PVC Gel Based Electro-Responsive Torsional Artificial Muscle

Artificial muscle mimics the characteristics of biological muscles based on smart materials that are transformed by various external stimuli such as electricity, heat, pressure, light, and humidity [1]. Because they have outstanding mechanical properties, remarkable flexibility, and excellent compliance, artificial muscles are driving advances in soft robotics. Among them, twisted-fiber artificial muscles (TFAMs) received tremendous attention from researchers nowadays. The common actuation mechanism of the developed TFAMs can be divided into two steps; 1) volumetric deformation (generally radial expansion) of pristine fiber, 2) geometrical effect of twisted structure [2]. TFAMs are reprogrammable by changing the amount and chirality of twist, and have demonstrated impressive performances including large strokes, fast response times, high specific energy density. However, most TFAMs are still in the research stage and have limitations in that they are difficult to use commercially.<br/>The use of electricity allows for quick and accurate control, is not affected by external environments unlike heat, light, or humidity, and only requires a small and lightweight battery rather than a heavy and noisy pump like pneumatic/hydraulic actuators. Nevertheless, in the TFAMs developed so far, electricity was used only for electrochemical reactions or Joule heating, which do not directly convert electrical energy into mechanical energy. Therefore, we developed a novel plasticized polyvinyl chloride (PVC) gel based electrically responsive torsional artificial muscle (PETAM) that uses the advantages of electricity and directly converts electrical energy into mechanical energy for more efficient actuation.<br/>PVC gel shows anodophilic creep deformation depending on the applied voltage due to the accumulation of plasticizers and charges toward anode surface [3]. Recently, Wang et al. analyzed the deformation of PVC gel using electrowetting [4]. They reported that as the applied voltage increases, the effective interfacial energy between the PVC gel and the cylindrical anode reduces, while adhesion energy between them increases. Extending this concept, we discovered the phenomenon that PVC gel infiltrate the gap between the anode composed of a bundle of microfibrils, such as carbon fiber by voltage resulting in the outer diameter of the anode increases.<br/>In this study, we proposed the modeling of the PVC gel infiltration on microfibril anode, and demonstrated PETAM by coating the twisted carbon fiber with PVC gel. The combined mechanism of the radial expansion of anode oriented from PVC gel infiltration and the effect of twist insertion allows PETAM to have torsional actuation. The fabricated PETAM was one-end tethered, electrically controllable, and exhibited a maximum stroke of 4.23 degrees/mm with an input voltage of less than 1kV. This result will lead to the development of electro-responsive TFAMs and bringing us one step closer to the artificial muscles that can be applied in real life.<br/><br/>References<br/>[1] S. M. Mirvakili, I. W. Hunter, Artificial muscles: Mechanisms, applications, and challenges. <i>Advanced Materials</i>, <b>30</b>, 1704407 (2018).<br/>[2] S. Aziz, G. M. Spinks, Torsional artificial muscles. <i>Materials Horizons</i>, <b>7</b>, 667-693 (2020).<br/>[3] M. Z. Uddin, M. Yamaguchi, M. Watanabe, H. Shirai, T. Hirai, Electrically induced creeping and bending deformation of plasticized poly (vinyl chloride). <i>Chemistry Letters</i>, <b>30</b>, 360-361 (2001).<br/>[4] Z. Wang, Y. Dong, Y. Zheng, Y. Wang, C. Li, Y. Hu, S. Cai, Electrowetting of a Soft Elastic Gel. <i>ACS Macro Letters</i>, <b>12</b>, 828-834 (2023).