Research ArticleAPPLIED SCIENCES AND ENGINEERING

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

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Science Advances  11 Oct 2019:
Vol. 5, no. 10, eaax5746
DOI: 10.1126/sciadv.aax5746

Abstract

Soft tubular actuators can be widely found both in nature and in engineering applications. The benefits of tubular actuators include (i) multiple actuation modes such as contraction, bending, and expansion; (ii) facile fabrication from a planar sheet; and (iii) a large interior space for accommodating additional components or for 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 contraction. Using multiple tubular actuators, we build a multifunctional soft gripper and an untethered soft robot.

INTRODUCTION

In nature as well as engineering applications, we can often find actuators of cylindrical shape, for instance, the trunk of the elephant, the arms of the octopus, and the tube feet of the starfish. Concentric tubular surgical robots (1, 2) and endoscopes (35) are representative examples from biomedical engineering, which can exhibit multimodal actuation. Those tubular actuators are usually made of stiff materials, in which a gear-and-shaft system is used to achieve their multimodal actuation.

Recent work has intensively explored soft continuum robots and has demonstrated their many unique and attractive characteristics, such as a large number of degrees of freedom and high biocompatibility (610). Notably, researchers have designed and fabricated various soft actuators with cylindrical shape to achieve various bioinspired movements, such as octopus-inspired robotic tentacles (11, 12), trunk-inspired manipulators (13), and worm-inspired architectures (14). However, most previously constructed soft actuators were either pneumatically or hydrodynamically driven, which often require a bulky external control system, complex internal channels for diverse actuation modes, and the prevention of fluid leakage (1517). There is a clear need for stimuli-responsive material to construct soft actuators (18, 19), which has the potential to simplify fabrication and assembly processes and reduce the complexity of the controlling.

Liquid crystal elastomer (LCE), a thermally driven actuating material that combines polymer network and liquid crystal mesogens, as shown in fig. S1 (A and B), has attracted much attention recently because of its unique properties, including large (~40%) and reversible actuation, high processability, and programmability (2025). As the temperature increases, liquid crystal mesogens transition from the nematic phase to the isotropic phase, leading to a notable and macroscopic deformation in the material. A variety of LCE-based actuating structures have been designed and successfully fabricated, which are often actuated by direct environmental heating (26, 27) or by light through photothermal and photochemical effects (2834). However, for most practical applications, electronically powered actuators provide notable convenience for system control and integration. A few recent studies have successfully integrated stretchable resistive heaters into LCE (3537), whose actuation can then be easily controlled by electrical potential. Here, we describe the design and fabrication of an LCE-based tubular actuator with multiple actuation modes and controlled by the externally applied electrical potential of several volts (1.0 to 3.0 V). To demonstrate the advantages of the soft tubular actuator in potential applications, we further fabricate an untethered soft robot with on-board power source and microcontroller, mainly driven by the LCE-based tubular actuators. These features are often highly desired in constructing compact and multifunctional devices.

RESULTS

Fabrication of LCE-based tubular actuator

Figure 1A and fig. S2 show the main steps of fabricating an LCE-based tubular actuator. We sandwiched three separate thin stretchable serpentine heating wires (as shown in fig. S3) between two layers of loosely cross-linked LCE films (1 mm, each), as shown in steps (i) and (ii). In step (iii), we made the tubular shape by rolling the sandwich-like structure and stretching it by λp. The LCE tubular actuator could be finally obtained once the entire structure was exposed to ultraviolet (UV) irradiation to fix the alignment of the liquid crystal mesogens inside.

Fig. 1 Design, fabrication, and operation principle of an LCE tubular actuator.

(A) Fabrication steps of an LCE-based tubular actuator: (i) Three serpentine heating wires were sandwiched between two layers of loosely cross-linked LCE films. (ii) The sandwiched structure was compressed slightly to promote adhesion. (iii) A tube was made by rolling the thin film. (iv) An LCE tubular actuator was obtained by stretching and then exposing the actuator under UV irradiation. (B) Principle of operation of LCE tubular actuators. Bending motion can be realized by applying an electrical potential to one of the heating wires (colored red); homogeneous contraction can be obtained by applying an electrical potential to all heating wires (colored red).

