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Electrically controlled liquid crystal elastomer–based soft tubular actuator with multimodal actuation

Electrically controlled liquid crystal elastomer–based soft tubular actuator with multimodal... SCIENCE ADVANCES RESEARCH ARTICLE APPLIED SCIENCES AND ENGINEERING Copyright © 2019 The Authors, some rights reserved; Electrically controlled liquid crystal elastomer–based exclusive licensee American Association soft tubular actuator with multimodal actuation for the Advancement of Science. No claim to 1 1 2 1 Qiguang He , Zhijian Wang , Yang Wang , Adriane Minori , original U.S. Government 1,2 1,2 Michael T. Tolley , Shengqiang Cai * Works. Distributed under a Creative Soft tubular actuators can be widely found both in nature and in engineering applications. The benefits of Commons Attribution tubular actuators include (i) multiple actuation modes such as contraction, bending, and expansion; (ii) facile NonCommercial fabrication from a planar sheet; and (iii) a large interior space for accommodating additional components or for License 4.0 (CC BY-NC). transporting fluids. Most recently developed soft tubular actuators are driven by pneumatics, hydraulics, or tendons. Each of these actuation modes has limitations including complex fabrication, integration, and nonuniform strain. Here, we design and construct soft tubular actuators using an emerging artificial muscle material that can be easily patterned with programmable strain: liquid crystal elastomer. Controlled by an externally applied electrical potential, the tubular actuator can exhibit multidirectional bending as well as large (~40%) homogenous contrac- tion. Using multiple tubular actuators, we build a multifunctional soft gripper and an untethered soft robot. INTRODUCTION vide notable convenience for system control and integration. A few In nature as well as engineering applications, we can often find ac- recent studies have successfully integrated stretchable resistive heaters tuators of cylindrical shape, for instance, the trunk of the elephant, into LCE (35–37), whose actuation can then be easily controlled by the arms of the octopus, and the tube feet of the starfish. Concen- electrical potential. Here, we describe the design and fabrication of tric tubular surgical robots (1, 2) and endoscopes (3–5) are repre- an LCE-based tubular actuator with multiple actuation modes and sentative examples from biomedical engineering, which can exhibit controlled by the externally applied electrical potential of several volts multimodal actuation. Those tubular actuators are usually made of (1.0 to 3.0 V). To demonstrate the advantages of the soft tubular ac- stiff materials, in which a gear-and-shaft system is used to achieve tuator in potential applications, we further fabricate an untethered soft their multimodal actuation. robot with on-board power source and microcontroller, mainly driven Recent work has intensively explored soft continuum robots and by the LCE-based tubular actuators. These features are often highly has demonstrated their many unique and attractive characteristics, such desired in constructing compact and multifunctional devices. as a large number of degrees of freedom and high biocompatibility (6–10). Notably, researchers have designed and fabricated various soft actuators with cylindrical shape to achieve various bioinspired RESULTS movements, such as octopus-inspired robotic tentacles (11, 12), Fabrication of LCE-based tubular actuator trunk-inspired manipulators (13), and worm-inspired architectures Figure 1A and fig. S2 show the main steps of fabricating an LCE-based (14). However, most previously constructed soft actuators were either tubular actuator. We sandwiched three separate thin stretchable ser- pneumatically or hydrodynamically driven, which often require a pentine heating wires (as shown in fig. S3) between two layers of loosely bulky external control system, complex internal channels for diverse cross-linked LCE films (1 mm, each), as shown in steps (i) and (ii). In actuation modes, and the prevention of fluid leakage (15–17). There is step (iii), we made the tubular shape by rolling the sandwich-like struc- a clear need for stimuli-responsive material to construct soft actuators ture and stretching it by l . The LCE tubular actuator could be finally (18, 19), which has the potential to simplify fabrication and assembly obtained once the entire structure was exposed to ultraviolet (UV) ir- processes and reduce the complexity of the controlling. radiation to fix the alignment of the liquid crystal mesogens inside. Liquid crystal elastomer (LCE), a thermally driven actuating ma- In nature, animals such as mammals can contract muscles in dif- terial that combines polymer network and liquid crystal mesogens, as ferent parts in their bodies to achieve multimodal actuation. Similarly, shown in fig. S1 (A and B), has attracted much attention recently be- the fabricated LCE-based tubular actuator contains three separate cause of its unique properties, including large (~40%) and reversible heating wires that can be controlled independently, as shown in actuation, high processability, and programmability (20–25). As the Fig. 1B. The electric current going through the metallic wires gener- temperature increases, liquid crystal mesogens transition from the ne- ates Joule heating and increases the temperature of the adjacent LCE. matic phase to the isotropic phase, leading to a notable and macro- Consequently, the heated LCE can contract in the longitudinal direc- scopic deformation in the material. A variety of LCE-based actuating tion. By applying an electrical potential onto different embedded structures have been designed and successfully fabricated, which are heating wires, we achieve various actuating modes. For example, if often actuated by direct environmental heating (26, 27) or by light only one of the heating wires generates Joule heating, the tube can through photothermal and photochemical effects (28–34). However, bend toward one direction, but if all three heating wires generate for most practical applications, electronically powered actuators pro- Joule heating, the tube can contract homogenously. Electrical potential–driven actuation of an LCE thin film Department of Mechanical and Aerospace Engineering, University of California, To better control the actuation of the tubular actuator, we first fab- San Diego, La Jolla, CA 92093, USA. Materials Science and Engineering Program, ricated and characterized the actuation behavior of an LCE artificial University of California, San Diego, La Jolla, CA 92093, USA. *Corresponding author. Email: shqcai@ucsd.edu muscle film. The fabrication of LCE artificial muscle film is similar to He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 1of7 | SCIENCE ADVANCES RESEARCH ARTICLE that of the LCE tubular actuator. A freestanding LCE artificial muscle Next, we quantitatively characterized the actuation behaviors of film contracted by 41% of its initial length as 3.0 V was applied, as LCE artificial muscle when different electrical potentials were applied. shown in Fig. 2A and movie S1. When we turned off the electrical In Fig. 2B, we showed the actuation strain of LCE artificial muscle ver- potential, the temperature of LCE artificial muscle gradually decreased sus time with four different levels of electrical potential (1.0, 1.5, 2.0, to room temperature, and the LCE artificial muscle recovered to its and 3.0 V) for 360 s. For a given electrical potential, the actuation original shape within 4 min. To quantitatively characterize the actua- strain of LCE artificial muscle increased at the beginning and then tion of the LCE-based artificial muscle, we define the actuation strain reached a plateau value either because the temperature field in the ar- as e =(L − l)/L × 100%, where L and l are the lengths of LCE artificial tificial muscle reached a steady state or because the actuation was at its muscle in the initial and actuated states, respectively. maximal value (41%). As we turned off the electrical potential, the Thermal images (taken by FLIR E75-42 Advanced Thermal Imag- ing Camera) in fig. S4A showed the surface temperature of LCE arti- ficial muscle film during the heating process when we applied 3.0 V to it. The highest temperature on the surface increased from 20° to 120°C within 30 s, and the actuation strain of the LCE artificial muscle reached 41%. We also noticed that nearly homogeneous temperature distribution could be generated on the surface of the artificial muscle via Joule heating and thermal diffusion after the 30 s of heating. If we applied the electrical potential onto the heating wires for a longer time, the surface temperature of the artificial muscle could be further increased to 220°C, while the actuation strain maintained its 41% con- traction. We had also shown, in fig. S1C, that the embedded heating wires of serpentine shape have a negligible influence on the actuation behavior of LCE films. Fig. 2. Thermomechanical characterizations of LCE artificial muscle film. (A) Optical images of reversible actuation of the artificial muscle: The film contracted by 40% of its initial length when an electrical potential of 3.0 V was applied. (B) Plot of actuation strain of LCE artificial muscle versus time with dif- ferent applied electrical potential ranging from 1.0 to 3.0 V. For a given electrical potential, actuation strain increased and then reached a plateau value (steady state). When the electrical potential increased from 1.0 to 2.0 V, the actuation strain at steady state increased from 15 to 41%. As the electrical potential further increased from 2.0 to 3.0 V, the actuation strain of LCE artificial muscle at steady state remained the same, while the response time (time for reaching the steady state) decreased from 100 to 30 s. (C and D) Actuation stress of the artificial Fig. 1. Design, fabrication, and operation principle of an LCE tubular actuator. muscle film (with fixed length) versus time with different electrical potentials rang- (A) Fabrication steps of an LCE-based tubular actuator: (i) Three serpentine ing from 1.0 to 3.0 V for 30 s. The actuation stress increased from 0.05 to 0.35 MPa heating wires were sandwiched between two layers of loosely cross-linked LCE as the electrical potential was increased from 1.0 to 3.0 V. (E) An LCE artificial films. (ii) The sandwiched structure was compressed slightly to promote adhesion. muscle film could lift a load of 3.92 N (the stress was 0.312 MPa) by 38% of its initial (iii) A tube was made by rolling the thin film. (iv) An LCE tubular actuator was length. (F) Actuation strain of the LCE artificial muscle film versus time under three obtained by stretching and then exposing the actuator under UV irradiation. different levels of applied stresses. As the load was increased, the time needed for (B) Principle of operation of LCE tubular actuators. Bending motion can be real- the LCE film to contract was almost unchanged, while the time needed for it to ized by applying an electrical potential to one of the heating wires (colored red); recover to its original length decreased. Inset: Maximal work density of LCE artificial homogeneous contraction can be obtained by applying an electrical potential to muscle under different applied stresses. Scale bars, 1 cm (A, C, and E). Photo credit: all heating wires (colored red). Qiguang He, University of California, San Diego. He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 2of7 | SCIENCE ADVANCES RESEARCH ARTICLE actuation strain decreased to zero within 240 s. When we increased the electrical potential onto one or two of the heating wires (same elec- electrical potential from 0.5 to 2.0 V, the maximal actuation strain rose trical potential for both heating wires) could generate bending ac- from 15 to 41%. As we incremented the electrical potential further tuation. Depending on which heating wires we distributedly activated, from 2.0 to 3.0 V, the maximal actuation strain maintained the same the tube could bend toward six different directions (red color in Fig. 3B value (41%), but the response time decreased. It took about 30 s for the indicates the activated heating wires). Similarly, by simultaneously LCE artificial muscle to reach the maximum contraction (41%) when applying the electrical potential to all the heating wires, the entire 3.0 V was applied compared with 150 s while 2.0 V was applied. As LCE tube was heated up, resulting in a homogenous contraction. shown in fig. S4B, as we increased the electrical potential from 1.0 to The LCE tubular actuator recovered to its original shape and geom- 2.0 V, the surface temperature of the LCE films in the steady state was etry after the electrical potential was turned off. increased from 55° to 120°C. Correspondingly, the actuation strain of On the basis of the measurement of the actuation strain on a single the LCE artificial muscle was increased from 16 to 41%, as shown in LCE artificial muscle thin film as described previously, we could quan- Fig. 2B. If we further increased the electrical potential from 2.0 to 3.0 V titatively predict the bending angle of the LCE tubular actuator, as and the maximal surface temperature increased from 120° to 220°C shown in Fig. 3C. In the experiment, we applied different electrical (fig. S4B), the actuation strain of LCE film stayed unchanged (Fig. 2B). potentials to an LCE artificial muscle thin film for 30 s and then turned We also showed that the performance of the LCE artificial muscle film off the electrical potentials. We measured the actuation strain of the almost remained unchanged after more than 50 and 100 cycles of actu- thin film as a function of time for different electrical potentials. The ation with an applied electrical potential of 3.0 and 2.0 V, respectively results are shown in Fig. 3D, together with the contraction of the LCE (fig. S5). Moreover, no delamination between LCE layer and heating wire tubular actuator when we applied the electrical potential onto all three was observed in cyclic heating and cooling process in the experiments. embedded heating wires. It can be seen that those two results are very By fixing the length of the LCE artificial muscle film (Fig. 2C), we similar to each other. studied its actuation stress of fixed length with different applied electrical To predict the bending angle of the tubular actuator when we ap- potentials ranging from 1.0 to 3.0 V for 30 s. For a given electrical plied the electrical potential only onto one of the heating wires, we potential, the actuation stress increased from zero to the maximum value adopted the model developed for calculating the deflection of a beam within 30 s and dropped to zero when we turned off the electrical caused by inhomogeneous thermal expansion (39). As shown in fig. S6, potential. As the electrical potential increased from 1.0 to 3.0 V, the max- when one of the three embedded heating wires was subjected to an imal actuation stress increased from 0.05 to 0.35 MPa, which is shown in electrical potential, we assumed that one-third of the tubular actuator Fig. 2D. We noted that the typical actuating stress generated by skeletal muscles of most mammals is also around 0.1 to 0.35 MPa (18, 38). We also measured the work density of LCE artificial muscle film by applying a constant load onto the LCE artificial muscle film and an elec- trical potential of 3.0 V (Fig. 2, E and F). In the experiment, we measured the contraction of the artificial muscle film with five different loads rang- ing from 0.49 to 3.92 N. We calculated the mechanical work done by the artificial muscle film by multiplying the magnitude of the load by the maximal displacement of contraction, as shown in Fig. 2E. The maximal work density can be further obtained by dividing the mechanical work by the volume of the LCE artificial muscle film. As shown in the inset of Fig. 2F, the maximal work density of the LCE artificial muscle film could reachashighas150 kJ/m when the applied stress was 0.31 MPa, which was comparable to that of dielectric