Tunable, Fast, Robust Hydrogel Actuators Based on Evaporation

Oct 27, 2017 - The heterogeneous GO-clay-PNIPAM hydrogels, which are cut into specific shape, can implement some funny 2D–3D deformations. Figure 5a...
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Tunable, Fast, Robust Hydrogel Actuators Based on Evaporation-Programmed Heterogeneous Structures Jinrong Wang, Jianfeng Wang, Zhuo Chen, Shaoli Fang, Ying Zhu, Ray H. Baughman, and Lei Jiang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03953 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Chemistry of Materials

Tunable, Fast, Robust Hydrogel Actuators Based on Evaporation-Programmed Heterogeneous Structures Jinrong Wang1, Jianfeng Wang*,2, Zhuo Chen1, Shaoli Fang3, Ying Zhu*,1, Ray H. Baughman3 and Lei Jiang1 1

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, BeiHang University, Beijing, 100191, China. 2 College of Materials Science and Engineering, Hunan University, Changsha 410082, China 3 Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX 75083, USA. ABSTRACT: The ability to topographically structure and fast controllably actuate hydrogel in two and three dimensions is the key for their promising applications in soft robots, microfluidic valves, cell and drug delivery, and artificial muscles. Inspired by evaporation-induced concentration differentiation phenomenon in the production process of beancurd sheet, we introduce a facile one-step evaporation process to create laminated layer/porous layer heterogeneous structure within graphene oxide-clay-poly(Nisopropylacrylamide) hydrogel in vertical direction and pattern the heterogeneous structure in lateral direction, forming tunable, fast and robust hydrogel actuators. The laminated layer/porous layer architecture is highly stable and robust without possibility of delamination. The evaporation-programmed heterogeneous structures tune thermoresponsive actuations from global bending/unbending for global heterogeneous structure to local bending/unbending and site-specific folding/unfolding for segmentpatterned heterogeneous structure, then to directional bending/unbending and chiral twisting/untwisting for stripe-patterned heterogeneous structure. These actuations are instant, reversible without detectable fatigue after many cycles.

1. INTRODUCTION Hydrogel actuators, which can reversibly change shape in response to external homogeneous stimuli,1-14 have attracted growing interests because of their promising applications in soft robots,15,16 microfluidic valves,17 cell and drug delivery,1820 and artificial muscles.21-23 Their shape changes arise from the mismatch of localized volume changes, which causes internal stress within hydrogels.24 Thus, design and control of the heterogeneous structure of hydrogel actuators are critical for manipulating their actuation behaviors. A traditional strategy is stepwise polymerization of a passive polymer hydrogel and an active polymer hydrogel, forming a bilayer structure.25-28 In general, such bilayer structure exhibits a slow bending/unbending deformation and tends to delamination along a weak interface after a large number of repetitive actuation, particularly in the case of large-scale bending. Other strategies to fabricate heterogeneous hydrogel actuators include local alteration of the cross-linking density of polyelectrolyte hydrogels by ionoprinting,29,30 creation of concentration gradient of charged particle inclusion by electrophoresis,31,32 and site-specific control of the orientation of magnetic platelet inclusion by rotating magnetic field.33 For these heterogeneous hydrogel actuators, the formation of sharp interface with weak interaction can be avoided because of containing the same polymer matrix. Their actuations can be tuned to a certain degree through manipulating their local heterogeneous structure. However, it cannot realize fast while complex tunable actuations. In addition, the process to manipulate local heterogeneous structures is complex, which limits their practical applications. Water evaporation from hot, dilute soybean milk can form a layer of film on the colloid surface, which is widely used to produce bean curd sheet in China and Japan (Figure 1a). At the beginning, water evaporates from the surface of hot