In nature, animals such as mammals can contract muscles in different parts in their bodies to achieve multimodal actuation. Similarly, the fabricated LCE-based tubular actuator contains three separate heating wires that can be controlled independently, as shown in Fig. 1B. The electric current going through the metallic wires generates Joule heating and increases the temperature of the adjacent LCE. Consequently, the heated LCE can contract in the longitudinal direction. By applying an electrical potential onto different embedded heating wires, we achieve various actuating modes. For example, if only one of the heating wires generates Joule heating, the tube can bend toward one direction, but if all three heating wires generate Joule heating, the tube can contract homogenously.

Electrical potential–driven actuation of an LCE thin film

To better control the actuation of the tubular actuator, we first fabricated and characterized the actuation behavior of an LCE artificial muscle film. The fabrication of LCE artificial muscle film is similar to that of the LCE tubular actuator. A freestanding LCE artificial muscle film contracted by 41% of its initial length as 3.0 V was applied, as shown in Fig. 2A and movie S1. When we turned off the electrical potential, the temperature of LCE artificial muscle gradually decreased to room temperature, and the LCE artificial muscle recovered to its original shape within 4 min. To quantitatively characterize the actuation of the LCE-based artificial muscle, we define the actuation strain as ε = (Ll)/L × 100%, where L and l are the lengths of LCE artificial muscle in the initial and actuated states, respectively.

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 different 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 muscle film (with fixed length) versus time with different electrical potentials ranging from 1.0 to 3.0 V for 30 s. The actuation stress increased from 0.05 to 0.35 MPa as the electrical potential was increased from 1.0 to 3.0 V. (E) An LCE artificial muscle film could lift a load of 3.92 N (the stress was 0.312 MPa) by 38% of its initial length. (F) Actuation strain of the LCE artificial muscle film versus time under three different levels of applied stresses. As the load was increased, the time needed for the LCE film to contract was almost unchanged, while the time needed for it to recover to its original length decreased. Inset: Maximal work density of LCE artificial muscle under different applied stresses. Scale bars, 1 cm (A, C, and E). Photo credit: Qiguang He, University of California, San Diego.

Thermal images (taken by FLIR E75-42 Advanced Thermal Imaging Camera) in fig. S4A showed the surface temperature of LCE artificial 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% contraction. 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.

Next, we quantitatively characterized the actuation behaviors of LCE artificial muscle when different electrical potentials were applied. In Fig. 2B, we showed the actuation strain of LCE artificial muscle versus time with four different levels of electrical potential (1.0, 1.5, 2.0, and 3.0 V) for 360 s. For a given electrical potential, the actuation strain of LCE artificial muscle increased at the beginning and then reached a plateau value either because the temperature field in the artificial muscle reached a steady state or because the actuation was at its maximal value (41%). As we turned off the electrical potential, the actuation strain decreased to zero within 240 s. When we increased the electrical potential from 0.5 to 2.0 V, the maximal actuation strain rose from 15 to 41%. As we incremented the electrical potential further from 2.0 to 3.0 V, the maximal actuation strain maintained the same value (41%), but the response time decreased. It took about 30 s for the LCE artificial muscle to reach the maximum contraction (41%) when 3.0 V was applied compared with 150 s while 2.0 V was applied. As shown in fig. S4B, as we increased the electrical potential from 1.0 to 2.0 V, the surface temperature of the LCE films in the steady state was increased from 55° to 120°C. Correspondingly, the actuation strain of the LCE artificial muscle was increased from 16 to 41%, as shown in Fig. 2B. If we further increased the electrical potential from 2.0 to 3.0 V and the maximal surface temperature increased from 120° to 220°C (fig. S4B), the actuation strain of LCE film stayed unchanged (Fig. 2B). We also showed that the performance of the LCE artificial muscle film almost remained unchanged after more than 50 and 100 cycles of actuation with an applied electrical potential of 3.0 and 2.0 V, respectively (fig. S5). Moreover, no delamination between LCE layer and heating wire was observed in cyclic heating and cooling process in the experiments.