elastomer membrane and mamma- lian skeletal muscle but higher than that of ionic polymer metal com- posites and piezoelectric actuators (18). Last, we would like to point out that, like other thermally respon- sive materials, the actuation bandwidth of LCE actuators depends primarily on the characteristic time of heat transfer in the material. The thermal conductivity of LCE is low, so their response time is rel- Fig. 3. Multimodal actuation of an LCE-based tubular actuator. (A) Real and atively slow, as shown in Fig. 2 (B, D, and F). The most effective way thermal images of an LCE tubular actuator during actuation: Activating one or to reduce the response time is to decrease the size of the material or, two heating wires resulted in bending motion; activating all three heating wires more precisely, the characteristic size of the heating area. However, resulted in homogeneous contraction. (B) Different actuation modes of the tubu- lar actuator. Six directional bending were realized by activating one or two this modification could adversely affect other actuation performance heating wires. Contraction was achieved by simultaneously activating all three of the LCE actuators, such as the actuation force and strain. There- heating wires. (C) Plot of bending angle of LCE tubular actuator versus time by fore, an optimized design will certainly be needed to achieve a desir- applying electrical potentials from 1.0 to 3.0 V. The heating time was 30 s, and the able combination of actuation speed, strain, and stress for specific cooling time was 270 s. Solid lines represent theoretical prediction, and dots repre- applications. sent experimental results. (D) Plot of contraction of LCE tubular actuator versus time through the activation of all three heating wires. The electrical potential Electrical potential–controlled multimodal actuation of a was set to 3.0 V, and the time of electrical potential on was set to 30 s; the time tubular actuator of electrical potential off was set to 270 s. Solid lines represent theoretical predic- We also characterized the multimodal actuation of an LCE tubular tion, and dots represent experimental results. Scale bars, 1 cm (A and B). Photo actuator. In Fig. 3 (A and B), we demonstrated that applying an credit: Qiguang He, University of California, San Diego. He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 3of7 | SCIENCE ADVANCES RESEARCH ARTICLE behaves just like the LCE artificial muscle thin film and that the other Mini, 3.0 V), electrical components [metal oxide semiconductor two-thirds of the tubular actuator acts like passive elastic material. field-effect transistors (MOSFETs) and wires)], battery (lithium/ Therefore, the bending angle of the tube can be computed as q ¼ polymer, 3.7 V), and LCE tubular actuators containing two separate M 4 4 T p l ,where l is the length of the tube; I ¼ E½ðd Þ ðd Þ ,known heating wires (for the detailed characterizations, see fig. S7, C and D) 0 0 o i I 64 as the bending stiffness of the tube with E as the modulus of the LCE and with an acrylic plate to build an untethered robot shown in Fig. 5A. d and d as the outer and inner diameter of the tube, respectively; M = In Fig. 5B, with automatically controlled LCE tubular actuators, o i T ∫ e ZdA as the thermal bending moment, with e as the actuation the untethered robot could walk on a flat surface. The robot started T T strain of the thin film and Z and A as denoted in fig. S6. With the input of to walk by simultaneously activating a pair of its diagonal tubular the actuation strain e measured for LCE thin film, as shown in Fig. 3D, actuators for 17 s to bend forward. After that, the other pair of tu- theaboveequationallows us to predict the bending angle of the tubular bular actuators was activated simultaneously for 18 s. In the last step, actuator as a function of time. The prediction agrees very well with the the electrical potential was turned off for all the tubular actuators for experimental measurements, shown in Fig. 3C. We observed that both 205 s, and all the actuators turned back to the original resting state. the length of the tube and the inner and outer diameters of the tube can The displacement in one period was around 8.5 mm (see Fig. 5D). be measured. There is no fitting parameter for the prediction. After 30 min of walking, the robot can move forward for a distance The position of the center of the unconstrained cross-sectional of about one body length (8 cm), as shown in movie S7. area of the tubular actuator is important to understand the actua- Figure 5C demonstrates three cycles of the untethered robot trans- tion performance. We plotted the trajectory in fig. S7 (A and B), in porting an item on the top. In the experiment, we placed a cardboard which one or two heating wires were subjected to electrical potentials. on top of the four tubular actuators and a weight of 10 g on the plate. The trajectory of the center point on the cross-sectional area of the During the operation, the two front actuators first bent forward grad- tubular actuator when we applied in one wire an electrical potential ually using around 45 s and then recovered back to the straight state of 3.0Vfor30sis showninfig.S7A andmovie S2.Whenthe power accompanied by contraction. During the recovery and contraction of supply delivered the same electrical potential (3.0 V) for 20 s to two the front actuators, the other two rear actuators bent forward also using heating wires, the trajectories of the actuator’s cross section have the about 45 s, which was followed by the state that the electrical potential displacements shown in fig. S7A and movie S3. When all three was turned off for all the actuators. These repetitive actions enabled the heating wires had the same electrical potential (3.0 V) for 30 s, we robottomovethe rigidplate andthe weight on topofthe plateforward observed a homogeneous contraction, as shown in movie S4. about 5 cm within 15 min, as shown in Fig. 5D and movie S8. Because of the thermal diffusion, when we applied high electrical The costs of transport (COTs) are estimated as 5.0 × 10 and 1.0 × potential (e.g., 3.0 V) to one or two heating wires, larger areas of the 10 for the robot walking and transporting experiments, respectively. tube could be heated up if the heating time was too long (e.g., 360 s), Compared with other untethered soft robots (40), the magnitude of which might induce unexpected geometrical distortion of the tube, as COTs of the LCE-based untethered robot is very large because of the shown in fig. S8. To address this problem, we adopted the following low energy efficiency of thermally driven actuating materials. strategy: We first applied a high electrical potential (V = 3.0 V) to the heating wires for 30 s to induce fast bending, and then the electrical potential was reduced to a lower value (V = 1.5 V) for the rest of time DISCUSSION to maintain the deformation, as shown in fig. S8. We show the cyclic Here, we have discussed the design and fabrication of an LCE-based bending of the LCE tubular actuator in fig. S9, when we turned on and tubular actuator, which can exhibit multidirectional bending and off, repeatedly, the electrical potential into one or two heating wires. homogenous contraction as a low electrical potential is selectively Because of the internal heating mechanism of the tubular actua- tor, we further show that the LCE tubular actuator could also work effectively in a water environment (fig. S10) with slightly increased electrical potential (6.0 V) due to the higher thermal conductivity and heat capacity of water as compared to air. Soft gripper based on LCE-based tubular actuator We next built a soft gripper (Fig. 4) using the tubular actuator dem- onstrated previously as the main building block. As shown in Fig. 4A, we could construct an electronically controlled soft gripper by using three tubular actuators. Three tubular actuators were first attached to a circular plate, which was further connected to an LCE artificial muscle film. We used a microcontroller to control the electrical