Figure 5. Characterization results of the circular-type actuators. (a)
Actuation strain as a function of the applied voltage and electric field
(amount of dielectric liquid 2 mL). (b) Actuation strain as a function
of the applied voltage and electric field (amount of dielectric liquid 4
mL). (c) Blocked force as a function of the applied voltage and electric
field. (d) Force-strain relationship at an applied voltage of 10 kV.
2.3. Circular-type biodegradable electrohydraulic soft actuators
Next, we characterized the circular-type actuators in terms of actuated
strain and blocked force. The tested actuators have different amounts of
soybean oil (dielectric liquid): 2 mL and 4 mL. The applied voltage
ranges from 0 to 10 kV with a step size of 1 kV. Figure 5a,b display the
measured strain as a function of the electric field under different
loads (0, 5, and 20 g) for actuators with different amounts of soybean
oil. Similar to the case of linear-type actuators, the actuation strain
increases with the voltage and reaches 18.6% (2 mL soybean oil) and
9.1% (4 mL soybean oil) at 10 kV under no load. The loading results in
reduced strain, similar to what is observed for the linear-type
actuator. Unlike the linear-type actuators, the circular ones allow the
application of voltage up to 10 kV. This is because, during the
actuation, the electrodes are always distant, that is, the electric
field strength does not exceed the breakdown strength of the pouch
layers due to the relatively large amount of soybean oil against the
entire volume of the actuator. The observed actuation strain in the
biodegradable circular actuator is 18.6% at 10 kV, which is within the
same range as a non-biodegradable actuator with identical dimensions
(25% at 10 kV[36]).
In the measured data (Figure 4a,b), a plateau and pull-in transition can
be seen, which are described as unique characteristics of actuators of
this type.[36] Pull-in transition is described as
a feature when the electrostatic force exceeds the threshold of the
restoring force. In Figure 5a and Figure 5b, pull-in transition can be
seen at 2 kV and 4 kV, respectively. This behavior has also been
reported in literature.[36] In the actuator, the
electrostatic force between the electrodes scales with the distance
between the electrodes. Short distance results in larger electrostatic
forces, leading to more actuation strain. The amount of dielectric
liquid determines the distance between the electrodes and hence the
actuation strain, which can be seen in the data shown in Figure 5a,b,
where different amounts of liquid (2 mL and 4 mL) are employed.
Next, we investigated the blocked force as a function of the applied
voltage. As plotted in Figure 5c, the force increases with the voltage
and reaches a value of 241 mN at 10 kV, corresponding to an actuation
pressure of 0.12 kPa. We further examined the blocked force at different
actuation strains, from which the force-strain characteristics of the
actuators were assessed at 10 kV. As shown in Figure 5c, the blocked
force reduces as the strain increases, which is also observed in
non-biodegradable actuators of the same type[42].
Further, to compare the actuation pressure, the non-biodegradable
circular-type actuator exhibits a value of 25 kPa at 8
kV[42]. This value is larger than that observed
for our actuator, which could be attributed to the difference in the
material properties of the dielectric liquids. For instance, a larger
dielectric constant leads to a larger electrostatic force and hence a
higher pressure. This implies that the output of biodegradable
electrohydraulic soft actuators can be increased using dielectric
liquids with a high dielectric constant. Nevertheless, the discussed
results confirm that the actuators developed in this study function
according to the working principle and their actuation performances are
comparable to those of the non-biodegradable ones; this proves the
successful implementation of our hypothesis.
3. Conclusion
In this study, we presented biodegradable electrohydraulic soft
actuators as electrically driven green robotic elements for
environmentally friendly soft robotics. We showed that the electrodes
used for the actuators can be tuned to achieve both softness and
conductivity suitable for electrically driven soft devices. We also
demonstrated the actuators in different forms; both exhibited actuation
performance comparable to non-biodegradable counterparts.
In the future, we will focus on the improvement of actuation performance
by investigating suitable biodegradable materials for every part of the
actuator, where both analytical and computational models are useful.
Since the actuator presented in this study consists of dielectric and
conductive materials, that is, essential components for electrically
driven soft robotic elements, applying the materials to existing working
principles may result in biodegradable soft robotic devices of various
forms, such as stretchable sensors, electroadhesive pads, and soft
pumps. Furthermore, the electrode fabricated in this study is
essentially edible, and it can be applied to realize a broad range of
devices in edible robotics.
Experimental Section