Inversing the actuation cycle of dielectric elastomer actuators for a facial prosthesis

Dielectric Elastomer Actuators (DEAs) are a type of smart material described as compliant capacitors. They show impressive performances as soft actuators, such as a high strain and fast response. Nonetheless, replicating natural muscle function with DEAs has posed a challenge since DEAs exhibit in-plane expansion, whereas natural muscles contract when stimulated. This publication aims to investigate the use of a normal configuration of DEAs to obtain a contractile movement for post paralysis facial reanimation, by inversing its actuation cycle: the voltage applied on the DEA will constantly be on to keep the DEA stretched and will be off when a contraction movement is wanted, for instance for smiling. Several difficulties linked to this solution need to be considered, such as the self-discharge rate of the DEA, linked to the leakage current flowing through the dielectric when a voltage is applied. The leakage current corresponds to a leakage of charge between the two electrodes and is suggested to influence the self-discharge rate but also the dielectric breakdown and the performance of the actuator. As DEAs present a fast self-discharging rate, the charging frequency of DEAs should be determined to avoid unwanted displacement leading to visible facial spasms. DEAs and their self-discharge rate are characterized, to determine the chosen charging frequency and duty cycle for facial reanimation. The goal is to have the minimum discharge between two actuation cycles. A discharge model was proposed and validated experimentally, allowing to determine a chosen frequency of 2 Hz and 50% duty cycle, leading to a discharge of less than 3% between two actuation cycles, and thus allows to consume 1.5% less energy over each cycle compared to a continuous actuation.