potential ap- plied to the actuator and, thus, the deformation of each tubular actu- ator. By selectively activating the heating wires in each tubular actuator of the gripper, the gripper can grasp and lift a vial (Fig. 4C and movie S5) and also twist its cap (Fig. 4B and movie S6) without additional external control. Fig. 4. A multifunctional soft gripper composed of three LCE tubular actuators. An untether soft robot (A) Schematic of the assembly of the soft gripper. (B) Soft gripper twisting the cap of We next designed and fabricated the untethered robot using the a vial. (C) Soft gripper grasping and lifting the vial (50 g). Scale bars, 1 cm (B and C). LCE tubular actuators. We integrated a microcontroller (Arduino Photo credit: Qiguang He, University of California, San Diego. He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 4of7 | SCIENCE ADVANCES RESEARCH ARTICLE Fig. 5. Electrically activated, untethered soft robot. (A) Schematic of the robot, mainly composed of a microcontroller, battery, and four LCE tubular actuators. (B) Frames from a video of the robot walking. Walking began from rest (all the actuators were in the deactivated state); then, a pair of diagonal tubular actuators was simultaneously activated for 17 s to bend forward. After that, the other pair of tubular actuators was activated simultaneously for 18 s. In the last step, the electrical potential to all actuators was turned off for 205 s, returning the actuators to their original states. (C) Frames from the video of the robot transporting an object on a plate of cardboard. During the operation, the two front actuators first bent forward for 45 s and then straightened without touching the top plate. During the straightening of the two front actuators, the two rear actuators bent forward for approximately 45 s; then, the electrical potential to all the actuators was turned off. Repeating this sequence enabled the robot to move the rigid plate and the weight on top forward about 5 cm within 15 min. (D) Plot of displacement of the untether robot (black curve) and displacement of the item on the top plate (red curve) versus time. Scale bars, 2 cm (B and C). Photo credit: Qiguang He, University of California, San Diego. applied to heating wires embedded in the LCE. In addition, on the methylpropiophenone (HHMP; Sigma-Aldrich; 98%), 2,2′-(ethylenedioxy) basis of the tubular actuators, we have further constructed soft grip- diethanethiol (EDDET; Sigma-Aldrich; 95%), pentaerythritol tetrakis pers that can grasp a vial and twist its cap. By integrating a battery, a (3-mercaptopropionate) (PETMP; Sigma-Aldrich; 95%), dipropyla- microcontroller, and the tubular actuators, we have built an untethered mine (DPA; Sigma-Aldrich; 98%), 3% hydrogen peroxide solution robot that can crawl on the ground and transport an item. (Sigma-Aldrich), sodium iodide (Sigma-Aldrich; 99%), and sodium The distributed actuation of a tubular LCE caused the multimodal thiosulfate pentahydrate (Fisher Scientific) were used as received with- actuation of the soft tubular actuator developed here. Because of the out further purification. The mechanical tests were conducted using embedded serpentine heating wires, we can fully activate the tubular the Instron Universal Testing Machine (5965 Dual Column Testing actuator by applying an electrical potential. Although LCE has been Systems; Instron) with 1000-N loading cell. The surface temperature intensively explored in recent years to design and fabricate various pre- of LCE artificial muscle was measured by thermal imaging technique viously unknown structures and devices (41–43), the multifunctional (FLIR E75-42 Advanced Thermal Imaging Camera). robot based on the tubular actuators, to our knowledge, is the first un- tethered robotic system only using LCE as the actuating material. Fabrication of loosely cross-linked LCE film Previous work in electrically controlled soft structures such as most We prepared the loosely cross-linked LCE following the previously dielectric elastomer actuators (DEAs) has shown much faster speed reported work with little modification (45). Liquid crystal mesogen and higher energy efficiency compared to the LCE-based tubular ac- RM257 (10.957 g, 18.6 mmol) was added into toluene, and the mixture tuators developed here. However, the LCE-based tubular actuator was heated at 85°C. The photoinitiator HHMP (0.077 g, 0.3 mmol) does not require any stiff frames to maintain large prestretch for was added into the mixture. Then, we added 3.076 g (16.9 mmol) of achieving large actuation (44). Furthermore, the tubular actuator also spacer EDDET, 0.244 g (0.5 mmol) of cross-linker PETMP, and 0.038 g uses a low-voltage power source (around three orders of magnitude (0.4 mmol) of catalyst DPA into the solution. After stirring and degas- lower than the electrical potential required to actuate DEAs). This fea- sing, we poured the mixture into the rectangular mold (1 mm thick- ture makes our tubular actuator compatible with most low-cost, com- ness) and put it at room temperature for 24 hours. Then, we placed the mercially available electronic devices and batteries. loosely cross-linked LCE film into an 85°C oven for solvent evapora- tion. After that, we obtained the loosely cross-linked LCE film. MATERIALS AND METHODS Fabrication of heating mesh Materials The fabrication process of the heating mesh is shown in fig. S3. First, a 1,4-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene 2-mm-thick layer of polyimide (PI) precursor was spin-coated onto the (RM257) (Wilshire Technologies; 95%), (2-hydroxyethoxy)-2- 20-mm-thick layer of copper. After that, the one-side coated copper He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 5of7 | SCIENCE ADVANCES RESEARCH ARTICLE Fig. S4. Temperature change of the LCE artificial muscle film during contraction and recovery. was cured under 300°C oven under nitrogen protection. Then, we Fig. S5. Cyclic actuation test of the LCE artificial muscle film with applied stress of 0.078 MPa. placed the PI-coated copper with the copper layer up onto a substrate, Fig. S6. Theoretical prediction of bending angle of LCE tubular actuator by using the actuation as shown in step (i). The photoresist AZ1512 was spin-coated onto the of LCE artificial muscle thin film. copper layer. Photolithography and wet etching define patterns of the Fig. S7. Characterizations of LCE tubular actuators. Fig. S8. The actuation behavior of LCE tubular actuator with different voltage controls. copper wires (step ii). Later, we removed the residual photoresist with Fig. S9. Cyclic test of LCE tubular actuator with different heating wires. acetone; then, another layer of PI was spin-coated on the patterned Fig. S10. The reversible actuation (bending motion and contraction) of LCE tubular actuator in copper layer and baked in 300°C oven under nitrogen protection. Af- the water environment. ter that, photolithography defined a mask for dry etching process with Movie S1. Reversible actuation of LCE thin film actuator. CF and O through the PI layer. Last, we obtained the heating mesh Movie S2. Reversible bending actuation of LCE tubular actuator (activate one heating wire). 4 2 Movie S3. Reversible bending actuation of LCE tubular actuator (activate two heating wires). by removing the photoresist with acetone. Movie S4. Homogeneous contraction of LCE tubular actuator (activate three heating wires). Movie S5. Soft gripper grasps the vial. Fabrication of LCE tubular actuator Movie S6. Soft gripper twists the cap. We showed the fabrication process of the LCE tubular actuator in Movie S7. Untether soft robot walks on the ground. Movie S8. Untether soft robot manipulates the weight. fig. S2. First, we sandwiched three heating wires between two layers of loosely cross-linked LCE film (39 mm × 30 mm × 1 mm). 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Kim, Z. Rao, Y. Li, W. Chen, J. Song, R. Verduzco, C. Yu, Soft crystal elastomer–based soft tubular actuator with multimodal actuation. Sci. Adv. 5, eaax5746 ultrathin electronics innervated adaptive fully soft robots. Adv. Mater. 30, 1706695 (2018). (2019). He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 7of7 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Science Advances Pubmed Central

