Large-Deformation Curling Actuators Based on Carbon Nanotube

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Large-Deformation Curling Actuators Based on Carbon Nanotube Composite: Advanced-Structure Design and Biomimetic Application Luzhuo Chen,† Mingcen Weng,† Zhiwei Zhou,‡ Yi Zhou,‡ Lingling Zhang,† Jiaxin Li,† Zhigao Huang,† Wei Zhang,*,† Changhong Liu,*,‡ and Shoushan Fan‡ †

Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, College of Physics and Energy, Fujian Normal University, Fuzhou 350007, China and ‡Tsinghua-Foxconn Nanotechnology Research Center and Department of Physics, Tsinghua University, Beijing 100084, China

ABSTRACT In recent years, electroactive polymers have been developed as actuator materials.

As an important branch of electroactive polymers, electrothermal actuators (ETAs) demonstrate potential applications in the fields of artificial muscles, biomimetic devices, robotics, and so on. Large-shape deformation, low-voltage-driven actuation, and ultrafast fabrication are critical to the development of ETA. However, a simultaneous optimization of all of these advantages has not been realized yet. Practical biomimetic applications are also rare. In this work, we introduce an ultrafast approach to fabricate a curling actuator based on a newly designed carbon nanotube and polymer composite, which completely realizes all of the above required advantages. The actuator shows an ultralarge curling actuation with a curvature greater than 1.0 cm
1 and bending angle larger than 360°, even curling into a tubular structure. The driving voltage is down to a low voltage of 5 V. The remarkable actuation is attributed not only to the mismatch in the coefficients of thermal expansion but also to the mechanical property changes of materials during temperature change. We also construct an S-shape actuator to show the possibility of building advanced-structure actuators. A weightlifting walking robot is further designed that exhibits a fast-moving motion while lifting a sample heavier than itself, demonstrating promising biomimetic applications. KEYWORDS: carbon nanotube . actuator . composite . biomimetic . electrothermal

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n the past decades, actuator materials have been widely studied.1
7 Among them, electroactive polymers (EAPs) were intensively used because of their attractive properties such as light weight, large deformation, cost-effectiveness, and flexibility. They have great potential in the applications of artificial muscles, biomimetic devices, switches, robotics, and so on. The actuators based on EAPs are commonly classified into two major groups according to different actuation mechanisms: electronic actuators and ionic actuators.1 Electronic actuators are mostly field-activated and can work in air. They have advantages of large strain and quick response, while the drawback is requiring high driving voltages (>1 kV).8
10 In contrast with electronic actuators, ionic actuators can be driven by much lower driving CHEN ET AL.

voltages (100 °C) to reach maximum actuation, which may not be convenient for practical applications. Meanwhile, the curing of PDMS usually requires several hours, slowing down the entire fabrication process. Therefore, new polymer materials are required to replace PDMS in the field of ETAs. Third, most of the reported bending ETAs have a bilayer structure and can only realize simple bending movements.21,23,25,27 Therefore, more ETAs which have sophisticated structure and could accomplish diverse actuation performance are still rare. Finally and most importantly, the actuation amplitude of the ETAs has huge room for improvement. For example, curling actuators that are able to form a tubular structure are still rare. In a word, it is an urgent need to develop new kinds of actuator materials to fabricate advancedstructure ETAs with ultralarge deformation under low driving voltages. Here, we introduce an ultrafast approach to fabricate a state-of-the-art curling actuator based on a superaligned CNT (SACNT) film and biaxially oriented polypropylene (BOPP) composite. The actuator simultaneously realizes all of the above requirements. It is able to perform an ultralarge curling actuation with a curvature greater than 1.0 cm
1 and a bending angle larger than 360°, even curling into a tubular structure. We also find that the ultralarge actuation is due not only to the mismatch in the coefficients of thermal expansion but also to the mechanical property changes of materials during temperature change. The driving voltage is optimized down to a low voltage

Figure 1. SACNT/BOPP actuators. (a) Schematic image illustrating the fabrication process of a SACNT film. (b) Optical photo showing a large-area free-standing SACNT film. (c) SEM image showing the aligned structure of SACNTs on the surface of a SACNT film. (d) Schematic image illustrating the bilayer structure of a SACNT/BOPP actuator.

