Recent progress on hydrogel actuators.

Hydrogels are known to be soft and wet smart materials that respond to external stimuli. Numerous hydrogel actuators have been developed that perform volume and shape changes, and reversible motions. Heterogeneity, periodicity, and integration of hydrogels with different properties are needed to realize biomimetic motions. In order to achieve specific actuations with predictable pathways, it is critical to fabricate hydrogel structures in a well-controlled manner. This review article summarizes recent progress in representative methods to fabricate hydrogel actuators by different methods, ranging from sequential synthesis, macroscopic supramolecular assembling, field-induced alignment, photolithography, ionoprinting, 3D printing, and gradient structuring. The advantages and limits of these methods are compared and analyzed. Finally, a brief perspective and conclusion are presented to point out some important issues for further studies.