Electrically controlled liquid crystal elastomer–based soft tubular actuator with multimodal actuation

Science Advances , Volume 5 (10) – Oct 11, 2019

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Abstract

SCIENCE ADVANCES RESEARCH ARTICLE APPLIED SCIENCES AND ENGINEERING Copyright © 2019 The Authors, some rights reserved; Electrically controlled liquid crystal elastomer–based exclusive licensee American Association soft tubular actuator with multimodal actuation for the Advancement of Science. No claim to 1 1 2 1 Qiguang He , Zhijian Wang , Yang Wang , Adriane Minori , original U.S. Government 1,2 1,2 Michael T. Tolley , Shengqiang Cai * Works. Distributed under a Creative Soft tubular actuators can be widely found both in nature and in engineering applications. The benefits of Commons Attribution tubular actuators include (i) multiple actuation modes such as contraction, bending, and expansion; (ii) facile NonCommercial fabrication from a planar sheet; and (iii) a large interior space for accommodating additional components or for License 4.0 (CC BY-NC). transporting fluids. Most recently developed soft tubular actuators are driven by pneumatics, hydraulics, or tendons. Each of these actuation modes has limitations including complex fabrication, integration, and nonuniform strain. Here, we design and construct soft tubular actuators using an emerging artificial muscle material that can be easily patterned with programmable strain: liquid crystal elastomer. Controlled by an externally applied electrical potential, the tubular actuator can exhibit multidirectional bending as well as large (~40%) homogenous contrac- tion. Using multiple tubular actuators, we build a multifunctional soft gripper and an untethered soft robot. INTRODUCTION vide notable convenience for system control and integration. A few In nature as well as engineering applications, we can often find ac- recent studies have successfully integrated stretchable resistive heaters tuators of cylindrical shape, for instance, the trunk of the elephant, into LCE (35–37), whose actuation can then be easily controlled by the arms of the octopus, and the tube feet of the starfish. Concen- electrical potential. Here, we describe the design and fabrication of tric tubular surgical robots (1, 2) and endoscopes (3–5) are repre- an LCE-based tubular actuator with multiple actuation modes and sentative examples from biomedical engineering, which can exhibit controlled by the externally applied electrical potential of several volts multimodal actuation. Those tubular actuators are usually made of (1.0 to 3.0 V). To demonstrate the advantages of the soft tubular ac- stiff materials, in which a gear-and-shaft system is used to achieve tuator in potential applications, we further fabricate an untethered soft their multimodal actuation. robot with on-board power source and microcontroller, mainly driven Recent work has intensively explored soft continuum robots and by the LCE-based tubular actuators. These features are often highly has demonstrated their many unique and attractive characteristics, such desired in constructing compact and multifunctional devices. as a large number of degrees of freedom and high biocompatibility (6–10). Notably, researchers have designed and fabricated various soft actuators with cylindrical shape to achieve various bioinspired RESULTS movements, such as octopus-inspired robotic tentacles (11, 12), Fabrication of LCE-based tubular actuator trunk-inspired manipulators (13), and worm-inspired architectures Figure 1A and fig. S2 show the main steps of fabricating an LCE-based (14). However, most previously constructed soft actuators were either tubular actuator. We sandwiched three separate thin stretchable ser- pneumatically or hydrodynamically driven, which often require a pentine heating wires (as shown in fig. S3) between two layers of loosely bulky external control system, complex internal channels for diverse cross-linked LCE films (1 mm, each), as shown in steps (i) and (ii). In actuation modes, and the prevention of fluid leakage (15–17). There is step (iii), we made the tubular shape by rolling the sandwich-like struc- a clear need for stimuli-responsive material to construct soft actuators ture and stretching it by l . The LCE tubular actuator could be finally (18, 19), which has the potential to simplify fabrication and assembly obtained once the entire structure was exposed to ultraviolet (UV) ir- processes and reduce the complexity of the controlling. radiation to fix the alignment of the liquid crystal mesogens inside. Liquid crystal elastomer (LCE), a thermally driven actuating ma- In nature, animals such as mammals can contract muscles in dif- terial that combines polymer network and liquid crystal mesogens, as ferent parts in their bodies to achieve multimodal actuation. Similarly, shown in fig. S1 (A and B), has attracted much attention recently be- the fabricated LCE-based tubular actuator contains three separate cause of its unique properties, including large (~40%) and reversible heating wires that can be controlled independently, as shown in actuation, high processability, and programmability (20–25). As the Fig. 1B. The electric current going through the metallic wires gener- temperature increases, liquid crystal mesogens transition from the ne- ates Joule heating and increases the temperature of the adjacent LCE. matic phase to the isotropic phase, leading to a notable and macro- Consequently, the heated LCE can contract in the longitudinal direc- scopic deformation in the material. A variety of LCE-based actuating tion. By applying an electrical potential onto different embedded structures have been designed and successfully fabricated, which are heating wires, we achieve various actuating modes. For example, if often actuated by direct environmental heating (26, 27) or by light only one of the heating wires generates Joule heating, the tube can through photothermal and photochemical effects (28–34). However, bend toward one direction, but if all three heating wires generate for most practical applications, electronically powered actuators pro- Joule heating, the tube can contract homogenously. Electrical potential–driven actuation of an LCE thin film Department of Mechanical and Aerospace Engineering, University of California, To better control the actuation of the tubular actuator, we first fab- San Diego, La Jolla, CA 92093, USA. Materials Science and Engineering Program, ricated and characterized the actuation behavior of an LCE artificial University of California, San Diego, La Jolla, CA 92093, USA. *Corresponding author. Email: shqcai@ucsd.edu muscle film. The fabrication of LCE artificial muscle film is similar to He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 1of7 | SCIENCE ADVANCES RESEARCH ARTICLE that of the LCE tubular actuator. A freestanding LCE artificial muscle Next, we quantitatively characterized the actuation behaviors of film contracted by 41% of its initial length as 3.0 V was applied, as LCE artificial muscle when different electrical potentials were applied. shown in Fig. 2A and movie S1. When we turned off the electrical In Fig. 2B, we showed the actuation strain of LCE artificial muscle ver- potential, the temperature of LCE artificial muscle gradually decreased sus time with four different levels of electrical potential (1.0, 1.5, 2.0, to room temperature, and the LCE artificial muscle recovered to its and 3.0 V) for 360 s. For a given electrical potential, the actuation original shape within 4 min. To quantitatively characterize the actua- strain of LCE artificial muscle increased at the beginning and then tion of the LCE-based artificial muscle, we define the actuation strain reached a plateau value either because the temperature field in the ar- as e =(L − l)/L × 100%, where L and