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Improving the Durability of Soft EPIC Actuators by Modifying Their Viscoelastic Properties by Using a Skin-Inspired Hybrid Polymer Film
Hyunwoo Kim1,Minjae Cho1,Chongyoung Chung1,Ki-Uk Kyung1
Korea Advanced Institute of Science and Technology1
Recently, a novel soft electrohydraulic actuator called “electrohydraulic actuator powered by induced interfacial charge (EPIC)” has been developed.  This actuator provides a high output force (16 times its body weight) at a relatively low operating voltage compared to existing electrohydraulic actuators such as the hydraulically amplified self-healing electrostatic (HASEL) actuator.  Based on the high performance of this actuator, various potential applications are expected, such as a soft varifocal lens, artificial muscles, and soft vibrotactile actuators. Indeed, a soft vibrotactile actuator based on EPIC technology has been developed and has achieved high acceleration performance (>3 G) over a broad frequency range (90–700 Hz).  However, despite the high potential of this actuator as a powerful soft transducer, it is plagued by durability issues, which are commonly encountered in the case of soft actuators.
In this work, we propose a novel approach to improve the durability of EPIC-based actuators. We have identified the main source of the durability problem as the viscoelastic characteristics of the polyvinyl chloride (PVC)-gel polymer film used in the EPIC actuator. Owing to the nature of its ionic intermolecular bonds, PVC-gel dissipates strain energy by breaking these bonds. This phenomenon can be described well by using the Maxwell model, in which an elastic component and a viscous component are connected serially. A material that follows the Maxwell model cannot maintain its internal stress when subjected to a step stretch owing to stress relaxation. The durability problem is probably caused by this relaxation phenomenon.
Therefore, we have developed a skin-inspired hybrid composite film to alter the mechanical properties of PVC-gel. The hybrid composite film is composed of a PVC-gel layer, an Elastosil P7670 layer, and a polyacrylamide (PAAM)-gel layer as an adhesive layer between the PVC-gel and Elastosil P7670 layers. Unlike PVC-gel, Elastosil P7670 is elastic owing to its covalent intermolecular bonds, meaning that this polymer can be described using the Voigt model, in which an elastic component and a viscous component are connected in parallel. Therefore, the Elastosil P7670 layer serves as an elastic layer in the hybrid film to compensate for the dissipative properties of the PVC-gel layer. These different materials were hybridized by means of benzophenone-assisted grafting polymerization of acrylamide (AAM).  We confirmed that this method can be utilized not only for fabricating hydrophilic–hydrophobic hybrids but also for fabricating mechanically different materials.
Finally, the fabricated PVC-gel and Elastosil P7670 hybrid composite film was compared with the PVC-gel film in terms of durability when used in an EPIC actuator. Each actuator was operated five times consecutively with frequency variation (1–400 Hz) at 2 kV. We compared the blocking force variation of these actuators at their resonant frequencies over five consecutive frequency-sweep runs. The results indicated that in the case of the hybrid film, the blocking force decreased by only 5.1%, whereas in the case of the PVC-gel film, it decreased by 18.4%. This result proves that the durability issues associated with the existing EPIC actuators are mainly related to the viscoelastic properties of the polymer film, and this durability problem can be solved by introducing an elastic component covering the entire strained area.
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