Reduced Graphene Oxide-Containing Smart Hydrogels with Excellent

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Reduced Graphene Oxide-Containing Smart Hydrogels with Excellent Electro-Response and Mechanical Property for Soft Actuators Chao Yang, Zhuang Liu, Chen Chen, Kun Shi, Lei Zhang, Xiao-Jie Ju, Wei Wang, Rui Xie, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 22, 2017

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ACS Applied Materials & Interfaces

Reduced Graphene Oxide-Containing Smart Hydrogels with Excellent Electro-Response and Mechanical Property for Soft Actuators Chao Yang,† Zhuang Liu,*,† Chen Chen,† Kun Shi,† Lei Zhang,† Xiao-Jie Ju,†,‡ Wei Wang,†,‡ Rui Xie,†,‡ and Liang-Yin Chu*,†,‡,§ †

School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R.

China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,

Sichuan 610065, P. R. China §

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing,

Jiangsu 211816, P. R. China

1 Environment ACS Paragon Plus


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ABSTRACT A novel reduced graphene oxide/poly(2-acrylamido-2-methylpropane sulfonic acid-coacrylamide) (rGO/poly(AMPS-co-AAm)) nanocomposite hydrogel that possesses excellent electro-response and mechanical property has been successfully developed.

The rGO

nanosheets that homogeneously dispersed in the hydrogels could provide prominent conductive platforms for promoting the ion transport inside the hydrogels to generate significant osmotic pressure between the external and internal of such nanocomposite hydrogels. Thus, the electro-responsive rate and degree of the hydrogel for both deswelling and bending performances become rapid and remarkable.

Moreover, the enhanced

mechanical properties including both the tensile strength and compressive strength of rGO/poly(AMPS-co-AAm) hydrogels are improved by the hydrogen bond interactions between the rGO nanosheets and polymer chains, which could dissipate the strain energy in the polymeric networks of the hydrogels.

The proposed rGO/poly(AMPS-co-AAm)

nanocomposite hydrogels with improved mechanical properties exhibit rapid, significant and reversible electro-response, which show great potential for developing remotely-controlled electro-responsive hydrogel systems, such as smart actuators and soft manipulators .

KEYWORDS Reduced graphene oxide; Stimuli-responsive materials; Hydrogels; Electro-response; Actuators


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INTRODUCTION Smart hydrogels have drawn considerable attention for myriad applications including optical systems,1 chemical valves,2 vehicles for drug delivery,3-5 biomimetic engineering,68

and tissue engineering.9-11

It is attributed to their changeable volume or

physical/chemical properties responding to environmental stimuli like temperature,12-15 pH,16-18 electric field,19-22 humidity,

23-25

light,26-28 special ions or molecules.29-32 Since the

electric stimuli can be remotely and facilely controlled with adjustable direction and strength, the electro-responsive hydrogels have attracted considerable interests.

The

hydrogels composed of polyelectrolyte, possessing electro-responsive swelling/shrinking or bending/unbending, are capable to accomplish the conversion of electric energy to mechanical energy for soft actuators.33,34 Typically, the most popular electro-responsive hydrogel is constituted of poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) polymer, which is polymerized by the strong acidic monomer, 2-acrylamido-2methylpropanesulfonic acid (AMPS). Such a monomer can completely ionize in aqueous solution over a broad pH range from 2 to 12, resulting in the good electro-responsive property of PAMPS hydrogel, which is not conspicuously affected by pH. Thus, compared with other weak electrolyte hydrogels like poly(acrylic acid)-based hydrogels whose responsive properties are influenced by environmental pH values, the PAMPS hydrogels show great application potentials.

For example, due to their electro-response, PAMPS

hydrogels have been extensively applied for fabricating soft biomimetic actuators, e.g., artificial muscle,35,36 crawler,20 “swimming” device,37,38 and robotic hand.39

In the

application of the hydrogels for soft actuators, the responsiveness and mechanical properties are two critical characteristics.

