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Thermo-Responsive Deformable Actuators Prepared by Local Electrochemical Reduction of Poly(N-isopropylacrylamide)/Graphene Oxide Hydrogels Xin Peng, Chen Jiao, Yaxin Zhao, Nan Chen, Yuqing Wu, Tianqi Liu, and Huiliang Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00022 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Nano Materials

Thermo-Responsive Deformable Actuators Prepared by Local Electrochemical Reduction of Poly(Nisopropylacrylamide)/Graphene Oxide Hydrogels Xin Peng, Chen Jiao, Yaxin Zhao, Nan Chen, Yuqing Wu, Tianqi Liu, Huiliang Wang* Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China KEYWORDS: deformable actuators, poly(N-isopropylacrylamide), graphene oxide, reduced graphene oxide, electrochemical reduction

ABSTRACT: Inspired by the shape deformations of plants with inhomogeneous structures, more and more attentions have been paid to prepare deformable actuators with inhomogeneous structures and precisely program their shape deformations. Here, we report a simple and novel method to prepare graphene oxide/reduced graphene oxide (GO/RGO) nanocomposite hydrogelbased deformable actuators by local electrochemical reduction of homogeneous poly(Nisopropylacrylamide)/GO (PNIPAM/GO) hydrogels. GO nanosheets in PNIPAM/GO hydrogels can be reduced in an electrolytic cell made of two indium tin oxide (ITO) glass plates, and local reduction can be realized by pasting patterned insulating stickers onto the cathodal ITO glass plate. The reduction of GO nanosheets leads to the breakage of hydrogen bonding between GO nanosheets and PNIPAM chains and hence lower cross-linking densities of the reduced regions.

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Therefore, the difference in the swelling/deswelling behaviors between the reduced and unreduced sides (or regions) enables the shape deformations of the PNIPAM/GO hydrogels, and their deformation degrees can be adjusted by changing their composition and thickness as well as the parameters of electrochemical reduction (e.g. applied voltage and reduction time). Very impressively, shape deformations from one-dimensional (1D) to two-dimensional (2D) and 2D to three-dimensional (3D) can be programmed by appropriate designing reduced regions on 1D hydrogel strips or 2D hydrogel sheets. As a representative example, a 3D “scorpion” with uplifted head/tail and an arched belly could deform from a 2D “scorpion” with the reduced head/tail on the same side and the reduced belly on the opposite side.


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INTRODUCTION

Numerous plants with inhomogeneous tissues (such as tendrils, pods and leaves) can undergo anisotropic shape deformations to change their geometric shapes for different functions such as spatial reorientation, seed dispersal and ingestion.1-3 Inspired by anisotropic shape deformations of the plants, several deformable actuators with inhomogeneous structures and anisotropic shape deformations have been developed.4-8 And these deformable actuators have been widely used to manufacture artificial muscles,9-10 soft robotics,11 soft machines,12-18 drug carriers19 and microfluidic valves.20 Up to now, three main types of inhomogeneous deformable actuators have been developed, i.e., bilayer actuators,12, 16-17, 21-24 actuators with gradient/different distributive component(s) across the thickness5,

15, 18, 25-27

and those with locally different responsive

properties.14, 18, 28-35 Recently, deformable actuators with locally different responsive properties have gained more and more attentions, due to the effective programming of complex shape deformations. These deformable actuators can be prepared through photolithographic,28-29, 32-33, 35 chemical printing,30 electrochemical ion-printing,14,

31

ion-transfer-printing (ITP)18 and ion-inkjet-printing (IIP)

methods.34 Photolithographic method uses patterned masks to control the irradiated regions and local irradiation doses, altering the local different components or cross-linking densities. Chemical printing methods use chemical solutions as inks to directly draw on a polymer material to locally change its chemical components. On the assistance of an electric filed, electrochemical ion-printing method locally introduces metal ions into the hydrogels containing polyelectrolyte(s) to change their local cross-linking densities. Very recently, our group used metal ions solutions as inks to directly print on the hydrogels containing a polyelectrolyte on the assistance of filter templates (ITP) or a digital inkjet-printer (IIP). These deformable actuators can undergo complex