soybean milk. The instantaneous concentration of soybean microparticles increases, corresponding to the decrease of interparticle distance and the increase of surface colloid viscosity. Once the interparticle distance is smaller than the diameter of soybean microparticles, the surface colloid would transit from a fluid-like to solid-like state.34 In this case, the crowded or jammed soybean particles cannot diffuse into the interior of soybean milk. In appearance, a layer of film forms on the surface of soybean milk, which is called as bean curd sheet. The interior of soybean milk still maintains a relaticely low particle concentration with the basical maintainence of colloid fluidity. This phenomenon is called as evaporationinduced concentration differentiation. Inspired by evaporation-induced concentration differentiation phenomenon in the production process of bean curd sheet, we develop a simple method of fabrication of tunable, fast, robust heterogeneous nanocomposite hydrogel actuators through simple evaporation of pre-gel solution containing graphene oxide (GO) nanosheets, clay nanoplatelets and N-isopropylacrylamide (NIPAM) monomers, followed by in situ radical polymerization of NIPAM. This process results in a unique heterogeneous structure with a dense laminated layer and a loose porous layer. Furthermore, the heterogeneous structure can be programmed on demand in plane direction through coverage of a designed mask on pre-gel solution to block the localized evaporation. This facile method of making hydrogel actuators provides new and impressive results demonstrating a number of advantages: i) robust structural stability without possibility of interfacial delamination, ii) enhanced tensile property with large extensibility, improved strength and modulus, iii) large actuation amplitude, superior to other film actuators, iv) fast actuation velocity, superior to other film actuators, v) high cycle stability without degradation after many cycles, vi) diverse actuation styles through designing heterogeneous

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patterns, including global bending, local bending, site-specific folding and global twisting, vii) controllable directions of bending and twisting. The facile approach would be highly useful in on-demand design and fabrication of highperformance hydrogel actuators for soft robot, micromanipulation and artificial muscle in the future. 2. EXPERIMENTAL SECTION 2.1. Materials: Graphite oxide (GO, XianFeng NANO Co., Ltd. platelet size = 0.5-5 µm in diameter and 0.8-1.2 nm in thickness), clay (Laponite XLG, [Mg5.34Li0.66Si8O20(OH)4]Na0.66, platelet size = 25-30 nm in diameter and 1 nm in thickness, Rockwood Ltd., UK), NIPAM (>98% purity, TCI), potassium peroxydisulfate (SigmaAldrich) and N,N,N`,N`-tetramethyl-ethylenediamine (SigmaAldrich) were used as received. 2.2. Preparation of global heterogeneous GO-clayPNIPAM hydrogels: GO dispersion in water (4 mg mL-1, 9.5 mL) obtained through bath ultrasonic was bubbled with nitrogen gas to remove oxygen, followed by addition of NIPAM (1.13 g) and clay (0.38 g) under stirring. Then, potassium peroxydisulfate solution (20 mg mL-1, 0.5 mL) and N,N,N`,N`-tetramethyl-ethylenediamine (10 µL) were added at ice-water bath temperature. The pre-gel was casted into a rectangular mold. The mold was transferred to a glass container, and nitrogen gas flow was applied to pass through the glass container for 4 h at 25 °C. Finally, the glass container was sealed for 20 h to complete polymerization, leading to global heterogeneous GO-clay-PNIPAM hydrogels. 2.3. Preparation of patterned heterogeneous GO-clayPNIPAM hydrogels: For patterned heterogeneous hydrogels, the formula of pre-gel is the same as that of global heterogeneous hydrogels. After the pre-gel was casted into a rectangular mold, a pre-designed mask was placed on the mold to cover the surface of pre-gel. Then, the mold was transferred to a glass container, and nitrogen gas flow was applied to pass through the glass container for 4 h at 25 °C. Finally, the glass container was sealed for 20 h to complete polymerization, leading to patterned heterogeneous GO-clay-PNIPAM hydrogels. The pattern was controlled through designing the geometry structure of mask, including segment mask and stripe mask. The geormetry size of masks is shown in Supporting Information, Figure S5. 2.4. Characterization: The cross-sectional and surface morphologies of lyophilized hydrogel were observed by SEM (JEOL, JSM-7500F). Tensile mechanical properties were measured using a Shimadzu AGS-X testing machine. Asprepared hydrogels were cut into rectangular strips with 3 mm in width and 30-40 mm in length, and coated with a layer of mineral oil to prevent water evaporation during tensile tests. The gauge length was 10 mm, and the loading speed was 50 mm min-1. Temperature-triggered actuation processes were recorded using a digital camera. 3. RESULTS AND DISCUSSION NIPAM is selected as monomer because poly(Nisopropylacrylamide) (PNIPAM) has well-defined lower critical solution temperature of 32 °C. Below and above this temperature, PNIPAM can quickly swell and shrink in water, Synthetic clay respectively.35,36

