CONTROLLING DIFFUSION

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