By fixing the length of the LCE artificial muscle film (Fig. 2C), we studied its actuation stress of fixed length with different applied electrical potentials ranging from 1.0 to 3.0 V for 30 s. For a given electrical potential, the actuation stress increased from zero to the maximum value within 30 s and dropped to zero when we turned off the electrical potential. As the electrical potential increased from 1.0 to 3.0 V, the maximal actuation stress increased from 0.05 to 0.35 MPa, which is shown in 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 electrical 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 ranging 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 reach as high as 150 kJ/m3 when the applied stress was 0.31 MPa, which was comparable to that of dielectric elastomer membrane and mammalian skeletal muscle but higher than that of ionic polymer metal composites and piezoelectric actuators (18).

Last, we would like to point out that, like other thermally responsive 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 relatively slow, as shown in Fig. 2 (B, D, and F). The most effective way to reduce the response time is to decrease the size of the material or, more precisely, the characteristic size of the heating area. However, this modification could adversely affect other actuation performance of the LCE actuators, such as the actuation force and strain. Therefore, an optimized design will certainly be needed to achieve a desirable combination of actuation speed, strain, and stress for specific applications.

Electrical potential–controlled multimodal actuation of a tubular actuator

We also characterized the multimodal actuation of an LCE tubular actuator. In Fig. 3 (A and B), we demonstrated that applying an electrical potential onto one or two of the heating wires (same electrical potential for both heating wires) could generate bending actuation. Depending on which heating wires we distributedly activated, the tube could bend toward six different directions (red color in Fig. 3B indicates the activated heating wires). Similarly, by simultaneously applying the electrical potential to all the heating wires, the entire LCE tube was heated up, resulting in a homogenous contraction. The LCE tubular actuator recovered to its original shape and geometry after the electrical potential was turned off.

Fig. 3 Multimodal actuation of an LCE-based tubular actuator.

(A) Real and thermal images of an LCE tubular actuator during actuation: Activating one or two heating wires resulted in bending motion; activating all three heating wires resulted in homogeneous contraction. (B) Different actuation modes of the tubular actuator. Six directional bending were realized by activating one or two heating wires. Contraction was achieved by simultaneously activating all three heating wires. (C) Plot of bending angle of LCE tubular actuator versus time by applying electrical potentials from 1.0 to 3.0 V. The heating time was 30 s, and the cooling time was 270 s. Solid lines represent theoretical prediction, and dots represent experimental results. (D) Plot of contraction of LCE tubular actuator versus time through the activation of all three heating wires. The electrical potential was set to 3.0 V, and the time of electrical potential on was set to 30 s; the time of electrical potential off was set to 270 s. Solid lines represent theoretical prediction, and dots represent experimental results. Scale bars, 1 cm (A and B). Photo credit: Qiguang He, University of California, San Diego.

On the basis of the measurement of the actuation strain on a single LCE artificial muscle thin film as described previously, we could quantitatively predict the bending angle of the LCE tubular actuator, as shown in Fig. 3C. In the experiment, we applied different electrical potentials to an LCE artificial muscle thin film for 30 s and then turned off the electrical potentials. We measured the actuation strain of the thin film as a function of time for different electrical potentials. The results are shown in Fig. 3D, together with the contraction of the LCE tubular actuator when we applied the electrical potential onto all three embedded heating wires. It can be seen that those two results are very similar to each other.

To predict the bending angle of the tubular actuator when we applied the electrical potential only onto one of the heating wires, we adopted the model developed for calculating the deflection of a beam caused by inhomogeneous thermal expansion (39). As shown in fig. S6, when one of the three embedded heating wires was subjected to an electrical potential, we assumed that one-third of the tubular actuator behaves just like the LCE artificial muscle thin film and that the other two-thirds of the tubular actuator acts like passive elastic material. Therefore, the bending angle of the tube can be computed as θ=MTIl0, where l0 is the length of the tube; I=π64E[(do)4(di)4], known as the bending stiffness of the tube with E as the modulus of the LCE and do and di as the outer and inner diameter of the tube, respectively; MT = ∫ εTZdA as the thermal bending moment, with εT as the actuation strain of the thin film and Z and A as denoted in fig. S6. With the input of the actuation strain εT measured for LCE thin film, as shown in Fig. 3D, the above equation allows us to predict the bending angle of the tubular actuator as a function of time. The prediction agrees very well with the experimental measurements, shown in Fig. 3C. We observed that both the length of the tube and the inner and outer diameters of the tube can be measured. There is no fitting parameter for the prediction.