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([Mg5.34Li0.66Si8O20(OH)4]Na0.66) is selected as crosslinker because it can efficiently cross-link PNIPAM molecules through hydrogen bond interaction between amide groups of PNIPAM and hydroxyl groups of clay, and help to improve the toughness of hydrogel.37 GO, containing carboxyl and hydroxyl groups on both basal plane and edge, also acts as cross-linker through hydrogen bond interaction with PNIPAM for increasing hydrogel strength.38 The preparation process of GO-clay-PNIPAM hydrogel actuators is illustrated in Figure 1b.

A homogeneous pre-gel solution consisting of GO, clay, NIPAM monomer, potassium peroxodisulfate initiator and N,N,N',N'-tetramethyl-ethylenediamine catalyst was poured into a rectangular mold. After the pre-gel solution was covered with a mask, nitrogen gas flow was applied at room temperature for 4 h. Water evaporation occured selectively at the site where the surface of pre-gel solution was exposed to nitrogen gas (the site without mask coverage) (Figure 1c). It increased the instantaneous concentration of GO and clay at the exposed surface, leading to a jammed solid-like surface layer, similar to the formation of bean curd sheet. With further water evaporation, the excluded-volume interaction between GO and clay induced the two-dimensional nanoplatelets to orient along pre-gel surface and co-assemble into layered structure for minimizing the free energy of system.39,40 However, the pre-gel beneath the jammed solid-like surface layer and at mask-covered site maintained relatively low concentration of GO and clay, which were distributed homogeneously and oriented randomly (Figure 1d). Then, the nitrogen gas-treated pre-gel solution stood for 20 h to complete the polymerization of NIPAM. In the process, the polymerized PNIPAM was cross-linked by clay and GO through hydrogen bond, which fixed the preformed heterogeneous structure. Through altering mask geometry, a serious of hydrogel actuators, including a global heterogeneous, two segment-patterned heterogeneous (different length of heterogeneous region) and four stripepatterned heterogeneous (different orientation direction of stripe) GO-clay-PNIPAM hydrogel actuators were prepared (Figure 1e). The hierarchical heterogeneous structure of the obtained hydrogel actuators were confirmed by SEM. At the site exposed to nitrogen gas, its cross section consists of a dense layer and a loose layer (Figure 1f). The dense layer exhibits a nanoscale laminated microstructure, while the loose layer shows well-defined honeycomb-like porous structure with nanometer-thick walls and microscale pore (Figure 1g, i). Because the laminated layer and porous layer contain the same PNIPAM matrix which was polymerized in a one-step process, the two layers are intimately connected without any interfacial delamination at microscopic level (Figure 1h). Energy-dispersive X-ray (EDX) analysis of the cross section proves the absence of phase separation of GO, clay and PNIPAM during the fabrication process (Supporting Information, Figure S1). X-ray diffraction (XRD) data further exclude the possibility of GO aggregation in the resultant hydrogel (Supporting Information, Figure S2). Differently, at the site covered by masks, its cross section has homogeneous porous structure without laminated layer (Supporting Information, Figure S3).

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Chemistry of Materials

Figure 1. Fabrication and structure of heterogeneous GO-clay-PNIPAM hydrogel actuators. (a) Evaporation-induced concentration differentiation in hot soybean milk, forming a layer of dense bean curd sheet on surface. (b) Preparation process of stripe-patterned heterogeneous hydrogel actuator. (c) Schematic illustration of microstructure evolution at the site exposed to nitrogen gas (the site without mask coverage). (d) Schematic illustration of microstructure evolution at the site covered by mask. (e) The used masks and corresponding hydrogel actuators, including global heterogeneous (1#), segment-patterned heterogeneous (2#, 3#) and stripe-patterned heterogeneous hydrogel actuators (4#-7#). (f-i) SEM images of heterogeneous hydrogel actuator at the site exposed to nitrogen gas. (f) Overall cross section. (g) Cross section of top dense layer. (h) Interface between dense layer and porous layer. (i) Cross section of bottom porous layer with an average pore diameter of 3.6 µm.