To achieve the versatility of these hydrogels, fast and

significant responsiveness and excellent mechanical properties are desired in most cases.40-45 For examples, these characteristics would contribute a conspicuous and rapid feedback in



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response to environmental stimuli, while the good deformability and extensibility would withstand the extraneous forces. Actually, conventional PAMPS polymeric hydrogels crosslinked by small molecules exhibit poor mechanical properties originating from the irregular distribution of the crosslinking points and the widespread distribution of the cross-linked chain lengths, which seriously restrict the application scope of such hydrogels.40 Meanwhile, the electro-responsive properties of PAMPS hydrogels are still coveted to be further enhanced for expansive uses. Thus, the development of PAMPS-based hydrogels possessing rapid and significant electro-response as well as excellent mechanical properties has a great significance on both science and technology. So far, the electro-responsive property of PAMPS hydrogels can be improved by the following strategies. One strategy is fabricating the PAMPS hydrogels with macroporous structure to improve the ionic transport in the polymer network. For example, Lin and coworkers have fabricated PAMPS hydrogels with macroporous structure employing polyethylene glycol 6000 (PEG6000) molecules as pore-forming agents.46

When the

hydrogels with sizes of 20 mm×4mm×2mm are immersed in 0.05 M NaCl solution under a voltage of 30 V , the bending curvature of the PAMPS hydrogels with macroporous structure introduced by PEG6000 could increase from 0.0297 mm-1 to 0.0454 mm-1within 5 min. Compared with the equilibrium bending curvature of control PAMPS hydrogels without the introduction of PEG6000 being 0.0401 mm-1, that of the macroporous PAMPS hydrogels at equilibrium state only increases to 0.0489 mm-1. Although the electro-responsive property of PAMPS hydrogels can be improved to a certain extent by forming the macroporous structure, the mechanical property of the hydrogel tends to weaken because of the macropores in the polymeric network.40,47

Alternatively, adding some electro-active components such as

polypyrrole (PPy) and polyaniline (PANI) can also improve the electro-responsive property of PAMPS hydrogels.48,49 For instance, Wu and co-workers have introduced PANI into


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PAMPS-based hydrogels to form interpenetrating polymer networks (IPN).49 Such PANIcontaining IPN hydrogel with size of 20 mm×4 mm×2 mm reaches its equilibrium bending curvature more rapidly in 0.2 M NaCl solution applied an electric field with 25 V voltage. The required time decreases from 20 min to 13 min compared with that of normal PAMPS hydrogels. Moreover, the interpenetrating polymer networks enable the hydrogels to improve the mechanical property.

Gangopadhyay and co-workers have fabricated an electrically

conducting semi-IPN PAMPS hydrogel with linear conducting PANI entrapped in the polymeric network.50 Compared with that of normal PAMPS hydrogel, the compressive stress of the semi-IPN one increases from 0.3 MPa to 0.75 MPa. In addition, the tensile stress increases from 0.012 MPa to 0.014 MPa, but the tensile fracture strain only increases from 40% to 92%.

The semi-IPN PAMPS hydrogels could achieve an increase in

compressive strength, while the enhancement of tensile strength is limited which may restrict their practical applications for soft actuators.

Thus, the preparation of PAMPS-based

hydrogels with excellent electro-responsiveness and mechanical property especially the tensile strength is still in a dilemma. Here, we report a novel nanocomposite PAMPS-based hydrogel with excellent electroresponse and mechanical property by introducing reduced graphene oxide (rGO) nanosheets into poly(2-acrylamido-2-methylpropanesulfonic acid-co-acrylamide) (poly(AMPS-co-AAm)) hydrogel. Compared with the normal PAMPS hydrogels, the rGO/poly(AMPS-co-AAm) nanocomposite hydrogels exhibit rapid and significant response to electric-stimulus due to the excellent electronic conductivity of rGO nanosheets enhancing the ion transport in the polymeric network. It has been reported that the rGO nanosheets exhibit excellent electronic conductivity,51-57 thus they can provide electric platforms to promote the ion transport in the rGO/poly(AMPS-co-AAm) nanocomposite hydrogels under electric field. The deformation behaviors of electro-responsive hydrogels can be explained by Flory’s theory of the osmotic