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shape deformations, as their deformable regions and degrees can be easily programmed by adjusting the hydrogel compositions and printed patterns. PNIPAM have been widely used to prepare thermo-responsive deformable actuators, since the sharp volume change of PNIPAM systems can be actuated at a mild lower critical solution temperature (LCST) around 33°C.36-38 The incorporation of GO/RGO nanosheets could enhance the mechanical properties of nanocomposite materials. Meanwhile, the photo-responsive property of the GO/RGO nanosheets also imparts the nanocomposite materials with remotecontrollable deformation under irradiation. Therefore, several actuators embedded with GO/RGO nanosheets have been prepared.39-42 Graphene oxide (GO) can be reduced to produce reduced graphene oxide (RGO) through thermal,43 chemical,44 light-assisted45-50 and electrochemical reductions.51-52 Among them, lightassisted reduction has been widely used for the fabrication of GO/RGO-based deformable actuators,

45-49

as the light-assisted reduction is convenient in creating a gradient or locally

different structure. For instance, Qu et al. successfully prepared GO/RGO-based fiber-type deformable actuators by local laser-assisted reduction of GO fibers.45 Later on, Sun et al. prepared GO/RGO bilayer deformable actuators by local reduction of GO papers through focused sunlight46 or UV light.47 Recently, Chen et al. used UV light to locally reduce GO nanosheets embedded in PNIPAM hydrogels to prepare deformable actuators.32 Electrochemical reduction is another facile and efficient method to produce RGO, since it requires a low applied voltage (lower than safe voltage of human body, 36 V) and a short reduction time (several minutes). Moreover, electrochemical reduction can be carried out at ambient or even lower temperatures. Therefore, this method has been widely used to prepare


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RGO-based products.51-52 Nevertheless, to our best knowledge, preparation of RGO/GO-based deformable actuators by electrochemical reduction is still unexplored. Here, we report a facile and efficient method to prepare GO/RGO nanocomposite hydrogelbased deformable actuators and program their shape deformations through local electrochemical reduction of PNIPAM/GO hydrogels. The reduced and the unreduced regions would undergo different deswelling behaviors upon heating, resulting in shape deformation of the hydrogels. Meanwhile, the deformation degrees of these deformable actuators can be adjusted by changing composition and thickness of the hydrogels and the parameters of electrochemical reduction (e.g. applied voltage and reduction time). In addition, shape deformations of these deformable actuators from 1D to 2D or 2D to 3D are programmed by appropriate designing reduced regions on 1D hydrogel strips or 2D hydrogel sheets.

RESULTS AND DISCUSSION

Electrochemical Reduction of PNIPAM/GO Hydrogels. Homogeneous PNIPAM/GO nanocomposite hydrogels used in this work were prepared with fixed concentrations of the monomer N-isopropylacrylamide (CNIPAM = 1.5 mol L-1) and the chemical cross-linker N,N’methylenebis(acrylamide) (CBIS = 1.5 × 10-3 mol L-1) and varying concentration of GO nanosheets (CGO = 1.0, 2.0 or 3.0 mg mL-1) by free radical polymerization.53 The as-prepared hydrogels could undergo isotropic shrinkage in hot water (Figure S1). The hydrogels were mostly made with a CGO of 3.0 mg mL-1, if not otherwise stated. To reduce GO nanosheets embedded in PNIPAM/GO hydrogels, an electrolytic cell is made by placing a PNIPAM/GO hydrogel between two indium-tin oxide (ITO) glass plates and a 10 V direct current voltage is applied to the electrolytic cell (Figure S2). On the cathode side, electrons would transfer across the ITO glass to the GO nanosheets and promote the following reaction:


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GO + aH2O + be- → RGO + bOH-, leading to the reduction of GO nanosheets. Meanwhile, GO nanosheets near the anode would not be reduced and hence keep intact (Figure 1). The hydrogel surface near the cathode with RGO is termed as the reduced surface, while the surface near the anode is called the as-prepared surface.

