Muscle-like hydrogels with fast isochoric responses and their applications as soft robots: a minireview

Hydrogels with abundant water and responsiveness to external stimuli have emerged as promising candidates for artificial muscles and garnered significant interest for applications as soft actuators and robots. However, most hydrogels possess amorphous structures and exhibit slow, isotropic responses to external stimuli. These features are far inferior to real muscles, which have ordered structures and endow living organisms with programmable deformations and motions through fast, anisotropic responses in complex environments. In recent years, this issue has been addressed by a conceptual new strategy to develop muscle-like hydrogels with highly oriented nanosheets. These hydrogels exhibit fast, isochoric responses based on temperature-mediated electrostatic repulsion between charged nanosheets rather than water diffusion, which significantly advances the development of soft actuators and robots. This minireview summarizes the recent progress in muscle-like hydrogels and their applications as soft actuators and robots. We first introduce the synthesis of muscle-like hydrogels with monodomain structures and the unique mechanism for rapid and isochoric deformations. Then, the developments of hydrogels with complex ordered structures and hydrogel-based soft robots are discussed. The morphing mechanisms and motion kinematics of the hydrogel actuators and robots are highlighted. Finally, concluding remarks are given to discuss future opportunities and challenges in this field.