Electrically Controlled Soft Actuators with Multiple and Reprogrammable Actuation Modes

New forms of soft actuators have enabled the construction of novel soft robots with various functionalities. Though previous researches have successfully fabricated soft actuators with diverse actuation modes, their actuation capabilities are often fixed once their fabrication is completed. Herein, an electrically controlled soft actuator with multiple and reprogrammable actuation modes is designed and fabricated. The soft actuator is composed of two layers of disulfide liquid crystal elastomer (ss‐LCE) film with embedded resistive heating wires of serpentine shape. The actuation mode of the actuator can be programmed by introducing a certain alignment of liquid crystal mesogens in the ss‐LCE film, which is achieved by the controlled deformation of the actuator at room temperature and the rearrangement of the polymer network through disulfide exchange reaction in the material. The actuation mode of the actuator can be easily erased by heating it up to 180 °C, and a new mode of actuation can be introduced by deforming the actuator to a new shape at room temperature. With the reprogrammable and multiple actuation modes, a reusable and general‐purpose soft actuator is demonstrated herein which can meet various requirements in constructing new soft active devices.

DOI: 10.1002/aisy.201900177 New forms of soft actuators have enabled the construction of novel soft robots with various functionalities. Though previous researches have successfully fabricated soft actuators with diverse actuation modes, their actuation capabilities are often fixed once their fabrication is completed. Herein, an electrically controlled soft actuator with multiple and reprogrammable actuation modes is designed and fabricated. The soft actuator is composed of two layers of disulfide liquid crystal elastomer (ss-LCE) film with embedded resistive heating wires of serpentine shape. The actuation mode of the actuator can be programmed by introducing a certain alignment of liquid crystal mesogens in the ss-LCE film, which is achieved by the controlled deformation of the actuator at room temperature and the rearrangement of the polymer network through disulfide exchange reaction in the material. The actuation mode of the actuator can be easily erased by heating it up to 180 C, and a new mode of actuation can be introduced by deforming the actuator to a new shape at room temperature. With the reprogrammable and multiple actuation modes, a reusable and generalpurpose soft actuator is demonstrated herein which can meet various requirements in constructing new soft active devices.
temperature, 100 and 140 C, can be found in Figure S5, Supporting Information.
In addition to the actuating material, how to apply external stimuli to trigger the actuation is crucial for an actuator in its applications. In the past, different strategies have been adopted to trigger the actuation of an LCE as thermally responsive material. One commonly adopted way to change the temperature of LCE is through controlling the environmental temperature. [28,29] Photothermal effects have also been used to realize remote control of local temperature change in LCE. [30,31] Compared with those heating strategies, Joule heating generated by resistive heating wires embedded in the material has shown Figure 1. Working mechanism, design, and the fabrication of reprogrammable ss-LCE-based actuator. a) Schematics of reversible actuation of ss-LCE with contraction mode and shear mode and the reprogrammability of ss-LCE based on disulfide exchange reaction. b) Fabrication process of a reprogrammable ss-LCE-based actuator: ① Stretch two ss-LCE films equal biaxially with the stretch ratio of around 1.3. The biaxial stretches in the two ss-LCE films are maintained for 1 day. ② Transfer heating wires from its glass substrate to a water-soluble tape. ③ Attach the tape with heating wires onto the biaxially prestretched ss-LCE film, dissolve the water-soluble tape away with only heating wires attached on ss-LCE film, and cover the thin film by another equal biaxially stretched ss-LCE film on top to get a sandwich structure. ④ Heat the sandwich structure up to 120 C for 5 mins, which causes biaxial contraction of the ss-LCE film and thus biaxially compresses the heating wires. ⑤ Place the whole structure under compression and in an oven of 180 C for 20 mins to form strong bonding between the two ss-LCE films. ⑥ Take the entire structure out from the oven and cool it down to room temperature, and then deform the entire structure at room temperature to program it with a desired actuation mode.
www.advancedsciencenews.com www.advintellsyst.com several advantages including simple control, low cost, and easy integration. [11] To construct an electronically controllable actuator, in this work, we integrate resistive heating wires of carefully selected shape with ss-LCE films.
To make a soft actuator with great stretchability along an arbitrary direction, we embedded heating wires with half-and-half Peano shape [32] into the actuator (Figure1b). The heating wires were fabricated through standard photolithography, as described in the Supporting Information and Figure S6 and S7, Supporting Information. To further enhance the stretchability of the heating wires, we attached the heating wires to an initially biaxially prestretched LCE film, and the heating wires were under biaxial compression after the prestretch in the LCE films was removed. Detailed fabrication steps are shown in Figure 1b and also in Experimental Section. With the fabricated actuator, we can program or reprogram it with various actuation modes, as discussed in the following paragraphs.
