Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel

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Applications of Polymer, Composite, and Coating Materials

Dual Salt- and Thermo-Responsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures Shengwei Xiao, Mingzhen Zhang, Xiaomin He, Lei Huang, Yanxian Zhang, Baiping Ren, Mingqiang Zhong, Yung Chang, Jintao Yang, and Jie Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06169 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 9, 2018

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ACS Applied Materials & Interfaces

Dual Salt- and Thermo-Responsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures Shengwei Xiao†♪ξ, Mingzhen Zhang¶ξ, Xiaomin He†, Lei Huang† , Yanxian Zhang¶, Baiping Ren¶, Mingqiang Zhong, Yung Changǁ, Jintao Yang*†, and Jie Zheng*¶ †

College of Materials Science& Engineering Zhejiang University of Technology, Hangzhou 310014, China ♪ School of Pharmaceutical and Chemical Engineering Taizhou University, Jiaojiang 318000, China ǁ

R&D Center for Membrane Technology and Department of Chemical Engineering Chung Yuan Christian University, Chung-Li, Taoyuan 320, Taiwan ¶

Department of Chemical and Biomolecular Engineering The University of Akron, Akron, Ohio 44325, USA

ξ the authors contributes equally to this paper. *Corresponding Author: J. Y. [email protected]; J.Z. [email protected]

KEYWORDS:

Bilayer Hydrogel; Double-Network

Zwitterion Materials; Stimuli-Response; Actuation;

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Abstract Development of smart soft actuators is highly important for fundamental research and industrial applications, but has proved to be extremely challenging. In this work, we present a facile, one-pot, one-step method to prepare dual-responsive bilayer hydrogels, consisting of a thermos-responsive poly(N-isopropyl acrylamide) (polyNIPAM) layer and a salt-responsive poly(3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonat) (polyVBIPS) layer. Both polyNIPAM and polyVBIPs layers exhibit a completely opposite swelling/shrinking behavior, where polyNIPAM shrinks (swells) but polyVBIPS swells (shrinks) in salt solution (water) or at high (low) temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS bilayer hydrogels enable to achieve fast and large-amplitude bidirectional bending in response to temperatures, salt concentrations, and salt types. Such bidirectional bending, bending orientation and degree can be reversibly, repeatedly, and precisely controlled by salt- or temperature-induced cooperative, swelling-shrinking properties from both layers. Based on their fast, reversible, bidirectional bending behavior, we further design two conceptual hybrid hydrogel actuators, serving as a six-arm gripper to capture, transport, and release an object and an electrical circuit switch to turn on-and-off a lamp. Different from the conventional two or multi-step methods for preparation of bilayer hydrogels, our simple, one-pot, one-step method and a new bilayer hydrogel system provide an innovative concept to explore new hydrogel-based actuators through combining different responsive materials that allow to program different stimulus for soft and intelligent materials applications.

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Introduction Stimuli-responsive hydrogels, considered as the most promising smart materials, have already impacted and will continue to impact on many fundamental research and industrial applications including artificial muscles, actuators, and soft robotics.1-10 Stimuli-responsive hydrogels usually can reversibly changes their shapes and volumes between the swollen and shrunken states in response to different external stimuli of pH, temperature, light, salt, and other (bio)chemical signals.11-24 Considering that conventional hydrogels are typically homogenous and isotropic materials, they usually exhibit a slow shape/volume deformation (bending, coiling, and twisting) caused by gel expansion and contraction under stimuli.25-28 Thus, design of hydrogel actuators with heterogeneous structures is critical for manipulating their actuation behaviors in a fast and controllable way. Hybrid hydrogels with a bilayer structure whose two layers possess different responsive properties are very promising design to achieve the fast, sensitive, and tunable actuation. The presence of two heterogeneous layers is expected to undergo different or completely opposite volume/shape changes, which are expected to amplify the mismatch of localized stress between the two layers, and thus induce rapid responsive shape/volume transition. Different methods, including chemical vapor deposition (CVD),25 electrophoresis,29 microfluidics,30 and bonding of two different homogeneous hydrogel,31, 32 have been developed for bilayer hydrogel actuators, where the two layers are either attached or integrated via a physically or chemically-linked adhesion. Among these methods, bonding of the two individual as-prepared hydrogels together is considered as the most straightforward and convenient for the preparation of bilayer hydrogels. This method exhibits several advantages: (1) each individual hydrogel layer can be separately designed and prepared with different geometries and properties (e.g. thickness, composition, network structure, mechanical properties, self-recovery/self-healing properties, etc.) and (2) a wide variety of physical and chemical interactions (e.g. van der waal force, hydrogen bonds, electrostatic attraction, and host/guest chemistry) can be used to realize the strong bonding at the two-layer interface.2,

31, 33-35

However, this method usually consists of a two-stepwise

polymerization for preparing two hydrogel layers and a bonding process to join the two layers 3



