A Photoresponsive Hydrogel with Enhanced Photoefficiency and the Decoupled Process of Light Activation and Shape Changing for Precise Geometric Control

Traditional shape-morphing hydrogels rely on structural implementation of inhomogeneity inside the material during fabrication to realize predetermined complex shape change upon activation. In recent years, several systems with reprogrammable shape-morphing capabilities have been developed. Among those, the photoresponsive hydrogels offer the best spatial and temporal control. However, for most photoresponsive hydrogels, upon light irradiation, they simultaneously deform, which requires the projection of the light pattern to be continuously adaptive to the deforming gel. It is impractical for complex 3D morphing. In this paper, by incorporating two photodissociable molecules that can form a reactive ion couple upon light activation into one hydrogel, the light irradiation process is decoupled with the morphing process, and the consumption of the reactive ion couple drives the reversible photochemical reaction forward. Consequently, the photochemical reaction efficiency is improved, and the photoresponsive molecules are locked in the activated state until a recovery stimulus is applied. Based on the proposed general scheme, a specific example is given by incorporating the triphenylmethane leucohydroxide and 2-nitrobenzaldehyde molecules into a polyacrylamide hydrogel. The swelling behavior is characterized, and the reprogrammable morphing with precisely controlled geometry is demonstrated.