. ●The Conductive hydrogel showed excellent sensitivity to stretching, bending, twist, and compression. ●The hydrogel exhibited Multiple-stimuli responsive shape memory behaviors.

Conflict of interest

The authors declare no conflict of interest.