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together (namely the three-step method), which is too somehow complex to easily and precisely manipulate local heterogeneous structures, thus limits their practical applications. So, the bonding process to join the two hydrogel together and to form a strong bilayer interface is challenging. To avoid this additional bonding process, a two-step method has been proposed, tested, and proved for the preparation of bilayer hydrogels. Briefly, once the first hydrogel layer was formed in the first step, the first hydrogel layer was then immersed into an aqueous solution containing all reactants used to form the second hydrogel layer, followed by in situ polymerization of the second hydrogel layer on the performed first hydrogel layer. During the second step, the immersion process would drive some recants to penetrate into the first hydrogel network and the subsequent polymerization would lead to the formation of a semi-interpenetrating network (semi-IPN) structure at the two-hydrogel interface. As a result, such semi-IPN structure at the hydrogel interface enables the two hydrogel layers to be adhered tightly one another. This two-step method has been used to fabricate several bilayer hydrogels including poly(dimethyl acrylamide)/poly(N-isopropyl acrylamide) (polyNIPAM),32, 36 poly(acrylic acid)/polyNIPAM,31, 37 and PolyMETAC/polyVBIPS bilayer hydrogels.38 These bilayer hydrogels showed fast shape/volume changes in response to temperature, ions, light, and pH. Despite these progresses, it is still highly desirable to further develop a simple, one-step method for the preparation of bilayer hydrogel actuators. Kim et al. proposed to use a microfluidic device to realize one-step preparation of a Janus bilayer hydrogel consisting of NIPAM-rich and NIPAM-poor layers. They injected NIPAM-rich and NIPAM-poor solutions into the microfluidic device, followed by UV polymerization to form Janus bilayer hydrogels.30 The resulting hydrogels exhibited a typical Janus characteristic and anisotropic thermo-responsive behavior. But, this one-step microfluidic-assisted method requires relatively complex equipment, handful skills, and unique Janus systems. Wang et al. recently developed a phase separation strategy to prepare bilayer hydro/organo hydrogels through the interfacial polymerization of immiscible hydrophilic and hydrophobic vinyl monomers.39 Due to the cooperative asymmetric swelling/shrinking of the hydrogel and organogel networks, the bilayer hydro/organo hydrogels showed a remarkable 4


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bending behavior in both aqueous solutions and organic solvents. However, this new design is only applied to the bilayer systems involving both hydrophilic hydrogel and hydrophobic organogel, and not to the bilayer hydrogel systems. So, the design and fabrication of bilayer hydrogels with high sensitivity, multi-responsiveness, and high mechanical properties still remain challenges.38, 40, 41 To overcome difficulties above, different from these multiple-step methods are time-consuming and required handful skills, we develop a simple one-pot method to fabricate tunable, fast, robust bilayer hydrogel actuators through in situ sequential radical polymerization in aqueous solution containing N-isopropyl acrylamide (NIPAM) monomer, VBIPS monomer, and cross-linker. Since NIPAM and VBIPS monomers have significant difference in polarization and polymerization activity, the two polymers are incompatible with each other in some physicochemical properties, resulting in a spontaneous phase separation and a unique pseudo-interpenetrating double-network structure at the interface to achieve high interfacial bonding strength42 . This one-pot process did not introduce any ancillary adhesives to bond the two bilayers, thus eliminating possible interfacial complexity that raises the outbreak of debonding. The resulting polyNIPAM/polyVBIPS bilayer hydrogels exhibited programmable and reversible shape changes in response to both salt- and temperature-stimuli. The bending degree, direction, and rate of bilayer hydrogels showed strong dependence on the salt concentration, counterion type, and temperature, demonstrating a controlled shape deformation triggered by both salt and temperature stimuli. Furthermore, we used polyNIPAM/polyVBIPS bilayer hydrogels to proof-design (i) a gripper to capture-transport-release small objective when switching from water to salt solution and from cold water to warm water and (ii) a sensor to control a circuit for turning on-and-off a light-emitting diode (LED). The knowledge developed from this simple, one-pot fabrication method and a new bilayer hydrogel system is possible to be expanded to other bilayer hydrogels, which feature smart actuators with programmable and versatile properties for many smart and intelligent soft-materials applications. Methods and Experiments 5



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Materials N-isopropylacrylamide (NIPAM, 97.0%) was purchased from Sigma-Aldrich and purified by recrystallization

from

hexanes.

