2.2. Linear-type biodegradable electrohydraulic soft actuators
We performed an electromechanical characterization of the linear-type biodegradable electrohydraulic soft actuator. In this test, strain as a function of the applied voltage was measured under different loading conditions (0, 5, and 20 g). Electrodes with a NaCl mass fraction of 10 wt% were used in the tested actuators. The applied voltage was limited to 1 kV, as it was found during a preliminary experiment that the actuators undergo electrical breakdown at voltages more than 1 kV. This is reasonable given the thickness (10 μm) and the dielectric strength of the film material (in the case of PLA, ~33 kV/mm[41]). When actuated, the film layers of the pouch come closer owing to the electrostatic forces between the electrodes, resulting in the growth of the electric field strength. Figure 4 shows the actuation and characterization results of the actuators. The relationships for other tested actuators with different amounts of NaCl in their electrodes are also shown in Figure S6. As shown in Figure 4b, the actuation strain increases from 0% to 3.4%. Contrarily, the strain decreases under a larger applied load. The maximum strains in the tested voltage range for loading of 0, 5, and 20 g are 3.4%, 2.0%, and 1.4% respectively. The simulated values calculated by an analytical model (see Supporting Information for the details) exhibit the same trend as the experimental data, which suggests that the fabricated biodegradable actuators are functioning properly according to the fundamental working principle employed in this study. Thus, it is possible to design biodegradable electrohydraulic soft actuators with the aid of models intended to be used for non-degradable actuators. However, there is a discrepancy between the simulation and the experiment on the strain at each voltage. This may result from the fact that the simulation assumes a two-dimensional motion of the actuator, as shown in the cross-sectional view of Figure 1c. The actuator also moves in three-dimensional motion, leading to actuation strains lower than the theoretical values. In addition, it also causes deformation of the sealed part of the pouch (5 mm width) that is mechanically passive.
At 1 kV, the measured strain of the biodegradable electrohydraulic soft actuators is 3.4%. This value is to that observed for non-biodegradable actuators of the same type; peano-HASEL exhibits an actuation strain of ~5.4% at 6 kV[37]. The result indicates that biodegradable materials can be incorporated into electrically driven soft actuators without compromising the actuation strain.