Figure 1. Schematic illustration of an electrolytic cell made of two ITO glass plates and a PNIPAM/GO hydrogel (left), and the electrochemical reduction of GO nanosheets near the cathode (right). To confirm the reduction of GO sheets, the reduced surface was characterized with Raman spectroscopy. As a comparison, the Raman spectrum of the as-prepared surface was also obtained. For the as-prepared surface, two distinct peaks corresponding to D and G bands are clearly observed at 1347 and 1610 cm-1, respectively, showing a D/G intensity ratio (ID/IG) of 0.94, while the Raman spectrum of a reduced surface (reduction time: 1 min) also contains both D (1331 cm-1) and G (1598 cm-1) bands, but its ID/IG is increased to 1.73 (Figure 2). The increased ID/IG suggests a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO, and can be explained if new graphitic domains were created that are smaller in


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size to the ones present in GO before reduction, but more numerous in number.54 Similar results have been reported for electrochemically reduced GO films by other groups.55-57 The inhomogeneous hydrogels containing both GO and RGO nanosheets are termed as PNIPAM/GO/RGO hydrogels.

D

Reduced surface As-prepared surface

G

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ID/IG=1.73 ID/IG=0.94

1000

1500

2000

2500

3000

-1

Raman shift (cm ) Figure 2. Raman spectra of as-prepared and reduced surfaces (reduction time: 1 min). There is not a direct convenient way to detect the depth of the gel layer with RGO sheets, due to the very low content of RGO and/or GO sheets that are homogeneously distributed in the gel matrix. It is reported that GO nanocomposite hydrogels become darker with the reduction of GO into RGO.32, 42 So the cross sections of the PNIPAM/GO and PNIPAM/GO/RGO hydrogels were observed and their transmittances were measured to know the reduced region of the PNIPAM/GO/RGO hydrogels. Unfortunately, we have not observed obvious color change when the gel was reduced for a short time. For instance, after being reduced for 1 min, the reduced side becomes only very slightly darker than as-prepared side (Figure 3a), and transmittance of the whole PNIAPM/GO/RGO hydrogel also decreases only slightly (Figure 3b). Only when the


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reduction time is increased to a long period of 240 min, the reduced side changes to black (ca. 200 µm) and the transmittance of the whole PNIAPM/GO/RGO hydrogel decreases significantly. It is very necessary to note that the hydrogel samples electrochemically reduced for a very short time ( ≤ 1min) can perform obvious shape deformations, which will be shown later. Therefore, the reduction of GO sheets in a thin layer of the hydrogel close to the cathode is sufficient for the purpose of preparing shape deformable actuators.

Figure 3. (a) Cross section photos and (b) transmittances of PNIPAM/GO and PNIPAM/GO/RGO hydrogels with different reduction times. Scale bar is 1 mm. In addition, local electrochemical reduction of the PNIAPM/GO hydrogels can be realized on the assistance of insulating stickers with different hollow patterns (Figure 4). For example, we cut a hollow heart pattern in an insulating sticker and then pasted it on an ITO glass which is


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used as the cathode. In the electrolytic process, only the hydrogel contacting the ITO glass in the patterned region can be reduced, while the hydrogel contacting the insulating sticker cannot. After being reduced for 1 min, no obvious pattern appears on the hydrogel sample, but after being reduced for 240 min, a black heart pattern appears (Figure 4b). Compared to the reported works, such as thermoresponsive polypeptide-based hydrogels with an anisotropic porous structure across the thickness reported by Wang et al.41 and bilayer-type photo-actuators combined by a PNIPAM/RGO active layer and a poly(acrylamide) passive layer reported by Kim et al.,42 our work could locally change responsive properties of the hydrogels, which could easily adjust the bending location/direction/degree and program the shape deformations of the hydrogels.

Figure 4. (a) Schematic illustration of the local electrochemical reduction of a PNIPAM/GO hydrogel on the assistance of an insulating sticker. (b) Photos are an ITO glass pasted with an insulating sticker with a hollow heart pattern (left), a hydrogel sample without a pattern (middle) and with a black heart pattern (right). Scale bar is 1 cm.