3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (VBIPS) was synthesized according to our previous report,43-48 N,N,N′,N′-tetramethylethylenediamine (TEMED), ammonium

persulfate

(APS),

N,N′-methylene-bis-acrylamide

(MBAA),

and

2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (photo-initiator, 98%) were purchased from Sigma-Aldrich, All other chemicals and solvents were commercially obtained at extra-pure grade and were used as received. Water used in these experiments was purified by a Millipore water purification system with a minimum resistivity of 18.0 MΩ cm. Preparation of Hydrogels To prepare the bilayer hydrogel, monomers (NIPAM and VBIPS) were first dissolved in water with a pre-specified amount of APS (initiator), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropio phenone (photoinitiator), MBAA (cross-linker), and TEMED (accelerator). The resulting solution was then purged by N2 and gently injected into a mold prepared by separating two glass slides with a certain thickness of Teflon spacer. The polymerization was carried out under a 365 nm UV light at room temperature for 2 h to obtain the hydrogel. Five different hydrogels were prepared in the same manner by changing the weight ratio of NIPAM to VBIPS (7:3, 6:4, 5:5, 4:6, and 3:7). Characterization The hydrogel was freeze-dried for chemical structure, composition, and morphology measurements. The chemical structure was characterized by Fourier transform infrared spectroscopic (FT-IR) measurement using a Thermo Nicolet (Nicolet 6700) with resolution at 4 cm −1 and scans at 32. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Axis Ultra DLD spectrometer (Kratos Analytical) to determine the composition of each layer. The morphology of the hydrogel was characterized by observing the fracture section of the freeze-dried hydrogel using Olympus Stereoscopic Microscope (SZX16). The cross section was prepared by freeze-drying and cut under liquid nitrogen. The mechanical properties of the 6


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hydrogels were evaluated by a universal tensile tester (Instron MOD EL5567, MA) with a 100 N load 170 cell and a 100 mm min

−1

of crosshead speed at room temperature. The hydrogel

specimens were cut into a dumb bell shape (ASTM-638-V) with 25 mm gauge length, 3.18 mm width, and 1.3 ± 0.1 mm thickness. The hydrogel was fixed in clamp and then stretched to breakage. The tensile stress (σ) was calculated as σ = F/A0, where F is the load and A0 is the cross-sectional area of the original specimen. The tensile strain (ε) was defined as the elongation (Δl) relative to the initial length (l 0), ε =Δl/l 0 × 100%. The process of hydrogel deformation was recorded by a digital camera (Canon FS100 A). Results and Discussion Fabrication and Characterization of PolyNIPAM/polyVBIPS Bilayer Hydrogels

Scheme 1. Schematic illustration of the preparation of polyNIPAM/polyVBIPS bilayer hydrogel. Scheme 1 shows a one-step, one-pot synthesis procedure of a polyNIPAM/polyVBIPS bilayer hydrogels in aqueous solution using sequential radical polymerization. Briefly, NIPAM and VBIPS monomers, along with a certain amount of cross-linker, initiator, and accelerator, were dissolved in water, and then injected into a mold for photo-polymerization for 2 h. Upon the polymerization, hybrid polyNIPAM/polyVBIPS hydrogels were formed with either single layer or double

layer

structure,

depending

on

NIPAM:VBIPS

weight

ratios.

When

polyNIPAM/polyVBIPS hydrogels were prepared at the weight ratios of NIPAM:VBIPS of 5:5, 4:6, and 3:7, two gel phases were sequentially formed and the bilayer structures can be clearly 7



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observed, where one layer (bottom layer) was opaque while the other layer (upper layer) was transparent in water (Scheme 1). However, in the cases of NIPAM : VBIPS ratios of 6:4 and 7:3, polyNIPAM/polyVBIPS hydrogels only exhibited a single layer.

Figure 1. SEM images and sulfur elemental scanning of polyNIPAM/polyVBIPS hydrogels, prepared at NIPAM:VBIPS weight ratios of (a) 3:7, (b) 4:6, (c) 5:5, (d) 6:4, and (e) 7:3. All scale bars are 50 μm. Figure 1 shows the typical SEM images of polyNIPAM/polyVBIPS hydrogels prepared at NIPAM:VBIPS ratios of 3:7, 4:6, 5:5, 6:4, and 7:3 and treated by freeze-drying method. As observed by the cross-section SEM images (Figure 1a-c), polyNIPAM/polyVBIPS hydrogels, prepared at NIPAM:VBIPS ratios of 3:7, 4:6, 5:5, showed a bilayer structure with an visible interface in between. In all three hydrogels, polyNIPAM layer (orange color) exhibited a large porous structure, while polyVBIPS layer (green color) displayed a compact structure. Both large and small porous size structures were observed at the interfacial area, indicating that the interfacial area is composed of the interpenetrating polymer chains from both hydrogels. Differently, as NIPAM:VBIPS ratios increased to 6:4 and 7:3, both bilayer structure and interfacial boundary disappeared, indicating a single layer structure. So, polyNIPAM/polyVBIPS hydrogels showed the composition-dependent network structure, i.e. the more VBIPS being polymerized, the higher tendency to form the bilayer structure. When the content of VBIPS is below a critical ratio of 50 wt%, all monomers would be consumed for the copolymerization with NIPAM, thus no VBIPS 8


