Photoresponsive Graphene Composite Bilayer Actuator for Soft

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Photoresponsive Graphene Composite Bilayer Actuator for Soft Robots Xiaodong Wang, Niandong Jiao, Steve Tung, and Lianqing Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09491 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

Photoresponsive Graphene Composite Bilayer Actuator for Soft Robots Xiaodong Wang1,2,3, Niandong Jiao1,3*, Steve Tung1,3,4, Lianqing Liu1,3* 1. State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China 2. University of the Chinese Academy of Sciences, Beijing 100049, China 3. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110016, China 4. Department of Mechanical Engineering, University of Arkansas, Arkansas 72701, USA * Corresponding authors E-mail: [email protected], [email protected] KEYWORDS. Soft actuator, photoresponse, biomimetic, superhydrophobic, composite materials

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ABSTRACT

Highly deformable and photoresponsive smart actuators are attracting increasing attention. Here, a high concentration of graphene is dispersed in polydimethylsiloxane (PDMS) by combining the advantages of various dispersion methods. The composite and pure PDMS layers are used to fabricate bilayer actuators with a high capacity for rapid deformation. The fabricated bilayer actuators exhibit novel and interesting properties. A bilayer actuator containing a 30 wt% graphene composite can be deflected by 7.9 mm in the horizontal direction under infrared laser irradiation. The graphene concentration in the composite influences actuator adjustment to deformation and its response speed, and the composite also exhibits superhydrophobicity. Based on its superhydrophobicity and large deformation capacity, the actuator made with 30 wt% graphene composite is used to construct a beluga whale soft robot. The robot can swim quickly in water at an average speed of 6 mm/s, and it can cover a distance of 30 mm in 5 s when irradiated just once with an infrared laser. Actuators fabricated with this method can be used in artificial muscle, bionic grippers, and various soft robots that require actuators with large deformation capacities.

1. INTRODUCTION In the context of smart materials, soft actuators are highly flexible and display excellent mechanical deformation properties when exposed to an external stimulus, such as a chemical agent,1 temperature,2 light,3 electricity,4 and humidity.5 The unique characteristics of these actuators grant them broad prospects for applications involving artificial muscles, microgrippers, and microrobots.6,7 Of particular interest are intelligent actuators comprised of light-responsive materials that can convert light energy into mechanical energy. The area exposed to light is adjustable, so actuators made of light-responsive materials can be precisely controlled. Both local and total deformation can be induced, and cable-free remote control is 2 ACS Paragon Plus Environment

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possible, which are very important attributes for practical applications.8 Because of these advantages, actuators constructed from light-responsive materials have received a great deal of attention in recent years. However, improvements in the photoresponsive deflection and deflection speed of these actuators are urgently needed. Some researchers have employed temperature-responsive hydrogels as raw materials in combination with various photosensitive materials to produce smart software actuators that respond to light stimulation. For instance, Wu et al. fabricated actuators with gold nanoparticles and temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogels that responded to infrared stimulation.9 However, actuators prepared with hydrogels are all operated in liquid or humid environments,10 which greatly limits their application. Because the deformation of hydrogels is caused by the movement of water molecules, their deformation speeds are low.11 In addition, the synthesis of light-responsive hydrogels is very complex.12 Photodeformable actuators that are operable in any environment are thus desirable. Researchers have recently discovered that azobenzene molecules can undergo isomerization from the trans- to cisconfiguration upon exposure to ultraviolet (UV) light, and that the molecules can convert back to the trans-configuration when exposed to white light.13 Photodeformable thin-film actuators based on the photoisomerism of the azobenzene molecule have been prepared with liquid crystal polymers. These have been employed in microrobots,14,15 microgrippers, and microfluidic mixing.16,17 However, the synthesis of macromolecular azobenzene chains is challenging.18 Actuators fabricated with commercial azobenzene have slow deformation speeds, and stimulation with two types of light is required to achieve deformation and recovery. These factors limit the application of azobenzene actuators. Carbon-based materials, such as graphene, display excellent photoresponsiveness and photothermal conversion characteristics. These materials can convert light energy into heat energy, and they are excellent thermal conductors.19 For these reasons, they are good light3 ACS Paragon Plus Environment