of 5 V. An S-shape actuator based on the SACNT/BOPP composite is also proposed as an advanced-structure actuator. Moreover, the actuator is quite powerful as it can lift a sample that is 7 times as heavy as itself. Finally, a biomimetic walking robot is elaborately designed, which demonstrates a fast-moving motion at a high speed of 39 mm min
1 while lifting a sample. RESULTS AND DISCUSSION Design and Fabrication of the SACNT/BOPP Bilayer. The SACNT film was fabricated through a roll-to-roll process, as shown in Figure 1a (see Methods for details). Then, the compact SACNT film with dimensions of 18 cm  20 cm was cut and detached from the rollers. As shown in Figure 1b, it is free-standing and composed of 400 layers of single-layer SACNT sheets with a total thickness of 7 μm. The scanning electron microscopy (SEM) image (Figure 1c) shows the surface of the SACNT film, which manifests that the SACNTs in the film are well-arranged along the same direction. Because of such well-organized structure, together with excellent conductivity of CNTs, the conductance of the SACNT film along the CNT alignment direction was measured to be up to 4  104 S m
1, leading to low driving voltage while made into actuators. Figure 1d schematically illustrates the bilayer structure of the SACNT/BOPP actuator. More fabrication details are described in the Methods section. Here, the newly designed SACNT/BOPP composite material is selected for the following reasons. First, the CTE of polypropylene (124 ppm K
1)31 is much larger than that of CNT (3 ppm K
1).32,33 Hence, when constructed into a bilayer structure, the composite is expected to show obvious bending actuation due to the large mismatch between CTEs. Second, compared with previously reported ETAs based on PDMS, the BOPP film can be obtained from mature commercial products, VOL. XXX

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ARTICLE Figure 2. Actuation performance of SACNT/BOPP actuators. (a) Optical photo showing the SACNT/BOPP actuator. (b) Optical photos showing the SACNT/BOPP actuator without a driving voltage (left panel) and with a driving DC voltage of 5 V (right panel). (c) Curvature as a function of time with different driving voltages (1, 1.5, 2,...4.5, and 5 V). (d) Temperature as a function of time with different driving voltages (1, 1.5, 2,...4.5, and 5 V). (e) Maximum curvature as a function of maximum temperature. (f) Electrical power density dependence of maximum curvature.

which means that there is no need to wait a few hours for the curing of PDMS, reducing the fabrication time to a few minutes. Compared with the reported BOPPbased actuator, which is externally thermally actuated and introduces a solution-casting process,34 the SACNT/BOPP actuator is electroactive and the fabrication process is totally solution-free as well as environmentally friendly without using any organic solvent. Therefore, this easy and ultrafast fabrication process is possible to scale up for industrial productions. Actuation Performance of the SACNT/BOPP Actuator. In order to investigate the actuation performance of the SACNT/BOPP actuator, both ends of the actuator were connected to two ultranarrow strips of copper foil (Figure 2a). The bending performance is quantified by the curvature of the actuator (Supporting Information Note 1). As shown in the left panel of Figure 2b, the top CHEN ET AL.

fixed end (4 mm) of a 70 mm long actuator was placed between two glass slides and suspended by a clip. Because of the initial imbalanced stress in two layers of the SACNT and BOPP film, the original SACNT/BOPP actuator was bending slightly toward the SACNT side with a small curvature of 0.056 cm
1. The right panel of Figure 2b shows the SACNT/BOPP actuator under an applied DC voltage of 5 V for 10 s. The actuator showed an exceptional bending of curling into a tubular structure with a current of 0.36 A. The curvature was up to 1.03 cm
1 with a bending angle of 389°. The extraordinary actuation performance is far beyond most reported ETAs with similar configurations (stripe-shaped or U-shaped). Besides ultralarge deformation, the driving voltage was down to 5 V due to the excellent conductivity of the SACNT film, which is also superior to that of most reported ETAs. The maximum VOL. XXX

