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.