Reversible Shape-Shifting of an Ionic Strength Responsive Hydrogel Enabled by Programmable Network Anisotropy

Reversible shape-shifting hydrogels exhibit great potential in diverse fields. Repeatable programmability for the shape transformation has newly been enabled in thermally responsive hydrogels via engineering of the chain orientation of the polymer network, which substantially promotes the transformation capability. However, diversified responsive behavior and the enabling mechanism require further investigation. Herein, we develop an ionic strength (IS) responsive hydrogel enabling the programmable reversible shape transformation based on a semi-interpenetrating network of poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA). Deformation of the hydrogel upon external force can be fixed due to crystallization of PVA that underwent cyclic freezing-thawing. Therefore, the chain orientation can be retained in the deformed area, enabling the programmable IS responsive actuation. In contrast to the thermally responsive actuation originated from the lower critical solution temperature phase transition, the IS responsive actuation does not accompany any phase change and the corresponding mechanism is proposed. Reversible bending providing an actuation angle as large as 80° can be achieved after optimization of the PVA content. The PVA crystals can be melted upon heating, and the responsive actuation can thus be reprogrammed. In addition, utilizing a digital light 3D printer, the hydrogels are further fabricated into arbitrary geometries, thus realizing more complex actuations. Overall, our work provides a general strategy to develop reversible shape-shifting hydrogels and paves the way for soft actuators.