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Figure 2f demonstrates the input power density dependence of the maximum curvature. The data were gained by inputting every certain power for 10 s. It is clear that by increasing the electrical power density, more electrical energy is converted into thermal energy, causing larger bending actuation of the SACNT/ BOPP actuator. Thus, the maximum curvature increases with the increase of input electrical power density, showing excellent controllability as an electromechanical actuator. The power density of the SACNT/BOPP actuator is in the same order as previously reported ETAs, while the actuation performance is much larger. For example, the SACNT/BOPP actuator exhibits a curvature up to 0.70 cm
1 with a power density of ∼24 mW mm
3, which is 2.4 times as much as a reported curvature of an ETA based on CNT/polymer composites (0.29 cm
1 with a power density of 25 mW mm
3).27 In addition, with the same input power density, the SACNT/BOPP actuator requires low actuation voltages, which is attributed to excellent conductivity of SACNT (Supporting Information Note 3). When the input power density increases to ∼31 mW mm
3, the curvature of the SACNT/BOPP actuator can be up to 1.03 cm
1. Figure S1 demonstrates that the temperature also increases with the increase of input electrical power density, which verifies that the actuation is due to an electrothermal heating process. Furthermore, the thickness of the SACNT/BOPP actuator (47 μm) is much thinner than that of the reported ETA (370 μm),27 which would result in better heat transfer and a lower temperature gradient across the BOPP layer. Since the temperature gradient leads to nonuniform heating of the BOPP layer, which counters the actuator performance, a lower temperature gradient in a thinner layer shortens the response time.25 In fact, the response time of the SACNT/BOPP actuator to reach maximum bending actuation (10 s) is much shorter than that of the above-mentioned ETA (50 s). The stability test of the SACNT/BOPP actuator was conducted by applying a 0.5 Hz square wave voltage (0
2 V) for 20 000 cycles. A laser displacement sensor was used to automatically record the free-end displacement of the actuator, replacing the measurement of the bending angle by a camera manually. Figure S2 (in Supporting Information) demonstrates that the actuation performance is quite repeatable. The degradation is not obvious after 20 000 cycles. Figure S3 shows a cross-sectional SEM of the actuator after cycling. It can be clearly seen that, after cycling, the SACNT layer and the BOPP layer are still tightly coupled to each other. There is no delamination between two layers. Hence, the stability test demonstrates the excellent reliability and stability of the SACNT/BOPP actuator. S-Shape Actuator: An Advanced-Structure Design. Most of the reported ETAs employ similar simple structures with two layers of materials having different CTEs, which results in monotonous bending actuations, that VOL. XXX

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temperature corresponding to a curvature of 1.03 cm
1 was measured to be 47.6 °C, which is much lower than that of the ETA based on PDMS (>150 °C) with maximum actuation,26 facilitating its practical applications. Meanwhile, the response speed is about 3 times as fast as the above-mentioned ETA based on PDMS, as the temperature change range becomes much narrower. Furthermore, the response speed can be accelerated by increasing driving voltages. For example, with a higher driving voltage of 6 V, the actuator was able to achieve the same large actuation performance in 4 s. When the voltage was cut off, the actuator recovered to its original state in 15 s. Figure 2c shows the curvature of the actuator with different driving voltages (1, 1.5, 2,...4.5, and 5 V) for 10 s, while the recovery time is 15 s. Obviously, the maximum curvature becomes larger when the driving voltage is higher. The corresponding temperature was measured synchronously in order to study the actuation mechanism, as shown in Figure 2d. It demonstrates that the temperature also increases with higher driving voltages. The temperature variation tendency is almost the same as that of the curvature, revealing that the actuation mechanism is due to an electrothermal effect. When a current passes through the SACNT/BOPP composite, the electrical energy absorbed by SACNTs is converted into thermal energy. As the two layers of the SACNT and BOPP films are coupled tightly, the entire actuator is heated instantaneously, resulting in thermal expansion of both the SACNT and BOPP films. Because the CTE mismatch between two material layers is quite large, the same temperature increase will give rise to larger expansion of the BOPP layer compared to that of the SACNT layer. Hence, the actuator is tremendously bent to the SACNT side and even curls into a tubular structure, which can potentially be applied in artificial muscles, drug delivery systems, and so on. Apart from the CTE mismatch between two material layers, the mechanical property changes of SACNT and BOPP during the temperature change will also determine the bending performance of the actuator. We develop a model to analyze the bending performance of the actuator based on the bimetal thermostat equation of Timoshenko35 (Supporting Information Note 2). From the modeling, the effect of the Young's modulus change with different temperature on the curvature is calculated. The modeling results show that with the increase of temperature, the curvature increment will be bigger. At about 21.5 °C, when the temperature increases by 1 °C, the curvature increment is 0.023 cm
1. At about 50 °C, the curvature increment increases to 0.030 cm
1. The modeling results are in accordance with our experiment results. Figure 2e shows that when the temperature increases, the acceleration of curvature increases, resulting in a larger bending performance of the actuator.

