Electro-responsive Homogeneous Polyelectrolyte Complexes

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Electro-responsive Homogeneous Polyelectrolyte Complexes Hydrogels from Naturally Derived Polysaccharides Qingye Liu, Ziye Dong, Zhenya Ding, Zhonglue Hu, Dan Yu, Yang Hu, Noureddine Abidi, and Wei Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00921 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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Fig. 1 Schematic illustration of the preparation of CHI/CMC polyelectrolyte complex hydrogels. Firstly, the CMC was dissolved homogeneously in LiOH/KOH/Urea solvent; Second, by dispersing chitosan powder into the solution, the obtained suspension was then subjected to the freezing-thawing process for 2 times, the uniform CHI/CMC polyelectrolyte complex solution will be obtained; Thirdly, the CHI and CMC chains form covalent cross-links induced by ECH to produce chemically cross-linked network. Finally, physical crosslinked domains were formed by immersing the hydrogels into the 70% ethanol solution (upper) and deionized water (lower), respectively. Correspondingly, the hydrogels were denoted as Egel and Wgel. These physical crosslinked domains result from multiple interactions, including electrostatic interactions, hydrogen bonding, hydrophobic interactions and the formation of crystalline hydrates within the chemically cross-linked network. 116x118mm (300 x 300 DPI)

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Fig. 2. The bending process of CHI/CMC polyelectrolyte hydrogel with different weight ratios in 0.1M NaCl solution at the dc voltage of 18 V. (a) 3:0, (b) 3:0.5, (c) 3:1 and (d) 3:2. The gel strips (pale white color) were aligned in a vertical direction and bended to the left. Bending angel is defined as the angle between gel strip and vertical direction. Blue dash lines were added for eye guidance. (e) Effects of dc electrical voltages on the bending angle of the CHI/CMC (3:2) polyelectrolyte hydrogel in 0.1M NaCl solution. (f) The variation of bending behaviors for the hydrogel strips under the dc electrical stimulus in different salt solutions with the same ionic strength of 0.1M. (g) Effects of pH on the bending angle of the CHI/CMC (3:2) polyelectrolyte hydrogel in 0.1M NaCl solution at the dc voltage of 18 V. The bending angles were measured by reading the angle of deviation from the vertical position. 64x44mm (300 x 300 DPI)


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Fig. 3 (a) The dependence of bending angle of the CHI/CMC (3:2) polyelectrolyte hydrogel on different ionic strength in NaCl solution at the dc voltage of 18 V. (b) Effect of ionic strength on the swelling ratio of CHI/CMC (3:2) polyelectrolyte hydrogel without the application of dc electrical field. (c) Schematic of the possible mechanisms in illustrating the bending behaviors of CHI/CMC PEC hydrogel in salt concentration by utilizing a dynamic enrichment/depletion model. The hydrogel strip (pink colored strip) can be identified as a bilayer membrane with one layer facing the cathode and the other facing the anode. Ionic distributions (black dots) are shown in the gel and the surrounding salt solution before and after the application of a dc electric field. 54x39mm (300 x 300 DPI)



Fig. 4 (a) Photographs of the Egel and Wgel with the weight ratio of CHI/CMC=3:2 compressed by a thumb. (b) Compressive stress-strain curves of the Egel and Wgel. (c) The typical consecutive loading-unloading curves with varying maximum compressions. (d) Representative loading-unloading compression curves (6 runs) of Egel with 60% maximum strain. 44x32mm (300 x 300 DPI)


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Fig. 5. SEM images of cross-sectional structures of Egel (a) and Wgel (b). (c) The XRD patterns and (d) ATRFTIR spectra of Egel and Wgel, respectively. 71x57mm (300 x 300 DPI)



Fig. 6. (a) The bending behaviors of CHI/CMC (3:2) hydrogel strip in a 0.1M NaCl solution under the reversible dc electrical stimulus for 20 min. SEM images of cross-sectional structures of CHI/CMC hydrogel strips without (b), and (c) with the application of reversible dc electrical stimulus. (d) The XRD patterns of CHI/CMC hydrogel strips. S-Egel and N-Egel represent the CHI/CMC hydrogel strips that were or not stimulated by the reversible dc electrical field, respectively. 60x43mm (300 x 300 DPI)


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Fig. 7. (a) Self-deformation upon time for the “+”-shaped PEC hydrogel in a 0.1M NaCl aqueous solution at dc voltage of 18 V. (b) The transformation process in the same electrical field by turning the hydrogel actuator with formed architecture upside down (figure a-4 to figure b-1). Twisted deformation process of the patterned PEC hydrogels in a 0.1M NaCl aqueous solution at the dc voltage of 18 V (c) and (e). The corresponding models for illustrating the multiple variations (d) and (f). In the fabricated hydrogels, the cyan color denotes the chitosan matrix, while the gray sections stand for the CHI/CMC composites inserted in the matrix with the ratio of 3:2. (g) Images of the movements of an adjacent object (left side of the red dash line) propelled by the bending actuation of PEC hydrogel strip (right side of the red dash line) in 0.1M NaCl aqueous solution at a dc voltage of 18 V. 121x244mm (300 x 300 DPI)



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Electro-responsive Homogeneous Polyelectrolyte Complexes Hydrogels from Naturally Derived Polysaccharides Qingye Liu1,2, Ziye Dong2, Zhenya Ding2, Zhonglue Hu 3, Dan Yu 4, Yang Hu5, Noureddine Abidi5, Wei Li2 *

1.

School of Chemical Engineering and Technology, North University of China, NO. 3 Xueyuan Road, Jiancaoping District, Taiyuan 030051, China

2.

Department of Chemical Engineering, Texas Tech University, 6th Street and Canton Ave, P. O. Box 43121, Lubbock, Texas 79409, USA

3.

Department of Mechanical Engineering, Texas Tech University, 805 Boston Ave, ME South 201, Lubbock, Texas 79409, USA

4.

Department of Critical Care Medicine, People’s Hospital of Zhengzhou University, No. 7 Weiwu Road, Jinshui District, Zhengzhou 450003, China

5.

