One-Pot and One-Step Fabrication of Salt-Responsive Bilayer

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Applications of Polymer, Composite, and Coating Materials

One-pot and One-step Fabrication of Salt-responsive Bilayer Hydrogels with 2D and 3D Shape Transformations Xiaomin He, Dong Zhang, Jiahui Wu, Yang Wang, Feng Chen, Ping Fan, Mingqiang Zhong, Shengwei Xiao, and Jintao Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06691 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

One-pot and One-step Fabrication of Salt-responsive Bilayer Hydrogels with 2D and 3D Shape Transformations Xiaomin He†, Dong Zhang¶, Jiahui Wu†, Yang Wang†, Feng Chen†, Ping Fan†, Mingqiang Zhong†, Shengwei Xiao*‡, Jintao Yang*† † College

of Materials Science& Engineering Zhejiang University of Technology, Hangzhou 310014, China ‡

¶

School of Pharmaceutical and Chemical Engineering Taizhou University, Jiaojiang 318000, China

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

*Corresponding Author: (J. Y.) [email protected]; (S.X.) [email protected] Keywords: one-step, one-pot, zwitterionic polymer, bilayer hydrogel, 3D shape transformation.

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ABSTRACT Bilayer hydrogels are one of most promising materials for use as soft actuators, artificial muscles, and soft robotic elements. Therefore, the development of new and simple methods for the fabrication of such hydrogels is of particular importance for both academic research and industrial applications. Herein, a facile, one-pot, and one-step methodology was used to prepare bilayer hydrogels. Specifically, several common monomers, including N-isopropyl acrylamide (NIPAM), acrylamide (AAm), and N-(2-Hydroxyethyl) acrylamide(HEAA), as well as two salt-responsive zwitterionic monomers,

3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate

(VBIPS),

and

dimethyl-(4-vinylphenyl) ammonium propane sulfonate (DVBAPS), were chosen and employed with different combinations and ratios to understand the formation and structural tunability of the bilayer hydrogels. The results indicated that a salt-responsive zwitterionic-enriched copolymer, that could precipitate from water, plays a dominant role in the formation of the bilayer structure, and that the ratio between the common monomer and the zwitterionic monomer had a significant effect on the structure. Due to the salt-responsive properties of polyVBIPS and polyDVBAPS, the resultant bilayer hydrogels exhibited excellent bidirectional bending properties in response to the salt solution. With the optimal monomer pair and ratio determined, the bend of the hydrogel could be reversed from ~ -360 o to ~ 266 o

in response to a switch between water and 1.0 M NaCl solution. Additionally, this method was further

used to fabricate small-scaled patterns with structural and compositional distinction in twodimensional hydrogel sheets. These two-dimensional hydrogel sheets exhibited complex and reversible three-dimensional shape transformations due to the different bending behavior of the patterned hydrogel stripes under the action of an external stimulus. This work provides greater insight into the mechanism of the one-step, one-pot method fabrication of bilayer hydrogels, demonstrates the ability of this method for the preparation of small-scale patterns in hydrogel sheets to endow the complex with three-dimensional shape transformation capability, and hopefully opens up a new pathway for the design and fabrication of smart hydrogels.

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INTRODUCTION The asymmetric bilayer structure is considered to be one of the most important motifs for the design of smart hydrogels. 1-4 Due to the differential swelling/shrinking of each layer under an external stimulus, the uniform volumetric expansion and contraction of the isotropic hydrogels is transferred into shape transformations such as bending, twisting, and folding. 5-7 These active motions endow the bilayer composite hydrogels with great potential for use in applications such as artificial muscles, actuators, and soft robotics. 8-10 Apart from their importance in practical applications, the principles of the design of bilayer hydrogels, such as asymmetric internal stress and mismatch of the strain between the two layers, as well as the osmotic flow of water, are also of interest for fundamental research. Due to the importance of bilayer hydrogels in both fundamental research and practical applications, much research concerning the development of new fabrication methods, structural design, and responsive polymer systems, has been conducted. 11-14 Concerning the fabrication of bilayer structures, employing two hydrogels with distinct composition and/or network structures into one composite is the most straightforward method.

4, 15-19

This method exhibits many advantages, including: (1) Each

layer of the hydrogel can be separately designed with optimized thickness, composition, and network structure; (2) a variety of interactions, such as van der waal forces, hydrogen bonds, electrostatic attractions, and host/guest chemistry can be used to achieve bonding, showing high versatility. However, the three-step process of this methodology, composed of two polymerizations and one bonding process, is complicated, and the interface is somewhat weak, becoming the main factor that leads to deterioration of the mechanical properties.

