Light-Driven Expansion of Spiropyran Hydrogels
The incorporation of molecular switches in organic structures is of great interest in the chemical design of stimuli-responsive materials that mimic the complex functions of living systems. Merocyanine dyes that convert to spiropyran moieties upon exposure to visible light have been extensively studied as they can be incorporated in hydrated covalent networks that will expel water when this conversion occurs and induce a volumetric shrinkage. We report here on a sulfonatebased water soluble photoswitch that in contrast to the well-known system triggers a volumetric expansion in hydrogels upon exposure to photons. Contraction is in turn observed under dark conditions in a highly reversible manner. The novel behavior of the photoswitch incorporated in the covalent network was also predicted by coarse grain simulations of the system’s chemical structure. Using pH control and polymeric structures that differ in lower critical solution temperature we were able to develop hydrogels with highly tunable volumetric expansion. Furthermore, the novel molecular function of the systems developed here led to materials with the negative phototaxis observed in plants, and can expand the potential use of hydrogels as sensors, soft robots, and actuators.