Fiber and Biopolymer Research Institute, Department of Plant and Soil Science, Texas tech University, 1001 East Loop 289, Mail Stop 45019, Lubbock, Texas 79409, USA

[*]

To whom correspondence and reprint requests should be addressed. E-mail: [email protected]

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Abstract Homogeneous polyelectrolyte complexes (PEC) hydrogels made from chitosan and carboxymethylcellulose were prepared in the LiOH/KOH/urea aqueous system through a freeze-thawing method. Following the treatments of sequential chemical and physical cross-linking, the resulting hydrogels with super-tough mechanical strength can operate as fast response actuators under electrical stimulus in salt aqueous solutions. The electromechanical behaviors of the hydrogels are strongly dependent on experimental parameters such as electric voltage, solvent constituents, pH, and ionic strength. It is proposed that the electromechanical deformation of hydrogel originates from a dynamic osmotic equilibrium effect taking place at the interface between the hydrogel and the surrounding medium, which is induced by the migration of ions throughout the gel network. In addition, programmable 3D shape transformations were obtained by using the PEC hydrogel with designed 2D geometric patterns. Moreover, the bending actuation behavior of the PEC hydrogel can propel an adjacent object to move forward. These hydrogels are expected to be used as underwater actuators for soft robotics and other smart biomimetic systems.

Keywords: Electro-responsive; Super-tough hydrogel; Polyelectrolyte complexes hydrogel; Natural polysaccharide; Programmed transformation

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Introduction Hydrogels with three-dimensional (3D) crosslinked network, are special types of soft polymers that have enormous capacity to absorb water. By changing their volume reversibly under certain external stimuli, i.e. light, heat, pH, solvent composition, humidity and external fields, some of the hydrogels can undergo desired, programmable 3D shape transformations and used as mechanical actuators1-8. Among those responsive materials, electro-sensitive hydrogels have attracted great attention in recent years, because electric stimulus can be simply and remotely controlled with adjustable strength and direction, and can be rapidly and precisely switched "on and off" 9-12. Since these hydrogels can directly convert electrical energy to mechanical response, they have been readily utilized in diverse applications, such as sorting switches, artificial muscles, mechanical pumps for 3D tissue engineering, micro-robotics, and other prosthetic devices5, 9-12. Exciting works have been reported on electro-responsive hydrogels. For example, Kwon and colleagues have developed an electroactive acrylate hydrogel actuator, which could be integrated into a microfluidic channel and used as a smart switch for sorting microparticles9. Migliorini et al have synthesized a novel electro-responsive hydrogel based on Na-4-vinylbenzenesulfonate. This hydrogel could generate low-voltage electro-mechanical actuation in physiological environments, demonstrating a promising material to be employed in the design of soft underwater actuators in biomimetic systems12. Rahimi et al have proposed a biocompatible hydrogel fabricated from acrylic acid and fibrin that can swell and shrink in response to an applied electrical field. Mechanical actuators made by this electrical-driven hydrogel were used to direct the alignment of smooth muscle cells and facilitate infiltration and distribution of cells in the hydrogel sheet11. Typically, electro-responsive hydrogels are made of synthetic polyelectrolytes and in most cases the hydrogels contain either positive or negative charged polymers12-15. Upon imposed voltage in a salted environment, these hydrogels can only achieve electro-responsive behaviors in relatively narrow pH ranges5, 10,14,16. Polyelectrolyte complexes (PEC) are aggregation of hydrated inhomogeneous nanocomposites or liquid-like coacervates generated when a pair of opposite charged polyelectrolytes are mixed and interact in an aqueous solution17-20. PEC hydrogels can be obtained by crosslinking PEC nanocomposites and reswelling them in an aqueous solution. Recently, electro-responsive hydrogels prepared from PEC have been developed using chitosan and carboxymethylcellulose and they have demonstrated bending behaviors in a salt solution with pH ranging from 6 to 1021. However, current PEC hydrogels are inhomogeneous and fragile as chemical crosslinking between PEC nanocomposites can only loosely link them together, hence, those hydrogels have demonstrated limited applications as actuators where high mechanical strength is required. In this work, we report a new type of homogeneously-structured electro-responsive PEC hydrogel with high mechanical strength and demonstrate its capability to be used as soft actuators. Naturally derived polysaccharides such as chitosan and carboxymethylcellulose, were chosen as starting materials owing to 3


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many inherently remarkable advantages including renewability, low-cost, biocompatibility and biodegradability. Those oppositely-charged polysaccharides were dissolved in an alkaline/urea aqueous solvent system at low temperature22-25, which formed homogeneous PEC hydrogels by effectively preventing the occurrence of coacervates and nanoprecipitation. Further, mechanical properties of the original PEC hydrogels were strengthened by a two-step crosslinking process to generate interpenetrating polymer networks: 1) chemical cross-linking by epichlorohydrin (ECH) and 2) physical cross-linking involving hydrogen bonding, hydrophobic interactions, electrostatic interactions and the formation of crystalline hydrates in an ethanol aqueous solution26, 27. The resulting super-tough hydrogels can operate as fast response actuators under electrical stimulus in salt aqueous solutions. The electromechanical behaviors of the hydrogels are strongly dependent on experimental parameters including electric voltage, solvent constituents, pH, and ionic strengths. It is proposed that the electromechanical deformation of the hydrogel originates from a dynamic osmotic equilibrium effect taking place at the interface between the hydrogel and the surrounding medium, which is induced by the migration of ions throughout the gel network. In addition, programmable 3D transformations such as flower, helix, V-, M-like shapes were obtained by using the PEC hydrogel with designed 2D geometric patterns. Furthermore, the bending behavior of the PEC hydrogel can actuate the movement of an adjacent object, revealing the actual loading ability. We anticipate these hydrogels to be used as underwater actuators for soft robotics and other smart biomimetic systems. Experimental Section Materials and Reagents All of the reagents were used as received unless otherwise noted. Chitosan (CHI) with degree of deacetylation over 85% and Mw of 1.5×104 was purchased from Polysciences Inc. (USA). Sodium 5 carboxymethylcellulose (CMC, Mw = 2.5×10 ) was obtained from Sigma-Aldrich. Epichlorohydrin (ECH) was supplied by Alfa Aesar (USA). Other chemicals including HCl, NaOH, NaCl, Na2SO4, KCl, NaNO3, CaCl2 from commercial sources in USA were of analytical grade and used without further purifications. Carbon electrodes (length ~10 cm, diameter ~0.5 cm) and direct current (dc) regulated power supply (TEK Power, TP300ST) were purchased from Amazon (USA). Preparation of robust CHI/CMC PEC hydrogels The CHI/CMC PEC hydrogels containing ECH as the crosslinker were prepared as follows4, 27. In brief, certain amount of CMC powder was dissolved in 100 g solvent consisting of LiOH/KOH/urea/H2O in the weight ratio of 4.5:7:8:80.5, with an intense agitation. A fixed amount of 3.0 g chitosan was then dispersed and dissolved in the solution via a freezing-thawing process, to achieve the desired weight ratio of CHI to CMC (i.e., 3:0, 3:0.5, 3:1, and 3:2). The freezing-thawing treatment was repeated twice to ensure complete dissolution. Then ECH was injected dropwise via a syringe into the stirred solution at a low temperature for 10 min to obtain a homogeneous pre-gel solution, the molar ratio of ECH-to-glucosamine unit was determined to be 1:1. After degassed 4