l are the lengths of LCE artificial tificial muscle reached a steady state or because the actuation was at its muscle in the initial and actuated states, respectively. maximal value (41%). As we turned off the electrical potential, the Thermal images (taken by FLIR E75-42 Advanced Thermal Imag- ing Camera) in fig. S4A showed the surface temperature of LCE arti- ficial muscle film during the heating process when we applied 3.0 V to it. The highest temperature on the surface increased from 20° to 120°C within 30 s, and the actuation strain of the LCE artificial muscle reached 41%. We also noticed that nearly homogeneous temperature distribution could be generated on the surface of the artificial muscle via Joule heating and thermal diffusion after the 30 s of heating. If we applied the electrical potential onto the heating wires for a longer time, the surface temperature of the artificial muscle could be further increased to 220°C, while the actuation strain maintained its 41% con- traction. We had also shown, in fig. S1C, that the embedded heating wires of serpentine shape have a negligible influence on the actuation behavior of LCE films. Fig. 2. Thermomechanical characterizations of LCE artificial muscle film. (A) Optical images of reversible actuation of the artificial muscle: The film contracted by 40% of its initial length when an electrical potential of 3.0 V was applied. (B) Plot of actuation strain of LCE artificial muscle versus time with dif- ferent applied electrical potential ranging from 1.0 to 3.0 V. For a given electrical potential, actuation strain increased and then reached a plateau value (steady state). When the electrical potential increased from 1.0 to 2.0 V, the actuation strain at steady state increased from 15 to 41%. As the electrical potential further increased from 2.0 to 3.0 V, the actuation strain of LCE artificial muscle at steady state remained the same, while the response time (time for reaching the steady state) decreased from 100 to 30 s. (C and D) Actuation stress of the artificial Fig. 1. Design, fabrication, and operation principle of an LCE tubular actuator. muscle film (with fixed length) versus time with different electrical potentials rang- (A) Fabrication steps of an LCE-based tubular actuator: (i) Three serpentine ing from 1.0 to 3.0 V for 30 s. The actuation stress increased from 0.05 to 0.35 MPa heating wires were sandwiched between two layers of loosely cross-linked LCE as the electrical potential was increased from 1.0 to 3.0 V. (E) An LCE artificial films. (ii) The sandwiched structure was compressed slightly to promote adhesion. muscle film could lift a load of 3.92 N (the stress was 0.312 MPa) by 38% of its initial (iii) A tube was made by rolling the thin film. (iv) An LCE tubular actuator was length. (F) Actuation strain of the LCE artificial muscle film versus time under three obtained by stretching and then exposing the actuator under UV irradiation. different levels of applied stresses. As the load was increased, the time needed for (B) Principle of operation of LCE tubular actuators. Bending motion can be real- the LCE film to contract was almost unchanged, while the time needed for it to ized by applying an electrical potential to one of the heating wires (colored red); recover to its original length decreased. Inset: Maximal work density of LCE artificial homogeneous contraction can be obtained by applying an electrical potential to muscle under different applied stresses. Scale bars, 1 cm (A, C, and E). Photo credit: all heating wires (colored red). Qiguang He, University of California, San Diego. He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 2of7 | SCIENCE ADVANCES RESEARCH ARTICLE actuation strain decreased to zero within 240 s. When we increased the electrical potential onto one or two of the heating wires (same elec- electrical potential from 0.5 to 2.0 V, the maximal actuation strain rose trical potential for both heating wires) could generate bending ac- from 15 to 41%. As we incremented the electrical potential further tuation. Depending on which heating wires we distributedly activated, from 2.0 to 3.0 V, the maximal actuation strain maintained the same the tube could bend toward six different directions (red color in Fig. 3B value (41%), but the response time decreased. It took about 30 s for the indicates the activated heating wires). Similarly, by simultaneously LCE artificial muscle to reach the maximum contraction (41%) when applying the electrical potential to all the heating wires, the entire 3.0 V was applied compared with 150 s while 2.0 V was applied. As LCE tube was heated up, resulting in a homogenous contraction. shown in fig. S4B, as we increased the electrical potential from 1.0 to The LCE tubular actuator recovered to its original shape and geom- 2.0 V, the surface temperature of the LCE films in the steady state was etry after the electrical potential was turned off. increased from 55° to 120°C. Correspondingly, the actuation strain of On the basis of the measurement of the actuation strain on a single the LCE artificial muscle was increased from 16 to 41%, as shown in LCE artificial muscle thin film as described previously, we could quan- Fig. 2B. If we further increased the electrical potential from 2.0 to 3.0 V titatively predict the bending angle of the LCE tubular actuator, as and the maximal surface temperature increased from 120° to 220°C shown in Fig. 3C. In the experiment, we applied different electrical (fig. S4B), the actuation strain of LCE film stayed unchanged (Fig. 2B). potentials to an LCE artificial muscle thin film for 30 s and then turned We also showed that the performance of the LCE artificial muscle film off the electrical potentials. We measured the actuation strain of the almost remained unchanged after more than 50 and 100 cycles of actu- thin film as a function of time for different electrical potentials. The ation with an applied electrical potential of 3.0 and 2.0 V, respectively results are shown in Fig. 3D, together with the contraction of the LCE (fig. S5). Moreover, no delamination between LCE layer and heating wire tubular actuator when we applied the electrical potential onto all three was observed in cyclic heating and cooling process in the experiments. embedded heating wires. It can be seen that those two results are very By fixing the length of the LCE artificial muscle film (Fig. 2C), we similar to each other. studied its actuation stress of fixed length with different applied electrical To predict the bending angle of the tubular actuator when we ap- potentials ranging from 1.0 to 3.0 V for 30 s. For a given electrical plied the electrical potential only onto one of the heating wires, we potential, the actuation stress increased from zero to the maximum value adopted the model developed for calculating the deflection of a beam within 30 s and dropped to zero when we turned off the electrical caused by inhomogeneous thermal expansion (39). As shown in fig. S6, potential. As the electrical potential increased from 1.0 to 3.0 V, the max- when one of the three embedded heating wires was subjected to an imal actuation stress increased from 0.05 to 0.35 MPa, which is shown in electrical potential, we assumed that one-third of the tubular actuator Fig. 2D. We noted that the typical actuating stress generated by skeletal muscles of most mammals is also around 0.1 to 0.35 MPa (18, 38). We also measured the work density of LCE artificial muscle film by applying a constant load onto the LCE artificial muscle film and an elec- trical potential of 3.0 V (Fig. 2, E and F). In the experiment, we measured the contraction of the artificial muscle film with five different loads rang- ing from 0.49 to 3.92 N. We calculated the mechanical work done by the artificial muscle film by multiplying the magnitude of the load by the maximal displacement of contraction, as shown in Fig. 2E. The maximal work density can be further obtained by dividing the mechanical work by the volume of the LCE artificial muscle film. As shown in the inset