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pressure.33 The proposed mechanism on the improvement of electro-responsive property in this study is different from those reported in the previous works that incorporation of graphene

oxide

(GO)

nanosheets

into

electro-responsive

poly(acrylic

acid) and

polyacrylamide-co-poly(acrylic acid) hydrogels,58-59 in which the electro-responsive performances are enhanced due to the carboxyl groups of GO nanosheets in the alkalescent aqueous solution. When applied a DC electric field, the electrons in the electrostatic double layer of the GO nanosheets get polarized, thus a secondary electric field that combines with and strengthens the first field is generated, which contributes to the improvement of the bending actuation performan.58 However, in our rGO/poly(AMPS-co-AAm) nanocomposite hydrogel networks, with the assistance of rGO nanosheets as conductive platforms, the ions directionally move out to quickly generate a remarkable osmotic pressure due to the strong acidic group of the propanesulfonic acid in the networks. The nanocomposite hydrogels are proposed with controllable and reversible electro-induced deswelling and bending behaviors under cyclic "on/off" electrical fields. Meanwhile, the tensile strength and compressive strength are both increased with addition of rGO nanosheets due to the composite structure of polymer matrix and rGO nanosheets which is beneficial for dissipating energy. The proposed rGO/poly(AMPS-co-AAm)

nanocomposite

hydrogels

possess

excellent

electro-

responsiveness and mechanical property, so that they have great prospects for the applications of remotely-controllable intelligent systems, like soft actuators, robots and artificial muscles.

EXPERIMENTAL SECTION Materials 2-Acrylamido-2-methylpropanesulfonic acid (AMPS) from Sigma-Aldrich, and potassium persulfate (KPS, K2S2O8), acrylamide (AAm), N,N’-methylenebisacrylamide (MBA),


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sodium chloride (NaCl), hydrazine hydrate and ammonia solution from Chengdu Kelong Chemical Reagents were used as received without further purification. Raw graphite (325 mesh) purchased from XF NANO Ltd was used to synthesize graphene oxide (GO) via the modified Hummers’method.60 Deionized water from a water purification system (Milli-Q Plus, Millipore) was used.

Preparation of rGO/Poly(AMPS-co-AAm) Nanocomposite Hydrogels The nanocomposite hydrogels were synthesized using thermal-induced polymerization method with AMPS and AAm as monomers, MBA as chemical cross-linking agent, GO nanosheets as additives, KPS as initiator. Typically, AMPS (0.7 M), AAm (0.3 M), MBA (0.05 M) and KPS (0.01 M) were dissolved in the GO suspension (5 mL). Next, hydrazine hydrate and ammonia solution were added to the reaction solution. The weight of hydrazine was about 70% to that of GO, the volume ratio of hydrazine solution to ammonia solution was about 1:7. After vigorously stirred and ultrasonic treatment, the reaction solution was put in a water bath for simultaneous polymerization and reduction at 65 °C for 8 hours. Subsequently, the resulting product was kept at 90 °C for 1 h for further reduction of GO. To study the influences of the rGO content on the electro-responsiveness and mechanical property, a series of rGO/poly(AMPS-co-AAm) nanocomposite hydrogels were prepared containing different rGO contents according the GO concentration in the reaction solution, which was typically 0.2, 0.5, and 1.0 mg·mL−1. The as-prepared rGO/poly(AMPS-co-AAm) nanocomposite hydrogels were labeled as “rGO-x”, in which “x” represents the mass concentration of GO (mg·mL−1). For comparison, poly(AMPS-co-AAm) hydrogels (labeled as “blank gel”) were also prepared by the similar method but without addition of GO nanosheets. After polymerization and reduction, these hydrogels were thoroughly purified with excessive water.



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SEM and TEM Characterizations and Raman Spectra of Hydrogels The microstructures of the proposed hydrogels were observed by field emission scanning electron microscope (FESEM, JSM-7500F, JEOL). The hydrogel samples were treated by freeze-drying method for SEM observation. The dispersion morphology of rGO nanosheets in the nanocomposite hydrogel was studied by transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN). To prepare samples for TEM observation, ultra-thin lamella of hydrogel was first cut with cryosection system (Leica FC6), and then collected on copper grids and subsequently dried at room temperature. Raman spectra were obtained from a Raman microscope (Horiba Jobin Yvon LabRAM HR).

The experimental samples were rGO power and rGO-0.5 hydrogel, which were

measured after lyophilization. A diode laser operated at 633 nm was used for excitation.

Mechanical Property Tests Measurements of the mechanical properties of the as-prepared hydrogels were performed with a testing machine (EZ-LX, Shimadzu) at 25 °C. For the tensile tests, the rod-like samples (the diameter is 4.3 mm, the length is 30 mm and the gauge length is 10 mm) were stretched under a constant rate of 100 mm/min.

For the compressive tests, cylindrical

samples with diameter of 12.6 mm and height of 10 mm were put between the self-leveling plates. The compressed rate on the samples was 100 mm/min.