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Thermo-Responsive Behaviors of Hydrogels. GO physically crosslink the PNIPAM hydrogels by forming hydrogen-bonding between the oxygen-containing groups on GO nanosheets and the acrylamide groups on PNIPAM chains. The reduction of GO to RGO reduces the number of oxygen-containing groups on GO nanosheets, leading to the breakage of hydrogen bonding and hence a lower cross-linking density in the reduced regions (Figure 1). PNIPAM hydrogels with different cross-linking densities would undergo different deswelling behaviors upon heating.27,

58

For comparison, deswelling behaviors of the as-prepared PNIPAM/GO

hydrogel sheets and the PNIPAM/GO/RGO hydrogel sheets with two reduced surfaces (reduction time: 1 min) in hot water were measured. After being immersed in 50ºC water for 30 s, water contents of the PNIPAM/GO hydrogels and the PNIPAM/GO/RGO hydrogels decrease from 92.3 wt% to 89.1 wt% and from 92.1 wt% to 81.1 wt%, respectively (Figure 5a). Therefore, the reduced regions with lower cross-linking densities would undergo faster deswelling upon heating. Due to the different deswelling behaviors of the as-prepared and reduced regions, shape deformable actuators can be prepared by reducing only one side of the PNIPAM/GO hydrogels. When one side of the PNIPAM/GO hydrogel (CPNIPAM = 1.5 mol L-1 and CGO = 3 mg mL-1) is reduced with a 10 V direct current voltage for 60 s, it can quickly bend towards the reduced side upon heating in 50ºC water, and its bending angle reaches 310º at 30 s. The bending angle is defined and measured according to our previous works (Figure S3).18 Then the bent hydrogel can return to its original state after being cooled in 25ºC water for 15 min (Figure 5b). This bending/unbending process is reversible and can be repeated for several times. These results suggest that the electrochemical reduction of one side of the PNIPAM/GO hydrogels for only 1 min can produce GO/RGO nanocomposite hydrogel-based actuators with obvious shape


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deformations, so the reduction time in the following studies is chosen to be 1 min, if not otherwise stated. 95

PNIPAM/GO hydrogel PNIPAM/GO/RGO hydrogel

b

90

85

350

50 °C 25 °C

300

Bending angle (°)

a Water content (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 200 150 100 50 0

80 0

3

6

9

12 15 18 21 24 27 30

0 0.5

t (s)

15.5 16

31 31.5

46.5

t (min)

Figure 5. (a) Deswelling behaviors of PNIPAM/GO hydrogels and PNIPAM/GO/RGO hydrogels with two reduced surfaces (reduction time: 1 min) in 50ºC water. (b) Reversible bending-unbending deformation of PNIPAM/GO/RGO hydrogels with one reduced surface in 50ºC water and 25ºC water. Yellow and blue zones indicate the bending of hydrogel strips in 50ºC water and the unbending in 25ºC water, respectively. Hydrogel size in (b): length = 2.5 cm, width = 1.5 mm and thickness = 1.4 mm. All experiments were conducted in triplicate and errors bars represent standard deviations. Controllable Deformation Degrees. Controllable deformation is important for practical applications of deformable actuators. The nature of the GO/RGO nanocomposite hydrogel-based deformable actuators allows us to control their bending degrees through two strategies, i.e., changing the composition (e.g. CGO) or thickness of the as-prepared hydrogels and the parameters of electrochemical reduction (e.g. applied voltage and reduction time). Figure 6a shows the bending-unbending behaviors of the GO/RGO nanocomposite hydrogelbased deformable actuators with varying CGO. Reduction of the PNIPAM/GO hydrogel with a higher CGO induces a larger difference in the cross-linking density between the as-prepared


