Multi-dimensional negative-stiffness thermoplastic bias mechanisms for high-stroke dielectric elastomer actuator arrays

A dielectric elastomer (DE) consists of a highly deformable polymer films sandwiched between flexible electrodes. When a voltage is applied between the electrodes, an electric field induces a compressive stress that leads to controlled deformation, making the DE operate as an electrostatic actuator. To achieve fully polymeric DE actuator (DEA) systems with large stroke and high flexibility, this work proposes a negative-stiffness multidimensional biasing mechanism based on thermoplastic polymer material. Such mechanism is not affected by the high inherent stresses of metal beams as well as by the highly hysteretic behavior of silicone, making the system suitable for soft mechatronic devices, wearables, micro-conveyors, and soft robots. The mechanism is shaped like an array of stress-free beams achieved through a thermoforming process, designed to exhibit a negative stiffness in its force-displacement behavior. If this negative stiffness matches the positive one of the DE, a significant increase in actuation stroke is achieved. By arranging multiple independent DEs in an array-like configuration onto a shared elastomer membrane, and coupling them with the proposed negative-stiffness biasing array, multi-dimensional cooperative actuator/sensor arrays can be created. After introducing the DE system concept, we provide details on the negative-stiffness mechanism manufacturing and characterization, in order to assess its suitability for DEA applications. The obtained results will provide fundamental guidelines for the future design of DE-based cooperative actuator/sensor systems.