Photochemically-Driven Shaping of Thin Hydrogel Sheets Containing Light-Sensitive Supramolecular Complexes

The ability to precisely define the shape or motion of responsive polymers has wide ranging applications from soft robotics to biomaterials. Among the available stimuli to select from, light represents a simple, remote method of prescribing shape change with a high degree of spatial and temporal control. Our group, and others, have previously leveraged photothermal effects in temperature-sensitive hydrogels to drive shape change, self-assembly, and collective motion. However, photothermally addressable materials require constant illumination to maintain a shape change and suffer from decreased resolution due to thermal broadening. These limitations have stimulated work by numerous groups on photochemically-responsive hydrogels. While azobenzene and related photoswitches offer attractive properties including highly tunable lifetimes and excellent photostability, historically their modest change in polarity upon photoswitching has limited the light-induced response that can be achieved. To enhance this effect, our group has studied photochemically driven shape changes in thin hydrogel disks using host-guest interactions between azobenzene and cyclodextrin. Patterned illumination of the disks with ultraviolet light results in localized deswelling, giving rise to shapes with either positive or negative Gaussian curvature. The reversibility of the host-guest complexation allowed for the shapes to be erased with visible light and repatterned an arbitrary number of times. In addition, we demonstrate azobenzene derivatives that enable both swelling and deswelling of hydrogels entirely with visible light, allowing for increased penetration depth and improving compatibility with materials for cell and tissue culture.