20

A two-step method, in which the second layer

hydrogels are fabricated in situ according to the first layer hydrogel, has been developed. 3-5 ,21 In this case, the solution containing the reactants partially penetrates the first hydrogel during the polymerization process. As a result, a semi-penetrable structure is formed at the interface, in which the two hydrogel sheets are stuck tightly together, even during swelling in solvents. Many systems, such as

poly(dimethyl

acrylamide)/poly(N-isopropyl

acrylamide)

(PNIPAM),

poly(acrylic

acid)/polyNIPAM, and polyMETAC/polyVBIPS bilayer hydrogels, have been prepared using this method. 4-5

4


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Although extensive studies have been directed towards the aforementioned three-step and two-step methods, the fabrication of a two-layer hydrogel in one step has not been widely reported. To the best of our knowledge, one of the only methods that can be regarded to be one-step is the microfluidic synthesis process which has been used to prepare Janus microhydrogels.

22-24

Based on the

characteristics of NIPAM, in which a supersaturated aqueous NIPAM solution will automatically separate into two immiscible phases, i. e., NIPAM-enriched and NIPAM-poor phases, Kim et al.

23

prepared Janus microhydrogels by injecting two distinct streams of NIPAM-enriched and NIPAMpoor solutions into a hydrodynamic focusing microfluidic device. Subsequently, UV irradiated polymerization and cross-linking reactions were conducted. These Janus hydrogels exhibited an anisotropic thermal responsive behavior. This method has been extended to many Janus systems, but sophisticated equipment and delicate techniques are required, and in addition, it is strictly limited to the fabrication of macro-scale hydrogels. Using the phase separation strategy, Wang et al 2 recently developed a binary hydro/organo macro copolymer (e.g. hydrogel and organogel) via an interfacial polymerization of hydrophobic and hydrophilic vinyl monomer solutions. Due to the cooperative asymmetric swelling/shrinking of the hydrogel and organogel networks, the copolymer showed remarkable bending behavior in both aqueous solutions and organic solvents. This intelligent design opens up a new pathway for the development of smart materials. However, only a hydrogel and organogel combination can currently be prepared due to difficulties associated with complete phase separation.

The three-dimensional (3D) shape transformation of planar hydrogel sheets is another new research direction for the preparation of responsive hydrogels with self-actuating abilities.

6, 11, 25-28

This

evolution in shape adaptiveness greatly extended the applications of smart hydrogels as more complex systems.

11, 30-32

Generally, the 3D shape transformation of planar hydrogels is induced by carefully

modulating small-scale internal stresses, for which an intricate design and integration of multiple small-scale components with different compositions or characteristics in one composite hydrogel, are needed.

4, 27, 33-36

Photolithography is an effective approach to creating small-scale multiple

components (e.g. patterned heterogeneity) in hydrogel sheets, but usually, multiple steps are required. For example, in the work reported by Nie et al., 36 four steps were carried out to form a patterned binary hydrogel in a primary gel. These steps included: 1) The first photoinitiated polymerization of NIPAM 5



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to prepare the primary gel; 2) Swelling by another pre-gel solution; 3) A second photo-initiated polymerization using a mask, and; 4) immersion in water to remove unreacted monomer and free polymer molecules. The distinct swelling and shrinking between the primary and binary hydrogels result in internal stress and leads to transformation from a planar sheet to a helix. Some new methods, 37

such as employing the use of locally reducing graphene oxide (GO) in a GO-PNIPAM composite

hydrogel sheet, and tuning the cross-linking density in a specific region of the same hydrogel network, have been developed to simplify the fabrication process. Therefore, a simple but versatile approach is also highly desirable for the fabrication of such hydrogel sheets.

In this study, a one-step and one-pot method was employed for the fabrication of bilayer hydrogels which showed 2D bending behavior, and planar hydrogels with patterned stripes of different structure that showed 3D shape transforming abilities. Three common monomers and two salt-responsive zwitterionic monomers were used to investigate the formation of bilayer structures. It was shown that all of the combinations between common monomers and zwitterionic monomers could form bilayer hydrogels, indicating the high versatility of this approach. The resultant bilayer hydrogels showed bending movement in response to a salt solution. The ratio and copolymerization behavior between the two monomers showed a significant effect on the bilayer structure and bending performance. Additionally, by employing a mask during the photo-initiated polymerization, a planar hydrogel with periodic stripes possessing a different bilayer structure (different thickness ratios and composition between the two layers) was prepared in one step and one pot. This hydrogel could undergo transformation from a planar sheet to a helix in response to a thermal stimulus and salt solution. The helical structure could be easily tuned by changing the size and angle of the stripes, as well as its environment. By designing sophisticated patterns, more complicated 3D shape transformations could be achieved.