The global heterogeneous GO-clay-PNIPAM hydrogel actuator exhibits fast, reversible and large-amplitude bending actuation in response to temperature change. When transferred from 20 °C to 40 °C water, the hydrogel strip with dimensions of 30 mm × 3 mm × 0.3 mm bent along the length direction of the strip with porous layer inwards and formed two connected concentric circles in 0.9 s (Figure 2a and Supporting Information, Movie 1). Once transferred back to 20 °C water, the strip recovered its original shape in 7.6 s (Figure 2b). The bending actuation arises from thermal response of PNIPAM in porous layer, which is contracted in warm water and expanded in cold water. However, the PNIPAM in laminated layer does not shrink/expand because high concentration of clay and GO completely restricts the thermal molecular motion of PNIPAM chains (Supporting Information, Figure S4).41 The strain mismatch between the two layers generates interfacial stress, which causes the hydrogel strip to bend toward the porous layer and unbend. The kinetics of bending and unbending movements is assessed by plotting the curvature of the formed circles against time, resulting in a bending curvature of 3.33 mm-1 in 0.9 s (Figure 2c). We compared the bending amplitude and bending velocity of the hydrogel actuator with previously reported film actuators in the literature (Figure 2d). As curvature of bending scales inversely with sample thickness, the bending amplitude is estimated to be ‘curvature × thickness’. The bending velocity is estimated to be ‘curvature × thickness ÷ time’. It is seen that our hydrogel actuator has both large bending amplitude (0.999 mm·mm-1)

and fast bending velocity (1.11 s-1), superior to previously reported bendable hydrogel film actuators, polymer film actuators, and comparable to the porous foam actuator, electrospun mat actuator (Supporting Information, Table S1). Moreover, the thermoresponsive bending/unbending actuation process is highly reversible and repeatable without fatigue. No changes in the bending curvature, response time and recovery time were observed after 100 cycles (Figure 2e). The superior bending actuation properties of the global heterogeneous GO-clay-PNIPAM hydrogel actuator are related to its unique hierarchical heterogeneous structure. The porous layer has high porosity and well-defined nanometerthick walls (Figure 1i). The thin pore wall shortens the diffusion length of water from the interior of wall to pore.42 The high porosity accelerates the transport of water from pore to external environment. The strong interface adhesion between porous layer and laminated layer, which was formed through one-step polymerization of NIPAM, rapidly transfers the contraction stress within porous layer to laminated layer. These factors work together and result in the fast bending velocity. The large bending amplitude is mainly attributed to large strain difference value between the porous layer and the laminated layer.43 When temperature rose across the lower critical solution temperature of PNIPAM, the porous layer contracted by 83%, while the laminated layer had no contraction, as confirmed by thickness measurement (Supporting Information, Figure S4).

Figure 2. Bending/unbending actuation of global heterogeneous GO-clay-PNIPAM hydrogel actuator (30 mm × 3 mm × 0.3 mm) in response to temperature change. (a) Bending actuation in response to temperature change from 20 to 40 oC. Photographs at 0 s corresponds to the state at 20 oC. (b) Unbending actuation in response to temperature change from 20 to 40 oC. Photographs at 0 s corresponds to the state at 40 oC. (c) Plot of curvature against time for the hydrogel actuator. (d) Comparison of bending amplitude and velocity of our hydrogel actuator (5) with previously reported hydrogel film actuators (1), polymer film actuators (2), porous foam actuator (3) and electrospun mat actuator (4). (e) Bending curvature, response time and recovery time in response to temperature for 100 cycles.