Electro-Induced Deswelling Behavior Tests The electro-induced deswelling behaviors of hydrogels were investigated by monitoring the weight change upon exposure to a DC electric voltage of 10V using a power supply. The equipment illustrated in Figure S1 was used to study the shrinking behavior of hydrogel.


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Fully swollen hydrogels with the size of 10.0mm×5.0mm×5.0mm were glued with two carbon electrodes, which were connected to a DC power supply. The contact area is 10.0 mm×5.0 mm. Water would be released from the hydrogels under the DC electric voltage. Weight measurements of the hydrogels were achieved by an electronic balance at various time intervals. The deswelling behaviors of the hydrogels were evaluated by the weight change of the hydrogels. The weight change percentage could be calculated as follows: Weight change percentage (%) = Wt/Wo × 100%

(1)

where Wt is the measured hydrogel weight at time t under electric field, W0 is the hydrogel weight at t=0 s (fully swollen state).

To investigate the reversibility of the

deswelling/swelling properties of such hydrogels, the rGO/poly(AMPS-co-AAm) nanocomposite hydrogels were alternately exposed to an electric field (10 V). The electric field was turned on for 2 min to reach the full deswelling of the hydrogel. Then, the hydrogels were immersed in water for 6 min without electric-stimulus. In addition, the electrical conductivity of nanocomposite hydrogels before and after the reduction of GO have been measured using a source meter (Model 2401, Keithley). The electrical conductivity (σ) was calculated using the following two equations: ρ=Rs×l (Ω.m)

(2)

σ=1/ρ (S/m)

(3)

where ρ is the bulk resistivity of the hydrogels, Rs is the sheet resistance and l is the thickness of the sample between two probes.

Electro-Induced Bending Behavior Tests The hydrogel samples of blank gel and rGO-0.5 hydrogel were used to investigate the bending behaviors. The size of both hydrogel samples was 20.0 mm×2.0 mm×1.0 mm (length×width×thickness). The methylene blue was used to dye the blank gel samples for



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easy observation. The equipment illustrated in Figure S2 was used to study the bending behaviors of hydrogel. Two carbon electrodes in NaCl solution (0.01 M) were parallelly placed with a distance of 44 mm, which were connected to a DC power supply to form a horizontal electric field. Further, the hydrogel strips were vertically put in the solution and placed centrally between the two electrodes, with 20.0 mm×2.0 mm side facing the electrodes. The top of the hydrogel strip was fixed by a clamp and ca. 15.0 mm in length was immersed in the solution with a 10 V supply voltage. We used a camera (E-PL5, Olympus) to record the bending behaviors of the hydrogel samples within 2 min.

Applications of Hydrogels as Soft Actuators The size of hydrogel strips was 25.0 mm × 3.0 mm × 0.5 mm (length × width × thickness). Using the same equipment illustrated in Figure S2, the hydrogel strip was immersed in 0.01 M NaCl solution with length of 17.0 mm, and was used to act as a “hydrogel cantilever”.

The applied electric voltage was 10 V.

The target objects were

polydimethylsiloxane (PDMS) blocks with different masses (0.2 g or 0.4 g). The whole actuating behaviors of these hydrogels were recorded with a camera. Then, rectangular hydrogels with sizes of 35.0 mm × 10.0 mm × 0.5 mm (length × width × thickness) were tied to a string by magnetic sheets to perform as a “hydrogel gripper”. The schematic diagram of the equipment of the hydrogels gripping motion was shown in Figure S3. The distance of the two parallel carbon electrodes was 34 mm. A substrate was put in the center of the two electrodes to form a platform. The PDMS strip with length of 15 mm as target object for gripping was put on the platform. The gripping behaviors of hydrogels were recorded with a camera. The applied electric voltage was 30 V.

RESULTS AND DISCUSSION


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Fabrication Strategy The fabrication strategy of the proposed nanocomposite hydrogels is illustrated in Figure 1. Firstly, the monomer solution containing AMPS, AAm, MBA, and GO nanosheets is treated by ultrasonic for 10 min to make GO nanosheets homogeneously dispersed in the solution (Figure 1a).

Then, the polymerization is initiated by KPS.