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monomers could emigrate from the polyNIPAM network to form the second layer. The formation of the bilayer structure is attributed to differences in polarity and polymerization activity between NIPAM and VBIPS. Computationally, our density functional theory (DFT) calculation showed that NIPAM and VBIPS monomers exhibited remarkably different dipole moment, 4.19 Debye and 15.22 Debye, respectively (Scheme 1). Experimentally, VBIPS showed significantly lower polymerization activity than NIPAM, as indicated by the fact that polyNIPAM hydrogels were formed in several minutes while polyVBIPS took several hours to polymerize into hydrogels (Figure S1). However, when blending VBIPS and NIPAM together in a certain ratios, we found that most of VBIPS monomers copolymerized with NIPAM monomers in the first polymerization and separated from the solution, forming the first hydrogel layer (polyVBIPS layer). The retained NIPAM monomers were then polymerized in second stage and formed the second hydrogel layer (polyNIPAM layer). Considering that polyVBIPS contains sulfur (S) element while polyNIPAM does not, elemental scanning (EDX) of sulfur element at the cross-section of the hydrogels was performed to qualitatively measure the composition of each layer. As shown in the sulfur intensity profiles in the bottom line of Figure 1, the hydrogels prepared at NIPAM:VBIPS ratios of 3:7, 4:6 and 5:5 exhibited a stepwise increase of sulfur intensity from the polyNIPAM layer to the polyVBIPS layer, while the hydrogels prepared at NIPAM:VBIPS ratios of 6:4 and 7:3 showed almost constant sulfur intensity distribution along the cross-section of the hydrogels, indicating that increase of NIPAM:VBIPS ratio indeed eliminates the bilayer structure, further confirming SEM results. Meanwhile, difference in sulfur intensity between the two layers indicates not only different cross-linking density between the two layers, but also the interpenetration networks at the two-layer interface.

9



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Figure 2. FTIR spectra of pristine polyNIPAM, polyVBIPS, and hybrid polyNIPAM/polyVBIPS hydrogels prepared at different NIPAM:VBIPS ratios. FTIR spectroscopy was used to characterize the chemical structure of both polyNIPAM and polyVBIPS layers of polyNIPAM/polyVBIPS hydrogels, as compared to pristine polyNIPAM and polyVBIPS hydrogels (Figure 2). As a control, polyNIPAM hydrogel had very strong absorption bands locating at 1365 cm-1 and 1386 cm-1 corresponding to the antisymmetric deformation of the isopropyl, while polyVBIPS hydrogel had characteristic peaks at 1511 cm-1 ,1424 cm-1, and 1036 cm-1 corresponding to the symmetric stretch of the phenyl group, C-N stretching vibration from imidazole group, and sulfonic acid group, respectively. In the cases of polyNIPAM/polyVBIPS hydrogels with NIPAM:VBIPS ratio of 6:4, the upper layer and bottom layer of the hydrogel showed almost identical spectra, indicating the uniform network structure across the whole hydrogel. In contrast, polyNIPAM/polyVBIPS hydrogel prepared at NIPAM:VBIPS ratio of 4:6 exhibited different spectra at 1511 cm-1 and 1424 cm-1 for the polyNIPAM layer and the polyVBIPS layer. In particularly, the characteristic absorptions at 954 cm–1, 1098 cm–1, 1344 cm– 1

, and 2882 cm–1 coming from the upper polyVBIPS layer were more pronounced than those from

the bottom polyNIPAM layer, confirming the bilayer structure.

10


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Figure 3. (a) XPS survey spectra of each side of bilayer hydrogels and the corresponding high resolution spectra of N1s for (b) polyNIPAM layer and (c) polyVBIPS layer of the bilayer hydrogel. X-ray photoelectron spectroscopy (XPS) was further performed to analyze and compare the chemical compositions of the two layers of bilayer hydrogels. The XPS survey spectrum (Figure 3a) shows characteristic peaks of carbon (C1s), nitrogen (N1s) oxygen (O1s), and S2p for both layers, in agreement with the expected chemical structure of both layers. However, comparison of the high-resolution spectrum of the N1s between polyNIPAM layer (Figure 3b) and polyVBIPS layer (Figure 3c) revealed obvious composition differences between the two layers. In both layers, the N1s region showed the two major peaks of ~396.2 eV and 398.1 eV, corresponding to nitrogen from -NHCO- and imidazolium, respectively. Clearly, the peak intensity ratio of the polyVBIPS layer, defined as the 398.1 eV peak vs. the 284.5 eV peak, was much higher than that of the polyNIPAM layer, indicating that more porous and compact polyVBIPS are formed in the upper layer.