Besides superior bending actuation properties, the global heterogeneous GO-clay-PNIPAM hydrogel actuator has excellent structural stability and robust mechanical property. To confirm excellent structural stability, we treat the heterogeneous hydrogel actuator with rigorous physical challenges, including freezing (-4 oC, 2h), ultrasonic treatment (100 W, 30 min), long-time immersion (20 oC, 5 months) and vacuum drying (20 oC, 2 h). As a result, the hydrogel actuator tolerates all of these challenges and maintains structural integrity without any interfacial delamination, indicating that the strong interface adhesion between the porous layer and the laminated layer. Furthermore, the rigorous physical challenges did not degrade its thermoresponsive actuation capability (Figure 3a, b). The bending curvature, response time and recovery time keep constant after freezing or ultrasonic

treatments (Figure 3c). The tensile stress-strain curve of global heterogeneous GO-clay-PNIPAM hydrogel actuator is shown in Figure 3d, e. It was stretched by 12.6 times without delamination before fracture, generating a tensile strength of 495 KPa and a tensile modulus of 105 Kpa. The robust mechanical properties is comparable to previously reported super stretchable hydrogels.44-47 To confirm the contribution of the added GO and the formed heterogeneous structure to the mechanical properties, we prepared global heterogeneous clay-PNIPAM hydrogel without GO, and global homogeneous GO-clay-PNIPAM hydrogel without dense laminated layer. The heterogeneous clay-PNIPAM hydrogel shows much lower tensile strength (54 KPa) and modulus (62 KPa) than heterogeneous GO-clay-PNIPAM hydrogel, indicating that, through hydrogen bond interaction with PNIPAM, GO

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provides significant reinforcement for the hydrogel. The homogeneous GO-clay-PNIPAM hydrogel also exhibits lower tensile strength (224 KPa) and modulus (28 KPa) than heterogeneous GO-clay-PNIPAM hydrogel, indicating that the dense laminated layer further strengthens the hydrogel.48-53 Segment-patterned heterogeneous GO-clay-PNIPAM hydrogel actuators exhibit fast, reversible, local bending/unbending and site-specific folding/unfolding in response to temperature change. The geometry structure of mask used for fabricating segment-patterned hydrogel is shown in supporting Information, Figure S5a, b. A threesegment hydrogel strip with two heterogeneous segments at each end and a homogeneous segment in between was demonstrated (Figure 4a). When transferred from 20 oC to 40

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o C water, the two heterogeneous segments at both ends fast bent, and the middle segment remained straight, forming a “B” shape structure. To achieve site-specific folding, we reduce the length of heterogeneous regions to the value much smaller than the length of homogeneous regions. A seven-segment hydrogel with four homogeneous segments connected with three narrow heterogeneous regions was demonstrated (Figure 4b). The three narrow heterogeneous regions folded like hinges in response to temperature change, resulting in a heartshape structure. These localized bending and site-specific folding actuations are highly reversible when these segmentpatterned heterogeneous hydrogel actuators were alternately immersed in 40 °C and 20 oC water.

Figure 3. Robust structural stability and tensile property of global heterogeneous GO-clay-PNIPAM hydrogel actuator. (a) Bending actuation of freezing-treated sample in response to temperature change from 20 to 40 oC. Photograph at 0 s corresponds to the state at 20 o C. (b) Bending actuation of ultrasonic-treated sample in response to temperature change from 20 to 40 oC. (c) Comparison of curvaturetime curves of as-prepared, freezing-treated and ultrasonic-treated samples, showing that the rigorous physical treatments have no effect on actuation behavior. The inserted photographs are the treated hydrogel strips without delamination. (d, e) Comparison of tensile stress-strain curves of heterogeneous GO-clay-PNIPAM hydrogel (curve 1) with homogeneous porous GO-clay-PNIPAM hydrogel (curve 2) and heterogeneous clay-PNIPAM hydrogel (curve 3).

