Friction Controllable Tendon-Driving Actuation by Using Hygroscopic Deformation of Polymer Tube

Abstract

There have been many studies on polymer actuators due to their advantages in mechanical property, reversibility, and reproducibility. Among them, the actuators based on volume control were mainly developed, which focus on the bending using water-swellable polymers such as Nafion and Aquivion along with non-swellable polymers. Although it is easy to fabricate, the actuator force is small. In this paper, we present a new type of polymer actuator based on friction force control of a tendon-driving mechanism using hygroscopic deformation of a polymer tube. The diameter of the water-swellable polymer tube can be controlled by adjusting the moisture content of solvent. This enables quantitative adjustment of the friction force applied to the wire passing through the tube. There are two types of mechanism: type of increasing the friction force on the wire when the tube is dried, and the type of confining the tube with a non-swellable tube, thereby increasing the friction force of the wire when the polymer tube swelled. In addition, the problem that most soft wearable devices shares is the bulkiness of actuators and batteries. In order to solve this problem, we applied the new type of actuator to the device that can implement the flexion/extension of the finger without electrical components. Two Nafion tubes were placed on the back and the palm of the hand, respectively, and the flexor wire and the extensor wire pass through each tube. Each wire has an elastic band at the end, applying a force to the wire. Although this mechanism requires time to inflate the tube and is difficult to control accurately, it has the advantage of being able to exert a large force using light and cost-effective elements.