Recent Advances in Stimuli-Responsive Shape-Morphing Hydrogels

Inspired by shape-morphing organisms in nature, researchers have developed various hydrogels with stimuli-responsive swelling, shrinking, bending, folding, origami, rolling, twisting, or locomotion. These smart hydrogels are usually created by patterning or 4D printing. The shape morphing of hydrogels allows the fabrication of helixing, twisting, and rolling microstructures, all of which are hard to reproduce directly by ordinary techniques. More importantly, under external stimuli (e.g., solvent, humidity, temperature, light, pH, and electric/magnetic fields), many hydrogels exhibit recoverable shape morphing and thus find promising applications in grippers, sensors, valves, soft robotics, etc. Since shape morphing determines the functions of hydrogels in a great number of cases, herein, recent advances of stimuli-responsive hydrogels are summarized, with their types, shape-morphing mechanisms, fabrication methods, shape-morphing modes, and extensive applications covered. The conclusion and perspectives are also presented to guide the design and fabrication of functional hydrogels.