The reduction of the GO

nanosheets happens at the same time to avoid of the aggregation problem of rGO nanosheets in the hydrogel networks, which is further reduced again (Figure 1b). For the rGO-containing poly(AMPS-co-AAm) hydrogel, the AAm groups are added to make the polymer chains soft.21 The hydrogel network is chemically crosslinked via the covalent bonds created by MBA molecules (Figure 1b). Additionally, the residual oxygen-containing groups on the edge of rGO nanosheets such as hydroxyl and carbonyl enable to form hydrogen bond interactions with the groups of poly(AMPS-co-AAm) chains (Figure 1c), which can physically crosslink the copolymer chains, thus increasing the mechanical properties of the nanocomposite hydrogels via dissipating the entheticforce.61 The rGO nanosheets exhibit excellent electronic conductivity, and thus provide electric platforms to promote the ion transport in the hydrogel under electric field,51-57 as a result, the rGO/poly(AMPS-co-AAm) hydrogels are expected to exhibit good electro-responsiveness.

Optical and Morphological Analyses of rGO/Poly(AMPS-co-AAm) Nanocomposite Hydrogels Optical images show the hydrogels are homogeneously fabricated by our strategy and the color of the hydrogel becomes darker with increasing the rGO content (Figure 2a), compared with the nanocomposite hydrogel that prepared by directly doping rGO nanosheets (Figure S4). Typical TEM images show the homodispersed distribution of rGO nanosheets in the rGO/poly(AMPS-co-AAm) hydrogel prepared by our strategy (Figure S5). The SEM images



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show the internal network structures of the hydrogel samples with different contents of rGO (Figure 2b). The porous structures of the hydrogels are caused by the ice crystals, which act as the pore-forming templates during the lyophilization. The network structures become denser and more compact with increasing the rGO content due to the increased physical crosslinking caused by the enhancement of interaction between rGO nanosheets and polymeric networks. The results indicate that the rGO content remarkably influences the internal network structures of hydrogels by forming physical crosslinking between rGO nanosheets and polymeric chains. Further, as shown in the Raman spectra (Figure 3), the G-band and D-band of rGO nanosheets (lower) and rGO-0.5 hydrogel (upper) are presented. Compared to the G-band and D-band positions of rGO nanosheets, the rGO-0.5 hydrogel shows a positive shift of Dband while a negative shift of G-band. The G-band negatively shifts from 1610 to 1606 cm-1, and the D-band of rGO-0.5 hydrogel positively shifts from 1329 to 1338 cm-1. The results imply the interaction between the rGO and polymeric chains.62,63

Mechanical Properties of rGO/Poly(AMPS-co-AAm) Nanocomposite Hydrogels The mechanical properties of the nanocomposite hydrogels are measured in relative shrinking states rather than fully swollen states, because the nanocomposite hydrogels are ususally considered to apply for applications with stimuli-induced bending, which is caused by the asymmetric shrinking of the hydrogel due to the osmotic pressure difference induced by electric-field-driven ion motion. Compared with blank gel samples, the rGO/poly(AMPS-coAAm) nanocomposite hydrogels exhibit excellent mechanical properties on both tensile strength and compressive strength. The blank gel is easy to be broken by elongating and compressing (Figure 4a1, Supplementary Movies S1). Excitingly, the composite hydrogel containing rGO, such as rGO-0.5 sample, displays a high performance in tensile strength. It


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can be even knotted and then stretched without damage (Figure 4a2, Supplementary Movie S2). Under the same compression, the composite hydrogel is much more tough than the blank hydrogel to withstand the high deformation (Figure 4b, Supplementary Movie S3). Upon removing of the compression, the rGO-0.5 hydrogel quickly recovers its original shape. The blank gel is broken at the compressive stress of 0.4 MPa (Figure 4c). By introducing rGO nanosheets in the network of rGO-0.5 hydrogel, its compressive stress at broken point is increased to 1.4 MPa. Further, the mechanical tensile stress tests of hydrogels with different contents of rGO are performed by using a universal testing machine. The stress-strain curves of blank gel, rGO-0.2, rGO-0.5 and rGO-1.0 hydrogels are shown in Figure 4d. Both the fracture stress and strain are increased with increasing the rGO content due to the incremental physical crosslinking. As a comparison, the tensile mechanical property of fully swollen rGO-0.5 hydrogel is decreased (Figure S6). As shown in Figure 4e, all the fracture stresses of the rGO/poly(AMPS-co-AAm) nanocomposite hydrogels are higher than that of the blank gel sample. The fracture stress of rGO-1.0 hydrogel is 96.68 kPa, which is four times larger than that of the blank gel. The tensile fracture strains of the hydrogels also increase gradually with increasing the rGO content (Figure 4f). The fracture tensile strain of rGO-1.0 hydrogel reaches 297.93%. The addition of rGO nanosheets makes the nanocomposite hydrogels more elastic, and the elastic modulus of the hydrogels will increase as the rGO content increases (Figure S7), which may imply the physical crosslinking mechanism between the rGO and the matrix. The results confirm that the hydrogen bond interactions between the rGO nanosheets and poly(AMPS-co-AAm) chains are expected to transfer load between the polymer chains and rGO nanosheets, resulting in the enhancement of mechanical properties. Additionally, the rGO nanosheets can twirl to parallel to the strain axis in the polymeric network during stretching process, causing the dissipation of strain.64,65



