Swelling Kinetics of Disulfide Cross-Linked Microgels

Hydrogels of 2-hydroxyethyl methacrylate (HEMA) cross-linked with N, N ‘-cystaminebis(acrylamide) (CBA) were prepared inside microchannels using an in situ photopolymerization for the study of chemical-responsive microgels. By chemically reducing disulfide bonds with dithiothreitol (DTT), the cross-link density of the hydrogel network decreased, leading to an observable swelling of the hydrogel. To maximize swelling response, acrylic acid (AA) was copolymerized with HEMA and CBA to afford a pH/chemically-responsive hydrogel. The combination of a decrease in cross-link density and a driving force for swelling (deprotonation of AA) led to a fast swelling response. Hydrogel swelling switched between half- and first-order kinetics depending on hydrogel composition and de-cross-linking conditions. A direct relationship between the hydrogel swelling rate and the rate of cross-link cleavage was found. A linear dependence between the square of the swelling rate and DTT concentration suggests the possibility of using responsive hydrogels as a quantitative chemical sensor.