Stripe-patterned heterogeneous GO-clay-PNIPAM hydrogel actuators exhibit fast, reversible, directional bending/unbending and chiral twisting/untwisting. The geometry structure and size of mask used for fabricating the stripe-patterned hydrogel is shown in Supporting Information, Figure S5c-f. The stripe pattern of the hydrogel consists of 1mm-wide heterogeneous regions and 1-mm-wide homogeneous regions. Their actuation style and direction are controlled through altering the orientation direction of stripes (Supporting Information, Movie 2). For hydrogel strip with stripes perpendicular to the length direction (orientation angle 90o), it bent along the length direction, forming a loop in 40 °C water (Figure 4c). It is due to that each heterogeneous stripe bent on a small scale perpendicular to stripe, which leads to large-scale bending of the whole hydrogel strip. For hydrogel strip with stripes oriented parallel to the length direction (orientation angle 0o), it bent along the width direction, forming a tube (Figure 4d). The small-scale bending direction of each heterogeneous stripe is also perpendicular to stripe. With an interesting configuration, when the stripes were oriented at 45o, the hydrogel strip self-twisted into a righthanded cylindrical helix (Figure 4e). On the other hand, hydrogel strip with stripes oriented at 135o self-twisted into a left-handed cylindrical helix (Figure 4f). These threedimensional helices completely recovered to their original flat

state when transferred to a 20 °C water. It should be noted that all of these directional twisting started perpendicular to the orientation direction of the stripes, and formed geometrical configurations with stripes along the axis of cylindrical helices and the patterned surface facing outside. The chiral twisting in this work is different from the previously reported stripe hydrogel sheet with alternating arrangement of two kinds of homogeneous hydrogel stripes, which cannot control the chirality of twisting through altering the orientation angle of stripes.54 The kinetics of twisting and untwisting movements is assessed by plotting the twisting angle of the formed helix against time, resulting in a twisting angle of 1080o in 1 s (Figure 4g). The twisting time is much shorter than the previously reported stripe-patterned hydrogels and magnetic field-oriented composite hydrogels, which take from several minutes to hours to complete twisting.33,54 As twisting angle scales inversely with sample thickness and with sample length, the twisting amplitude is estimated to be ‘twisting angle × thickness ÷ length’. The twisting velocity is estimated to be ‘twisting amplitude ÷ time’. The twisting amplitude and twisting velocity are compared with previously reported twistable film actuators in the literature, as shown in Figure 4h. It is seen that our hydrogel actuator has both large twisting amplitude (20.77 o·mm·mm-1) and fast twisting velocity (20.77 o -1 ·s ), obviously superior to previously reported twistable film actuators (Supporting Information, Table S2).

Figure 4. Controllable actuation of segment-patterned and stripe-patterned heterogeneous GO-clay-PNIPAM hydrogel actuators (30 mm × 3 mm × 0.5 mm) in response to temperature change. (a) A three-segment hydrogel actuator with two heterogeneous segments (length: 20, 20 mm) at each end and a homogeneous segment in between (length: 20 mm), showing localized bending. (b) A seven-segment hydrogel actuator with four homogeneous segments (length: 10, 20, 20, 10 mm) connected by three short heterogeneous segments (length: 5, 5, 5 mm), showing site-specific folding. (c-f) Stripe-patterned hydrogel actuators with 1 mm-wide homogeneous stripes and 1 mm-wide heterogeneous stripes oriented at different angles, θ, relative to the length direction of hydrogel strip, showing directional bending and chiral twisting. (c) θ= 90°, (d) θ= 0°, (e) θ= 45°, (f) θ= 135°. (g) Plot of twisting angle against time for stripe-patterned hydrogel actuator with orientation angle 45°. (h) Comparison of twisting amplitude and twisting velocity of the hydrogel actuator in (e) with previously reported twistable film actuators. The gradual red arrows represent the transferring of hydrogels from 20 to 40 oC water. The homogeneous red arrows represent the time-lapse actuation in 40 oC water.

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Chemistry of Materials

The heterogeneous GO-clay-PNIPAM hydrogels, which are cut into specific shape, can implement some funny 2D-3D deformations. Figure 5a shows that, after put into 40 oC water, a on-demand shaped segment-patterned heterogeneous hydrogels bent locally, leading to the deformation from plane to the shape of clench fist. Figure 5b shows that a on-demand shaped segment-patterned heterogeneous hydrogels folded site-specifically, generating deformation from plane to cube. In addition, the heterogeneous hydrogels are highly attractive material candidates for temperature-controlled underwater

manipulators. As a demonstration, a cross-shaped global heterogeneous hydrogel encapusulated a colored stone in warm water, just like petal closure (Figure 5c). Figure 5d shows that the cross-shaped hydrogel gripped a plastic block in warm water, 57 times heavier than drying weight itself, just like video game prize claw. All of these deformations and actuations are highly reversible and fast when these shaped hydrogels were alternately immersed in 40 °C and 20 oC water.

