Hydrogel-Based Artificial Muscles: Overview and Recent Progress

Artificial muscles are promising as an intelligent material that can replace conventional power systems to reproduce the motion of a living organism. Among various building materials, hydrogels have great advantages as artificial muscles, such as responsiveness to a wide range of stimuli, large volume deformation, and sensitivity. However, conventional hydrogel actuators that generate only isotropic volume change in response to an external stimulus cannot be considered as artificial muscles because complex deformations are not possible. Instead, programmed complex deformations originated from a rationally designed anisotropic structure are desired in artificial muscles. Herein, recent approaches for constructing stimuli-responsive hydrogels with an anisotropic structure, thereby enabling a directional complex motion to mimic a muscle are reviewed. First, stimuli-responsive shape-deforming hydrogels as a main building matrix are categorized according to stimulating external signals. The representative methods for modulating the isotropic structure of a hydrogel into various anisotropic structures, such as multilayer structure, linearly oriented structure, and patterned structure are discussed. Finally, the recent strategies to achieve muscle-like motion of hydrogels by combining stimuli responsiveness and anisotropic structures for translation from elementary contraction and expansion to programmed deformations, such as multidirectional bending, directionally amplified deformation, and compositive programmed motion, are covered.