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Salt- and Thermal-Induced Actuation Behavior of PolyNIPAM/polyVBIPS Bilayer Hydrogels PolyNIPAM is a well-known thermo-responsive polymer with a low critical soluble temperature (LCST) of ~32 oC. Thus, the most appealing feature of polyNIPAM-based hydrogels is their volume phase transition across the lower critical solution temperature (LCST), i.e. polyNIPAM hydrogel swells at temperatures < LCST while shrinks at temperatures > LCST. In addition, the presence of ions also influences the LCST of polyNIPAM, i.e. LCST will decrease in the presence of ions, leading to the volume shrink at polyNIPAM. As shown in Figure 4, for zwitterionic polyVBIPS, we have reported its anti-polyelectrolyte property,44, 47 i.e. polyVBIPS shrinks in water, but swells in salt solution. Meanwhile, polyVBIPS chains adopt coil conformation at temperatures < UCST, but extended conformation at temperatures > UCST. Taken together, both polyVBIPS hydrogel and polyNIPAM hydrogel can change their volumes in response to temperature or salt solution, but in a complete opposite way, which is expected to achieve bi-directional bending to an extremely large extent.

Figure 4. Schematic illustration for bending behavior of the polyNIPAM/polyVBIPS bilayer hydrogel in response to temperature and salt. To test our hypothesis, Figure 5 shows the shape transitions (bending) of polyVBIPS/polyNIPAM hydrogels with single or double layer structures in response to salt solution (from pure water to 1.0 M NaCl solution) and temperature (pure water from 25 oC to 55 o

C). In Figure 5a, to define the bending angle, we located one edge of a hydrogel on the original

point in a coordinate system where a vertical tangent was applied to the edge of the hydrogel. Upon 12


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the bending, a bending angle is determined between a vector connecting the original point and another bending edge of the hydrogel and a vertical tangent. Accordingly, a negative bending angle is defined as the bilayer hydrogel bends toward the polyVBIPS side, while a positive bending angle is vice versa. In some cases, if the hydrogel bend more than two circles, the bending angle is more than ±360o. It can be clearly seen in both Figure 5b and Figure 5c that polyVBIPS/polyNIPAM single-layer hydrogels, prepared at 7:3 and 6:4 ratios of NIPAM and VBIPS, retained their original straight shapes and were insensitive to external stimuli, while all bilayer hydrogels exhibited large and sensitive shape adaptivity in response to salt solution. For instance, the bilayer hydrogel prepared at NIPAM:VBIPS ratio of 5:5 exhibited a large bending angle of ~-180o (bending toward polyVBIPS layer) in water due to the swelling of polyNIPAM (hydrophilic characteristic) and non-swelling of polyVBIPS (hydrophobic characteristic). When placing this bilayer hydrogel into 1.0 M NaCl solution, the hydrogel with a bending angle of -180o bent towards the opposite polyNIPAM layer and finally reached to a bending angle of ~200o. Such large bending degree is attributed to the synergistic effect of the swelling of polyVBIPS layer and the shrinking of polyNIPAM layer in salt solution. Similar solvent-induced bending behaviors for other two bilayer hydrogels were also observed (Figure 5b). In Figure 5c, bilayer hydrogels also showed temperature-induced bending behaviors. The hydrogel (prepared at NIPAM:VBIPS ratio of 5:5) in 25 oC of water bent ~350o towards polyVBIPS side, but upon immersing the hydrogel in 55 oC of water, it exhibited an opposite bending towards polyNIPAM side with a bending angle of ~130o. Increase of temperature leads to the shrinkage of polyNIPAM layer (above LCST) and the swelling of polyVBIPS (above UCST) simultaneously, both of which offer cooperative driving force to facilitate the bending behavior of bilayer hydrogels. Moreover, bilayer hydrogels also showed the component-dependent bending behaviors. As NIPAM:VBIPS ratios decreased from 5:5, 4:6, to 3:7, bilayer hydrogels underwent more pronounced bending changes from -180o to 220o, from -380o to 210 o, and from -310o to 340o when switching from water to 1.0 M NaCl solution, as well as from -230o to 164o, from -384 o to 200o, and from -350o to 370o when switching from 25 oC of water to 55 oC of water. In addition, we tested and measured the bending behaviors 13



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in two ways: (1) one is fixed while the other end is free to bend and (2) both ends are free to bend. No significant difference in bending angles was observed for the two testing cases (data no shown). Considering in some cases a large bending angle is expected where the hydrogel can be bended in several circles, so we conducted all bending experiments for the hydrogels with both free ends.

Figure 5. (a) Scheme for the definition of bending angle. Bending behavior of polyNIPAM/polyVBIPS bilayer hydrogels, prepared at different NIPAM:VBIPS ratios, actuated by (b) salt solution and (c) temperature. All scale bars are 5 mm. To further quantify the bending kinetics of bilayer hydrogels in response to the changes from water to salt solution and from cool water to hot water, Figure 6 shows the time-dependent saltand thermal-induced bending of bilayer hydrogels. In Figure 6a, three bilayer hydrogels experienced a two-stage bending, i.e. bilayer hydrogels initially adopted a negative curvature in water, once placing them into 1.0 M NaCl solution, they rapidly changed its initial bending of -180o, -350o, and -380 o to a straight shape of 0o within ~1 min (the first stage), then continuously and gradually increased its bending to a positive curvature (210o-340o) in ~10 min. (the second stage). So, the salt-induced bending rates decreased from ~3.3o /s - ~0.8o/s at the first stage to ~0.8o/s - ~0.55o/s at the second stage. Overall, the trend of bending of negative 14