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PNIPAM/GO hydrogel and the PNIPAM/GO/RGO hydrogel, leading to their larger difference in the deswelling behaviors (decrease of water contents) upon heating (Table S1). As the bending degrees and rates are depended on the different deswelling degrees of the reduced and asprepared regions, so the deformable actuators with more GO nanosheets would undergo faster bending and reach higher bending angles. For example, the bending angles of the deformable actuators with 1.0, 2.0 and 3.0 mg mL-1 GO nanosheets increase to 95º, 123º and 340º, respectively, after being immersed in 50ºC water for 30 s. Meanwhile, hydrogels with higher bending angle need longer time to unbend to their original state. Moreover, the PNIPAM/GO/RGO hydrogels with different thicknesses also undergo different bendingunbending behaviors (Figure 6b). The hydrogel with a thickness of 2.2 mm shows an extremely slow bending rate in the whole heating process, whose bending angle is only 24º even after being heated for 30 s, while the thinner hydrogel (1.4 mm thickness) bends much faster and reaches a high bending angle of 340º at 30 s. Note that the thinnest hydrogel (0.6 mm thickness) undergoes three bending stages in the whole heating process. Firstly, the hydrogel bends very quickly, and its bending angle increases almost linearly to 411° in 9 s, then it keeps almost no change till 21 s, and finally the hydrogel strip unbends and its bending angle gradually decreases to 322° at 30 s, due to the shrinkage of its unreduced side. On the other hand, changing the parameters of electrochemical reduction, e.g. applied voltage and reduction time, is also very effective in adjusting the bending-unbending behaviors of the GO/RGO nanocomposite hydrogel-based deformable actuators. A higher applied voltage leads to the reduction of more GO nanosheets, and hence lower transmittances of the hydrogels (Figure S4) and lower cross-linking densities of the reduced regions. Therefore, the hydrogels reduced


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with higher applied voltages would undergo faster bending and reach higher bending angles (Figure 6c). A longer reduction time also leads to similar results (Figure S5 and Figure 6d). 400

50 °C

Bending angle (°)

350

25 °C

-1

b

450

CGO (mg mL ) 1 2 3

300 250 200 150 100

12.5 min 15.5 min

50

Thickness (mm) 0.6 1.6 2.2

400

Bending angle (°)

a

350 300 250 200 150 100

10.5min

10.5 min

0

110 10 20 30 150 40 270 50 390 60 510 70 630 80 750 90 870 100 990

0

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10 20 30 150 40 50 390 60 510 70 630 80 750 90 100 110 870 990 90 270

t (s)

t (s) 400

50 °C

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Applied voltage (V) 8 9 10

250 200 150 100

12.5 min

15.5 min

50 9.5min

0 0

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50 °C

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300

Reduction time (s) 30 45 60

250 200 150 13.5 min

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15.5 min

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t (s)

d Bending angle (°)

c

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Bending angle (°)

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0

110 10 20 30 150 40 270 50 390 60 510 70 630 80 750 90 870 100 990

t (s)

Figure 6. Factors affecting the bending-unbending behaviors of GO/RGO nanocomposite hydrogel-based deformable actuators. (a) GO concentration (CGO), (b) hydrogel thickness, (c) applied voltage and (d) reduction time. Yellow and blue zones indicate the bending of the hydrogel strips in 50ºC water and the unbending of the hydrogel strips in 25ºC water, respectively. Hydrogel size: length = 2.5 cm, width = 1.5 mm and thickness = 1.4 mm. Reduction time is 1 min, if not otherwise stated. All bending deformations were performed in 50ºC water for 30 s, and unbending deformations were performed in 25ºC water All experiments were conducted in triplicate and errors bars represent standard deviations. Programming Complex Shape Deformations. Local altering responsive properties of deformable actuators is an effective strategy to program their complex shape deformations. By


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appropriate designing the locations and patterns of reduced regions, complex shape deformations of GO/RGO nanocomposite hydrogel-based deformable actuators from 1D to 2D and 2D to 3D can be easily achieved. Figure 7 and Movie S1 show the shape deformations from 1D to 2D. When a half of one side of a hydrogel strip is electrochemically reduced and hence is endowed with bending ability, the hydrogel strip would deform into a “J” shape upon heating (Figure 7a). A hydrogel reduced and cut according to the illustration in Figure 7b would deform into a “X” shape upon heating, due to the opposite bending direction of top and bottom parts of the hydrogel sample (Figure 7b). A hydrogel strip with three reduced parts on different sides, as illustrated in Figure 7c, would deform into a “Ω” shape upon heating (Figure 7c).

