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