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curvaturestraightpositive curvature was consistent for all bilayer hydrogels, but the degree of bending is depended on their compositions. When comparing the bending kinetics of bilayer hydrogels in different environments, the hydrogels exhibited the faster bending in response to temperature changes. In Figure 6b, for the same bilayer hydrogels, bending rates of the hydrogels with NIPAM:VBIPS ratios of 5:5, 4:6, 3:7 were ~3o/s, ~3.9o/s, and ~6.5 o/s at the first stage (negative curvaturestraight) and ~0.5o/s, ~0.9o/s, and ~1.1o/s at the second stage (straightpositive curvature), respectively. Particularly, at the first stage thermal-induced bending rates were as almost twice as faster than salt-induced ones. Also, bilayer hydrogels showed higher thermally

induced

bending

rates

as

VBIPS

content

increased,

demonstrating

the

composition-dependent bending behaviors. The higher bending rate is likely attributed to copolymerization of more VBIPS monomers with NIPAM monomers, which leads to the increase of polarity incompatibility between the two layers, phase separation and shape change.

Figure 6. Bending kinetics of polyNIPAM/polyVBIPS bilayer hydrogels, prepared at different NIPAM/VBIPS ratios, in response to the environmental changes (a) from water to 1.0 M NaCl solution and (b) from cool water at 25 oC to hot water at 55 oC.

To further understand the bending behavior of bilayer hydrogels, we examined the simultaneous effect of both temperature and salt concentration changes on the bending behavior of bilayer hydrogels prepared at a NIPAM:VBIPS ratio of 4:6. To test this simultaneous effect, the bilayer hydrogel was first immersed into cold water, then transferred to 1.0 M NaCl solution at 55 o

C. As shown in Figure 7, a combination of salt and temperature stimuli led to the larger bending 15



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angle of 300oC towards polyNIPAM layer, which was much higher than that of ~210oC under either 1.0 M NaCl or 55oC water, as well as the faster bending rate of ~4.9o/s (particularly at the initial stage from negative curvature to straight conformation) than that of ~2.6o/s triggered by individual stimuli.

Fgiure 7. Bending kinetics of polyNIPAM/polyVBIPS bilayer hydrogels, prepared at a NIPAM/VBIPS ratio of 4:6, in response to in response to salt, temperature, and a combination of salt and temperature stimuli.

Due to zwitterionic nature of polyVBIPS layer, the dependence of salt concentrations and salt types on the bending behavior of polyNIPAM/polyVBIPS bilayer hydrogels was examined. In Figure 8a, the bilayer hydrogel initially bent with ~-370o as a closed circle in water. When switching to NaCl solution, as NaCl concentration increased from 0.05 M to 0.53 M (similar to sea water concentration), the hydrogel gradually changed its bending from a cycle structure (-370o), a half-cycle structure (-180o), a curved structure (-50o), to a linear structure (0o). Continuous increase of NaCl concentration induced the opposite and positive curvature of ~180o at 0.53 M, ~380o at 1.0 M and ~440o at 6.1 M. Furthermore, Figure 8b-c shows the effect of various salts on the bending behavior of the bilayer hydrogels. Clearly, different cations and anions induced different bending of the bilayers. In the case of anion effect, three monovalent anions of Cl−, Br−, and NO 3− induced the positive bending angles of ~380o, ~210o and ~210o, while divalent anion of SO42- caused the negative bending angle of -420o. Such anion-induced bending difference could be 16


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due to Hofmeister anion effect.44, 49 Cl−, Br−, and NO3 − are typical chaotropic anions and they can interact strongly with imidazolium groups in polyVBIPS, which leads to the weaker inter-/intrachain dipole-dipole interaction and the stronger interaction between sulfonate groups and water molecules, and thus the higher hydration (swelling) and more extend conformation of polymer chains. As a result, the hydrogels bend towards the polyNIPAM layer with the positive curvature. Differently, SO42- as a typical kosmotropic anion possesses high hydration capacity, which competes with water-polyVBIPS interactions and leads to the shrinking of this layer. As a result, the hydrogel further bended towards the polyVBIPS layer to form an even smaller circle than that in water. In Figure 8c, we also examined the effect of bending behaviors of polyNIPAM/polyVBIPS bilayer hydrogels in the presence of different cations (K+, Na+, Mg2+, and Ca2+) and a common anion of Cl- at 1.0 M. All cations actuated the bending of the hydrogels toward the opposite direction relative to that of the hydrogels in water solution. The bending angles of the hydrogel in K+, Na+, Mg2+, and Ca2+ were ~385o, ~380o, ~88o, and ~240o, respectively, as compared to ~-370o in water, where monovalent cations show the stronger actuation than divalent cations in the ordering of K+ > Na+ > Ca2+ > Mg2+.

