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.