Zero-order release from glassy hydrogels. I. Enigma of the swelling interface number

Abstract

In vitro release kinetics of structurally related benzoic acids and other active ingredients from glassy hydrogels are reported. It is shown that a zero-order release from swellable glassy poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels can be achieved by a judicious choice of the active ingredient. The criteria for the swelling controlled zero-order release proposed in the past have been examined. The equilibrium swelling interface number (Sw_e) is a rather conservative criterion. It is shown that swelling controlled zero-order release can be realized for values of the swelling interface number up to 0.6. The Deborah numbers for release (De_R) for the systems which follow zero-order kinetics are in good agreement with the predictions based on the time dependent diffusion coefficient to account for the role of the molecular relaxation processes.