Figure 7. 2D shapes deformed from 1D hydrogel strips. (a) A “J” shape deformed from a hydrogel with one reduced part. (b) A “X” shape deformed from a hydrogel with two reduced parts. (c) A “Ω” shape deformed from a hydrogel with three reduced parts. Top: schematic illustrations of the reduced regions (in red) on hydrogel strips and the deformed shapes of the gel


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samples upon heating. Bottom: photos showing the shape deformation processes of the 1D hydrogel strips in 50ºC water. All scale bars are 1 cm. 3D shapes can also be easily obtained through shape deformations of 2D hydrogel sheets with reduced regions on one or two surfaces. When only one surface of the hydrogel sheets is reduced, reduced regions would bend to the same direction and the hydrogel sheets would deform into some interesting 3D shapes. For example, when the petals of a flower shaped hydrogel sheet are reduced as the illustration in Figure 8a, bending of the reduced petals induces the hydrogel sheet deform into a blooming flower shape upon heating (Figure 8a and Movie S2). Figure 8b shows the shape deformation of a hydrogel “lily”. We firstly reduced one surface of a rectangular hydrogel sheet and cut the reduced region into six parts. Then scrolled the hydrogel sheet with the reduced surface towards outside to assemble a “lily bud” consists of 3 inner petals embraced by 3 outer sepals. After being immersed in hot water, all petals bend towards outside to actuate the blooming of the hydrogel “lily” (Movie S3). When different regions on two surfaces of a hydrogel sheet are reduced, the hydrogel would deform into more interesting 3D shapes. We cut a “scorpion” shaped hydrogel sheet and reduce it according to the illustration in Figure 8c (the same side of head/tail and the opposite side of belly are reduced). Upon heating, the planar “scorpion” deforms into a 3D “scorpion” with uplifted head/tail and an arched belly, as its head and tail bend towards the same direction but the belly bends towards the opposite direction (Figure 8c and Movie S4).


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Figure 8. 3D shapes deformed from 2D hydrogel sheets. (a) A 3D blooming flower shape deformed from a 2D flower shaped hydrogel sheet. (b) A blooming hydrogel “lily” shape deformed from a scroll of rectangular hydrogel sheets. (c) A 3D “scorpion” with uplifted head/tail and an arched belly deformed from a 2D “scorpion” shaped hydrogel sheet. Top: schematic illustrations of the gel samples with designed patterns, red regions indicate reduced regions. Bottom: photos showing the shape deformation processes of the 2D hydrogel sheets in 50ºC water. All scale bars are 1 cm.

CONCLUSION

In this work, we employed electrochemical reduction technique to prepare GO/RGO nanocomposite hydrogel-based deformable actuators and program their shape deformations.


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Homogeneous PNIPAM/GO hydrogels can be reduced in an electrolytic cell made of two ITO glass plates. And local reduction can be realized by pasting patterned insulating stickers on cathodal ITO glass plates. After being reduced, oxygen containing groups on RGO nanosheets are reduced, leading to breakage of hydrogen bonding between PNIPAM chains and RGO nanosheets, and hence lower cross-linking densities and faster deswelling rates of the reduced regions. Therefore, GO/RGO nanocomposite hydrogel-based deformable actuators can be prepared by local reducing PNIPAM/GO hydrogels. Moreover, deformation degrees of these deformable actuators can be controlled by changing the composition (CGO) and thickness of hydrogels as well as the parameters of electrochemical reduction (e.g. applied voltage and reduction time). In addition, by selective reducing 1D hydrogel strips and 2D hydrogel sheets, many interesting 2D and 3D shapes can be obtained. Such as a 3D “scorpion” with uplifted head/tail and an arched belly can be obtained by reducing the head/tail on the same side and the belly on the opposite side. Due to easy preparation of these GO/RGO nanocomposite hydrogel-based deformable actuators and programming of their shape deformations, they can be an ideal candidate for many applications, such as soft robots, soft machines and artificial muscles, etc. In addition, the electrochemical reduction technique can be extended to other smart materials containing GO nanosheets, leading to other stimuli-responsive deformable actuators.