EXPERIMENTAL SECTION Materials. N-isopropyl acrylamide (NIPAM), acrylamide (AAm), N-(2-Hydroxyethyl) acrylamide (HEAA), N,N,N′,N′-tetramethyl ethylenediamine (TEMED), ammonium persulfate (APS), N,N′-methylene-bisacrylamide (MBAA), and 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (photo-initiator, 6


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98%) were purchased from Sigma-Aldrich. Before use, NIPAM was purified by recrystallization from hexanes. 3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (VBIPS) and dimethyl(4-vinylphenyl) ammonium propane sulfonate (DVBAPS) was synthesized according to our previous report. 5, 14, 38-43 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, a pre-specified amount of monomer(0.3 g/ml), cross-linking agent (MBAA, 1.0 mg/ml), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (photo-initiator, 5 mg/ml), APS (thermos-initiator, 3.0 mg/ml), and accelerator (TEMED, 0.7 mg/ml) were dissolved in water to prepare the pre-gel solution. The resultant solution was purged by N2 for 10 min to remove the oxygen, and then gently injected into a mold prepared by separating two glass slides with a 1.0 mm Teflon spacer. The dimension of the mold is 60×25×1 mm (length×width×thickness). The system was placed at room temperature for 3 h thermal polymerization, and then irradiated by a 365 nm UV light for 3 h of photopolymerization. After the polymerization, the resultant hydrogel was taken out from the mold and immersed in pure water to remove the free polymers and unreacted monomers, and cut into a strip with size of 30×2 mm (length×width) for bending/unbending behavior measurements. To prepare the planar hydrogel sheet with periodic stripes, a mask was first placed on the top of the glass mold. The system was directly placed under a 365 nm UV light for 3 h, where both photopolymerization and thermal polymerization occurred at the exposed area. Subsequently, the mask was removed and the polymerization was continued for 3 h. The post-treatment of the planar hydrogel sheet was similar to that of the bilayer hydrogels.

Characterization. A similar recipe as to that of the pre-gel solution but without a cross-linker was prepared. The copolymers in the bottom layer and upper layer were collected, and the composition was characterized by 1H-NMR (Bruker, ANANCE III 500M). The hydrogel was freeze-dried for morphology measurements. The freeze-dried hydrogel was brittle fractured in liquid nitrogen, and the cross-section was observed by SEM (FEI Nova Nano 450). The bending and 3D transformation of the hydrogel were 7



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recorded by digital camera (Canon FS100A) and the bending angles were calculated using Protractor software. RESULTS AND DISCUSSION

Figure 1. (a) Schematic illustration of the fabrication of the bilayer hydrogel. (b) Images of the pre-gel solution, formations of the first layer and second layer, as well as the image of the bilayer hydrogel in water; (c) 1H-NMR spectra of the copolymers; (d) SEM image and (e) EDS measurement for the distribution of element S across the thickness of the polyHEAA/polyVBIPS bilayer hydrogel.

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3.1 Fabrication of Bilayer Hydrogels. Figure 1a represents the preparation of bilayer hydrogels by a one-pot method. In this case, the pre-gel solution underwent two sequential polymerizations, i.e., thermal polymerization and photo-polymerization, where it was found that an opaque hydrogel layer at the bottom was first formed at the bottom after thermal-polymerization, and the upper layer solution was transformed into hydrogel by the subsequent-photo polymerization, forming a bilayer structure. Herein, the system consisting of HEAA and VBIPS, with a weight ratio of 4:6 (HEAA:VBIPS), was chosen to demonstrate the process of formation of the bilayer structure, as shown in Figure 1b. When the hydrogel was placed in water, the hydrogel curved, and two distinct layers (opaque layer and semitransparent layer) were observed. Considering the poor solubility of polyVBIPS in water, 39 we speculated polyVBIPS might be the primary component in the bottom layer (e.g. the opaque layer in water). To confirm this hypothesis, a solution with a similar recipe to that of the pre-gel solution, but without the crosslinking agent, was employed and polymerized as an analogous system. The sediment formed in the first step and the subsequently formed polymer, respectively corresponding to the bottom layer and upper layer in the bilayer hydrogel, were collected and characterized by 1H NMR analysis (performed with D2O salt solution as a solvent). As shown in Figure 1c, the resonances for the CH2N group at δ = 5.46 ppm and for CH2OH at δ =3.47 ppm are characteristic resonances of polyVBIPS and polyHEAA, respectively. It can be seen that the characteristic resonance of polyHEAA is difficult to observe in the spectrum of the copolymers of the bottom layer, indicating the polyVBIPS-dominated composition of the bottom layer. Conversely, in the spectrum of the upper layer, the characteristic resonance peaks corresponding to polyVBIPS were greatly reduced, and resonance peaks corresponding to polyHEAA became significant. These results, although not quantitative, suggested a significant difference in the compositions of the two layers. The bilayer structure was further characterized by observing the morphology of the freeze-dried hydrogels using SEM, as shown in Figure 1d. Clearly, two layers with distinct morphologies were observed, in which the bottom layer containing the polyVBIPS-enriched composition showed a much smaller pore size compared with the upper layer. In parallel, energy-dispersive X-ray spectrometry (EDS) of sulfur elements at the crosssection of the hydrogels was performed, as shown in Figure 1e. An obvious stepwise increase in sulfur intensity was observed from the polyHEAA layer to the polyVBIPS, further indicating the difference in composition between the two layers. 9