responsive additives for composites.20 By combining graphene and other carbon-based materials with polymers, researchers have fabricated bilayer actuators that are highly deformable when stimulated by external light.21 For example, Hu et al. prepared a bilayer actuator with a high deformation capacity and fast response time by combining reduced graphene oxide (RGO) with carbon nanotubes (CNT) and PDMS. They then used the actuator to construct a light-driven crawler-type robot.22 And they also fabricated a bilayer photoactuator with CNTs and PDMS and used it to make a jumping robot.23 Chen et al. built a bilayer actuator with graphene oxide and a biaxially oriented polypropylene (BOPP) composite. Under irradiation with infrared light, the actuator displayed significant bending capability.24 Bilayer actuators that contain a carbon-based layer and a polymer layer are capable of substantial and rapid deformation. However, the carbon material binds weakly and readily detaches during preparation and deformation. The carbon-based layer in the actuator is very thin, and the manufacturing process is complex and costly. Thus, these actuators are not wellsuited for large-scale production and application. To overcome this problem, scientists have fabricated photoresponsive composites of carbon-based materials and polymers with excellent properties. For example, Balaji et al. mixed graphene with PDMS to prepare composites capable of photo-induced deformation.25 Jia et al. fabricated grating structures using PDMS and graphene composites that could be controlled with infrared light, thereby demonstrating their potential for applications in biochemical sensing.26 Wang et al. manufactured a caterpillar-like robot using graphene and PDMS composites, which could move when stimulated with UV light.27 Liu et al. reported using graphene and PDMS composites to fabricate bionic robotic platforms and a cell trainer.28,29 However, the graphene/PDMS composites used in these studies contained low concentrations of graphene by mass. This resulted in lower energy conversion efficiency and thermal conductivity than desired. The deformability of the composites fabricated in these studies was low, so they were not able to meet the demands of significant 4 ACS Paragon Plus Environment

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actuator deformation. These disadvantages greatly limit the prospects for application of graphene composites in actuators. Thus, the fabrication of highly responsive graphene composite actuators with good deformability with simple methods is a hot research topic. In this work, we combined graphene and PDMS to manufacture highly deformable bilayer actuators. We integrated multiple existing dispersion methods to improve the dispersion of graphene in PDMS, thereby increasing the concentration of graphene by mass in the composite. Because the composite and pure PDMS had different coefficients of thermal expansion (CTEs), we were able to construct a bilayer actuator with a strong deformation response to infrared light. The actuator utilized infrared energy efficiently, so it was highly deformable under lowintensity infrared light. Moreover, bonding between the two layers of the actuator was strong, so it was highly stable. The bilayer actuator was also superhydrophobic, and a beluga whalelike robot fabricated with the actuator swam rapidly in water. The actuator prepared in this study has excellent prospects for applications in soft robotics and microrobotics. 2. THEORETICAL ANALYSIS OF BILAYER ACTUATOR The bilayer actuator consisted of two layer membranes. The lower membrane was active layer 2, and the upper membrane was active layer 1 (Figure 1a). Under infrared irradiation, the upper membrane converted light energy into heat energy and transmitted it to the lower layer. The CTE of the lower layer was larger than that of the upper layer, so the bilayer actuator could bend upwards. According to the beam theory,30,31,47

6 E1 E2 a1a2  a2  a1  2  1  T 1  . R  E a 2 2   E a 2 2  2 E E a a  2a 2  3a a  2a 2  1 1 2 2 1 2 1 2 1 1 2 2

(1)

R in Equation (1) is the deformation curvature radius of the bilayer film; E1 and E2 are the elastic moduli of active layers 1 and 2, respectively; a1 and a2 are the thicknesses of layers 1 and 2 respectively; 1 and 2 are the CTEs of layer 1 and layer 2, respectively; and T is the 5 ACS Paragon Plus Environment


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temperature change. Deflection of the actuator is thus determined by several variables. These can be simplified to

a1 E  m, 1  n , and a1  a2  h a2 E2

(2)

Combining Equation (1) and (2) yields

6  2  1  T 1  m  1  R 1   2  h 3 1  m   1  mn   m 2   mn     2

(3)

Equation (3) indicates that actuator deflection is greater when T and the difference between the thermal expansion coefficients are large, and

1 R   2  1  T

(4)

According to Equation (4), the larger the difference between 1 and 2, the larger the deformation of the bilayer actuator. PDMS has a very high CTE (3 × 10-4 K-1) and a low Young’ modulus, so we selected it to make the active layers of the actuator. In contrast, graphene undergoes almost no thermal expansion, and its Young’ modulus is very high.32 The CTE of a PDMS/graphene mixture is lower than that of pure PDMS, while its Young’s modulus is higher. As the graphene concentration of the mixture is increased, its CTE decreases, and the radius of curvature under light irradiation becomes smaller. In other words, an actuator will become more deflective as graphene is added. Bilayer actuators with different deflective behaviors can thus be obtained by adjusting the concentration of graphene in PDMS. However, due to the inherent properties of graphene, it tends to aggregate into clusters and is difficult to disperse in highly viscous polymers.33 A variety of methods have been used to disperse graphene in PDMS, such as high-speed shearing.34,35 However, the concentration of graphene in composites mixed by 6 ACS Paragon Plus Environment

