A Variable Degree-of-Freedom and Self-sensing Soft Bending Actuator Based on Conductive Liquid Metal and Thermoplastic Polymer Composites

This paper presents a soft actuator embedded with conductive liquid metal and shape memory epoxy (SME) which function together to enable self-sensing, tunable mechanical degrees of freedom (DoF), and variable stiffness. We embedded thermoplastic shape memory epoxy in the bottom portion of the actuator. Different sections of the SME could be selectively softened by an implanted conductive silver yarn located at different positions. When an electric current passes through the conductive silver yarn, it induces a phase transition that changes the epoxy from stiff state to compliant state. Each section of SME could be softened within 5 s by applying a current of 200 mA to the silver yarn. To acquire the strain curvature, eGaIn was infused into a microchannel surrounding the chambers of the soft actuator. A spiral-shaped eGaIn sensor was also attached to the tip of the actuator to perceive the contact with reliable dynamic force response. Systematic experiments were performed to characterize the stiffness, tunable DoF, and sensing property. We show the ability of the soft composite actuator to support a weight of 200g at the tip (as a cantilever) while maintaining the shape and the ability to recover its original shape after large bending deformation. In particular, seven different motion patterns could be achieved under the same pneumatic pressure of the actuator due to selectively heating the SME sections. A gripper which was fabricated by assembling two actuators to a base was able to grasp the weight up to 56 times of a single actuator through an appropriate motion pattern. For demonstration purposes, the gripper was used to grasp various objects by adjusting the DoF and stiffness with real-time feedback of the bending strain and the contact force.