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Based on structural and compositional characterization, we can see that the composition of the copolymer changes as the copolymerization proceeds; this is the main reason for the formation of the bilayer structure. To understand this process, pristine HEAA hydrogel, polyVBIPS hydrogel, and polyHEAA/VBIPS bilayer hydrogel were prepared in the same manner and the gel times were compared, as shown in Figure S1. It is interesting to note that both the pristine polyHEAA hydrogel and polyVBIPS were formed rapidly, particularly for HEAA which was formed in 108 s. However, a significantly longer time (~ 2.5 h) was required to form the first hydrogel layer when the weight ratio for HEAA and VBIPS was 4:6. The reduced polymerization rate and formation of the VBIPS-enriched copolymer is likely due to the difference in the activities of HEAA and VBIPS as well as their respective radical species. Due to its conjugated structure, VBIPS has a high reaction activity but the activity of its corresponding radical is low. HEAA exhibits the opposite behavior compared with VBIPS, i.e., high radical activity and low monomer reactivity. Therefore, once the radical is formed, regardless of it being a HEAA radical or VBIPS radical, it prefers to reacts with VBIPS due to the high reactivity of VBIPS. Upon reaction, VBIPS radicals with low reactivity are formed; VBIPS monomers readily polymerize compared with HEAA monomers, therefore, VBIPS-enriched copolymers are formed first. Due to the low solubility of polyVBIPS in water, these copolymers precipitate from the solution, forming the first layer. As the reaction proceeds, the ratio of VBIPS monomer to HEAA monomer decreases. As a result, copolymers with high HEAA content were formed in the subsequent polymerization, which becomes the second layer.

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Figure 2. (a) Images of the hydrogels prepared from different monomer pairs and ratios in water. (b) Illustration of the calculation of the curvature angle. (c) Thickness of each layer and curvature angles of the hydrogels prepared from different monomer pairs and ratios. Note: NIPAM, AAm, HEAA, VBIPS, and DVBAPS were abbreviated as N, A, H, V, and D. Numbers represent weight ratios. Scale bar = 6mm.

To further illustrate the process of bilayer structure formation, several combinations of various monomers with different ratios were used to study the monomer- and ratio-dependence of this process. Herein, four monomer pairs, HEAA/VBIPS, AM/VBIPS, NIPAM/VBIPS, and NIPAM/DVBAPS, with a combination of four monomer ratios ranging from 3:7 to 6:4 for each monomer pair, were used. The resultant hydrogels were observed to investigate how the monomer pairings and ratios affected the structure of the bilayer hydrogel, as shown in Figure 2a. At first glance, we can see that the variations in the monomer pair and ratio not only determine the success or failure of the formation of the bilayer structure, but also significantly affect the appearance and thickness of each layer of the bilayer hydrogels. Overall, a high content of zwitterionic monomer facilitates the formation of the bilayer structure, and increases the thickness of the polyzwitterionic-enriched layer, leading to a significant distinction in appearance between the two layers. For example, for polyNIPAM/polyVBIPS hydrogels, when the weight ratios of NIPAM: VBIPS are 5:5, 4:6, and 3:7, distinct bilayer structures are formed, whereas a ratio of 6:4 resulted in a single layered structure. For the bilayer hydrogels, both the appearance and thickness of each layer showed a strong dependence on monomer ratio. The higher content of zwitterionic monomer, the greater the transparency of the upper layer, showing a more distinct bilayer structure. To understand the effect of the monomer ratio on the thickness of each layer in the bilayer hydrogels, the thicknesses of the bottom layer (polyzwitterionic-enriched layer) and upper layer were measured, as shown in Figure 2c. It is clear that following an increase in the amount of zwitterionic monomer, the thickness of the bottom layer gradually increases, and correspondingly, the thickness of the upper layer gradually decreases. This result can be explained by the fact that more zwitterionic-enriched copolymers are formed as the bottom layer when the zwitterionic monomer content is high. Analogous systems without a crosslinking agent showed a similar trend, i.e., a high zwitterionic monomer ratio leads to a thicker precipitation layer (Figure S2). To investigate the effect of the monomer ratio on composition, the copolymers obtained in the first polymerization step from the solutions with different monomer ratios (HEAA: VBIPS of 3:7 and 6:4) were collected by dialysis and characterized by NMR, as shown in Figure S3. It can be seen that the composition of the 11