The position of the center of the unconstrained cross-sectional area of the tubular actuator is important to understand the actuation performance. We plotted the trajectory in fig. S7 (A and B), in which one or two heating wires were subjected to electrical potentials. The trajectory of the center point on the cross-sectional area of the tubular actuator when we applied in one wire an electrical potential of 3.0 V for 30 s is shown in fig. S7A and movie S2. When the power supply delivered the same electrical potential (3.0 V) for 20 s to two heating wires, the trajectories of the actuator’s cross section have the displacements shown in fig. S7A and movie S3. When all three heating wires had the same electrical potential (3.0 V) for 30 s, we observed a homogeneous contraction, as shown in movie S4.

Because of the thermal diffusion, when we applied high electrical potential (e.g., 3.0 V) to one or two heating wires, larger areas of the tube could be heated up if the heating time was too long (e.g., 360 s), which might induce unexpected geometrical distortion of the tube, as shown in fig. S8. To address this problem, we adopted the following strategy: We first applied a high electrical potential (V1 = 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 (V2 = 1.5 V) for the rest of time to maintain the deformation, as shown in fig. S8. We show the cyclic bending of the LCE tubular actuator in fig. S9, when we turned on and off, repeatedly, the electrical potential into one or two heating wires.

Because of the internal heating mechanism of the tubular actuator, 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 demonstrated 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 applied to the actuator and, thus, the deformation of each tubular actuator. 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.

(A) Schematic of the assembly of the soft gripper. (B) Soft gripper twisting the cap of a vial. (C) Soft gripper grasping and lifting the vial (50 g). Scale bars, 1 cm (B and C). Photo credit: Qiguang He, University of California, San Diego.

An untether soft robot

We next designed and fabricated the untethered robot using the LCE tubular actuators. We integrated a microcontroller (Arduino Mini, 3.0 V), electrical components [metal oxide semiconductor field-effect transistors (MOSFETs) and wires)], battery (lithium/polymer, 3.7 V), and LCE tubular actuators containing two separate heating wires (for the detailed characterizations, see fig. S7, C and D) with an acrylic plate to build an untethered robot shown in Fig. 5A.

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.

In Fig. 5B, with automatically controlled LCE tubular actuators, the untethered robot could walk on a flat surface. The robot started to walk by simultaneously activating a pair of its diagonal tubular actuators 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 was turned off for all the tubular actuators for 205 s, and all the actuators turned back to the original resting state. The displacement in one period was around 8.5 mm (see Fig. 5D). After 30 min of walking, the robot can move forward for a distance of about one body length (8 cm), as shown in movie S7.

Figure 5C demonstrates three cycles of the untethered robot transporting an item on the top. In the experiment, we placed a cardboard on top of the four tubular actuators and a weight of 10 g on the plate. During the operation, the two front actuators first bent forward gradually using around 45 s and then recovered back to the straight state accompanied by contraction. During the recovery and contraction of the front actuators, the other two rear actuators bent forward also using about 45 s, which was followed by the state that the electrical potential was turned off for all the actuators. These repetitive actions enabled the robot to move the rigid plate and the weight on top of the plate forward about 5 cm within 15 min, as shown in Fig. 5D and movie S8.

The costs of transport (COTs) are estimated as 5.0 × 104 and 1.0 × 105 for the robot walking and transporting experiments, respectively. Compared with other untethered soft robots (40), the magnitude of COTs of the LCE-based untethered robot is very large because of the low energy efficiency of thermally driven actuating materials.

