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Natural muscle provides excellent motilities for animals. As the basic unit of the muscle system, the skeletal muscle fibers function as a soft linear actuator. Inspired by the muscle fibers, researchers have developed various soft active devices with linear actuation. This paper reviews several soft linear actuators, such as the dielectric elastomer, thermal responsive hydrogels, pneumatic artificial muscle, and conducting polymers. The actuation mechanisms and performances of these soft linear actuators are summarized. Based on the dielectric elastomer, we propose a design of a hybrid system with linear actuation, driven by both the electric motor and dielectric elastomer cone. The electromechanical behaviors of the dielectric elastomer cone have been investigated in both experiment and finite element analysis. This work may guide the further design of soft actuators and robots.

Biological muscles are composed of aligned actuatable fiber units to generate directional linear output, inspiring the design of microstructures in artificial muscles. However, synthetic soft elastomers generally possess isotropic mechanical properties; therefore, a promising artificial muscle (dielectric elastomer actuator) generally outputs nondirectional force or deformation. In this work, we report a muscle-mimetic anisotropic dielectric elastomer fiber with directional output driven by electric field. The millimeter-in-diameter fibers are fabricated by rolling elastomeric ultrathin films of triblock copolymer that are pretreated to generate aligned nanostructures and network strands. We theoretically and experimentally reveal that a moderate orthogonal modulus ratio of radial to axial directions (2.8) leads to highly directional actuation, enhancing linear axial strain by 100%. Assembling the anisotropic fine fibers into bundles is able to scale up force output and enables variable motion modes such as extension, bend, and rotation.