Investigation of the Strain-Dependent Electrical Impedance of Fiber-Reinforced Electrodes in Dielectric Elastomer Transducers

Soft robotics is a rapidly advancing field, and dielectric elastomer actuators (DEAs) have emerged as a highly promising technology among actuator concepts. These DEAs offer significant advantages, including substantial deformation, rapid response times and sensing of their strain and stress state. An innovative approach involves incorporating anisotropic layers, such as fibers, to improve actuator performance. Fiber-reinforced DEAs have gained prominence due to their ability to increase force output and eliminate the need for additional support structures. Carbon fibers, in particular, with their high stiffness and electric conductivity, not only serve as excellent reinforcement but also as electrode materials. Furthermore, DEAs exhibit the capability of using their electrodes for self-sensing, with the electrodes and dielectric acting as resistive or capacitive sensors. Despite various methods for implementing sensing and control capabilities, the understanding of the piezoresistivity and strain-dependent impedance change of carbon fiber-reinforced DEAs is lacking. Our work aims to bridge this gap by employing tensile testing with electrical impedance spectroscopy, exploring the behavior of carbon particle- and carbon fiber-filled elastomer electrodes. The study aims to validate assumptions regarding their equivalent circuits and assess how circuit parameters vary with different electrode configurations and matrix materials. Key results include the pure resistive, yet non-monotonic impedance-strain behavior of carbon fiber electrodes as well as the drastic increase in complex conductance with higher viscosity matrix for isotropic electrodes. Both have major implications for the sensing properties of DEAs.