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Figure 8. Bending behaviors of bilayer hydrogels prepared at NIPAM:VBIPS ratio of 3:7 in response to (a) NaCl concentrations, and different salt types containing (b) different anions of NO3−, Br−, Cl−, and SO42− with the same Na + at 1 M and (c) different cations of K+, Na +, Mg2+, and Ca2+ with the same Cl− at 1.0 M. Since both polyNIPAM and polyVBIPS are sensitive to LCST and UCST temperatures, respectively, we studied the bending behavior of different polyNIPAM/polyVBIPS hydrogels at a range of temperature from 15 oC to 60 oC, which cover the LCST of 32 oC and UCST of 55 oC. In Figure 9, three bilayer hydrogels, prepared at NIPAM:VBIPS ratios of 5:5, 4:6, and 3:7, exhibited similar bending trends from negative bending angles to positive ones as temperature increased from 15 oC to 60 oC, showing S-shape-like curve. Overall, at a low temperature range of 15-30 oC that are lower than LCST of polyNIPAM, three bilayer hydrogels almost maintained their initial bending independent of temperatures. But, at a middle temperature range of 35-45 oC that are higher than LCST of polyNIPAM, the hydrogels experienced large bending changes in an opposite direction. Such bending actuation mainly arises from thermal response of the polyNIPAM layer, which is contracted in warm water but expanded in cold water. As the temperature was above 55 18


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o

C, the bending angles almost kept unchanged again. Moreover, different bilayer hydrogels

showed different the temperature-dependent bending angles, indicating composition effect. Different compositions would affect both LCST and UCST values of the resulting bilayer hydrogels, leading to different temperature-triggered bending.

Figure 9. Thermo-induced bending behavior of the polyNIPAM/polyVBIPS bilayer hydrogels prepared at NIPAM:VBIPS ratios of 5:5, 4:6 and 3:7. Reversible and Repeatable Salt- and Thermal-Induced Bending of PolyNIPAM/polyVBIPS Bilayer Hydrogels Controllable and programmable actuation requires the reversible and recoverable behaviors for practical soft actuators and robots. Here, we further examined the reversible bending of bilayer hydrogels in response to solvent and temperature changes. Figure 10a shows the reversible bending of the bilayer hydrogel when putting the hydrogel in and out of water and 1.0 M NaCl solution. The bilayer hydrogel initially bent -370o in water and ~365o in 1.0 M NaCl solution. When repeatedly switching the solution between water and NaCl solutions, the hydrogel can completely recovered its initial bending of -370o in water and ~365o in NaCl solution within only ~5 min in 5 cycles, demonstrating a fast and reversible actuation. Consistently, as shown in Figure 10b, the bilayer hydrogel also exhibited the temperature-induced bending of -350o in 25 oC water and ~380o in 55 oC water, which was highly reversible and repeatable. No changes in the bending curvature, response time, and recovery time were observed within 5 cycles, supporting that the 19



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reversible actuation has a perfect repeatability.

Figure 10. Reversible bending of polyNIPAM/polyVBIPS bilayer hydrogels with 1.0 mm thickness prepared at a NIPAM:VBIPS ratio of 3:7 with the environments switched (a) between in water and in 1.0 M NaCl solutions and (b) between 25 oC and 55 oC in water.

In addition, high mechanical property is also highly desirable for the actuating hydrogels to maintain their efficient action in response to external stimuli.50-52 Figure 11a shows typical stress-strain curves of three different polyNIPAM/polyVBIPS bilayer hydrogels in water and in salt solution. It can be seen that salt-treated hydrogels exhibited much higher tensile stress, but the lower tensile strain, than the corresponding water-treated hydrogels. Regardless in water or in salt solution, increase of polyNIPAM content led to high mechanical strength. In Figure 11b, the salt-treated bilayer hydrogels exhibited excellent compressive mechanical strength, which can be compressed by as high as 2.25 MPa at a fracture strain of 60%. In contrast, the water-treated bilayer hydrogels were still weak and can not sustain even 0.2 MPa compression. Taking NIPAM:VBIPS ratio of 4:6 hydrogel as an example, the salt-treated hydrogels (2.179 MPa) showed ~60 times higher compression stress than the water-treated hydrogels (0.034 MPa). Thus, while both composition and environment affect the mechanical properties of bilayer hydrogels, the salt-treated bilayer hydrogels generally possess the stronger mechanical properties than the water-treated ones. Such stress enhancement should be attributed to the synergistic effect of the Hofmeister ions and the changes of LCST of polyNIPAM. 20


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Figure 11. Comparison of (a) tensile stress-strain and (b) compression stress-strain curves of polyNIPAM/ polyVBIPS bilayer hydrogels in water and in 1.0 M NaCl solution.