of Fig. 2F, the maximal work density of the LCE artificial muscle film could reachashighas150 kJ/m when the applied stress was 0.31 MPa, which was comparable to that of dielectric elastomer membrane and mamma- lian skeletal muscle but higher than that of ionic polymer metal com- posites and piezoelectric actuators (18). Last, we would like to point out that, like other thermally respon- sive materials, the actuation bandwidth of LCE actuators depends primarily on the characteristic time of heat transfer in the material. The thermal conductivity of LCE is low, so their response time is rel- Fig. 3. Multimodal actuation of an LCE-based tubular actuator. (A) Real and atively slow, as shown in Fig. 2 (B, D, and F). The most effective way thermal images of an LCE tubular actuator during actuation: Activating one or to reduce the response time is to decrease the size of the material or, two heating wires resulted in bending motion; activating all three heating wires more precisely, the characteristic size of the heating area. However, resulted in homogeneous contraction. (B) Different actuation modes of the tubu- lar actuator. Six directional bending were realized by activating one or two this modification could adversely affect other actuation performance heating wires. Contraction was achieved by simultaneously activating all three of the LCE actuators, such as the actuation force and strain. There- heating wires. (C) Plot of bending angle of LCE tubular actuator versus time by fore, an optimized design will certainly be needed to achieve a desir- applying electrical potentials from 1.0 to 3.0 V. The heating time was 30 s, and the able combination of actuation speed, strain, and stress for specific cooling time was 270 s. Solid lines represent theoretical prediction, and dots repre- applications. sent experimental results. (D) Plot of contraction of LCE tubular actuator versus time through the activation of all three heating wires. The electrical potential Electrical potential–controlled multimodal actuation of a was set to 3.0 V, and the time of electrical potential on was set to 30 s; the time tubular actuator of electrical potential off was set to 270 s. Solid lines represent theoretical predic- We also characterized the multimodal actuation of an LCE tubular tion, and dots represent experimental results. Scale bars, 1 cm (A and B). Photo actuator. In Fig. 3 (A and B), we demonstrated that applying an credit: Qiguang He, University of California, San Diego. He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 3of7 | SCIENCE ADVANCES RESEARCH ARTICLE behaves just like the LCE artificial muscle thin film and that the other Mini, 3.0 V), electrical components [metal oxide semiconductor two-thirds of the tubular actuator acts like passive elastic material. field-effect transistors (MOSFETs) and wires)], battery (lithium/ Therefore, the bending angle of the tube can be computed as q ¼ polymer, 3.7 V), and LCE tubular actuators containing two separate M 4 4 T p l ,where l is the length of the tube; I ¼ E½ðd Þ ðd Þ ,known heating wires (for the detailed characterizations, see fig. S7, C and D) 0 0 o i I 64 as the bending stiffness of the tube with E as the modulus of the LCE and with an acrylic plate to build an untethered robot shown in Fig. 5A. d and d as the outer and inner diameter of the tube, respectively; M = In Fig. 5B, with automatically controlled LCE tubular actuators, o i T ∫ e ZdA as the thermal bending moment, with e as the actuation the untethered robot could walk on a flat surface. The robot started T T strain of the thin film and Z and A as denoted in fig. S6. With the input of to walk by simultaneously activating a pair of its diagonal tubular the actuation strain e measured for LCE thin film, as shown in Fig. 3D, actuators for 17 s to bend forward. After that, the other pair of tu- theaboveequationallows us to predict the bending angle of the tubular bular actuators was activated simultaneously for 18 s. In the last step, actuator as a function of time. The prediction agrees very well with the the electrical potential was turned off for all the tubular actuators for experimental measurements, shown in Fig. 3C. We observed that both 205 s, and all the actuators turned back to the original resting state. the length of the tube and the inner and outer diameters of the tube can The displacement in one period was around 8.5 mm (see Fig. 5D). be measured. There is no fitting parameter for the prediction. After 30 min of walking, the robot can move forward for a distance The position of the center of the unconstrained cross-sectional of about one body length (8 cm), as shown in movie S7. area of the tubular actuator is important to understand the actua- Figure 5C demonstrates three cycles of the untethered robot trans- tion performance. We plotted the trajectory in fig. S7 (A and B), in porting an item on the top. In the experiment, we placed a cardboard which one or two heating wires were subjected to electrical potentials. on top of the four tubular actuators and a weight of 10 g on the plate. The trajectory of the center point on the cross-sectional area of the During the operation, the two front actuators first bent forward grad- tubular actuator when we applied in one wire an electrical potential ually using around 45 s and then recovered back to the straight state of 3.0Vfor30sis showninfig.S7A andmovie S2.Whenthe power accompanied by contraction. During the recovery and contraction of supply delivered the same electrical potential (3.0 V) for 20 s to two the front actuators, the other two rear actuators bent forward also using heating wires, the trajectories of the actuator’s cross section have the about 45 s, which was followed by the state that the electrical potential displacements shown in fig. S7A and movie S3. When all three was turned off for all the actuators. These repetitive actions enabled the heating wires had the same electrical potential (3.0 V) for 30 s, we robottomovethe rigidplate andthe weight on topofthe plateforward observed a homogeneous contraction, as shown in movie S4. about 5 cm within 15 min, as shown in Fig. 5D and movie S8. Because of the thermal diffusion, when we applied high electrical The costs of transport (COTs) are estimated as 5.0 × 10 and 1.0 × potential (e.g., 3.0 V) to one or two heating wires, larger areas of the 10 for the robot walking and transporting experiments, respectively. tube could be heated up if the heating time was too long (e.g., 360 s), Compared with other untethered soft robots (40), the magnitude of which might induce unexpected geometrical distortion of the tube, as COTs of the LCE-based untethered robot is very large because of the shown in fig. S8. To address this problem, we adopted the following low energy efficiency of thermally driven actuating materials. strategy: We first applied a high electrical potential (V = 3.0 V) to the heating wires for 30 s to induce fast bending, and then the electrical potential was reduced to a lower value (V = 1.5 V) for the rest of time DISCUSSION to maintain the deformation, as shown in fig. S8. We show the cyclic Here, we have discussed the design and fabrication of an LCE-based bending of the LCE tubular actuator in fig. S9, when we turned on and tubular actuator, which can exhibit multidirectional bending and off, repeatedly, the electrical potential into one or two heating wires. homogenous contraction as a low electrical potential is selectively Because of the internal heating mechanism of the tubular actua- tor, we further show that the LCE tubular actuator could also work effectively in a water environment (fig. S10) with slightly increased electrical potential (6.0 V) due to the higher thermal conductivity and heat capacity of water as compared to air. Soft gripper based on LCE-based tubular actuator We next built a soft gripper (Fig. 4) using the tubular actuator dem- onstrated previously as the main building block. As shown in Fig. 4A, we could construct an electronically controlled soft gripper by using three tubular