We first programmed the soft actuator with the contraction mode, which is one of the most important actuation modes for an actuator. The programming process can be found in the Experimental Section. To quantitatively characterize the actuation performance determined by Joule heating of the heating wires, we performed isotonic (constant load) and isometric (constant displacement) tests to measure the actuation strain and stress, as shown in Figure 2a. We first measured the surface temperatures of the actuator with different applied voltages (2, 3, and 4 V for 120 s) using infrared (IR) camera (FLIR E75-42), Figure 2b shows the maximum temperature on the surface of the actuator as a function of time. Once the voltage was turned on, the surface temperature started increasing and then reached a plateau value. The plateau value of the maximal surface temperature increased from 83 to 140 and to 191 C, when we successively increased the voltage from 2 to 3 and to 4 V. When we turned off the voltage, the surface temperature gradually decreased to room temperature within 180 s. Although a higher voltage resulted in a relatively higher surface temperature with a shorter period, we had to make sure that the actuating temperature of the actuator is below 180 C to prevent erasing the programmed actuation.
To measure the actuation strain of the actuator, we applied three different levels of voltages (2, 3, and 4 V) for 120 s. The actuator was subjected to 0.196 N axial load (20 g weight) to keep it straight, as shown in Figure 2a. The actuation strain can be defined as ε ¼ LÀl L Â 100%, where L is the length of the actuator in the initial state, whereas l is the length in the actuated state. As shown in Figure 2c, the actuation strain increased and reached the maximal value after 60 s for all three different voltages. Specifically, when the voltage was 3 V, 23% actuation strain could be generated within 45 s. In addition, the response of the soft actuator was faster as we increased the applied voltage. An average strain rate of 0.15%, 0.75%, and 1.5% s À1 could be realized as the applied voltage was 2, 3, and 4 V, respectively. Furthermore, the actuation stress could be measured by fixing the length of the actuator (Figure 2a) while applying different levels of voltage (2, 3, and 4 V), as shown in Figure 2d. Similarly, for a given value of voltage, the actuation stress increased from zero to the maximum value after the voltage was turned on and gradually dropped to zero when the voltage was turned off. Specifically, a maximum actuation stress of 0.2 MPa could be produced by the actuator when the voltage is 3 V. The actuator could also generate cyclic actuation when the voltage was turned on and off cyclically, as shown in Figure S8, Supporting Information. The actuation stress generated by the actuator depended on the magnitude as well as the duration of the voltage when the voltage was turned on.
We next demonstrated the reprogrammability of the actuator. The actuator was first programmed with uniaxial contraction mode along X direction, as shown in Figure 2e. We then heated the actuator to 180 C in an oven for 20 mins to erase its contractive actuation in X direction and reprogrammed it with contractive actuation along Y direction. We further erased the contractive actuation in Y direction and repeated the aforementioned reprogramming and erasing processes. The active contraction along two orthogonal directions can be alternatively introduced into the actuator, as shown in Figure 2f. Before reprogramming, the actuator programmed for the first time could generate an actuation stress of 0.17 MPa in isometric condition with an applied voltage of 3 V, and its actuation strain under the isotonic condition could reach 28% after the voltage was turned on for 120 s. After each reprogramming, both actuation strain and stress of the actuator maintained almost an unchanged state, indicating its very robust reprogrammability.
We then demonstrated the multimode programmability of the actuator. In the experiment, we programmed an actuator with three different actuation modes: contraction mode, bending mode, and shear mode. The detailed steps of programming an ss-LCE actuator are shown in the Experimental Section. As shown in Figure 3a, the actuator with the contraction mode could lift a weight of 60 g up by 6 mm (80 times of its own weight) with an applied voltage of 3 V. Figure 3b shows its actuation strain as a function of time. It took 45 s for the actuator to reach the actuation strain of 25%, which was comparable with the case shown in Figure 2b with the weight of 20 g.
As shown in Figure 3c, the actuator with bending actuation can be achieved by attaching a strain-limiting layer (Kapton tape, 3M Company) to a contractive actuator. As the voltage was applied to the actuator, bending actuation could be produced, as shown in Figure 3c. The bending angle shown in Figure 3d was measured as a function of time. The actuator was not straight as it was fabricated and had an initial bending angle of 20 . When we applied a voltage of 3 V, the bending angle of the actuator increased from 20 to 160 within 40 s. After we turned off the voltage, its bending angle gradually reduced to its initial value within 180 s.
We also studied the actuator with the shear mode, as shown in Figure 3e. In the experiment, two acrylic plates were attached to the two parallel edges of the actuator. One of the plates was fixed, and the other one was allowed to move horizontally along the glass slider. When the voltage was turned on, large shear deformation could be generated by the actuator (Figure 3e). When the voltage was turned off, the actuator gradually recovered back to its initial shape. In Figure 3f, we show the shear angle of the actuator as a function of time when the voltage was first turned on for 90 s and then turned off for 150 s.