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high-speed shearing is not high. Ultrasonication36 and vacuum37 have also been used to disperse graphene, as have various solvents. These include isopropanol,38 acetone,39 dimethyl formamide (DMF),40 n-hexane,41 tetrahydrofuran (THF),42 and ethyl acetate.43 Addition of organic solvents can reduce the viscosity of PDMS and other polymers, thereby making the dispersion of graphene and other carbon-based materials less difficult. We devised a new dispersion technique by combining ultrasonic dispersion, vacuum dispersion, and solvent addition. A larger quantity of graphene was dispersed into PDMS than could be with any of the individual dispersion techniques in less time and at a lower cost. 3. EXPERIMENTAL SECTION 3.1. Materials and Reagents Polydimethylsiloxane (PDMS) were purchased from Dow Corning (U. S. A). The PDMS model is Sylgard™ 184 silicone elastomer and its viscosity is 5500 mPA.s. Graphene were purchased from Jiangnan Graphene Research Institute, the diameter of graphene we chose was 10 μm,44,45 as shown in Figure S1. The n-hexane were obtained from Liaoning Xinxing Reagent Co., Ltd (China). 3.2. Preparation of Graphene/PDMS Composites The preparation of graphene/PDMS composites is shown schematically in Figure 1b. PDMS (2g) base liquid and curing agent (0.2g) were added to a beaker in a 10:1 ratio. Graphene was then weighed (Weighing according to experiment) and added to the beaker. The graphene and PDMS were stirred with glass rod for two minutes. And then the n-hexane (40mL) was weighed and poured into the beaker to reduce the viscosity of PDMS. The temperature of the mixture was adjusted to 69 °C, and it was placed in an ultrasonicator for mixing. Once the nhexane in the mixture had almost completely evaporated. The beaker containing the composite 7 ACS Paragon Plus Environment


material was placed in a water bath to prevent curing. After allowing the mixture to stand for two minutes, the graphene/PDMS composite material was finally obtained. A photograph of the prepared composite material is shown in Figure 1c. Due to the presence of high concentration of graphene, we directly add curing agent before dispersing, so as to avoid uneven mixing of the mixture and curing agent. So before ultrasound, we put the PDMS base liquid, curing agent and graphene in a beaker. Ultrasonication directly promoted the dispersion of graphene in PDMS. The n-hexane boiled as the temperature rose during ultrasonication, generating a large number of n-hexane bubbles that carried away a significant amount of heat. Following addition of n-hexane, PDMS became much less viscous, so solidification of PDMS was hindered. The n-hexane bubbles contributed to composite mixing as they rose and oscillated locally due to ultrasonication, thereby promoting the dispersion of graphene in PDMS. And similar to vacuum pressure dispersion, when bubbles burst on the surface of composite, the force generated by the bursting of n-hexane bubbles on the composite surface generated a local fluidic flow, which also promoted the dispersion of graphene. With this combination of conditions, a large quantity of graphene was dispersed in the PDMS.


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Figure 1. (a) Schematic illustration of a bilayer structure. (b) Fabrication process for the graphene/PDMS composite materials. (c) Photograph of the graphene composite material.

3.3. Fabrication of the Bilayer Actuator The process used to prepare the bilayer film actuator is shown schematically in Figure 2a. First, the slide was wiped clean with alcohol, and polyimide (PI) tape was affixed to it to form a rectangular groove. The PDMS base liquid and curing agent in a 10:1 ratio were mixed for 10 min. After static degassing, the PDMS mixture was poured into the groove and smoothed with a spatula to obtain a uniform film. The PDMS film was placed on a heating plate and cured at 80 °C for 4 h, and a PDMS film with uniform thickness was obtained. PI tape was affixed to the top of the cured PDMS layer to form a rectangular groove. The graphene/PDMS composite was poured into the groove, spread evenly, and then cured at 80 °C for 4 h to form the complete bilayer actuator. A photograph of the bilayer actuator is shown in 9 ACS Paragon Plus Environment


Figure 2b, which illustrates its excellent flexibility. A cross-sectional SEM image of the bilayer structure is shown in Figure 2c. Due to the presence of PDMS, the composite layer and the pure PDMS layer were tightly joined during the curing process. The graphene in the composite made good contact with the PDMS, and the distribution of graphene was relatively uniform. This method of fabricating light response actuators was quite simple and could be performed quickly.

Figure 2. (a) Fabrication process for the bilayer actuators. (b) Photograph of a bilayer actuator. (c) Cross-sectional SEM image of the bilayer (scale bar = 40 µm).

3.4. Analysis of Composite Layer Morphology and Deflection The SEM image of the PDMS film in Figure 3a shows that the pure PDMS film was quite smooth, and that surface morphology changed following the addition of graphene. The surface morphologies of composites containing different graphene concentrations are shown in Figure 3b–3d. Graphene at 15 wt%, 25 wt%, and 30 wt% in the composites made good contact with 10 ACS Paragon Plus Environment

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PDMS, and it was uniformly dispersed. Graphene was evenly mixed in PDMS and formed stable structures, so the composite contained a network that enabled good thermal conductivity. The heat generated by infrared radiation could thus be transferred to the PDMS layer quickly.