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by centrifugation at 4300 RPM at 4 °C for 5 min, the 15 mL transparent PEC solution was cast into a petri dish with desired thickness to obtain a PEC pre-gel sheet. This pre-gel sheet was then maintained at 4 °C for 12 h for the completion of chemical cross-linking reaction of ECH with the hydroxyl groups on the polymer chains. Finally, the obtained hydrogels were removed from the mold and immersed into a 70% aqueous ethanol solution at 4 °C for 12 h, which terminated the chemical crosslinking reaction and simultaneously induced physical cross-linking. After through washing with 70% aqueous ethanol solution, the CHI/CMC PEC hydrogels denoted as Egel were achieved. Reference samples of chemically cross-linked hydrogels were obtained from deionized water using the same procedure and referred to as Wgel. Preparation of patterned hydrogel actuators The actuators were prepared by a molding process. The flower-like hydrogel actuators shown in Fig.6 were directly tailored from the CHI/CMC thin sheets with approximately 1.5 mm in thickness. For other programed transformations, the actuators were fabricated by patterning two types of hydrogels shown in Fig.7. First, the polydimethylsiloxane (PDMS) strips (~5 mm in width and ~1.5 mm in thickness) were aligned with designed pattern in a petri dish. Then 4 wt% CHI alkali aqueous solution as the matrix was cast into the petri dish and filled the space among the covered PDMS strips. After completely cured at room temperature for 1 h, the PDMS strips were carefully peeled off. Second, the transparent CHI/CMC PEC solution was injected into the vacant spaces and the patterned actuators were obtained by further curing at 4 °C for 12 h. Different sections were interconnected at the contact interfaces via chemical cross-linking. Finally, the fabricated actuators were immersed into a 70% ethanol aqueous solution for further solidification and detached from the mold until the extra alkali in the hydrogel actuators was completely removed. Characterization of PEC hydrogels The morphologies of dry hydrogel samples were characterized by using scanning electron microscopy (SEM, Hitachi S-4300, Japan) at an accelerating voltage of 5 kV. FT-IR spectra were recorded in the wavenumber range from 4000 to 600 cm−1 using universal attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Perkin-Elmer, USA) at room temperature. The hydrogels were freeze-dried in a conventional freezer dryer and then dried at 80 °C in a vacuum chamber to eliminate water from the samples. The XRD patterns of the dried hydrogel samples were recorded in reflection mode on a Rigaku RINT 2000 diffractometer equipped with a CuKα radiation source ( λ = 1.542 Å) operated at 40 kV and 30 mA; the samples were scanned at 1° min–1 and at a step size of 0.05° in 2θ. Mechanical testing Compressive measurements on the hydrogels with varying maximum compressions were performed by using a universal tensile-compressive tester (Instron 5900, USA). A cylindrical hydrogel with a diameter of 10 mm and a height of 10 mm was placed on the lower plate and compressed by the upper plate at a strain rate of 1 mm·min-1. The load and displacement data were collected during the experiments. For the Wgel, the solvent exchange treatment was performed before the compressive test by immersing the Wgel into a 70% ethanol aqueous solution for five 5


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days. Cyclic tests were carried out by performing subsequent trials immediately after the initial loading. Swelling behaviors. The gravimetric method22 was employed to measure the swelling ratios of the hydrogel strips in NaCl solution with different ionic strength of 0.01, 0.1, 0.2, 0.6, 1.0, 2.5 and 5.0 M. The temperature was set as 37 °C to simulate body temperature for exploring potential biomedical applications. The swelling ratio (SR) was calculated as SR= (Ws - Wd ) / Wd × 100%

(1)

where Ws is the weight of the swollen gel at 37 °C and Wd is the weight of the gel in the dry state. After 24 h culture in NaCl solution, the hydrogel samples were taken out from the aqueous solution and weighed after removing the excess water on the surfaces with a filter paper. Electromechanical behavior of PEC actuators in dc electric field A schematic diagram of the equipment used for studying the electrical response of the hydrogels is shown in Fig. 4. All hydrogel strips (~40 mm in length, ~5 mm in width, and ~1.5 mm in thickness) for bending investigation were first swollen to equilibrium in as-prepared salt solution with different ionic strength. Two parallel carbon electrodes (10.0 cm apart) were immersed in the salt solutions (pH = 6), and the hydrogel stripe was mounted centrally between them. Upon application of a dc electric field, the degree of bending was measured by reading the angle of deviation from the vertical position. The bending angle towards the anode is defined as the positive value, whereas the bending angle towards the cathode is defined as the negative. The 3D shaped deformation processes were performed in NaCl aqueous solution with the ionic strength of 0.1 M, and the transformation behaviors were recorded with a digital camera (Nikon J1, Japan). For the load bearing test, the cuboid object (2.12 g) with the dimensions of 30 mm × 17 mm × 3.0 mm was fixed by a flexible thread and hung in the salt solution. The PEC hydrogel was tailored into a strip with the size of ~35 mm in length, ~10 mm in width and ~2.0 mm in thickness, and parallelly mounted to the center of the container (5.0 mm apart between the object and the PEC hydrogel strip, as shown in Fig.7g). The object movement propelled by the bending actuation of the PEC hydrogel in 0.1M NaCl aqueous solution at dc voltage of 18 V was recorded with a digital camera (Nikon J1, Japan). Results and Discussion Preparation of homogeneous super-tough CHI/CMC PEC hydrogels As a native polysaccharide, CHI is a polycation, which is usually dissolved in acidic aqueous solution for further manipulation28. However, in acidic conditions, if negatively charged CMC is introduced into the solution of CHI, nanoaggregates form and precipitate due to the electrostatic interactions between CHI and CMC 17. As a result, no homogeneous PEC hydrogels can form. In order to overcome the limitation of the conventional methods, we have used an alkali/urea aqueous solution to dissolve

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Fig. 1. Schematic illustration of the preparation of CHI/CMC polyelectrolyte complex hydrogels. Firstly, the CMC was dissolved homogeneously in LiOH/KOH/Urea solvent; Second, by dispersing chitosan powder into the solution, the obtained suspension was then subjected to the freezing-thawing process for twice, the uniform CHI/CMC polyelectrolyte complex solution will be obtained; Thirdly, the CHI and CMC chains form covalent cross-links induced by ECH to produce chemically cross-linked network. Finally, physical crosslinked domains were formed by immersing the hydrogels into the 70% ethanol solution (upper) and deionized water (lower), respectively. Correspondingly, the hydrogels were denoted as Egel and Wgel. These physical crosslinked domains result from multiple interactions, including electrostatic interactions, hydrogen bonding, hydrophobic interactions and the formation of crystalline hydrates within the chemically cross-linked network.