Finally, we showed the possibility of transforming the actuator from one actuation mode to another. We first programmed a pair of actuators with shear mode and then combined them into one actuator to generate bishear actuation, as shown in Figure 3g.  www.advancedsciencenews.com www.advintellsyst.com www.advancedsciencenews.com www.advintellsyst.com The free end of the new actuator was able to pull a spring by 4.8 mm with a voltage of 3 V applied to both two actuators for 90 s. We next took one of the actuators out and reprogrammed it with contraction mode. As shown in Figure 3g, after reprogramming, the actuator could lift up a skeleton arm within 60 s with applied voltage of 3 V. Such significant transformation of actuation mode in an actuator can be very useful in many engineering applications.
In conclusion, we have successfully constructed an ss-LCE-based soft actuator, which exhibits various actuation modes after programming or reprogramming. We have also characterized different actuation modes such as contraction, bending, and shear of the ss-LCE actuator. Most existing soft actuators have fixed actuation modes after they fabricated. In contrast, we have shown that both erasing and reprogramming an actuation mode in the newly developed soft actuator are facile. Moreover, the actuation of the soft actuator can be electrically controlled with a low voltage, which enables its simple integration with most existing control systems with low costs. We hope the newly developed reprogrammable soft actuator can find its wide applications in constructing diverse soft active structures.

Experimental Section
Fabrication of ss-LCE Actuator: We first fabricated heating wires of half-and-half Peano shape on a glass substrate using photolithography ( Figure S6, Supporting Information, and Supporting Information for fabrication details). Meanwhile, by following our previous work, [15] we synthesized two ss-LCE thin films with the thickness of %0.25 mm and then cut them into a square shape of 4 cm Â 4 cm. Details of synthesizing an ss-LCE film can be found in Supporting Information.
We then stretched both ss-LCE films equal biaxially with the stretch ratio %1.3 and fixed the stretch by adhering the four edges of each film to an acrylate plate using VHB tapes (4910, 3M company). To attach the heating wires onto the surface of one of the ss-LCE films, we first transferred the heating wires from the glass substrate to a water-soluble tape (Arc-Zone) and then attached the tape onto the ss-LCE film using stick glue. The ss-LCE film together with the water-soluble tape and heating wires was left untouched for 1 day. We next cut out the central area of the ss-LCE film and took it away from the acrylate plate. We then dissolved the water-soluble tape on the surface of the ss-LCE film using deionized (DI) water, while the heating wires still attached on the surface of the ss-LCE film. After the entire film was fully dried, we covered it by the other stretched ss-LCE film on top to obtain a sandwich structure. We then heated up the sandwich structure using a hotplate with a temperature of 120 C for 5 mins, which caused the biaxial contraction of the entire film and thus increased stretchability of heating wires. After that, we compressed the sandwich structure using two glass slides and multiple clips and then put it in an oven of 180 C for 20 mins to form bonding between two ss-LCE films. Finally, we took the entire structure out from the oven and cooled it down to room temperature. To connect the actuator with external power supply, we carefully peeled off small pieces of the LCE film to expose the metal contact pads ( Figure S7, Supporting Information) of the heating wires and then soldered electrical wirings onto them for the connection of external power supply. The final actuator was of 3 cm Â 3 cm with a thickness of %0.5 mm. We can then program the actuator as discussed in the following paragraphs.
Programming of ss-LCE Actuator into Different Modes: The programming of actuators with different modes shares a similar procedure: first, the actuator was heated to 180 C for 20 mins and cooled down to room temperature; then, a certain deformation was applied to the actuator and maintained for 24 h. The heating time of 20 mins and the time for holding the deformation (24 h) were determined through parametric studies ( Figure S3, Supporting Information). Detailed information on different modes of programming is described as follows. 1) Contraction mode: we first heated the actuator up to 180 C and cooled it down. Then, we fixed one end of the actuator using Kapton tape (3M company) and manually applied a uniaxial stretch with the stretch ratio of 1.5 to the actuator. We then fixed the other end using Kapton tape to hold the stretch for 24 h. 2) Bending mode: We first programmed the actuator with a uniaxial stretch of 1.5, as described earlier. Then, we adhered Kapton tape with the width of 8 mm and length of 3 cm to one surface of the actuator to form a bilayer structure. 3) Shear mode: We first heated the actuator up to 180 C and cooled it down. Then we sheared an actuator with a shear angle of 30 using a simple customized device, as shown in Figure S9, Supporting Information. We then held the shear deformation in the actuator for 24 h.
Characterization of ss-LCE Actuator: We used an external power supply (Keysight E3642A) to apply voltage to the actuator. We measured the surface temperature of the actuator during its actuation using Advanced Thermal Imaging Camera (FLIR E75-42). We measured the actuation strain of the actuator by processing the photos taken by the digital camera (Canon 80D) using ImageJ. We measured the actuation stress of the actuator using the Instron Universal Testing System (5965 Dual Column Testing Systems; Instron) with 10 N loading cell.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.