Towards artificial muscle implants: structured reinforcement of dielectric elastomers

Abstract Dielectric elastomer actuators (DEAs) are a recent type of smart materials that show impressive performances as soft actuators, making them a promising technology for the development of implantable artificial muscles and soft robotic devices. Notably, they are explored as implants for the restoration of facial movements post paralysis. However, implementing DEAs that mimic natural muscles has been proven difficult, as DEAs provide in-plane expansion when actuated, while natural muscles contract upon stimulation. Multiple solutions can be found in literature, namely stack DEAs and fiber-reinforced DEAs. The fibers used for DEAs to achieve contractile motion rely on a fishnet design, where the angle between the fibers, the spacing, mechanical properties as well as the fiber dimensions can be set by establishing a fiber analytical model. Contraction has only been achieved with DEAs based on acrylic elastomer and pre-stretched with rigid frames, thus making them unsuitable as soft implants. This work introduces the first silicone-based, non-pre-stretched DEAs presenting in-plane contractile behavior by embedding such soft structured fiber sheets in the actuators. Fiber-reinforced DEAs were shown to achieve modest contractile strains, particularly with optimal fiber angles between 55∘ and 65∘, enhancing their ability to mimic muscle-like behavior. A peak occurs at approximately 60∘, where the maximum contraction of −0.6% was achieved, resulting in a 3% error from the model. The low contractile strains of silicone-based DEAs indicate that further optimization is needed for real-world applications.