2. Results and Discussion
The linear and circular biodegradable electrohydraulic soft actuators developed in this study are shown in Figure 1a,b (see also Supplementary Video S1). Both actuators have a sandwich structure composed of two layers of dielectric film made of polylactic acid (PLA) and polybutylene adipate-co-terephthalate (PBAT), forming a pouch encapsulating soybean oil as the dielectric liquid. PLA/PBAT film is known as a biodegradable substrate with good processability.[38] The pouch is fabricated by welding two layers of dielectric film. On the pouch, electrodes are attached, which consist of a mixture of gelatin, glycerol, and sodium chloride (NaCl). The electrode material has an adhesive nature because of which it can be attached to the pouch without the need for glue. The entire surface of the device is covered by corn starch powder to keep the surface non-sticky. The integration of these materials and ingredients makes the actuator fully biodegradable.Figure 2 shows a biodegradation process of the circular-type actuator. The device gradually degrades in the soil with time. The electrode material is not only biodegradable but also edible owing to the ingredients used. Therefore, the conductive material synthesized in this study may be useful in “edible robotics.”[18] The geometry of our actuators is similar to those of the peano-HASEL actuator and circle planar HASEL actuator available in the literature.[36,37] Since the dielectric liquid used is soybean oil, which provides no self-healing ability, we cease to use the abbreviation HASEL in the rest of the paper to avoid any confusion on the functionality.
The working principle of the linear and circular-type actuators is identical and is shown in Figure 1c,d . When voltage is applied, electrodes are charged, resulting in an electrostatic pressure. This pressure squeezes the pouch containing the soybean oil, which is placed in the areas where the electrodes do not overlap. The oil transferred in the non-electrode areas inflates the pouch, causing deformation of the entire structure. In the linear-type actuator, the actuated deformation occurs as contraction. In the circular-type actuator, it leads to expansion. In the linear-type actuator, the dimensions of the pouch and electrode area are 40 mm × 20 mm and 38 mm × 10 mm, respectively. In the circular-type actuator, the inner diameter of the pouch and the diameter of the electrode are 50 mm and 25 mm, respectively.