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-dimensional (3D) deformations and thermoresponsive actuation of polymer hydrogels in a well-defined manner. Precise infiltration of Fe³⁺ with periodic patterns is additionally used to cross-link the local polymer network through ionoprinting with a patterned electrode array. The patterned Fe³⁺ cross-linking generates periodic undulations in cross-link density, stiffness, and thermoresponsiveness. The internal stress induces 3D helical structures with tunable chirality and dimensions. The differential thermoresponsiveness imbues a fourth dimension to the shape deformations. Moreover, sequential ionoprinting generates well-defined in-plane periodic distributions of differential modulus and responsiveness, leading to 3D/4D umbrella-like origami upon temperature triggers.