Solvato-morphologically controlled, reversible photo-actuated hydrogels, operative in neutral environments

In this study photo-actuator hydrogels operating in neutral environments were generated using N-isopropylacrylamide-co-acrylated-spiropyran-co-acrylic acid, p(NIPAAM-co-SPA-co-AA) copolymer, in a 100-1-5 mole ratio. Acrylic acid dissociates in neutral environments, triggering spiropyran (SP) moiety conversion to the more hydrophilic protonated merocyanine form (MC-H ⁺ ). MC-H ⁺ allows water uptake, causing hydrogel expansion. Exposure to white light causes the expanded hydrogel to contract due to the photonic-conversion of MC-H ⁺ to the more hydrophobic SP form. Different ratios of deionised water: organic solvent (tetrahydrofuran (THF), dioxane and acetone) were used as the polymerisation solvent. Varying the polymerisation solvent mixtures resulted in hydrogels with different pore sizes and therefore different swelling/shrinking properties and photo-actuation kinetics.