In this talk I will present a mathematical model  describing the dynamics of a substance between a two-layer media of different properties and extents.  The model incorporates drug diffusion, dissolution and solubility  in the polymer coating, coupled with diffusion, convection and reaction in the biological tissue.  Each layer contains bound and free drug phases so that the resulting model is a coupled two-phase two-layer system of partial differential equations.  The model is used as predictive tool for a fundamental understanding of drug transport in the skin, possibly  enhanced by an electrical field, as in iontophoresis. The results show the roles of the different model parameters, to evaluate drug release efficacy and to assess an optimal control strategy of  delivery systems.