New kinetic model of drug release from swollen gels under non-sink conditions

A kinetic model of drug release from slabs of swollen gels under non-sink conditions is presented. The model is based on the mass transfer of drug from the interfacial region to an external solution of finite volume. Mass transfer is coupled with drug diffusion from the bulk gel towards the interfacial region together with a non-equilibrium adsorption–desorption of drug to the polymer chains. The coupled kinetic equations obtained for the whole mentioned process allowed us to describe the time evolution of drug concentration released to the external volume, without the needs of keeping perfect sink conditions. The remaining bulk drug concentration can also be recovered. The model is tested with experimental data of teophylline release from xanthan gum gels placed in a steady-state Franz cell. It is observed that a pseudo steady-state concentration of free (non-adsorbed) drug is reached at the interfacial region after a short transitory time, t0. Besides, the kinetic features of the model have been simulated for several different parameters like slab dimensions, drug diffusion coefficient in the bulk gel, the equilibrium constant for the drug partition between gel and solution and the initial drug loading in the gel.