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is, bending to only one side with smaller CTE.21,23,25,27 As an improvement, Kim et al. reported a CNT/PDMS sandwich structure, which showed a bidirectional bending.22 They also fabricated a twistable actuator. However, the reported ETAs are mostly fabricated through a casting and drying (curing) process, which takes time and is only suitable for simple layer-by-layer structures. Because top-layer materials are in the form of solution or viscous fluid during the casting process, it is not easy to constrain them on certain surface areas of bottom-layer materials. Here, we demonstrate an S-shape SACNT/BOPP actuator as an initial trial for various advanced-structure actuators. The length of the S-shape actuator is also 70 mm. Different from the actuator described above with BOPP film on only one side of the SACNT film, the BOPP films in the S-shape actuator are elaborately designed to be attached to both sides of the SACNT film from top to bottom alternately. The left panel of Figure 3a shows the schematic structure of the actuator without driving voltages. In the fix-end part (10 mm) and the lower part (30 mm) of the actuator, BOPP films are attached to the right side. On the other hand, in the upper part (30 mm) of the actuator, BOPP film is attached to the left side. The schematic structure of the actuator under a driving voltage is shown in the right panel of Figure 3a. Similar to the test of the actuator described above, the S-shape actuator was suspended with the top fix end (4 mm) placed between two glass slides (top panel in Figure 3b). The rest of the parts of Figure 3b show the optical images of the S-shape actuator with different driving voltages (3, 4, and 5 V) for 10 s. With a driving voltage of 3 V, the actuator showed a slight wavy shape. When the voltage was increased to 4 V, an initial small S-shape could be observed. Finally, an obvious S-shape was formed with a voltage of 5 V, similar to the schematic shape shown in the right panel of Figure 3a. Figure 3c quantifiably demonstrates the curvatures of both upper and lower parts of the S-shape actuator with different driving voltages (3, 4, and 5 V). The curvatures of both upper and lower parts increase with the increase of driving voltages. The curvature variation tendencies of the upper and lower parts are quite similar. When the bending actuations become greater, the curvatures of the upper part become smaller than that of the lower part. The reason for the slight difference is that when the bending actuations become greater, the gravity of the lower part will partly counteract the internal bending stress in the upper part. Thus, the bending actuation of the upper part recedes compared to the lower part. The easy and ultrafast fabrication process in this work provides a possible solution to fabricate top-layer materials on certain surface areas of bottom-layer materials precisely with excellent controllability. By elaborate design, more advanced-structure actuators are expected to be realized.

Figure 3. S-shape SACNT/BOPP actuators. (a) Side-view schematic images illustrating the S-shape SACNT/BOPP actuator without a driving voltage (left panel) and with a driving voltage (right panel). (b) Optical photo series showing the S-shape SACNT/BOPP actuator with different driving voltages (3, 4, and 5 V). (c) Curvatures of upper and lower parts of the S-shape SACNT/BOPP actuator as a function of time with different driving voltages (3, 4, and 5 V).