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Electro-responsive Properties of rGO/Poly(AMPS-co-AAm) Nanocomposite Hydrogels When the rGO/poly(AMPS-co-AAm) nanocomposite hydrogels are placed in contact with both electrodes under electric fields (as illustrated in Figure S1), the shrinking deformation of the hydrogels occurs. The electric field not only produces the motion of free ions that causes the electro-osmosis of water due to the asymmetrical distribution of ions between interior and exterior of the hydrogel, but also engenders a force on the negatively charged sulfonic acid groups in the polymer network. The stress gradient makes the hydrogel shrink.33,66-68 The electro-induced deswelling behaviors of hydrogels containing different contents of rGO are investigated by monitoring the weight lose under an electric field with DC voltage of 10V. The water is released from the hydrogel matrix, which is monitored at various time intervals.

The reduction effect of GO mainly depends on treatment

temperature,69 so the resulted mixture has been kept at 90 °C for 1 h for further reduction of GO.

The reduction of GO can significantly increase the electrical conductivity of the

nanocomposite hydrogels (Figure S8), thus providing better electric platforms to promote the ion transport in the nanocomposite hydrogels under electric field. Therefore, the electroresponsive property of rGO/poly(AMPS-co-AAm) nanocomposite hydrogels could be improved by further reducing the rGO nanosheets (Figure S9). As shown in Figure 5a, the rGO content in the network affects the deswelling behaviors of hydrogels under voltage of 10 V. There is an improvement of the electro-induced deswelling property as the content of rGO increases from 0 to 1.0 mg mL−1. For example, rGO-1.0 hydrogel displays a relatively rapid deswelling, and the weight loss ratio is approximately 71% within 5 min. While, the weight loss ratio of blank gel only achieved to be ~35% within 5 min. It is thus confirmed that the rGO/poly(AMPS-co-AAm) nanocomposite hydrogels deswell faster than the blank gel due to the rGO nanosheets in the networks, which promote the ion transport in the hydrogel with DC voltage of 10 V. When the applied voltage increases from 5 V, 10 V to 15


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V, the rGO-0.5 hydrogel exhibit faster and more significant deswelling behavior (Figure 5b). Moreover, to investigate the reversibility and repeatability of the swelling/shrinking behaviors, the rGO-0.5 hydrogels are alternately exposed to an electric field (10 V). The electric field with DC voltage of 10 V is applied to the hydrogels for 2 min. After removing the electric field, the hydrogels are put in water to swell for 6 min. As shown in Figure 6, when applying the electric field for 2 min, hydrogels shrink to 58~68% of original weight; when removing the electric field then immersing the shrunken hydrogels in water for 6 min, the rGO-0.5 hydrogels can re-swell. The results confirm the excellent reversible electroinduced swelling/shrinking properties of such nanocomposite hydrogels. Under the noncontact electric field in an electrolyte solution (Figure S2), the rGO/poly(AMPS-co-AAm) hydrogel strips show bending deformation. Because the transport of mobile ions under the electric field causes osmotic pressure difference between the exterior and interior of the hydrogels, which drives the bending of rGO/poly(AMPS-co-AAm) hydrogels toward the cathode.

When the PAMPS-based hydrogels are loaded with

noncontact electric field in an electrolyte solution such as NaCl, the free ions in the solution will transport to the counter-electrodes.