Figure 3. SEM images showing the surface morphology of PDMS and graphene/PDMS composites containing different concentrations of graphene. (a) Surface of a pure PDMS film. (b) Composite with 15 wt% graphene. (c) Composite with 25 wt% graphene. (d) Composite with 30 wt% graphene (scale bar = 20 µm).

4. RESULTS AND DISCUSSION A schematic diagram of bilayer actuator deformation induced by infrared irradiation is shown in Figure 4a. Graphene absorbs a certain amount of the infrared light46 and converts it to heat energy. Heat is transferred to the pure PDMS layer via layer-to-layer transmission. During the heat transfer process, PDMS in the composite layer also expands. However, because it does not contain graphene, deformation of the pure PDMS layer due to thermal expansion is 11 ACS Paragon Plus Environment


greater than that of the composite layer. Under stress, the bilayer actuator deflects to the side on which thermal expansion is lower. The composite layer is very tightly bound to the pure PDMS layer, so the bilayer actuator deflects to the composite layer side. With the decrease of graphene concentration in the composite layer, the heat is transferred to the PDMS layer decreases, resulting in the decrease of thermal expansion and the deflection of the bilayer actuator. To verify that deflection of the bilayer actuator could be enhanced by a high concentration of graphene, three bilayer actuators were prepared with different graphene concentrations, and the magnitude of deflection under infrared irradiation ( = 980 nm) at various intensities was measured. The bilayer actuators were cut into strips 20 mm long and 3 mm wide. Each PDMS and composite layer had a thickness of 60 µm, and the total thickness of the bilayer film was 120 µm. A 5 mm × 5 mm area on each bilayer actuator was then irradiated. Under infrared irradiation at 18.1 mW/mm2, deflection was observed with actuators at all three graphene concentrations, and horizontal deflection became larger as the graphene concentration increased (Figure 4b). The relationship between temperature and, graphene concentration and light intensity of actuator is shown in the Figure S2. At a graphene composite concentration of 30 wt%, horizontal deflection of the bilayer actuator reached 7.9 mm (see Movie S1, Supporting Information). When irradiation was stopped, the actuator returned to its initial position. We noted that recovery of the actuator made with the 30 wt% graphene composite was faster than that of the actuator made with 25 wt% graphene composite. This was due to greater upward deflection by the actuator made with 30 wt% graphene composite during infrared irradiation. When illumination stopped, it recovered more quickly due to the recovery force and gravity of the film. Because they contained different concentrations of graphene, the total infrared power absorbed by the composite films differed at the same illumination intensity. Heat conduction in the composites occurred at different speeds, so the deflection speeds of the actuators differed. 12 ACS Paragon Plus Environment

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We found that deflection was very fast at all three graphene concentrations, occurring within 1 s of illumination with infrared light. Therefore, we had to record deflection of the actuators within 1 s of irradiation. To ensure accuracy of measurement, we irradiated at several different light intensities and recorded the date of each experiment. Under illumination at each intensity, the bilayer actuator made with 30 wt% graphene composite exhibited the largest deflection within 1 s (Figure 4c) and thus the fastest response speed. From these results, we concluded that the bilayer actuator made with 30 wt% graphene composite had the best deflection characteristics. This bilayer actuator exhibited the largest deflection and the fastest deflection speed, which was consistent with the theoretical bilayer actuator model expressed by Equation 1.47 We further analyzed the properties of the bilayer actuator made with 30 wt% graphene composite. Repeated measurements with the same bilayer actuator indicated good reproducibility. The deflection and recovery speed were reproduced under a light intensity of 18.1 mW/mm2 over five cycles of the deflection process (Figure 4d). The infrared laser intensity had a great influence on the deflection of the actuators. The maximum deflection of the bilayer actuator made with 30 wt% graphene composite under illumination at different intensities is shown in Figure 4e. According to the graph, the maximum deflection increases with the increase of illumination intensity, and the fitted curve indicated that the increment of maximum deflection decreases with the increase illumination intensity. The composite layer of the bilayer actuator was exposed to more light power at higher irradiation intensities. Thus, more heat was transferred from the composite layer to the pure PDMS layer, and thermal expansion of the pure PDMS layer was greater. This increased deflection of the bilayer actuator in the vertical direction, so the increase in the horizontal direction was smaller. The deflection of a bilayer actuator made with 30 wt% graphene composite under infrared laser irradiation at 19.9 mW/mm2 is depicted in Figure 4f and Movie S2 (Supporting Information). 13 ACS Paragon Plus Environment


Based on Equation (4), the deflection of an actuator is determined by the ratio of the thickness of its two layers. Under infrared light, the expansion of PDMS results in the deflection of the bilayer actuator. The thickness of the PDMS active layer influences the behavior of the bilayer actuator during this process. With this in mind, we fabricated PDMS layers of different thicknesses to examine the effect of PDMS thickness on the bilayer actuator, while the thickness of the graphene/PDMS composite layer was held constant. When the concentration of graphene in the composite was 30 wt%, horizontal deflection increased in smaller increments as illumination intensity was increased, while the increments of vertical deflection increased (Figure 4e).