EXPERIMENTAL SECTION

Materials. N-isopropylacrylamide (NIPAM) (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) was recrystallized from n-hexane before usage. Potassium persulfate (KPS), N,N,N’,N’tetramethylenediamine

(TEMED)

and

N,N’-methylenebisacrylamide

(BIS)

(Sinopharm

Chemical Reagent Co., Ltd., Shanghai, China) were used without further purification. Indium-tin


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oxide (ITO) glass plates were purchase from South China Science and Technology Co., Ltd., Shenzhen, China. Preparation of Homogeneous PNIPAM/GO Hydrogels. PNIPAM/GO hydrogels were prepared according to our previous work.53 NIPAM (1.5 mol L-1), GO (1.0, 2.0 and 3.0 mg mL1

), BIS (1.5×10-3 mol L-1) and KPS (0.01 mol L-1) were dissolved in deionized water. After being

deoxygenated by bubbling with high-purity nitrogen for 15 min in an ice-water bath, TEMED (0.01 mol L-1) was added into the solution. Then the solution was injected into a mold made of two glass plates with a silicone-rubber spacer (0.5, 1 or 1.5 mm thickness). Finally, the molds were sealed and then kept at 25°C for 48 h. As-prepared hydrogels were swollen in 25°C deionized water to achieve equilibrium swelling. The thicknesses of equilibrium swollen hydrogel were 0.6, 1.4 and 2.2 mm, respectively. 1.4 mm thick hydrogels were mostly used in work, if not otherwise stated. Electrochemical Reduction of PNIPAM/GO Hydrogels. A PNIPAM/GO hydrogel was sandwiched between two ITO glass plates, then a direct current voltage was applied across the ITO glass plates for different times. Characterization. Raman scattering measurements were performed at room temperature on a microscopic confocal Raman spectrometer (LavRAM Aramis, Horiba Jobin Yvon, France) with a 633 nm He-Ne laser. Cross sections of the hydrogel samples were observed with an optical microscope (SMART-POL, OPTEC, China) equipped with a digital camera. Optical transmittances of the hydrogel samples were recorded with a UV-vis spectrophotometer (UV2450, Shimadzu, Japan). Deswelling

Behaviors

of

Hydrogels.

As-prepared

PNIPAM/GO

hydrogels

and

PNIPAM/GO/RGO hydrogels with two reduced surfaces (reduction time: 1 min) were immersed


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in 50°C water, and their water contents, defined as (ms-md)/ms×100%, where ms and md are weights of swollen and dried samples, respectively, were measured. Thermosensitive Deformations of PNIPAM/GO/RGO Hydrogels. Thermosensitive deformations of 1D hydrogel strips and 2D hydrogel sheets immersed in 50°C water were recorded with a digital camera. And their bending angles were defined and measured according to our previous work (Figure 3).18

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ******. Figures S1 showing the isotropic shrinkage of a PNIPAM/GO homogeneous hydrogel in 50°C water. Figure S2 showing the schematic diagram of the electrochemical reduction process. Figure S3 showing the definition of bending angle. Figure S4 showing transmittances of PNIPAM/GO and PNIPAM/GO/RGO hydrogels reduced with varying applied voltages. Figure S5 showing transmittances of PNIPAM/GO and PNIPAM/GO/RGO hydrogels with varying reduction times. Table S1 showing water contents of PNIPAM/GO with varying CGO and PNIPAM/GO/RGO hydrogels with two reduced surfaces before and after being immersed in 50ºC water for 30 s. (PDF). Movie S1 for the shape deformation processes of the 1D hydrogel strips in 50ºC water. (AVI).


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Movie S2 for the shape deformation process from a 2D flower shaped hydrogel sheet to a 3D blooming flower shape in 50ºC water (AVI). Movie S3 for the shape deformation process from a hydrogel “lily bud” to a 3D blooming “lily” in 50ºC water (AVI). Movie S4 for the shape deformation process from a 2D “scorpion” sheet to a 3D “scorpion” with uplifted head/tail and an arched belly in 50ºC water (AVI).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ORCID Huiliang Wang: A-5374-2009 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21274013), and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).

ABBREVIATIONS


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GO, graphene oxide; RGO, reduced graphene oxide; ITO, indium tin oxide; NIPAM, Nisopropylacrylamide; PNIPAM, poly(N-isopropylacrylamide); KPS, potassium persulfate; TEMED, N,N,N’,N’-tetramethylenediamine; BIS, N,N’-methylenebisacrylamide.