actuators. Three tubular actuators were first attached to a circular plate, which was further connected to an LCE artificial muscle film. We used a microcontroller to control the electrical potential ap- plied to the actuator and, thus, the deformation of each tubular actu- ator. By selectively activating the heating wires in each tubular actuator of the gripper, the gripper can grasp and lift a vial (Fig. 4C and movie S5) and also twist its cap (Fig. 4B and movie S6) without additional external control. Fig. 4. A multifunctional soft gripper composed of three LCE tubular actuators. An untether soft robot (A) Schematic of the assembly of the soft gripper. (B) Soft gripper twisting the cap of We next designed and fabricated the untethered robot using the a vial. (C) Soft gripper grasping and lifting the vial (50 g). Scale bars, 1 cm (B and C). LCE tubular actuators. We integrated a microcontroller (Arduino Photo credit: Qiguang He, University of California, San Diego. He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 4of7 | SCIENCE ADVANCES RESEARCH ARTICLE Fig. 5. Electrically activated, untethered soft robot. (A) Schematic of the robot, mainly composed of a microcontroller, battery, and four LCE tubular actuators. (B) Frames from a video of the robot walking. Walking began from rest (all the actuators were in the deactivated state); then, a pair of diagonal tubular actuators was simultaneously activated for 17 s to bend forward. After that, the other pair of tubular actuators was activated simultaneously for 18 s. In the last step, the electrical potential to all actuators was turned off for 205 s, returning the actuators to their original states. (C) Frames from the video of the robot transporting an object on a plate of cardboard. During the operation, the two front actuators first bent forward for 45 s and then straightened without touching the top plate. During the straightening of the two front actuators, the two rear actuators bent forward for approximately 45 s; then, the electrical potential to all the actuators was turned off. Repeating this sequence enabled the robot to move the rigid plate and the weight on top forward about 5 cm within 15 min. (D) Plot of displacement of the untether robot (black curve) and displacement of the item on the top plate (red curve) versus time. Scale bars, 2 cm (B and C). Photo credit: Qiguang He, University of California, San Diego. applied to heating wires embedded in the LCE. In addition, on the methylpropiophenone (HHMP; Sigma-Aldrich; 98%), 2,2′-(ethylenedioxy) basis of the tubular actuators, we have further constructed soft grip- diethanethiol (EDDET; Sigma-Aldrich; 95%), pentaerythritol tetrakis pers that can grasp a vial and twist its cap. By integrating a battery, a (3-mercaptopropionate) (PETMP; Sigma-Aldrich; 95%), dipropyla- microcontroller, and the tubular actuators, we have built an untethered mine (DPA; Sigma-Aldrich; 98%), 3% hydrogen peroxide solution robot that can crawl on the ground and transport an item. (Sigma-Aldrich), sodium iodide (Sigma-Aldrich; 99%), and sodium The distributed actuation of a tubular LCE caused the multimodal thiosulfate pentahydrate (Fisher Scientific) were used as received with- actuation of the soft tubular actuator developed here. Because of the out further purification. The mechanical tests were conducted using embedded serpentine heating wires, we can fully activate the tubular the Instron Universal Testing Machine (5965 Dual Column Testing actuator by applying an electrical potential. Although LCE has been Systems; Instron) with 1000-N loading cell. The surface temperature intensively explored in recent years to design and fabricate various pre- of LCE artificial muscle was measured by thermal imaging technique viously unknown structures and devices (41–43), the multifunctional (FLIR E75-42 Advanced Thermal Imaging Camera). robot based on the tubular actuators, to our knowledge, is the first un- tethered robotic system only using LCE as the actuating material. Fabrication of loosely cross-linked LCE film Previous work in electrically controlled soft structures such as most We prepared the loosely cross-linked LCE following the previously dielectric elastomer actuators (DEAs) has shown much faster speed reported work with little modification (45). Liquid crystal mesogen and higher energy efficiency compared to the LCE-based tubular ac- RM257 (10.957 g, 18.6 mmol) was added into toluene, and the mixture tuators developed here. However, the LCE-based tubular actuator was heated at 85°C. The photoinitiator HHMP (0.077 g, 0.3 mmol) does not require any stiff frames to maintain large prestretch for was added into the mixture. Then, we added 3.076 g (16.9 mmol) of achieving large actuation (44). Furthermore, the tubular actuator also spacer EDDET, 0.244 g (0.5 mmol) of cross-linker PETMP, and 0.038 g uses a low-voltage power source (around three orders of magnitude (0.4 mmol) of catalyst DPA into the solution. After stirring and degas- lower than the electrical potential required to actuate DEAs). This fea- sing, we poured the mixture into the rectangular mold (1 mm thick- ture makes our tubular actuator compatible with most low-cost, com- ness) and put it at room temperature for 24 hours. Then, we placed the mercially available electronic devices and batteries. loosely cross-linked LCE film into an 85°C oven for solvent evapora- tion. After that, we obtained the loosely cross-linked LCE film. MATERIALS AND METHODS Fabrication of heating mesh Materials The fabrication process of the heating mesh is shown in fig. S3. First, a 1,4-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene 2-mm-thick layer of polyimide (PI) precursor was spin-coated onto the (RM257) (Wilshire Technologies; 95%), (2-hydroxyethoxy)-2- 20-mm-thick layer of copper. After that, the one-side coated copper He et al., Sci. Adv. 2019; 5 : eaax5746 11 October 2019 5of7 | SCIENCE ADVANCES RESEARCH ARTICLE Fig. S4. Temperature change of the LCE artificial muscle film during contraction and recovery. was cured under 300°C oven under nitrogen protection. Then, we Fig. S5. Cyclic actuation test of the LCE artificial muscle film with applied stress of 0.078 MPa. placed the PI-coated copper with the copper layer up onto a substrate, Fig. S6. Theoretical prediction of bending angle of LCE tubular actuator by using the actuation as shown in step (i). The photoresist AZ1512 was spin-coated onto the of LCE artificial muscle thin film. copper layer. Photolithography and wet etching define patterns of the Fig. S7. Characterizations of LCE tubular actuators. Fig. S8. The actuation behavior of LCE tubular actuator with different voltage controls. copper wires (step ii). Later, we removed the residual photoresist with Fig. S9. Cyclic test of LCE tubular actuator with different heating wires. acetone; then, another layer of PI was spin-coated on the patterned Fig. S10. The reversible actuation (bending motion and contraction) of LCE tubular actuator in copper layer and baked in 300°C oven under nitrogen protection. Af- the water environment. ter that, photolithography defined a mask for dry etching process with Movie S1. Reversible actuation of LCE thin film actuator. CF and O through the PI layer. Last, we obtained the heating mesh Movie S2. Reversible bending actuation of LCE tubular actuator (activate one heating wire). 4 2 Movie S3. Reversible bending actuation of LCE tubular actuator (activate two heating wires). by removing the photoresist with acetone. Movie S4. Homogeneous contraction of LCE tubular actuator (activate three heating wires). Movie S5. Soft gripper grasps the vial. Fabrication of LCE tubular actuator Movie S6. Soft gripper twists the cap. We showed the fabrication process of the LCE tubular actuator in Movie S7. Untether soft robot walks on the ground. Movie S8. Untether soft robot manipulates the weight. fig. S2. First, we sandwiched three heating wires between two layers of loosely cross-linked LCE film (39 mm × 30 mm × 1 mm). 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