  In this paper we investigate the extent to which variable porosity
drug-eluting coatings can provide better control over drug release than
coatings where the porosity is constant throughout. In particular, we aim to
establish the potential benefits of replacing a single-layer with a two-layer
coating of identical total thickness and initial drug mass. In our study, what
distinguishes the layers (other than their individual thickness and initial
drug loading) is the underlying microstructure, and in particular the effective
porosity and the tortuosity of the material. We consider the effect on the drug
release profile of varying the initial distribution of drug, the relative
thickness of the layers and the relative resistance to diffusion offered by
each layer's composition. Our results indicate that the contrast in properties
of the two layers can be used as a means of better controlling the release, and
that the quantity of drug delivered in the early stages can be modulated by
varying the distribution of drug across the layers. We conclude that
microstructural and loading differences between multi-layer variable porosity
coatings can be used to tune the properties of the coating materials to obtain
the desired drug release profile for a given application.
