Mathematical modelling of variable porosity coatings for controlled drug release

- Variable porosity coatings for drug-releasing medical implants are proposed.
- A theoretical description of drug transport in coupled porous media of variable porosity is given.
- Semi-analytical and analytical solutions are derived.
- We show that a contrast in microstructure between layers can be used to tune desired release profile.
- The quantity of drug delivered at early times can be modulated by varying initial drug distribution.

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.