While, the crosslinked polymer networks will

prevent the transport of the sulfonic groups, only counterions of the sulfonic groups are mobile and can transport to the counter-electrodes. Thus, an ionic concentration gradient will be created along the direction of electric fields due to the ion motions, which leads to the osmotic pressure difference inside hydrogels.21,68 The osmotic pressure of the hydrogel side facing the anode is greater than that of the opposite side facing the cathode, so the hydrogel will bend to the cathode side.21 To investigate the electro-induced bending behaviors of the hydrogels containing different rGO contents, hydrogel samples are immersed in electrolyte and put centrally between two electrodes (Figure S2). The curvature value is used to evaluate the bending behaviors of hydrogels. Compared with the blank gel (Figure 7a), the rGO-0.5



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hydrogel bends faster and more significant (Figure 7b, Supplementary Movie S4). As shown in Figure 7c, both the rate and degree of the bending deformation are increased as the rGO content increases. The curvature value of rGO-1.0 hydrogel reaches 0.088 mm-1, while that of blank gel is only 0.05 mm-1. The results indicate that the incorporation with rGO nanosheets could improve the electro-responsiveness including both bending rate and bending degree of hydrogels.

Demonstrations of rGO/Poly(AMPS-co-AAm) Nanocomposite Hydrogels as Actuators The rGO/poly(AMPS-co-AAm) nanocomposite hydrogels exhibit reversible bending behaviors in electrolyte solutions, so they are highly attractive and promising to be used as soft actuators by alternately applying electric field. To demonstrate the applications, the hydrogels are designed as “soft cantilevers” and “soft grippers” based on their electroinduced bending behaviors. The blank gel and rGO-0.5 hydrogel are first cut into strips and then used as “hydrogel cantilevers”. The hydrogel strips are vertically immersed in the solution with the top fixed, which are placed centrally between the two electrodes. PDMS blocks (0.2 g or 0.4 g) are used as target objects for lifting. The lifting behaviors of blank gel and rGO-0.5 hydrogel for lifting the 0.2 g PDMS blocks are shown in Figures 8a and 8b, respectively (Supplementary Movie S5). By applying an electric voltage of 10 V, both blank gel and rGO-0.5 hydrogel can bend toward the cathode side and lift the PDMS block. Compared with the blank gel, the rGO-0.5 hydrogel could lift the block (0.2 g) faster and higher. When lifting a heavier block (0.4 g), the blank gel could not afford efficient driving force to achieve the lifting of the block (Figure 8c). While the rGO-0.5 hydrogel could lift the 0.4 g heavy load successfully (Figure 8d, Supplementary Movie S6). By taking into account of buoyant force, the real output force of the nanocomposite hydrogel upon electro-responsive


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bending is calculated as ca. 0.38 mN. The results show the actuating performances of rGOcontaining hydrogels are better than that of the blank gel due to the incorporation of rGO nanosheets in the hydrogels improves both electro-responsive property and mechanical property. The blank gel and rGO-0.5 hydrogel are also used as “hydrogel grippers”. First, the hydrogels are cut into rectangles, and then connected to a string by using magnetic sheets. The “hydrogel gripper” is hung between two parallel carbon electrodes to pick up the target object, remotely controlled by applying electric field (Figure S3). The length of the target object is 15 mm. As shown in Figure 8e, under the electric voltage of 30 V, the blank gel still could not generate enough bending deformation to capture the target object within 30 s. However, the rGO-0.5 hydrogel could sufficiently bend to pick up the target object within only 12 s (Figure 8f, Supplementary Movie S7).

CONCLUSION In summary, rGO/poly(AMPS-co-AAm) nanocomposite hydrogels with excellent electroresponse and mechanical property have been developed by a two-step reduction method to avoid the aggregation of rGO nanosheets in the hydrogel networks. The incorporation of rGO nanosheets could physically crosslink the poly(AMPS-co-AAm) chains, resulting in effective dissipation of strain and transfer of load between the polymer networks and the rGO nanosheets. The mechanical properties of the nanocomposite hydrogels are significantly improved on both tensile strength and compressive strength.

The excellent electronic

conductivity of rGO nanosheets enables to promote the ion transport in the hydrogels, thus causing improved electro-responsive properties including both deswelling/swelling and bending/unbending

performances.