CHI and CMC to fabricate the PEC hydrogels, adapted from the methods originally developed by Zhang et al23. The preparation process and network structure of PEC hydrogels was shown in Fig.1. Specifically, in the alkali/urea solution, both CHI and CMC were dissolved and no precipitation was observed. Meanwhile, in order to avoid the possible formation of heterogeneous structure in hydrogel generated by the incomplete dissolution, the freezing-thawing process was repeated twice to ensure that the polysaccharide chains were fully dispersed in the aqueous solution29. After 30 minutes, a translucent homogeneous PEC hydrogel was formed. However, this hydrogel was fragile and not stable in acidic conditions. As the goal was to develop PEC gels that have strong mechanical properties and suitable for application in a wide pH ranges, we have performed a two-step crosslinking treatment to further strengthen the PEC hydrogels. First, ECH was introduced to the CHI/CMC solution to trigger condensation reactions between -OH groups on CHI and CMC molecules. Second, the formed gel was immersed into a 70% aqueous ethanol solution to form double 7


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network by physical cross-linking of polymer chains with hydrogen bonds, hydrophobic interactions, electrostatic interactions and the formation of crystalline hydrates in the polymer networks26, 27. This process also removed extra alkali and urea. The resulting PEC hydrogels with both chemical and strong physical crosslinking processes was denoted as Egel and the PEC hydrogels with negligible physical crosslinking was denoted as Wgel. As indicated in Fig. 1, Egels and Wgels show different morphologies and swelling behaviors. Egels were translucent while Wgels appeared more transparent, regardless of the weight ratio between CHI and CMC. Both gels showed an increasing tendency of SR with the increment of CMC content (Supporting Information Fig. S1). As the weight ratios of CHI/CMC varied from 3:0 to 3:2, SR of Egels and Wgels increased from 5 to 10 and 20 to 65, respectively. Further, for gels with the same CMC content, SR of Egels was 4-6 times smaller in comparison to that of the Wgels. Usually, the inhomogeneous structure and weak mechanical property for PEC hydrogels are the main factors that strongly constraint their wide applications. These drawbacks are generated by the fast complexation reaction at the interface of two solutions17, 18. Hence, materials based on PEC are usually limited to thin films via the layer-by-layer assembly30-32. Thus, a worthwhile endeavor was made to seek an easy way to fabricate the PEC hydrogels with high toughness. Recently, Gong’s group19 has successfully developed a method to improve the mechanical strength of the PEC hydrogels. By polymerizing one of the polyelectrolyte from its monomers in the presence of another oppositely-charged polymer, a new class of physical crosslinked hydrogels with high toughness were obtained. However, this approach has only be performed in the synthetic acrylate or methacrylate derivatives systems. The manipulation of two oppositely-charged natural polyelectrolyte chains remains a significant challenge. It has been proved that the presence of salt during PEC formation could drastically reduce the aggregation, but a further increase in salt concentration may also cause secondary aggregation leading to macroscopic flocculation33. Therefore, the employed method here will open a new avenue for the construction of novel PEC hydrogels with homogeneous structure and high toughness, especially for the hydrogel materials originated from natural ionic polysaccharides, such as alginate, chitosan, chondroitin sulfate or hyaluronan et al. Our approach is expected to be applied in processing the advanced PEC materials combined with other methods. The electro-mechanical behavior of CHI/CMC PEC hydrogels The CHI/CMC PEC hydrogels displayed interesting bending behaviors when a dc electric field was applied to the hydrogel strips in NaCl solution (pH = 6). Since fast response of gel is preferred for future applications as actuators, we focused on the deformation of gels in the first 60s. As shown in Fig. 2a-d, the PEC hydrogels with ratio of CHI/CMC from 3:0 to 3:2 showed enhanced electrical sensitivity. Specifically, the hydrogel stripe quickly bended toward anode and the bending curvature almost increased linearly with time, reaching a bending angle from 0 to 60ºin about 60 s 8



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(detailed data shown in supporting information Fig S2). Gel stripe with CHI/CMC ratio of 3:2 had demonstrated the faster response with a maximum bending angle and was selected for further study. It can be inferred that the composition of the hydrogel network plays an important role in modulating the electrical response. Basically, the electro-mechanical actuation is often induced by asymmetric swelling and shrinking34, which is a result of the difference in osmotic pressure applied to opposite sides of the hydrogels caused by the translational entropy of free ions in the gel network. As previously reported, the electric-response of PEC hydrogels also strongly depends on the polymer concentrations, crosslinking density, charge density and external ionic strength5, 14, 35. We have chosen a few parameters to further investigate the response of the gel stripe of with CHI/CMC ratio of 3:2.

Fig. 2. The bending process of CHI/CMC polyelectrolyte hydrogel with different weight ratios in 0.1M NaCl solution at the dc voltage of 18 V. (a) 3:0, (b) 3:0.5, (c) 3:1 and (d) 3:2. The gel strips (pale white color) were aligned in a vertical direction and bended to the left. Bending angel is defined as the angle between gel strip and vertical direction. Blue dash lines were added for eye guidance. (e) Effects of dc electrical voltages on the bending angle of the CHI/CMC (3:2) polyelectrolyte hydrogel in 0.1M NaCl solution. (f) The variation of bending behaviors for the hydrogel strips under the dc electrical stimulus in different salt solutions with the same ionic strength of 0.1M. (g) Effects of pH on the bending angle of the CHI/CMC (3:2) polyelectrolyte hydrogel in 0.1M NaCl solution at the dc voltage of 18 V. The bending angles were measured by reading the angle of deviation from the vertical position.