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copolymers highly depended on the monomer ratio. With a monomer ratio of HEAA : VBIPS=3:7, VBIPS is the predominant monomer in the copolymers, while HEAA becomes predominant when the monomer ratio is 6:4. In the case of a monomer ratio of 6:4, the copolymers did not precipitate due to the low VBIPS content, which resulted in a very poor bilayer structure. When comparing the hydrogels prepared from different monomer types, it was found that the monomer type also affected the formation and structure of the bilayer hydrogels. When the monomer pairs were NIPAM/VBIPS and HEAA/VBIPS, distinct bilayer structures could be formed with three monomer ratios, whereas for the AAm/VBIPS pair, only two monomer ratios formed a bilayer structure. In addition to the difficulties associated with the formation of the bilayer structure, the bilayer structure in polyAAm/polyVBIPS hydrogels could not be readily distinguished. This is likely due to the high copolymerization tendency between AAm and VBIPS, which leads to only a slight difference between the two layers. The effect of the zwitterionic monomer on the bilayer structure was studied by choosing two types of zwitterionic monomer, VBIPS and DVBAPS, and the common monomer NIPAM. Both the NIPAM/VBIPS pair and the NIPAM/DVBAPS pair lead to the formation of distinct bilayer structures when the monomer ratios are 5:5, 4:6, and 3:7. These results indicate that the type of salt-responsive monomer has a slight effect on the bilayer structure and proves the prominent role of the zwitterionic monomer in the formation of the bilayer structure. Due to the difference in swelling properties bet、ween the two layers, once placed into water, the bilayer hydrogels bend towards the polyzwitterionic-enriched layer and show a curved shape. The bending angles of the bilayer hydrogels were calculated following the method shown in Figure 2b and presented in Figure 2c. A strong correlation between the curved shape and structure of the bilayer hydrogel was observed. Although a high content of the zwitterionic monomer, for example, a 3:7 ratio, resulted in the formation of a distinct bilayer structure, the hydrogels did not show the largest curve angle. This result is reasonable since for bilayer actuators, an optimal thickness ratio between two layers is needed to achieve maximum deformation. For this reason, polyNIPAM/polyVBIPS polyNIPAM/polyDVBAPS, and polyHEAA/polyVBIPS hydrogels showed large bending angles in water when the weight ratio between the common monomer and zwitterionic monomer was 4:6, whereas maximum bending in water for the polyAAm/polyVBIPS hydrogel occurred when the monomer ratio was 5:5. 12


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Figure 3. (a) Images, (b) thickness of each layer, and curvature angles of the polyNIPAM/polyVBIPS and polyNIPAM/polyDVBAPS bilayer hydrogels from different content of thermal initiator. The monomer ratio was 4:6. Scale bar = 6mm.

To obtain a bilayer structure, the formation of zwitterionic-enriched copolymers and their precipitation process are the most important steps, during which the transformation from solution to gel also occurs. In other words, the gel formation process, related to the polymerization rate, is competitive with the precipitation process in the formation of bilayer structure; if the polymerization rate is high and the gel is formed very quickly, precipitation does not occur fast enough to form the bottom layer. On the other hand, however, this may enable a method for the tuning of the structure of the bilayer hydrogels. To confirm this, different amounts of initiator that could lead to different polymerization rates, i.e. gel formation rate, were used to investigate the kinetic dependency of the bilayer structure. As shown in Figure 3, an increase in thermal initiator content caused a decease in the bottom polyzwitterionic-enriched layer, reducing the distinction between the two layers, and even leading to the formation of a single layer. Using the polyNIPAM/polyVBIPS system as an example, when the thermal initiator was 1 mg, the thickness of the bottom layer was ~ 1.1 mm. An increase in the amount of thermal initiator content to 3 mg decreased the thickness to ~ 0.9 mm, and 5 mg thermal 13



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initiator content further decreased the thickness to ~ 0.7 mm. At the same time, the transparency of the upper layer deceased following an increase in the thermal initiator content (Figure 3a). When the thermal initiator content was increased to 7 mg, a hydrogel with a single layer structure and entirely white color was obtained. Due to the effects of the thermal initiator on the bilayer structure, an optimal content of 3 mg initiator was also chosen to achieve the maximum curvature in water.

Figure

4.

Bending

behavior

of

(a)

polyNIPAM/polyVBIPS,

(b)

polyNIPAM/polyDVBAPS,

(c)

polyHEAA/polyVBIPS, and (d) polyAAm/polyVBIPS bilayer hydrogels, prepared at different monomer ratios, actuated by 1.0 M salt solution.