Figure 4. (a) Schematic diagram of bilayer actuator deformation under infrared irradiation. (b) Horizontal deflection profile of bilayer actuators with composites containing various graphene concentrations under irradiation at 18.1 mW/mm2. (c) The deflection of actuators made with different concentrations of graphene under infrared irradiation for 1 s. (d) Repeatable deformation of the bilayer actuator under 980 nm NIR irradiation. (e) Maximum deflection as a function of light intensity and the


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intensity of the infrared laser used ranges from 5.4 mW/mm2 to 19.9 mW/mm2. (f) The deflection process of a bilayer actuator with a graphene concentration of 30 wt% under infrared laser irradiation at 19.9 mW/mm2.

To verify the effect of PDMS film thickness on deflection, we fabricated a bilayer actuator with a length of 10 mm and a width of 1 mm. At this size, the bilayer actuator exhibited greater deflection in the vertical direction. Vertical deflection of the bilayer actuator is plotted against time in Figure 5a. In accordance with previous results, the prepared film was stable, and deflection in the vertical direction reached 8.3 mm. We then recorded the maximum deflection of actuators made with PDMS layers with different thicknesses. To assess the reliability of our measurements, we measured the maximum deflection at three light intensities. As the PDMS film thickness increased, the deflection of the bilayer actuator first increased, then decreased (Figure 5b). Deflection of the bilayer actuator was greatest when the thicknesses of the pure PDMS and graphene/PDMS composite layers were equal (1:1). At a given light intensity, the maximum vertical deflection of the bilayer actuator remained essentially constant. This indicated that reproducibility was good, and it confirmed the thickness of the PDMS layer influenced the maximum deflection of the actuator. With a 60 µm-thick composite film, we used a PDMS layer of approximately equal thickness to ensure that maximum deflection was observed. Deflection of the bilayer actuator made with a 60 µm-thick PDMS film under irradiation at 18.1 mW/mm2 is illustrated in Figure 5c and Movie S3 (Supporting Information).



Figure 5. (a) Vertical deflection profile of a bilayer actuator 10 mm in length and 1 mm wide under irradiation at 18.1 mW/mm2. (b) The deflection of bilayer actuators made with PDMS layers with different thicknesses. (c) The deflection process of a bilayer actuator under infrared irradiation at 18.1 mW/mm2.

5. Application of the Bilayer Actuator 5.1. A Fast Swimming Beluga Whale Robot We designed a soft robot that mimicked the motion of a swimming beluga whale. A bilayer actuator was used to construct the tail of the robot, which served as the driving unit. The head was comprised of solidified PDMS. The movement cycle of a beluga whale is shown in Figure 6a. The whale is propelled through the water by the vertical motion of its tail fin. Based on this swimming mode, we situated the pure PDMS layer of the bilayer actuator facing upward. This way, infrared light illumination would induce the tail to deflect downward. When illumination stopped, the tail would return to its initial upward state. This cycle generated the driving force that propelled the robot forward. In pure water, our beluga whale robot exhibited a very high swimming speed and good acceleration. As shown in Figure 6b, the tail deflected downward 16 ACS Paragon Plus Environment

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within 1 s. At this point, displacement of the robot was 0 mm. Following acceleration induced by the external force, the robot could move rapidly. The total distance covered by the robot was 30 mm over 5 s at an average speed of 6 mm/s. We recorded time and displacement when the robot was illuminated with infrared light once. As shown in Figure 6c, the robot moved quickly within 5 s. This can be viewed in Move S4 (Supporting Information). Water resistance reduced the robot’s speed until it finally stopped moving. Based on the information in Figure 6b, the robot would move the instant it was exposed to infrared illumination if its motion was caused by the photothermal effect of water. In the experiment, the displacement of the robot was 0 mm when it was irradiated for 1 s. For this reason, the motion of the robot was attributed to the deformation of the bilayer actuator. As shown in Movie S5 (Supporting Information), the robot demonstrated continuous and rapid motion when the tail was irradiated twice with the laser. The robot was able to swim quickly, because the surface properties of the composite material changed with the increasing graphene concentration. When the concentration of graphene in the composite layer was 30 wt%, the composite layer had superhydrophobic properties (Figure 6d) and the contact angle was >150°.