We firstly studied the effect of applied voltages on the bending behaviors of the gel strips. As shown in Fig. 2e, for the selected range of voltage from 9 to 27 V, an approximate linear dependence of electric voltage on the bending rate and angles was observed, also the bending angles at the same time points in 60s increased as higher voltage was applied. It is easy to understand that ions in the ambient solution transfer

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more quickly as the electric voltage increases, thereby resulting in the increases in both bending rate and angle. Secondly, the ionic species also facilitate the electro-mechanical performance of the CHI/CMC PEC hydrogels. As shown in Fig. 2f, for gel strips in the different electrolyte solutions with the same cation but different anionic species, such as Na2SO4, NaCl and NaNO3, the maximum steady-state bending angle gradually decreased. This observation can be explained by the Hofmeister effect8, 36. The Hofmeister effect is related to the influence of the salts on the interactions between ions and polymer chains, where the anions have a more pronounced effect than cations and are usually ordered as: CO32- > SO42- > S2O32- > H2PO4- > F- > CH3COO- > Cl- > Br- > NO3- > I- > ClO4- > SCN- 37, 38. The ions on the left side of the series are kosmotropes (well-hydrated ions), and those on the right side are chaotropes (poorly hydrated ions). The poorly-hydrated chaotropes ions could enhance water-polymer interactions, resulting in a higher swelling degree for the hydrogel in the solution38. The hydrogel strips with higher hydration will allow more cations to migrate through the matrix, leading to a smaller osmotic pressure difference, and thereby a smaller maximum steady-state bending angle. On the other hand, for gel strips in the different electrolyte solutions with the same anion but different cationic species, i.e. NaCl, KCl, and CaCl2, due to the facts that the hydrated radii of the cations increased as the order of Ca2+ > Na+ > K+39, and the hydrated radii, however, vary inversely with their mobility, it caused a slower maximum bending speed but a larger maximum steady-state bending angle (shown in Fig. 2f ). One remarkable property of PEC hydrogels is that it can act as either a polyanionic or polycationic hydrogel depending on the pH of the solution. Thus, the electro-mechanical behavior of the CHI/CMC PEC hydrogel over pH range of 2-14 was tested. As indicated in Fig. 2g, the gel strips behave as polycationic materials and bend toward the anode in acidic solution, as a result of the protonation of amino group (-NH2). However in a basic solution, the -COOH groups in the gel network are easy to ionize into -COO− and cause the PEC gel strips to behave as polyanionic materials. As a consequence, they bend toward the cathode. Hence, we have confirmed that the CHI/CMC PEC hydrogel is a kind of amphoteric hydrogel with good electromechanical responses over a wide pH range. The electrical actuation mechanism for CHI/CMC PEC hydrogels Although the actuation mechanism of polyelectrolyte films has not been fully explored, it is generally accepted that the deformation of a hydrogel membrane under a dc electric field is mainly due to the voltage-induced motion of ions and the concomitant osmotic pressure differences across the boundaries of the polymer gel with the surrounding salt solution5, 12, 40. When an ionic polymer gel is placed in a salt solution, the swelling equilibrium and the distribution of the ions in the gel and the surrounding solution described as the Donnan equilibrium were first established41, 42. However, after an external electric field was applied, the mobile cations migrate preferentially from the anode towards the cathode due to the gel permeability, and a new Donnan equilibrium will be re-built due to the re-distribution of the ions between the gel and the surrounding solution to maintain electroneutrality and establish equal 10



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chemical potential between the phases5. According to Flory’s theory43, for the given system, the resulting osmotic contribution from this equilibrium is given by the equation below:

π = RT ∑（C g − C s ）

(2)

where R is the universal gas constant, T is the absolute temperature, and Cg and Cs are the concentrations of ions in the gel and in the solution, respectively. Simply, in this case, the charged hydrogel strips can be considered to be a bilayer membrane with one layer facing the anode and the other facing the cathode (see Fig. 3). Hence, the osmotic pressure difference at the interface of the gel closest to the anode, π1, can be written as:

π 1 = RT ∑（C g 1 − C s 1）

(3)

The osmotic pressure difference between the gel and the outside solution occurred at the cathode side, π2, can be expressed as:

π 2 = RT ∑（C g 2 − C s 2）

(4)

The change of osmotic pressure as the main driving force for the bending of hydrogel strip thereby can be referred to:

∆π = π 1 − π 2

(5)

Without the effect of the electric field, the gel could swell to equilibrium, since the distribution of ionic species across the inner gel and the external solution is uniform. At the moment, the hydrogel strip hangs straight down with Δπ = 0. When the electric field is turned on, the original swelling equilibrium is broken by the movement of the mobile ions under the influence of the electric field. An osmotic pressure difference of Δπ > 0 could enable the hydrogel to bend toward the cathode. The opposite bending behavior would occur when ∆π < 0. In our case, the Na+ could transfer across the chitosan/CMC PEC hydrogel membrane from the anode to the cathode side once the electrical field is turned on, as indicated in Fig. S3, large amount of NaCl salt crystals can be observed in the gel matrix. Based on this observation, we have tested the electro-responses of our gels in NaCl solutions with different ionic strength ranging from 0.1 to 0.6 M, and observed interesting bending behaviors. As shown in Fig. 3a, the bending angle at 60 s increased as the ionic strength increased from 0.1 to 0.2 M, while gradually decreased as the ionic strength further increased to 0.6 M. Since it is known that the ionic strength affects the swelling ratio of PEC hydrogels, we also tested the SR of our gels in NaCl solution in the same range (Fig. 3b). Herein we have proposed a dynamic enrichment/depletion model to interpret the bending behaviors of PEC hydrogels, which has been used to describe the actuation mechanism of polyionic gels in saline solutions44. The schematic mechanism was illustrated in Fig. 3c.

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Fig. 3 (a) The dependence of bending angle of the CHI/CMC (3:2) polyelectrolyte hydrogel on different ionic strength in NaCl solution at the dc voltage of 18 V. (b) Effect of ionic strength on the swelling ratio of CHI/CMC (3:2) polyelectrolyte hydrogel without the application of dc electrical field. (c) Schematic of the possible mechanisms in illustrating the bending behaviors of CHI/CMC PEC hydrogel in salt concentration by utilizing a dynamic enrichment/depletion model. The hydrogel strip (pink colored strip) can be identified as a bilayer membrane with one layer facing the cathode and the other facing the anode. Ionic distributions (black dots) are shown in the gel and the surrounding salt solution before and after the application of a dc electric field.