DISCUSSION

Here, we have discussed the design and fabrication of an LCE-based tubular actuator, which can exhibit multidirectional bending and homogenous contraction as a low electrical potential is selectively applied to heating wires embedded in the LCE. In addition, on the basis of the tubular actuators, we have further constructed soft grippers that can grasp a vial and twist its cap. By integrating a battery, a microcontroller, and the tubular actuators, we have built an untethered robot that can crawl on the ground and transport an item.

The distributed actuation of a tubular LCE caused the multimodal actuation of the soft tubular actuator developed here. Because of the embedded serpentine heating wires, we can fully activate the tubular actuator by applying an electrical potential. Although LCE has been intensively explored in recent years to design and fabricate various previously unknown structures and devices (4143), the multifunctional robot based on the tubular actuators, to our knowledge, is the first untethered robotic system only using LCE as the actuating material.

Previous work in electrically controlled soft structures such as most dielectric elastomer actuators (DEAs) has shown much faster speed and higher energy efficiency compared to the LCE-based tubular actuators developed here. However, the LCE-based tubular actuator does not require any stiff frames to maintain large prestretch for achieving large actuation (44). Furthermore, the tubular actuator also uses a low-voltage power source (around three orders of magnitude lower than the electrical potential required to actuate DEAs). This feature makes our tubular actuator compatible with most low-cost, commercially available electronic devices and batteries.

MATERIALS AND METHODS

Materials

1,4-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (RM257) (Wilshire Technologies; 95%), (2-hydroxyethoxy)-2-methylpropiophenone (HHMP; Sigma-Aldrich; 98%), 2,2′-(ethylenedioxy) diethanethiol (EDDET; Sigma-Aldrich; 95%), pentaerythritol tetrakis (3-mercaptopropionate) (PETMP; Sigma-Aldrich; 95%), dipropylamine (DPA; Sigma-Aldrich; 98%), 3% hydrogen peroxide solution (Sigma-Aldrich), sodium iodide (Sigma-Aldrich; 99%), and sodium thiosulfate pentahydrate (Fisher Scientific) were used as received without further purification. The mechanical tests were conducted using the Instron Universal Testing Machine (5965 Dual Column Testing Systems; Instron) with 1000-N loading cell. The surface temperature of LCE artificial muscle was measured by thermal imaging technique (FLIR E75-42 Advanced Thermal Imaging Camera).

Fabrication of loosely cross-linked LCE film

We prepared the loosely cross-linked LCE following the previously reported work with little modification (45). Liquid crystal mesogen RM257 (10.957 g, 18.6 mmol) was added into toluene, and the mixture was heated at 85°C. The photoinitiator HHMP (0.077 g, 0.3 mmol) was added into the mixture. Then, we added 3.076 g (16.9 mmol) of spacer EDDET, 0.244 g (0.5 mmol) of cross-linker PETMP, and 0.038 g (0.4 mmol) of catalyst DPA into the solution. After stirring and degassing, we poured the mixture into the rectangular mold (1 mm thickness) and put it at room temperature for 24 hours. Then, we placed the loosely cross-linked LCE film into an 85°C oven for solvent evaporation. After that, we obtained the loosely cross-linked LCE film.

Fabrication of heating mesh

The fabrication process of the heating mesh is shown in fig. S3. First, a 2-μm-thick layer of polyimide (PI) precursor was spin-coated onto the 20-μm-thick layer of copper. After that, the one-side coated copper was cured under 300°C oven under nitrogen protection. Then, we placed the PI-coated copper with the copper layer up onto a substrate, as shown in step (i). The photoresist AZ1512 was spin-coated onto the copper layer. Photolithography and wet etching define patterns of the copper wires (step ii). Later, we removed the residual photoresist with acetone; then, another layer of PI was spin-coated on the patterned copper layer and baked in 300°C oven under nitrogen protection. After that, photolithography defined a mask for dry etching process with CF4 and O2 through the PI layer. Last, we obtained the heating mesh by removing the photoresist with acetone.