Figure 6. (a) The movement cycle of the beluga whale. (b) Photographs showing the displacement of the beluga whale robot. The fin size was 9 mm × 3 mm, and the body was 6 mm long × 4 mm wide. (c) Time-dependent displacement of the beluga whale robot. (d) The composite layer showing superhydrophobic characteristics. The volume of the water droplet was 1 μL. At right is a SEM image of a composite with a graphene concentration of 30 wt% (scale bar = 8 µm).

An SEM image of the 30 wt% graphene composite is shown at the right of Figure 6d. The surface of the composite film had protuberances and micropores, which conferred its superhydrophobic properties. Hydrophobic materials and superhydrophobic materials can trap bubbles.48 Thus, when the robot was immersed in water, a layer of air was present between the composite layer and the water. When the bilayer actuator bended under infrared light, the water exerted great thrust on the robot through the action of surface tension due to the air layer. This enabled the robot to swim fast. To verify this was the case, we added a 10% solution of sodium dodecyl sulfate (SDS) to the robot’s tank, which reduced the surface tension of the water. A plot of the swimming data following addition of SDS is shown in Figure 7a. When the tail 18 ACS Paragon Plus Environment

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deflected under light illumination, the robot swam forward 20 mm in 38.03 s. The relationship between displacement and time is illustrated in Figure 7b. It can be clearly seen that the motion of the robot after addition of SDS differed from that shown in Figure 6c. The motion of the robot was essentially uniform and can be viewed in Movie S6 (Supporting Information).

Figure 7. (a) Photographs showing the motion of the beluga whale robot following addition of SDS. (b) Time-dependent displacement of the robot following addition of SDS.

The swimming speed of the robot was different under the two conditions, which proved that fast swimming was caused by the deflection of the bilayer actuator and the superhydrophobic effect. It also established that the robot’s movement was not caused by heat. Based on these findings, we validated the principle underlying the robot’s ability to swim fast. The kinetic parameters of mini-robotic motion in the presence and absence of superhydrophobic effects are summarized in Table 1. Compared to the robot lacking superhydrophobicity, the robot with superhydrophobicity took less time to swim the same distance. Movie S4 (Supporting Information) shows that in this state, the tail of the robot beats the water only once, and the total displacement of the robot could reach 41 mm. Thus, superhydrophobicity greatly improved the robot’s swimming efficiency and conserved energy. Moreover, the robot could



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swim long distances quickly when it was repeatedly illuminated under infrared light (Movie S5, Supporting Information). Table 1. Kinematic parameters in two states. Robot State

Displacement (mm)

Time(s)

Average Velocity (mm/s)

Beat Times

Superhydrophobic (No SDS)

20

1.23

16.26

1

Non-superhydrophobic (SDS)

20

38.04

0.53

about 30 times

5.2. Rapid Swimming Mechanism of the Beluga Whale Robot Once we established that our robot's ability to swim rapidly was due to large deformation of the bilayer actuator and the superhydrophobic effect, we analyzed the swimming mechanism in more detail. A cross-sectional diagram of the bilayer membrane microstructure is shown in Figure 8a. We assumed the superhydrophobicity of the bilayer membrane was conferred by its microstructure. When the beluga whale robot was immersed in water, the superhydrophobic effect caused air to be trapped in the microstructural cavity by the composite layer. We performed a force analysis of the microstructure when the bilayer actuator was stationary. In its initial state, the microstructure was in equilibrium. Gravity and buoyancy were equal in the vertical direction, and the net force in the horizontal direction was zero. The microstructure was subjected to buoyant force (F0) and surface tension (FS). Since the motion of the mini-robot was caused mainly by surface tension, we examined variations in FS.

Fs1 cos 1  Fs1 cos 1  Fs 2 cos  2  Fs 2 cos  2  0 ,

(5)

Where  is the angle between the surface of the water and air at the point of contact between the three phases. FS can be written as

FS   L .

(6)


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In Equation (6), γ is the surface tension coefficient of water, and L is the length of the microstructural. The surface tension coefficient of water varies with its temperature. Since the PDMS was transparent and absorbed little infrared energy, the composite material was directly irradiated when the bilayer actuator was illuminated with infrared light. The composite material converted the infrared light energy to heat, some of which was transmitted to the PDMS layer to realize deformation of the bilayer actuator. The remaining heat was transmitted through the composite layer to the water. This increased the temperature of the water at the interface with the composite layer microstructure and changed the water's surface tension. The air/water interface moved into the cavity and gradually transitioned from the Cassie state to the Wenzel state.49,50 The infrared laser intensity had a Gaussian distribution, so the thermal energy of the microstructure decreased gradually with increasing distance from the central point of infrared irradiation. The surface tension decreased with increasing temperature,51 and a surface tension gradient formed in the microenvironment from the center to the edge of the irradiated spot. As shown in Figure 8b, the direction of the resultant force acting on the bilayer membrane microstructure changed.

