Programmable and Reversible 3D/4D Shape Morphing Hydrogels with Precisely Defined Ion Coordination

Precise and programmable control of reversible deformations of hydrogels has important implications for bionics. This work reports on programmable three-dimension (3D) deformations and thermo-responsive actuation of polymer hydrogels in a well-defined manner. Precise infiltration of Fe3+ with periodic patterns is used to additionally crosslink the local polymer network through ionoprinting with a patterned electrode array. The patterned Fe3+ crosslinking generates periodic undulations in crosslink density, stiffness, and thermo-responsiveness. The internal stress induces 3D helical structures with tunable chirality and dimensions. The differential thermo-responsiveness entitles a fourth dimension to the shape deformations. Moreover, a sequential ionoprinting generates welldefined in-plane periodic distributions of differential modulus and responsiveness, leading to 3D/4D umbrella-like origami upon temperature triggers.