Release kinetics of pendant substituted bioactive molecules from swellable hydrogels: role of chemical reaction and diffusive transport

A series of substituted benzoic acids was covalently linked with 2-hydroxyethyl methacrylate (HEMA). These monomers were copolymerized with HEMA. The kinetics of the release of the substituted benzoic acid from the glassy hydrogel discs in both alkaline and acidic media was investigated. Depending upon the nature of the substituent, the release kinetics could be controlled by (1) the velocity of the penetrating medium, (2) the rate of the hydrolysis step, or (3) the diffusion of the active ingredient released during hydrolysis. This experimental work provides perhaps the first experimental evidence of swelling controlled zero order release of an active ingredient covalently linked to a hydrogel. The theoretical framework developed would be useful in the development of controlled release delivery systems of pragmatic applications in the future