The

rGO/poly(AMPS-co-AAm)

nanocomposite

hydrogels exhibit repeatable and reversible responsive deformation by cyclically applying



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electric fields. The responsive rate and degree are increased with increasing the rGO content of hydrogels. Based on the electro-induced bending property, the proposed nanocomposite hydrogels are capable to be developed as soft robots like cantilevers and grippers. Such nanocomposite hydrogel systems show great potential in myriad applications for remote manipulation and transportation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.orgor from the author. Schematic illustration of the device for studying the electro-induced deswelling behaviors of hydrogels; Schematic illustration of the device for studying the electro-induced bending behaviors of hydrogels; Schematic illustration of the device to perform a “hydrogel gripper”; Optical image of nanocomposite hydrogel prepared by directly doping rGO nanosheets; TEM image of rGO-0.5 hydrogel; Tensile stress-strain curve of the fully swollen rGO-0.5 hydrogel; Effect of rGO concentration on the elastic modulus of hydrogel; Effect of the reduction of GO on the electrical conductivity of hydrogel; Effect of the reduction of GO on the electroinduced deswelling behavior of hydrogel. (PDF) Movie S1 of tensile test performed on the blank gel (AVI) Movie S2 of tensile test performed on therGO-0.5 hydrogel (AVI) Movie S3 of compressive test performed on the blank gel and rGO-0.5 hydrogel (AVI) Movie S4 of electro-responsive bending behaviors of blank gel and rGO-0.5 hydrogel (AVI)


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Movie S5 of lifting behaviors of blank gel and rGO-0.5 hydrogel for the load with weight of 0.2 g (AVI) Movie S6 of lifting behaviors of blank gel and rGO-0.5 hydrogel for the load with weight of 0.4 g (AVI) Movie S7 of the gripping behaviors of blank gel and rGO-0.5 hydrogel for the target object with length of 15 mm (AVI)

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Z. Liu). * E-mail: [email protected] (L.-Y. Chu). Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge support from the National Natural Science Foundation of China (21490582, 81621062), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).



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FIGURES

Figure 1. Schematic illustration of fabrication process and structure of the proposed rGO/poly(AMPS-co-AAm) nanocomposite hydrogels. (a, b) The polymerization of hydrogel is simultaneous with the reduction of graphene oxide. (c)The rGO/poly(AMPS-co-AAm) nanocomposite hydrogels are chemically crosslinked by MBA, and the hydrogen bond interactions between rGO nanosheets and polymer chains result in the physical crosslinking.


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Figure 2. Optical (a) and SEM (b) images of hydrogels with different contents of rGO. (b1) blank gel, (b2) rGO-0.2, (b3) rGO-0.5 and (b4) rGO-1.0. Scale bars in SEM images are 100 µm.



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Figure 3. Raman spectra of rGO powder and the rGO-0.5 hydrogel.


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Figure 4. Mechanical properties of hydrogels. (a, b) Tensile test (a) and compressive test (b) of blank gel and rGO-0.5 hydrogel. (c, d) Compressive stress-strain curves (c) and tensile stress-strain curves (d) for blank gel and rGO-containing hydrogels. (e, f) Effects of rGO concentration on the tensile fracture stress (e) and tensile fracture strain (f) of hydrogels.



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Figure 5. (a) Effect of rGO concentration on the deswelling behaviors of hydrogels with different rGO contents with an applied voltage of 10V. (b) Effect of applied voltage on the deswelling behaviors of rGO-0.5 hydrogels.


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Figure 6. Reversible deswelling/swelling behaviors of rGO-0.5 hydrogel under cyclic “on/off” electric fields (10 V, on: 2 min, off: 6 min).



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Figure 7. Effect of rGO content on the bending behaviors of rGO/poly(AMPS-co-AAm) composite hydrogels with an applied voltage of 10V. (a, b) Optical photographs of the electro-responsive bending behaviors of blank gel (a) and rGO-0.5 hydrogel (b). (c) Electroresponsive change of the curvature of hydrogels with different rGO contents.


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Figure 8. Applications of hydrogels as “hydrogel cantilevers” (a-d) and “hydrogel grippers” (e, f). (a, b) The lifting behaviors of blank gel (a) and rGO-0.5 hydrogel (b) cantilevers for lifting PDMS block with weight of 0.2 g. (c, d) The lifting behaviors of blank gel (c) and rGO-0.5 hydrogel(d) for lifting PDMS block with weight of 0.4 g. (e, f) The gripping behaviors of blank gel (e) and rGO-0.5 hydrogel (f) grippers for gripping a target object with length of 15 mm.



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Graphic for TOC


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Reduced Graphene Oxide-Containing Smart Hydrogels with Excellent

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