CONTROLLING DIFFUSION

In this paper I first briefly look at diffusion in several controlled release devices. I have found that these exploit complicated diffusion al mechanisms. These cannot be understood using the conventional descr iption of diffusion. So, a general theory of multicomponent diffusion which can describe them is considered. An overview follows of the pres ent knowledge on diffusivities of small and large permeants in dense p olymers, swollen polymers, charged polymers, gels and porous media. Th e paper ends with a brief discussion of the limitations of diffusion t heory in swelling polymers.