Concurrent reaction and diffusion in photo-responsive hydrogels

A hydrogel consists of a polymer network imbibing a solvent. It can undergo large deformation like a soft solid originating from the polymer network underpinned by the crosslinks, as well as exchange matter with the environment like a porous medium. Hydrogels with guest species undergoing a photoisomerization reaction can respond to light in the form of a volumetric change due to the different hydrophobicity of the photoisomers. In this work, we have combined hydrogel elasticity with chemical reaction under the framework of non-equilibrium thermodynamics to study the response kinetics of the photo-responsive hydrogels. As a result, the kinetic behavior of the photo-responsive hydrogels is governed by both the reaction of the photoisomerization and the diffusion of the solvent molecules. Our model allows us to investigate different kinetic processes in photo-responsive hydrogels, including reaction-limited, diffusion-limited, and coupled reaction-diffusion processes.