Immediately after the electric field was applied, a temporary enrichment of cations in the local solution nearby the solution/gel interface facing the anode occurred, which should be ascribed to the significant shielding effect in high salt concentration solution45. This leads to a smaller swollen state and lower migration rate of ions in the gel than in the bulk solution. Therefore, the concentration of the salt at the anode side of the solution nearby the gel surface (Cs1) increases while that at the cathode side of the solution near the gel surface (Cs2) decreases. Because of the inhomogeneous distribution of ions in both sides, Cg1 - Cs1 < 0, Cg2 - Cs2 > 0, the changes in local ionic strength could in turn induce the local shrinking/swelling near the gel faces, causing the gel bending toward the anode with the osmotic pressure difference of π1 - π2 < 0. With further increment in NaCl concentration higher than 0.2 M, however, the positive maximum steady-state bending angle decreased. It has been demonstrated that the ratio of the salt concentration in the gel to the bulk solution increases as the external salt concentration increases44. Therefore, one possible reason is that the concentration of mobile ions in the hydrogel network is almost the same as that of the 12



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external solution in high ionic strength, and the exchange of the amount of ions between the gel and the bulk solution would finish in a minute even under the electrical stimulus. Another possible explanation lies in that the gels can be regarded as neutral gels in the strong screening regime, which may decrease the sensitivity of the gel to the external field46. Additionally, for a neutral gel, based on the C* theorem, the shear modulus (or the elastic modulus) should be inversely proportional to the equilibrium swelling degree, under a variation of the excluded volume parameter resulting from an interchange of solvent46. Hence, the lower equilibrium swelling degree in concentrated salt solution will produce higher elastic modulus, which could act to resist the osmotic pressure difference, eventually leading to a relative smaller steady-state bending angle. It is noted that certain critical point exists in the sensitivity for these electroactive hydrogels during the bending process, which is strongly depended on the electrolyte concentration5, 21, 47. For example, with the number of charge in the gel being blocked in Na2SO4 solutions with concentration higher than 0.05 M, the tough electroactive hydrogels cross-linked by functional triblock copolymer micelles could display slower response rate and smaller bending angles47. This “responsive window” can provide us with significant information, by utilizing which, the fabricated actuators are expected to perform their activities with the highest sensitivity and efficiency. Mechanical properties of CHI/CMC PEC hydrogels Compression strength: Since the Egel and Wgel were derived from different solvent systems, we exchanged the water solvent in the Wgel into the 70% ethanol solution for normalized comparison. Remarkably, the Egel (i.e. CHI/CMC, 3:2) was robust enough and could withstand the applied large compression without any deformation and rupture, demonstrating its excellent mechanical strength (Fig. 4a), whereas the Wgel (CHI/CMC = 3:2) was brittle and completely collapsed when subjected to the compression of a thumb. As shown in Fig. 4b, the compressive fracture stress of Egel was 4.64 MPa which is 40 times higher than that of the Wgel (0.129 MPa), and the corresponding fracture strain of Egel was 71.5% greatly larger than that of Wgel (46.7%). Although the water entrapped in the Wgel has been replaced by the ethanol aqueous solution, the high swollen state could still not be suppressed due to the strong electrostatic repulsion between CMC molecular chains. It can be inferred that a permanent expanded hydrogel network occurred in the Wgel. Energy Dissipation: Egel displayed an excellent performance of energy dissipation under loading-unloading cycles (Fig. 4c and d). These curves were obtained immediately after the loading-unloading cycles. It was presented that the energy dissipation of the Egel was effective after unloading without serious deformations during the loading-unloading cycles (Fig. 4d). Additionally, the pronounced hysteresis of the Egel was demonstrated by the cyclic stress-strain curves, which showed that the stress increases with the applied strain up to 70% (Fig. 4c). This substantial hysteresis may be due to the internal fracture of irreversible covalent bonds between the polymer chains occurring at different strain values during the 13


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loading-unloading cycles, similar to the “Mullins effect” observed in filled rubbers and the “sacrificial bonds” in double-network hydrogels27, 48-50. Shape recovery property: The Egel also exhibits significant shape recovery property. Although the energy dissipation steadily increased up to 70% strain, the compressive stress almost returned to the original state after unloading for each cycle (Fig. 4c). Moreover, the loading-unloading cycles showed that neither serious plastic deformation nor strength degradation occurred in the Egel at a set strain of 60% (see Fig. 4d), indicating that the Egel exhibits excellent shape-recovery property after the first cycle. This recovery behavior may be attributed to some reversible physically cross-linked interactions, such as hydrogen bonding and electrostatic attractions. It is reported that the high toughness and excellent shape-recovery property of PEC hydrogels originate from both well-dispersed chemically cross-linked domains and physically cross-linked domains within hydrogel networks26, 27. In our Egels, the irreversible covalent cross-links in the intertwined polymer network connected by ECH serve as “sacrificial bonds” that break into small clusters to efficiently disperse mechanical stress. Additionally, the crystallite hydrates of each polymer chains in the physically cross-linked domains can act as “load carriers” that effectively absorb energy and sustain large deformations. The percolation network of interconnected rigid polymer chains acts as a “scaffold” to transfer mechanical stresses across chemically and physically cross-linked domains via strong hydrogen bonds between entangled chains. This effect could further be enhanced by the electrostatic attractions between opposite charged polymer chains19. Since the inter-macromolecular complex formation occurs through the electrostatic interaction of the cationic groups from CHI with the anionic ones from CMC, the presence of the characteristic bands at 1589 cm-1 is expected. The intensity of the band was gradually enhanced with the increment of CMC content (shown in Fig. S4), revealing the increasing interactions51. As a concern, the Egel would suffer a long time to recover to its original shape under high stress, which is undesirable in applications as sensitive actuators. As shown in Fig. 4c and d, the hysteresis and generated plastic deformation in uniaxial compression were becoming more and more obvious with the increase of applied maximum deformation, which would give rise to an increasing recovery duration. However, it is noteworthy that the hysteresis and plastic deformation can dramatically decreased when the actuation behaviors were conducted under a smaller stress. It is considered that only a few irreversible covalent bonds were destroyed in the gel network during deformation process, preferentially leading to a swift resilience. Hence, the imposed maximum stress on the Egel strip during the bending process was investigated. The mechanics of gel bending in solution between parallel electrodes was analyzed by utilizing a three-point bending test model5: (6) where Y is the deflection of bending, W and L are the width and length of the gel beam, respectively, E is the Young’s Modulus which can be obtained from the Fig. 4b, and σ is the applied stress. According to the above equation, in the case of the bending 14



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angle of 60°, the applied stress in the electrical field was approximately 0.21 MPa, which is equivalent to the enforced stress under a 30% compress strain. As shown in Fig. S5, a substantially smaller hysteresis and better recovery behavior were observed at a set strain of 30%. It is expected if the bending angle under the electrical stimuli is confined to a degree less than 60°, the Egel could be readily used as a fast-responsive actuator. It is common for the hydrogels to exhibit a hysteresis loop upon cyclical loading, thus resulting in long recovery time52, 53. In order to improve the mechanical property aiming to obtain good resilience, a hybrid hydrogel by introducing the ductile polymer chains as the second cross-linked network would be the potential solution to eliminate the large hysteresis and permanent deformation48, 54. Furthermore, by embedding the PEC microgels into the other polymer network, the fabricated hydrogel with better resilience could also be obtained55, 56.