3.2 Salt-responsive Bending Behavior of Bilayer Hydrogels. In our previous studies, 5, 14, 38-43 we have reported the strong “anti-polyelectrolyte effect” salt-responsive properties of polyDVBAPS and polyVBIPS, which allow for the polymers to be insoluble in water and adopt a collapsed chain conformation, but in a salt solution adopt a stretched chain conformation and being soluble. Therefore, polyVBIPS- and polyDVBAPS-based hydrogels shrink in water and swell in salt solutions. However, for polyAAm-, polyHEAA-, and polyNIPAM- hydrogels, they either show no response to salt solution, i.e., no change in volume, or show opposite response behaviors, i.e., swelling in water while shrinking in salt solution. Figure S4 further shows the swelling/contracting behavior of the pristine hydrogels in water and 1 M NaCl solution. It can be seen that when the as-prepared hydrogels were placed in water, polyAAm, polyHEAA, and polyNIPAM hydrogels gradually swelled to ~ 200%, ~ 350%, and ~ 200% 14


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of the original weight, respectively, whereas polyVBIPS and polyDVBAPS shrunk to ~ 68% and 82% of their original weight, respectively. However, when the swollen hydrogels were placed into 1 M NaCl solution, polyAAm and polyHEAA hydrogels showed a slight increase in weight, but the polyNIPAM hydrogel shrunk to 30 % its original weight, indicating the high salt-responsive shrinkage properties of polyNIPAM. Contrary to the polyNIAPAM hydrogel, polyDVBAPS and polyVBIPS swelled in the salt solution due to the strong anti-polyelectrolyte effect, and their weights became ~500 % and ~ 380 % of their original values, respectively. When the hydrogels were placed into water again, significant swelling and shrinking were observed for polyAAm, polyHEAA, polyNIPAM, polyDVBAPS and polyVBIPS hydrogels, respectively. In the bilayer hydrogel, the difference in composition between the two layers leads to a difference in volumetric change, which then endows the hydrogels with a bi-directional bending capability. Figure 4 shows the bending behavior of the bilayer hydrogel in water and 1 M NaCl solution. It is clear that the bilayer hydrogels exhibit bending towards the polyzwitterionic-based layer in water due to the swelling of the other layer and slight shrinking of the polyzwitterionic-based layer, where the bending angle is negative. In 1.0 M NaCl solution, the swelling of the polyzwitterionic layer and the non-swelling or shrinkage leads to bending behavior opposite to that of the polyzwitterionic layer. In this case, the bending angle is positive. It was shown that both the bending in water and salt solution strongly depends on the type of the monomer type and ratio. It was observed that an optimal monomer ratio is needed to give a large bending angle. For example, as shown in Figure 4c, the bending angles of the polyHEAA/polyVBIPS bilayer hydrogel with a HEAA:VBIPS ratio of 3:7 in water and in salt solution were ~ -236 o and ~ 51 o, respectively. With HEAA:VBIPS ratios of 4:6 and 5:5, the bending in water increased to ~ -282 o and ~ -280 o, respectively, but the bending in salt solution decreased to ~ 16 o and ~ 44 o. It appears that only the polyHEAA/polyVBIPS bilayer hydrogel with a 3:7 HEAA:VBIPS ratio showed a curvature in both water and salt solution. For the polyNIPAM/polyDVBAPS, polyAAm/polyVBIPS, and polyNIPAM/polyVBIPS bilayer hydrogels, the optimal monomer ratios that provided large bending angles in both water and salt solution were 3:7, 3:7 and 3:7, and the large bending angles were ~ -259 o,

~ -71 o, and ~ -360 o in water and ~ 134 o, ~ 152 o, and ~ 266 o in salt solution, respectively. By

comparing the bending angles of the hydrogels prepared from the optimal monomer ratios, the effect of the monomer type on the bending angles can be elucidated. Among the polyHEAA/polyVBIPS, polyAAm/polyVBIPS, and polyNIPAM/polyVBIPS hydrogels that were prepared from recipes using 15



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VBIPS as the same salt-responsive monomer, the polyNIPAM/polyVBIPS bilayer hydrogel has the largest bending angles in both water and salt solution, while the bending angles of the polyAAm/polyVBIPS hydrogel were very low. It is well known that polyNIPAM hydrogels also show a volume change in response to the salt solution and shrink in salt solution. Thus, the simultaneous but opposite swelling/shrinking behaviors lead to large bending angles. As for polyAAm/polyVBIPS, the small bending angle could be ascribed to the small difference between the two layers. The effect of the type of polyzwitterionic species on the bending angles was investigated by comparing the bending behaviors of the polyNIPAM/polyVBIPS and polyNIPAM/polyDVBAPS bilayer hydrogels. Larger bending angles are observed for polyNIPAM/polyVBIPS, which is most likely due to the more prominent salt-responsive properties of polyVBIPS.