Fabrication of LCE tubular actuator

We showed the fabrication process of the LCE tubular actuator in fig. S2. First, we sandwiched three heating wires between two layers of loosely cross-linked LCE film (39 mm × 30 mm × 1 mm). Then, the whole structure was compressed, forming a sandwich-like structure. In step (iii), we rolled the sandwich-like structure to a tube and stretched it along the longitudinal direction by λp = 1.8 to align the liquid crystal mesogens. Next, we obtained the LCE tubular actuator by exposing under UV irradiation for 30 min, as shown in step (iv). Last, we soldered the electrical wires to the heating wires.

Preparation of soft gripper

We glued three fabricated LCE tubular actuators (embedded with three heating wires) to the acrylic plate. Arduino Mini (3.0 V) was used to control the on/off of the heating mesh, MOSFET (IRLB3034PBF) was used to magnify the signal, and an electrical power source (Dr. Meter PS-305DM) was used to provide the electrical energy.

Preparation of untethered robot

We fixed four fabricated LCE tubular actuators (embedded with two heating wires) to the acrylic plate and controlled the actuator using Arduino Mini (3.0 V). A MOSFET (IRLB3034PBF) was used to magnify the signal, and a lithium/polymer battery (uxcell DC 3.7 V, 1300 mAh) was used as the power source for the system.

Characterization of LCE artificial muscle/LCE tubular actuator

We characterized the actuation strain of LCE artificial muscle by analyzing the videos taken by the digital camera (Canon 60D) using ImageJ. The surface temperature of LCE artificial muscle during actuation was measured by thermal imaging technique (FLIR E75-42 Advanced Thermal Imaging Camera). The actuation stress of LCE artificial muscle was measured using the Instron Universal Testing System (5965 Dual Column Testing Systems; Instron) with 1-kN loading cell. During the mechanical measurement, we fixed the strain of LCE artificial muscle between two loading cells.

We characterized the actuation (bending and contraction) of the LCE tubular actuator by analyzing the videos taken by the digital camera (Canon 60D) using ImageJ. The surface temperature of the LCE tubular actuator during actuation was characterized using thermal imaging technique (FLIR E75-42 Advanced Thermal Imaging Camera).

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaax5746/DC1

Fig. S1. Chemical component, fabrication process, and actuation performance of an LCE thin film.

Fig. S2. Process of the fabrication of an LCE tubular actuator.

Fig. S3. Design, fabrication, and morphology of heating wire.

Fig. S4. Temperature change of the LCE artificial muscle film during contraction and recovery.

Fig. S5. Cyclic actuation test of the LCE artificial muscle film with applied stress of 0.078 MPa.

Fig. S6. Theoretical prediction of bending angle of LCE tubular actuator by using the actuation of LCE artificial muscle thin film.

Fig. S7. Characterizations of LCE tubular actuators.

Fig. S8. The actuation behavior of LCE tubular actuator with different voltage controls.

Fig. S9. Cyclic test of LCE tubular actuator with different heating wires.

Fig. S10. The reversible actuation (bending motion and contraction) of LCE tubular actuator in the water environment.

Movie S1. Reversible actuation of LCE thin film actuator.

Movie S2. Reversible bending actuation of LCE tubular actuator (activate one heating wire).

Movie S3. Reversible bending actuation of LCE tubular actuator (activate two heating wires).

Movie S4. Homogeneous contraction of LCE tubular actuator (activate three heating wires).

Movie S5. Soft gripper grasps the vial.

Movie S6. Soft gripper twists the cap.

Movie S7. Untether soft robot walks on the ground.

Movie S8. Untether soft robot manipulates the weight.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: We thank Q. Wang for helping with the thermal measurement and acquiring IR images. Funding: The authors acknowledge the support from ONR through grant no. N00014-17-1-2062 and the NSF through grant no. CMMI-1554212. This work was performed, in part, at the San Diego Nanotechnology Infrastructure (SDNI) of UCSD, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the NSF (grant no. ECCS-1542148). Author contributions: Q.H., Z.W., and S.C. conceived the research. Q.H., Z.W., and Y.W. performed the experiments and analyzed the experimental results. Q.H., A.M., M.T.T., and S.C. composed the manuscript. All authors reviewed the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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