Fs 2 cos  2  Fs 4 cos  4  Fs 3 cos 3  Fs 5 cos 5

(7)

On the right side of the irradiated spot, the resultant force on the microstructure was directed to the right. Due to its high degree of deformation, the bilayer actuator deflected upward under the resultant force generated by the surface tension gradient on the left side of the irradiated spot. Therefore, the total surface tension on the mini-robot's composite layer was directed to the right. The microstructure was subjected to a large amount of force due to water surface tension, which drove the rapid movement of the robot.



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As the robot moved, the air/water interface of the microstructure gradually returned to its initial state. The resultant force on the microstructure due to surface tension was gradually reduced, and the net force (F) approached zero according to Equation (8).

F  Fs 2 cos  2  Fs 4 cos  4  Fs 3 cos 3  Fs 5 cos 5  0

(8)

This was consistent with the experimental data shown in Figure 7b, in which acceleration gradually decreased. Due to the viscosity of the water, the robot finally stopped moving.

Figure 8. (a) Cross-sectional diagram of the bilayer membrane microstructure in the initial state. (b) The air/water interface gradually changes from the Cassie state to the transition state.

The rapid swimming mechanism of the beluga whale robot could be explained with a simple model. The actual movement process certainly could have been affected by a variety of factors, such as the morphological characteristics of the superhydrophobic structure and environmental temperature. This is a good topic for future research. Rapid movement of the robot was achieved through a combination of significant bilayer actuator deformation and the superhydrophobic effect, and the robot’s movement could be made more efficient. These findings can inspire new ideas for the development of efficient microrobots. 6. CONCLUSIONS We combined various dispersion methods to disperse large quantities of graphene in PDMS. Photoresponsive bilayer actuators were then fabricated with the graphene composites. 22 ACS Paragon Plus Environment

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This fabrication method was simple, quick to perform, convenient, and economical. Additionally, it yielded actuators with excellent stability. The bilayer actuators had large deformation capacities, fast response speeds. The prepared bilayer actuators could be used to simulate movements involving significant deformation in nature. A beluga whale soft robot built with an actuator containing a 30 wt% graphene composite layer swam rapidly in water at an average speed of 6 mm/s over a distance of 30 mm in 5 s when irradiated just once with an infrared laser. This was possible due to the superhydrophobic effect and the large degree of bilayer actuator deformation. The total displacement of the robot reached 41 mm. Thus, incorporation of the bilayer actuator greatly improved the efficiency of the robot’s movement and conserved energy. Bilayer actuators fabricated with this method may provide novel insights into motion caused by significant deformation. This method could also be used to economically produce new intelligent materials with excellent properties for artificial muscle, functional grippers, soft robots, and other applications. ASSOCIATED CONTENT Supporting Information. Additional experimental details Movie S1: Deflection of a bilayer actuator built with a 30 wt% graphene composite layer under 18.1 mW/mm2 infrared laser irradiation. The actuator dimensions are 20 mm × 3 mm (length × width). Movie S2: Deflection of a bilayer actuator built with a 30 wt% graphene composite layer under 19.9 mW/mm2 infrared laser irradiation. The actuator dimensions are 20 mm × 3 mm (length × width).



Movie S3: Deflection of a bilayer actuator fabricated with 60-m thick PDMS film under 18.1 mW/mm2 and 19.9 mW/mm2 infrared laser irradiation. The actuator demonstrates a large deformation capacity. The sample dimensions are 10 mm × 1 mm (length × width). Movie S4: The beluga whale soft robot is built with an actuator containing a 30 wt% graphene composite layer. The whale can swim rapidly in water after being illuminated once with infrared light in its initial position. The total displacement of the robot is 41mm. Movie S5: The beluga whale soft robot exhibits continuous and rapid motion in water when its tail is irradiated twice with the infrared laser. When the whale robot is repeatedly illuminated, it can swim over long distances quickly. Movie S6: The beluga whale soft robot swimming in an aqueous SDS solution under continuous infrared irradiation. AUTHOR INFORMATION Corresponding Author *Niandong Jiao. E-mail: [email protected] *Lianqing Liu. E-mail: [email protected]