Fig. 4 (a) Photographs of the Egel and Wgel with the weight ratio of CHI/CMC=3:2 compressed by a thumb. (b) Compressive stress-strain curves of the Egel and Wgel. (c) The typical consecutive loading-unloading curves with varying maximum compressions. (d) Representative loading-unloading compression curves (6 runs) of Egel with 60% maximum strain.

Structural characterization of CHI/CMC PEC hydrogels To understand the difference in the macroscopic morphology and swelling ratios between the Egels and Wgels, we have characterized their microstructures using SEM, XRD and FT-IR. First, the cross-sectional morphology of freeze-dried hydrogel samples were investigated. As shown in Fig. 5a and b, the Egel exhibited compact wrinkled patterns surrounding the solid pores with the size of ~10 µm. However, the Wgel with the pore size of 20~40 µm, showed a more open and loose structure, which 15


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may be due to the fact that the electrostatic repulsions caused by the excess -COO− groups in CMC chains had enlarged the space in the networks of the hydrogels22, 57. Therefore, compared to Egels with compact microstructure, Wgels with larger pore size could allow water molecules to more easily diffuse in and out, leading to their higher swelling ratios. Second, as indicated in Fig. 5c, Egels showed higher crystallinity as comparing to Wgels. Specifically, the value of FWHM (Full Width at Half Maxima) for the main characteristic peak centered around 20ºin the XRD pattern of Egels and Wgel are 5.08º and 10.96º, respectively. This result indicates that polymer chains have the propensity to crystallize in ethanol aqueous solution, and these crystallite hydrates formed aggregations via strong hydrogen bonds26, 27.

Fig. 5. SEM images of cross-sectional structures of Egel (a) and Wgel (b). (c) The XRD patterns and (d) ATR-FTIR spectra of Egel and Wgel, respectively.

Further evidence of stronger hydrogen binding in Egels were obtained from FT-IR spectrometry, where the interactions between functional groups on polymer chains can be identified58, 59. Particularly, both Egels and Wgels showed two broad characteristic peaks at 3364 and 3286 cm−1 for O-H and N-H stretching, one peak at 2917 cm−1 attributed to C-H stretching vibration in pyranose ring, one peak at 1589 cm-1 corresponding to the interaction between -NH3+ and -COO− groups (Fig. 5d). In Egels, the intensities of all characteristic peaks were greatly enhanced, and the C-H stretching vibrations peak assigned to –NHCOCH3 was shifted from 2875 to 2850 cm−1. These fingerprints indicated that strong hydrogen bonding and electrostatic interactions exist between the polycationic and polyanionic chains in Egels.

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Reversible actuation behavior for CHI/CMC PEC hydrogels The reversible actuation behavior was examined by applying a reversible dc electric voltage to the PEC hydrogels for a certain time. Fig. 6a shows that the hydrogels could bend back and forth between the electrodes quickly as the gel strips were imposed in a reversible dc electric field. The pattern of each cycle is fairly similar, indicating the good reversibility. Interestingly, the PEC hydrogels exhibited different cross-sectional SEM images before and after the effect of electrical stimulus. As shown in Fig. 6b and c, without alternating electro-stimuli, a continuous macroporous structure occurred in the PEC hydrogel due to the pre-equilibrium swollen state of gels in salt solution. Whereas the hydrogel exhibited a compact architecture with the layer-by-layer assembled pattern after suffering from the alternating electrical stimulus for a certain amount of time. It is speculated that such feature may generate from the electro-driven force, which facilitates the anisotropic orientation for the changed polymer chains. Similar phenomenon on the electric field directed re-arrangement of polymer chains were also observed. For example, alternating electric fields can be used to effectively align poly(3-hexylthiophene) organogels over both micrometer and nanometer scales60. The short stimulation of cyclic mechanical changes of the polyacrylic acid/fibrin hydrogel induced by alternating electrical currents can provide better cell penetration and alignment in the tissue construct11. Further evidence could also be collected from the XRD patterns (shown in Fig. 6d). Both of the PEC hydrogels show characteristic peak centered around 20ºin the XRD patterns. The difference is that after being subjected to the electrical field, the PEC gel strips (denoted as S-Egel) showed higher diffraction peak intensity than the one without the stimulation (denoted as N-Egel). It is suggested that more microcrystalline fragments may exist in the S-Egel matrix. Besides, the enhanced degree of ordering for the molecular chain arrangement under the electrical stimulus (Fig. 6c) may also contribute to higher diffraction intensity, which usually relates to mono-distribution of the periodicity between polymer backbone chains61, 62. Still, great endeavor will be needed to explore the mechanism of the anisotropic orientation.

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Fig. 6. (a) The bending behaviors of CHI/CMC (3:2) hydrogel strip in a 0.1M NaCl solution under the reversible dc electrical stimulus for 20 min. SEM images of cross-sectional structures of CHI/CMC hydrogel strips without (b), and (c) with the application of reversible dc electrical stimulus. (d) The XRD patterns of CHI/CMC hydrogel strips. S-Egel and N-Egel represent the CHI/CMC hydrogel strips that were or not stimulated by the reversible dc electrical field, respectively. Multiple electromechanical performance for the programmed PEC actuator Under an electrical field, the PEC networks can achieve asymmetric swelling/shrinking volume changes as well as bending behaviors, due to the osmotic pressure difference. Based on this, the electro-responsive PEC hydrogels with designed patterns can perform various deformations, which are expected to be widely used for engineering soft actuators. As presented in Fig. 7a, the hydrogel with a tailored shape of “+” can be considered as an electro-sensitive flower. Arising from the bending movement of four hydrogel petals in the electrical field, the blooming hydrogel flower gradually closed. Meanwhile, the reversible actuation process (shown in Fig. 7b) could take place by turning the deformed hydrogel flower toward opposite direction. Benefitting from such reversibility, this hydrogel flower is expected to be further fabricated into a soft gripper for cargo transportation. Furthermore, by designing the geometric pattern and size, this PEC gels hydrogel can also demonstrate desired, programmable 3D transformations (shown in Fig.7c and e).