Weightlifting Walking Robot: A Biomimetic Application. We also study the load-carrying capacity of the SACNT/ BOPP actuator. The actuator can lift an object that was much heavier than itself (Figure S4 in Supporting Information). The actuator has a dimension of 70 mm  18 mm  47 μm with a weight of 53 mg. A foam sample with a weight of 369 mg was placed on the actuator (Figure S4a). When a DC voltage of 5 V was applied for 10 s, the actuator could lift up the sample. The loadcarrying capacity is associated with the curvature of the actuator. As the bending curvature decreases, the loadcarrying capacity is increased (Table S1 in Supporting Information). The heaviest load was almost 7 times as heavy as the actuator (Figure S4b) with a curvature of 0.18 cm
1, demonstrating the powerful electromechanical performance of the SACNT/BOPP actuator. In the end, we would like to leverage excellent properties of the SACNT/BOPP actuator in biomimetic applications. Besides lifting samples at a resting state, the SACNT/BOPP actuator can be further designed as a weightlifting walking robot, which is capable of weightlifting while moving forward at the same time. Figure 4a shows the schematic structure of the robot. One end of the SACNT/BOPP composite film is tailored into a zigzag shape. There is an additional BOPP film attached to the zigzag area of the SACNT side, as shown in the enlarged area of Figure 4a. Figure 4b shows that a foam sample with the weight of 73 mg is placed on the robot with the weight of 48 mg. When a driving DC voltage of 5 V was applied for 10 s, the zigzag end of the robot slid forward while the flat end remained still, resulting in the robot bending and VOL. XXX

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ARTICLE Figure 4. Weightlifting walking robot. (a) Schematic image illustrating the structure of the walking robot (the enlarged image of the zigzag end is shown in the red circle). (b) Optical photo series showing the moving process of the weightlifting walking robot. (c) Schematic image illustrating the force analysis of the weightlifting walking robot during the bending actuation (0
10 s). An additional BOPP film is attached below the zigzag area. (d) Schematic image illustrating the force analysis of the weightlifting walking robot during the stretching recovery (10
20 s).

lifting up the sample, which is 1.5 times as heavy as itself (0
10 s). The force analysis is shown in Figure 4c. Because the robot is almost symmetrical, the normal force (N) acting on either end is assumed to be equal to one-half of gravity force (G/2). As the contact area between the zigzag area and the substrate is the additional BOPP film below the zigzag area, the maximum static friction acting on the zigzag end is f1 = μBOPPN, where μBOPP is the friction coefficient between the BOPP film and the substrate. On the other hand, the maximum static friction acting on the flat end is f2 = μCNTN, where μCNT is the friction coefficient between the SACNT film and the substrate. As the BOPP surface is smoother than the SACNT surface, it would give the result of μBOPP < μCNT. Hence, the comparison between two static frictions is given by f1 = μBOPPN < f2 = μCNTN. As a result, the flat end of the robot would remain static, while the zigzag end is driven by the bending actuation and moves forward. After the voltage was cut off, the zigzag end remained still while the flat end slid forward. The robot recovered to the original state and moved forward by 13 mm (10
20 s), exhibiting a biomimetic walking motion. The force analysis is shown in Figure 4d. In this case, the zigzag cross section contacts the substrate, showing a rough contact mode, which is different from the smooth contact between the BOPP film of the zigzag area and the substrate in the previous process. CHEN ET AL.

Thus, the friction coefficient, μzigzag, is assumed to be much larger than μCNT, resulting in the comparison result between two static frictions of f10 = μzigzagN > f20 = μCNTN. Therefore, the zigzag end of the robot would remain still, while the flat end slides forward with the stretching of the robot, completing the entire walking motion process. Afterward, another walking process was repeated (20
40 s). With periodic electrical stimulus, the robot shows a continuous walking motion, which biomimetically mimics the motion model of animals such as limbless worms. The robot here exhibits a high walking speed of 39 mm min
1, which is achieved while the robot is weightlifting a sample heavier than itself, showing excellent mechanical property together with fast-moving ability. Obviously, the walking speed and performance are highly dependent on the carrying load. If there is no carrying load, the walking speed can be increased to 57 mm min
1, which is 15 times as fast as a reported walking robot with a speed of 3.8 mm min
1.36 CONCLUSIONS In summary, SACNT/BOPP actuators are fabricated by an easy and ultrafast fabrication approach. Driven by a low DC voltage of 5 V, the actuator exhibits remarkable ultralarge curling actuation, in which the curvature is greater than 1.0 cm
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the SACNT/BOPP composites. With further development, this newly designed composite will have great potential in the fields of high-performance artificial muscles, ultrasensitive switches, drug delivery systems, biomimetic robotics, and so on.