Notes The authors declare no competing financial interest.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61573339, 91748212, U1613220, 61573341, 91848201) and the CAS/SAFEA International Partnership Program for Creative Research Teams. REFERENCES [1] Zhang, L. D.; Naumov, P.; Du, X. M.; Hu, Z. G.; Wang, J. Vapomechanically Responsive Motion of Microchannel-Programmed Actuators. Adv. Mater. 2017, 29 (37), 1702231. [2] Behl, M.; Kratz, K.; Noechel, U.; Sauter, T.; Lendlein, A. Temperature-memory polymer actuators. Proc. Natl. Acad. Sci. USA 2013, 110 (31), 12555-12559. [3] Zeng, H.; Wani, O. M.; Wasylczyk, P.; Priimagi, A. Light-Driven, Caterpillar-Inspired Miniature Inching Robot. Macromol. Rapid Commun. 2018, 39 (1), 1700224. [4] Park, J. H.; Lee, S. W.; Song, D. S.; Jho, J. Y. Highly Enhanced Force Generation of Ionic Polymer−Metal Composite Actuators via Thickness Manipulation. ACS Appl. Mater. Interfaces 2015, 7 (30), 16659-16667. [5] Shin, B.; Ha, J.; Lee, M.; Park, K.; Park, G. H.; Choi, T. H.; Cho, K. J.; Kim, H. Y. Hygrobot: A self-locomotive ratcheted actuator powered by environmental humidity. Sci. Rob. 2018, 3 (14), eaar2629. [6] Mirvakili, S. M.; Hunter, I.W. Artificial Muscles: Mechanisms, Applications, and Challenges. Adv. Mater. 2018, 30 (6), 1704407. [7] Raman, R.; Bashir, R. Biomimicry, Biofabrication, and Biohybrid Systems: The Emergence and Evolution of Biological Design. Adv. Healthcare Mater. 2017, 6 (20), 1700496. [8] Han, D. D.; Zhang, Y. L.; Ma, J. N.; Liu, Y. Q.; Han, B.; Sun, H. B. Light-Mediated 25 ACS Paragon Plus Environment


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Figure 1. (a) Schematic illustration of a bilayer structure. (b) Fabrication process for the graphene/PDMS composite materials. (c) Photograph of the graphene composite material. 254x184mm (300 x 300 DPI)

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Figure 2. (a) Fabrication process for the bilayer actuators. (b) Photograph of a bilayer actuator. (c) Crosssectional SEM image of the bilayer (scale bar = 40 µm). 245x180mm (300 x 300 DPI)


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Figure 3. SEM images showing the surface morphology of PDMS and graphene/PDMS composites containing different concentrations of graphene. (a) Surface of a pure PDMS film. (b) Composite with 15 wt% graphene. (c) Composite with 25 wt% graphene. (d) Composite with 30 wt% graphene (scale bar = 20 µm). 234x159mm (300 x 300 DPI)



Figure 4. (a) Schematic diagram of bilayer actuator deformation under infrared irradiation. (b) Horizontal deflection profile of bilayer actuators with composites containing various graphene concentrations under irradiation at 18.1 mW/mm2. (c) The deflection of actuators made with different concentrations of graphene under infrared irradiation for 1 s. (d) Repeatable deformation of the bilayer actuator under 980 nm NIR irradiation. (e) Maximum deflection as a function of light intensity and the intensity of the infrared laser used ranges from 5.4 mW/mm2 to 19.9 mW/mm2. (f) The deflection process of a bilayer actuator with a graphene concentration of 30 wt% under infrared laser irradiation at 19.9 mW/mm2. 245x175mm (300 x 300 DPI)


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Figure 5. (a) Vertical deflection profile of a bilayer actuator 10 mm in length and 1 mm wide under irradiation at 18.1 mW/mm2. (b) The deflection of bilayer actuators made with PDMS layers with different thicknesses. (c) The deflection process of a bilayer actuator under infrared irradiation at 18.1 mW/mm2. 234x144mm (300 x 300 DPI)



Figure 6. (a) The movement cycle of the beluga whale. (b) Photographs showing the displacement of the beluga whale robot. The fin size was 9 mm × 3 mm, and the body was 6 mm long × 4 mm wide. (c) Timedependent displacement of the beluga whale robot. (d) The composite layer showing superhydrophobic characteristics. The volume of the water droplet was 1 μL. At right is a SEM image of a composite with a graphene concentration of 30 wt% (scale bar = 8 µm). 240x165mm (300 x 300 DPI)


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Figure 7. (a) Photographs showing the motion of the beluga whale robot following addition of SDS. (b) Timedependent displacement of the robot following addition of SDS. 189x90mm (300 x 300 DPI)



Figure 8. (a) Cross-sectional diagram of the bilayer membrane microstructure in the initial state. (b) The air/water interface gradually changes from the Cassie state to the transition state. 255x82mm (300 x 300 DPI)


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TOC: Photoresponsive smart actuators with high deformation capability are receiving increasing attention. Composites with high graphene concentrations are obtained by combining various dispersion methods and used to fabricate highly deformable bilayer actuators. A beluga whale soft robot built with an actuator containing a 30 wt% graphene composite layer swims rapidly in water due to a combination of superhydrophobicity and high deformation. 54x49mm (300 x 300 DPI)


Photoresponsive Graphene Composite Bilayer Actuator for Soft

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