To gain a deeper understanding of the effect the that the type of monomer and ratio of monomer had on the bending behavior of the bilayer hydrogels, the kinetics of the bending which occurs from switching from water to a 1.0 M NaCl solution, was analyzed, as shown in Figure S5. The significant difference between the two layers and the synergistic swelling/shrinking of the two layers that induces the large bending angles, similarly led to high bending rates. For example, among the polyHEAA/polyVBIPS bilayer hydrogels, the bilayer with a 3:7 ratio of HEAA: VBIPS exhibited the highest bending rate, from negative curvature to straightness. This is because in this hydrogel, the polyVBIPS-enriched layer is thicker and distinct from the polyHEAA-based layer. On the contrary, polyAAm/polyVBIPS showed very low bending rates. Compared to the polyHEAA/polyVBIPS and polyAAm/polyVBIPS bilayer hydrogels that show that the bending is driven by the swelling/shrinking of one layer, polyNIPAM/polyVBIPS and polyNIPAM/polyDVBAPS showed much higher bending rates due the synergistic action of the two layers. In particular, for the polyNIPAM/polyVBIPS bilayer hydrogel consisting of a 3:7 ratio of NIPAM:VBIPS, negative bending from ~ -360 o to being straight was observed over 200 s; positive bending to ~ 266 o occurred in 570 s.

The average bending rate of

the whole process was ~ 48.8o /min.

Furthermore, the reversibility and reliability of the bilayer hydrogels in repeated bi-directional bending actions were examined. Figure S6 shows the reversible bending of the bilayer hydrogels in response to repeatedly switching between water and 1.0 M NaCl solution. All the bilayer hydrogels 16


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exhibited negative curvature in water and upon immersion into salt solution changed to a positive bending shape. When placed into water again, the hydrogels returned completely to their original shapes over a period of time. The bending angles, rates, and recovery times were almost similar over 5 cycles, indicating the high repeatability of the bending properties of the bilayer hydrogels.

Figure 5. (a) Illustration of the fabrications of planar hydrogels with periodic stripe patterns by a photomask; (b) images of the helical structure of the planar polyNIPAM/polyVBIPS hydrogels with exposed stripes of consistent width, unexposed stripes of different widths, and different θ angles; (c) shape transformation of planar hydrogel in response to the water/salt-solution and cold water/warm water switch; (d) helical structure of the planar hydrogel in salt solutions consisting of different anions and cations; (e) hydrogel stripes with capability of transforming into a heart shape by careful design of the patterns. The planar hydrogel used in (c) and (d) is polyNIPAM/polyVBIPS hydrogel with 4 mm exposed stripes. Scale bar = 6mm.

3.3 Planar hydrogel sheets with 3D shape transformation. On the basis of the above studies, we have found that the polymerization rate is also a key factor in dictating the formation of a bilayer structure because it competes with the precipitation process of the polyzwitterionic-enriched copolymers. When the polymerization rate is high, the polyzwitterionic-enriched copolymers do not have enough time to precipitate, hardly forming a bottom layer. Thus, a very poor bilayer or even only a single layer structure is formed. Inspired by this phenomenon, we envisaged that different 17



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polymerization rates, locally controlled in specific small-scale regions, could lead to hydrogels consisting of multiple components with different compositions and structures. In response to environmental change, the different regions would exhibit a site specific change in shape, leading to a transformation of the 3D shape. In order to realize this hypothesis, a combination of photopolymerization and thermal polymerization was employer, and additionally, masks with periodic stripes were used. In this case, a high polymerization rate in the UV-radiated regions resulted in the formation of a hydrogel with a poor bilayer structure, whereas slow polymerization in the regions without UV-radiation resulted in a good bilayer structure. Therefore, a planar hydrogel sheet consisting of alternative stripes of different bilayer structures, was fabricated in one-step and in one-pot. The stripe size, interval between the stripes, and the angle of the stripe can be easily tuned by the mask design. Figure 5a shows the digital images of the prepared planar hydrogel sheets with a 4:6 ratio of NIPAM:VBIPS. It is clear that alternative stripes with distinct appearances are formed in which the bilayer hydrogel stripes show white and opaque colors; the poor bilayer hydrogel stripes are more transparent. When the hydrogels are placed in water, the mismatch in the shape change between the stripes leads to a planar-to-helix shape transformation. In addition, the shape of the helix showed a strong dependence on the stripe size and the angle θ. For a rectangular sheet with a fixed length of 55 mm, θ= 30 o and exposed stripes of 4 mm, upon decreasing the width of the unexposed stripes from 6 mm to 2 mm, the morphology of the helix becomes increasingly compact, and the number of the turns, N, gradually increases from 2 to 5. As for the effect of the angle θ, when the θ is 0 o and 90 o, the hydrogel curves into a multi-roll structure along the length and width directions, respectively. Whereas when θ=30 o and θ=60 o, helices with different morphology are formed; θ=30 o resulted in a much more compact helical structure compared with θ=60 o (Figure 5b). It has been shown that both polyNIPAM and polyVBIPS show salt- and thermal-responsive properties, but their responsive behaviors differ to each other; polyNIPAM shrinks (swells) but polyVBIPS swells (shrinks) in salt solution (water) or at high (low) temperatures. Due to these responsive synergistic behaviors, the bilayer polyNIPAM/polyVBIPS hydrogels exhibited large, rapid, and reversible bending deformation in response to switching between salt solution and water or between hot and cold water. Herein, using these characteristics, reversible transitions between two helices with differing handedness of the helix, variation in pitch and number of turns, as well as different appearance, was achieved. As shown in Figure 5c, when a hydrogel (widths of exposed stripes 18