METHODS

Acknowledgment. This work was jointly supported by the National Natural Science Foundation of China (51202031, 51173098, 11404059, 11504051), the Natural Science Foundation of Fujian Province (2014J01175, 2015J01008), the National Basic Research Program of China (2012CB932301, 2011CBA00200), and the Undergraduate Training Programs for Innovation and Entrepreneurship of China (201510394003).

Fabrication of the SACNT/BOPP Actuator. The SACNTs were fabricated by a solution-free approach, which has been developed since 2002.37
40 As shown in Figure 1a, a single-layer SACNT sheet was initially drawn out from a SACNT array on an 8 in. silicon wafer substrate. Jiang et al. have described in detail the drawing process of SACNTs.40 After that, the SACNT sheet was pressed by two close cylindrical rollers tightly. With the rolling of the cylindrical rollers, more layers of SACNT sheets were pressed and attached on the lower roller. A counter was applied to precisely control the layer number of the SACNT film. BOPP films were purchased commercial products. The total thickness of the BOPP film is 40 μm, coated with acrylic as adhesive. The function of the acrylic adhesive is to combine the SACNT and BOPP films firmly. The fabrication process of the SACNT/BOPP actuator is as follows. First, the compact SACNT film with dimensions of 18 cm  20 cm  7 μm was cut and detached from the rollers. Second, a small piece of SACNT film (70 mm  18 mm  7 μm) was cut from the large SACNT film and spread on a piece of flat glass. Finally, a BOPP film with dimensions of 70 mm  18 mm  40 μm was attached to the SACNT film with acrylic as adhesive. Then, a SACNT/BOPP actuator was achieved. The dimensions of the entire actuator are 70 mm  18 mm  47 μm (length  width  thickness). Fabrication of the S-Shape Actuator. The first two steps of SACNT film preparation are the same as those described above in the fabrication of the SACNT/BOPP actuator. After that, a BOPP film with dimensions of 10 mm  18 mm  40 μm was first attached to one end of the SACNT film, which formed the fix-end part of the S-shape actuator. Then, another BOPP film with dimensions of 30 mm  18 mm  40 μm was attached to the other end of the SACNT film, which formed the lower part of the S-shape actuator. Finally, the composite film was turned over, and a third BOPP film with dimensions of 30 mm  18 mm  40 μm was attached to the middle of the SACNT film, which formed the upper part of the S-shape actuator. Then, a complete S-shape actuator was achieved. Fabrication of the Walking Robot. First, a SACNT/BOPP actuator was prepared as described above. Second, the composite film was turned over, and a BOPP film with dimensions of 3 mm  18 mm  40 μm was attached to one end of the SACNT side of the film. Finally, the end with the additional BOPP film was tailored into a zigzag shape. Then, a walking robot was achieved. In the SACNT/BOPP actuators (including S-shape actuator) and the walking robot, both ends of the composite film were connected to two ultranarrow strips of copper foil. Silver paste was coated on both ends between the SACNT film and copper electrodes to enhance conductivity. Physical Characterization of the SACNT/BOPP Actuator. A laser sight infrared thermometer (Optris LS) with a temperature resolution of 0.1 °C was used to measure the temperature of the actuator. The temperature data were obtained from the SACNT layer's surface of the actuator. The emissivity coefficient was set to be 0.95. The bending angle of the actuator was captured by a digital camera. An optical laser distance sensor (Leuze ODSL9) was used to measure the displacements in the stability test. A DC power supply (Lvyang YB1732A(5A)) was used to provide the DC voltage. A Keithley 2410 source meter was used as the power supply in the stability test. SEM images were performed by a field emission scanning electron microscope (Hitachi SU8010). Conflict of Interest: The authors declare no competing financial interest.

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constructed as a demonstration of advanced-structure actuators. Based on the weightlifting property, a prototype of a weightlifting walking robot with high moving speed is elaborately designed and constructed to demonstrate promising biomimetic applications of

Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05413. Curvature calculation, mechanical modeling, discussion of actuation performance, electrical power density dependence of maximum temperature, the stability test, crosssectional SEM image after cycling, load-carrying capacity, and bending curvature dependence of load-carrying capacity (PDF)

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