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Table of Contents Graphic


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Figure 1. Schematic illustration of an electrolytic cell made of two ITO glass plates and a PNIPAM/GO hydrogel (left), and the electrochemical reduction of GO nanosheets near the cathode (right). 750x379mm (96 x 96 DPI)


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Figure 2. Raman spectra of as-prepared and reduced surfaces (reduction time: 1 min). 288x232mm (300 x 300 DPI)



Figure 3. (a) Cross section photos and (b) transmittances of PNIPAM/GO and PNIPAM/GO/RGO hydrogels with different reduction times. Scale bar is 1 mm. 500x550mm (96 x 96 DPI)


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Figure 4. (a) Schematic illustration of the local electrochemical reduction of a PNIPAM/GO hydrogel on the assistance of an insulating sticker. (b) Photos are an ITO glass pasted with an insulating sticker with a hollow heart pattern (left), a hydrogel sample without a pattern (middle) and with a black heart pattern (right). Scale bar is 1 cm. 597x534mm (96 x 96 DPI)



Figure 5. (a) Deswelling behaviors of PNIPAM/GO hydrogels and PNIPAM/GO/RGO hydrogels with two reduced surfaces (reduction time: 1 min) in 50ºC water. (b) Reversible bending-unbending deformation of PNIPAM/GO/RGO hydrogels with one reduced surface in 50ºC water and 25ºC water. Yellow and blue zones indicate the bending of hydrogel strips in 50ºC water and the unbending in 25ºC water, respectively. Hydrogel size in (b): length = 2.5 cm, width = 1.5 mm and thickness = 1.4 mm. All experiments were conducted in triplicate and errors bars represent standard deviations. 291x242mm (300 x 300 DPI)


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Figure 5. (a) Deswelling behaviors of PNIPAM/GO hydrogels and PNIPAM/GO/RGO hydrogels with two reduced surfaces (reduction time: 1 min) in 50ºC water. (b) Reversible bending-unbending deformation of PNIPAM/GO/RGO hydrogels with one reduced surface in 50ºC water and 25ºC water. Yellow and blue zones indicate the bending of hydrogel strips in 50ºC water and the unbending in 25ºC water, respectively. Hydrogel size in (b): length = 2.5 cm, width = 1.5 mm and thickness = 1.4 mm. All experiments were conducted in triplicate and errors bars represent standard deviations. 298x242mm (300 x 300 DPI)



Figure 6. Factors affecting the bending-unbending behaviors of GO/RGO nanocomposite hydrogel-based deformable actuators. (a) GO concentration (CGO), (b) hydrogel thickness, (c) applied voltage and (d) reduction time. Yellow and blue zones indicate the bending of the hydrogel strips in 50ºC water and the unbending of the hydrogel strips in 25ºC water, respectively. Hydrogel size: length = 2.5 cm, width = 1.5 mm and thickness = 1.4 mm. Reduction time is 1 min, if not otherwise stated. All bending deformations were performed in 50ºC water for 30 s, and unbending deformations were performed in 25ºC water All experiments were conducted in triplicate and errors bars represent standard deviations. 305x240mm (300 x 300 DPI)


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Figure 7. 2D shapes deformed from 1D hydrogel strips. (a) A “J” shape deformed from a hydrogel with one reduced part. (b) A “X” shape deformed from a hydrogel with two reduced parts. (c) A “Ω” shape deformed from a hydrogel with three reduced parts. Top: schematic illustrations of the reduced regions (in red) on hydrogel strips and the deformed shapes of the gel samples upon heating. Bottom: photos showing the shape deformation processes of the 1D hydrogel strips in 50ºC water. All scale bars are 1 cm. 579x379mm (96 x 96 DPI)


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Figure 8. 3D shapes deformed from 2D hydrogel sheets. (a) A 3D blooming flower shape deformed from a 2D flower shaped hydrogel sheet. (b) A blooming hydrogel “lily” shape deformed from a scroll of rectangular hydrogel sheets. (c) A 3D “scorpion” with uplifted head/tail and an arched belly deformed from a 2D “scorpion” shaped hydrogel sheet. Top: schematic illustrations of the gel samples with designed patterns, red regions indicate reduced regions. Bottom: photos showing the shape deformation processes of the 2D hydrogel sheets in 50ºC water. All scale bars are 1 cm. 548x596mm (96 x 96 DPI)


Thermoresponsive Deformable Actuators Prepared ... - ACS Publications

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