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and unexposed stripes are 6 mm and 4 mm, respectively, and θ=30 o) that adopts a right-handed helix in water is placed into 1 M NaCl solution, the helix unrolls immediately, and then becomes planar in shape with ripples, within 20 s. Subsequently, over 6 mins, the hydrogel rolls in the opposite direction and transforms into a left-handed helix with a loose structure (large pitch). At the same time, the whole hydrogel becomes more transparent. Similarly, in water at a temperature of 55 C, transition from a right-handed helix to a planar shape, and then to a right-handed helix, also occurred in 20 s and 5 min, but the helical structure in this case becomes more compact, and the white color is retained due to shrinkage of the polyNIPAM layer. Inspired by the ion-specificity of the polyzwitterion, i.e., specific responsive properties to different ions, we further tuned the transformation of the shape of the planar hydrogels by using different ions. Figure 5d shows the helical morphology of the planar hydrogel in the presence of different anions and cations with a consistent concentration of 1 M. The hydrogels adopted different helical shapes in the presence of different ions. In the presence of the common Cl- anion, regardless of the type of cation, the hydrogels exhibited a left-handed helical structure, indicating the prominent role of the anion in triggering the salt-responsive behavior of the polyzwitterion. However, the structural characteristics of the helix, such as the diameter, the number of turns, and the pitch, varies with the type of cation. K+ induced a loose and thick helix with the least number of turns, Na+ and Mg2+ induced a loose but thin helix, and Ca2+ gave a very compact helical structure. The effect of the anion on the helical structure was investigated by placing the hydrogel into 1 M salt solutions with different anions and Na+ as the common cation. It is interesting to note that right-handed helices, with different pitches and number of turns, were formed in Cl-, NO3-, and Br-, while the left-handed helix is retained when in water and became more compact in SO42+. The differences in the transition of the shape are believed to result from the different interactions of the cations with the polyzwitterion. 13 In addition to the planar hydrogels with patterned stripes that exhibited a 3D helix structure, we also developed hydrogels with multiple 3D shape transformation capabilities by designing complicated patterns in the hydrogels. As shown in Figure 5e, for a flower-shaped hydrogel, different sections were selectively exposed, whereas the petals were unexposed and showed good bilayer structure with large bending ability. Conversely, the receptacle was exposed and showed a poor bilayer structure and low bending ability. As a result, the petals curled in water and salt solution, whereas the receptacle almost retained its original shape, similar to a real-world flower. 19



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CONCLUSION In summary, this study provides insight into the mechanism of bilayer hydrogel formation in onestep and one-pot using three common monomers and two zwitterionic monomers. The zwitterionicenriched copolymers and their precipitation were demonstrated as the key factors that determine the bilayer structure. The monomer pair and ratios, as well as the initiator content, had a significant effect on the bilayer structure. Specific monomer ratios and initiator content were determined to achieve the optimal thickness between the two layers and thus obtain large bending transformations. By manipulating the competitive relationship between the precipitation of the zwitterionic-enriched copolymer and gel formation rate, planar hydrogels consisting of periodic stripes with different structures were fabricated by simply using a patterned mask; bilayer hydrogel stripes with different structures were formed in the UV exposed and unexposed areas. The specific change in shape on different stripes led to a 3D transformation of the planar hydrogel into a helical structure in response to salt solution or thermal stimulus. By changing the width and angle of the stripes, as well as the ions in the salt solution, many helices with various morphologies, such as varying number of turns, pitch, and direction of the helical spiral, could be obtained. Furthermore, some programming and complicated 3D transformations could also be conducted by employing a sophisticated pattern design. We believe that these results provide a new method for the design of smart actuators which can undergo 3D transformations.

SUPPORTING INFORMATION. The supporting information includes figures showing gel formation of pristine hydrogels, images of the copolymers formed with different monomer pairs and monomer ratios, 1H-NMR spectra of copolymers from HEAA:VBIPS=3:7 and 6:4, swelling/deswelling behaviors of pristine hydrogels, bending kinetics and reversibility of the bending of the bilayer hydrogels.

ACKNOWLEDGEMENTS. 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). 20


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One-Pot and One-Step Fabrication of Salt-Responsive Bilayer

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