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Fig. 7. (a) Self-deformation upon time for the “+”-shaped PEC hydrogel in a 0.1M NaCl aqueous solution at dc voltage of 18 V. (b) The transformation process in the same electrical field by turning the hydrogel actuator with formed architecture upside down (figure a-4 to figure b-1). Twisted deformation process of the patterned PEC hydrogels in a 0.1M NaCl aqueous solution at the dc voltage of 18 V (c) and (e). The corresponding models for illustrating the multiple variations (d) and (f). In the fabricated hydrogels, the cyan color denotes the chitosan matrix, while the gray sections stand for the CHI/CMC composites inserted in the matrix with the ratio of 3:2. (g) Images of the movements of an adjacent object (left side of the red dash line) propelled by the bending actuation of PEC hydrogel strip (right side of the red dash line) in 0.1M NaCl aqueous solution at a dc voltage of 18 V. We incorporated the PEC hydrogel strips into the pure chitosan hydrogel matrix with periodic alignment, where the PEC strips displayed an oblique angle of 70ºwith 19


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respect to the long axis of the sheet (Fig. 7c and d). Due to the different sensitivity to external electrical stimulus (Fig. 2), the transformation of the planar sheet could yield a 3D helical architecture. Meanwhile, different geometric designs were achieved by embedding the PEC strips into the matrix in a line, as indicated in Fig. 7e and f. This would result in such deformed shapes as V-, M-like and other intermediate variations. Furthermore, the load bearing ability of PEC gel stripes was also explored tentatively to reveal the practical application as soft actuators. As shown in Fig.7g, the thin PEC hydrogel strip could propel an adjacent object (2.12g in weight) to move forward in two minutes, where the driving force originated from the bending actuation process. Since the bended hydrogel strip under the electrical stimuli can display well-resisting force to support the deflection shift of the object, it is expected that this kind of PEC hydrogel with appropriate tailored size and better designed equipment could be applied as underwater robot for cargo transportation. In addition, but not limited to the water aqueous system, the PEC hydrogel could also undergo similar reversible actuation behaviors in the mixed ethanol/water aqueous solution, demonstrating the enlarged applications (see Fig. S6). Conclusions In this work, we prepared a new homogeneous CHI/CMC PEC hydrogel with super-tough mechanical strength in an alkali aqueous system. These PEC hydrogels have displayed highly sensitive responses to the dc electrical stimulus, which is strongly dependent on electricity voltage, solvent constituents, pH, and ionic strength in the aqueous solution. Due to the asymmetric deformation of gel strips resulting from uneven osmotic stress at the both sides of hydrogel, this PEC hydrogel can perform various desired, programmable 3D shape transformations, such as flower, helix, V-, M-like shapes and other intermediate variations. Moreover, the bending actuation behavior of the PEC hydrogel can propel an adjacent object to move forward. This work paves the way to use biopolymer-derived PEC hydrogels as controllable electromechanical actuators integrated in biomimetic smart systems. Further, this PEC hydrogel has potential to be employed in the design and fabrication of smart soft robotics that can be remotely controlled. Supporting Information Fig. S1. Swelling ratios of the original CHI/CMC PEC hydrogels obtained from 70% ethanol aqueous solution (Egels) and deionized water (Wgels). Fig. S2. Time-dependence of bending angles for the hydrogel strips with different CHI/CMC ratio in a 0.1M NaCl solution at the dc voltage of 18 V. Fig. S3. SEM images of cross-sectional structures of a CHI/CMC hydrogel strip under reversible dc electrical stimulus for 20 min in a 0.1M NaCl solution. NaCl crystals in the hydrogel matrix were indicated by the white arrow. Fig. S4. ATR-FTIR spectra of PEC hydrogel with different CHI/CMC weight ratios. Fig. S5. Representative loading-unloading compression curves (6 runs) of Egel with 20



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30% maximum strain. Fig. S5. The reversible bending responses of CHI/CMC (3:2) polyelectrolyte hydrogel strip in a 70% v/v ethanol aqueous solution with NaCl concentration of 0.1 M under the dc voltage of 18 V. ORCID Qingye Liu: 0000-0002-9427-6416 Ziye Dong: 0000-0002-0419-8523 Zhenya Ding: 0000-0002-5388-8803 Zhonglue Hu: 0000-0001-7529-8751 Dan Yu: 0000-0003-0514-003X Noureddine Abidi: 0000-0001-5642-6332 Wei Li: 0000-0002-4738-1475 Acknowledgements QL gratefully acknowledges the financial supports from Natural Science Foundation of China (No. 51603195). WL thanks New Faculty Startup Funds from Texas Tech University. We also greatly appreciate Prof. Jingtai Zhao’s assistance in XRD testing and analyzing. References 1. Meng, H.; Li, G., Reversible switching transitions of stimuli-responsive shape changing polymers. J. Mater. Chem. A 2013, 1, 7838-7865. 2. Kempaiah, R.; Nie, Z., From nature to synthetic systems: shape transformation in soft materials. J. Mater. Chem. B 2014, 2, 2357-2368. 3. Qin, C.; Feng, Y.; Luo, W.; Cao, C.; Hu, W.; Feng, W., A supramolecular assembly of cross-linked azobenzene/polymers for a high-performance light-driven actuator. J. Mater. Chem. A 2015, 3, 16453-16460. 4. Duan, J.; Liang, X.; Zhu, K.; Guo, J.; Zhang, L., Bilayer hydrogel actuators with tight interfacial adhesion fully constructed from natural polysaccharides. Soft matter 2017, 13, 345-354. 5. Morales, D.; Palleau, E.; Dickey, M. D.; Velev, O. D., Electro-actuated hydrogel walkers with dual responsive legs. Soft Matter 2014, 10, 1337-1348. 6. Richter, A.; Paschew, G.; Klatt, S.; Lienig, J.; Arndt, K.-F.; Adler, H.-J. P., Review on hydrogel-based pH sensors and microsensors. Sensors 2008, 8, 561-581. 7. Ionov, L., Hydrogel-based actuators: possibilities and limitations. Mater. Today 2014, 17, 494-503. 8. Xiao, S.; Yang, Y.; Zhong, M.; Chen, H.; Zhang, Y.; Yang, J.; Zheng, J., Salt-Responsive Bilayer Hydrogels with Pseudo Double Network Structure Actuated by Polyelectrolyte and Anti-polyelectrolyte Effects. ACS Appl. Mater. Inter. 2017, 9, 20843–20851. 9. Kwon, G. H.; Choi, Y. Y.; Park, J. Y.; Woo, D. H.; Lee, K. B.; Kim, J. H.; Lee, S.-H., Electrically-driven hydrogel actuators in microfluidic channels: fabrication, characterization, and biological application. Lab. Chip 2010, 10, 1604-1610. 10. Yang, C.; Wang, W.; Yao, C.; Xie, R.; Ju, X.-J.; Liu, Z.; Chu, L.-Y., Hydrogel walkers with electro-driven motility for cargo transport. Sci. Rep. 2015, 5,13622. 21


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Table of Content

A polysaccharide-based homogeneous polyelectrolyte complexes (PEC) hydrogel with super-tough mechanical strength, can perform various 3D shape transformations and propelling movements under electrical stimulus in aqueous salt solutions.

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Electro-responsive Homogeneous Polyelectrolyte Complexes

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