Figure 5. Application demonstrations of heterogeneous GO-clay-PNIPAM hydrogels. (a) clench fist-like deformation, (b) deformation from plane to cube, (c) encapsulation of a colored stone in a cross-shaped hydrogel, like petal closure. (d) capture of a block of plastic by a cross-shaped hydrogel, like prize claw.

Finally, the heterogeneous GO-clay-PNIPAM hydrogel with fast actuation velocity is demonstrated as sensor for smart water temperature alarm switch application. As a proof of concept, a sensor device was designed and fabricated with global heterogeneous GO-clay-PNIPAM hydrogel as sensing element, as shown in Figure 6a, b. The GO-clay-PNIPAM hydrogel strip with a metal slice on one end worked as a switch. In cold water, the hydrogel strip is flat. The circuit is

in the state of disconnection with the warning lamp being extinguished. When water temperature surpassed 32 oC, the hydrogel strip bent upwards instantly, resulting in the contact between the two electrodes above the hydrogel. The circuit was switched on, which brightened the warning lamp (Figure 6c, d). Once the water temperature restores to original state, the warning lamp was extinguished again.

Figure 6. Demonstration of global heterogeneous GO-clay-PNIPAM hydrogel as smart water temperature alarm switch. The hydrogel strip with a metal slice at one end acts as a switch to detect water temperature change. (a, b) Schematic representation and photograph of the smart switch circuit in cold water. The hydrogel strip is flat, and the circuit is in the state of disconnection. (c, d) Schematic representation and photograph of the smart switch circuit in warm water. The hydrogel strip bends quickly, causing the metal slice to connect the circuit and trigger warning damp.

4. CONCLUSIONS In summary, inspired by evaporation-induced concentration differentiation phenomenon in the production process of beancurd sheet, we created and patterned the heterogeneous structure within GO-clay-PNIPAM hydrogel through a facile evaporation process, generating a series of tunable, fast and robust hydrogel actuators. Multiple types of actuations were demonstrated, from two-dimensional directional bending, localized bending and site-specific folding to threedimensional chiral twisting, all of which were guided by evaporation-programmed heterogeneous structures. The heterogeneous structure, consisting of intimately connected laminated layer and porous layer, imparted the hydrogel actuators with large actuation amplitude, superior actuation velocity, high cycle stability, robust structural stability and mechanical property. We expect that the developed approach can be applied to other nanocomposite hydrogel actuators with incorporation of functional nanoparticles for applications, such as soft robot, micromanipulation and artificial muscle.

°C water and 40 °C water. Masks with different shapes and sizes for fabricating segment-patterned and stripe-patterned heterogeneous GO-clay-PNIPAM hydrogel actuators. Data collection for Figure 2d and Figure 4h in the main text. Bending/unbending actuation of global heterogeneous GO-clayPNIPAM hydrogel actuator (avi); Controllable actuation of stripepatterned heterogeneous GO-clay-PNIPAM hydrogel actuators (avi). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Jianfeng Wang,* Ying Zhu,* Email: [email protected], [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

ASSOCIATED CONTENT

The authors declare no competing financial interest.

Supporting Information. Element mapping of cross section of GO-clay-PNIPAM hydrogel; XRD curves of GO, clay and heterogeneous GO-clay-PNIPAM hydrogel; Optical microscope images of global heterogeneous GO-clay-PNIPAM hydrogel at 20 °C water and 40 °C water; SEM of the homogeneous porous structure of the hydrogel covered by mask. Optical microscope images of global heterogeneous GO-clay-PNIPAM hydrogel at 20

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51403008, 51273008, 51473008), the National Basic Research Program (2012CB933200), 111 Project (B14009) and China Scholarship Council.

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