PolyNIPAM/polyVBIPS Bilayer Hydrogels Used as Prototype Soft Actuators Actuation force is also another important parameter to assess the performance of hydrogel actuators. Here, we roughly estimated the actuation force of the hydrogel by lifting an object of 0.75 g (~23 times of the dried hydrogel weight of 0.033 g), which was fixed at one end of the hydrogel. As shown in Figure S4, the initial length of hydrogel and object was 4.9 cm in 25 oC water, and this length was then reduced to 4.6 cm after ~10 min due to the shrinkage of polyVBIPS layer. But, when immersing the hydrogel into 1.0 M NaCl solution, the increased mechanical strength of the hydrogel led to a larger actuation force under which the object was lifted ~1.2 cm. Considering the much higher actuation force in salt solution than in water solution, we further designed and fabricated a six-arm gripper made of the polyNIPAM/polyVBIPS bilayer hydrogel, in which upper layer consists of polyVBIPS and the bottom layer consists of polyNIPAM (Figure 12). The gripper arms bent upwards in 25 oC water due to the cooperative driving forces from the swelling of the polyNIPAM layer and the shrinkage of the polyVBIPS layer. When transferring the gripper to salt solution or warm water, the gripper gradually bent its arms downward to grasp the rubber and hold tightly for further transport back to 25o water, where the gripper unfolded and released the rubber, as well as recovered to its original upward arms. Movie S1 and Movie S2 in Supporting Information recorded the entire process of capturing, transporting, and releasing the rubber in response to solvent and temperature changes, respectively. By weighting the rubber that 21



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the hydrogel grasped, we found that the gripper can grasp, transfer and release the object with the weight of 2.3 g (~20 times of the dried hydrogel of 0.115g).

Figure 12. (a) Scheme for a programmable six-arm gripper actuated by salt and temperature. (polyVBIPS layer is colored as blue while polyNIPAM layer as pink). (b) A entire actuation procedure for the gripper to capture, transport, and release a rubber in response to solvent (left) and temperature (right) changes. All scale bars are 20 mm. Based on bidirectional bending behavior of polyNIPAM/polyVBIPS bilayer hydrogel under different environments, we further prepared a hydrogel actuator as sensing elements to precisely control its bending direction for turning on-and-off the circuit. It can be seen in Figure 13 that the bilayer hydrogel actuator bent towards polyVBIPS side in 25 oC water to reach a switch that turns on a green lamp. When putting the bilayer hydrogel strip in 1.0 M NaCl solution or warm water (55 o

C), the hydrogel bent towards polyNIPAM side to form a new electric circuit, resulting in

switching off the green lamp but switching on the red lamp. Once the water temperature restored to original state, the red lamp was extinguished again while the green lamp was switched on. Thus, bidirectional bending of this actuator induced by either salt increase (1.0 M NaCl for ~100 s) or heating (55 °C water for ~60 s) enables to realize an electrical switch at a conceptual level.

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Figure 13. (a) Conceptual schematic of a hydrogel-mimic electrical switch. (b) Experimental demonstration of polyNIPAM/polyVBIPS bilayer hydrogel as a smart switch to turn on-and-off two lamps in response to solvent and temperature changes. Conclusions In this work, we presented a facile, one-pot method to fabricate polyNIPAM/polyVBIPS bilayer hydrogels by taking advantage of large differences in polymerization activity and polarity between two monomers and polymers. Both polyNIPAM and polyVBIPS layers showed swelling/shrinking behavior in response to salt solution and temperature changes, but in a completely opposite way. As a result, by combining thermos-responsive polyNIPAM and salt-responsive polyVBIPS together and tuning polyNIPAM:polyVBIPS ratios, the resulting polyNIPAM/polyVBIPS bilayer hydrogels can achieve different bidirectional bending orientation, curvature, rate in response to different temperatures, salt types, and salt concentrations. Such bidirectional and programmable bending can be reversibly and repeatedly controlled by the cooperative swelling (shrinkage) of the polyNIPAM layer and the shrinkage (swelling) of the polyVBIPS layer, thus acting as promising candidates for soft actuators. As a proof-of-concept, we further prepared two hydrogel actuators with rapid response and precise control of the acting direction, including a six-arm gripper to capture, transport, and release an object and a hydrogel 23



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switch to turn on-and-off a circuit. This facile method and new bilayer hydrogel system would be highly useful in on-demand design and fabrication of smart hydrogel-based actuators with programmable and versatile properties. SUPPORTING INFORMATION Supporting Information includes the equilibrium water content of polyNIPAM/polyVBIPS bilayer hydrogels at different NaCl concentrations and temperatures, and the bending angle of polyNIPAM/polyVBIPS bilayer hydrogels of different thicknesses. The two movies record a gripper to capture, transfer, and release an small object in response to solvent and temperature changes. Acknowledgement. J.Y. thanks financial support from Natural Science Foundation of China (No.51673175), Natural Science Foundation of Zhejiang Province (LY16E030012), and Zhejiang Top Priority Discipline of Textile Science and Engineering (2015KF06). J.Z. thanks financial support from NSF (DMR-1607475 and CMMI-1825122).

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FIGURE OF CONTENT

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Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel

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