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.National Research Foundation (NRF)This study was supported by the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme through the Marine and Offshore Program. This study was also supported under the RIE2020 Industry Alignment Fund-Industry Collaboration Projects (IAF-ICP) Funding Initiative, as well as cash and in-kind contribution from the industry partner, HP Inc., through the HP-NTU Digital Manufacturing Corporate Lab