Drug-eluting stents (DESs) are
drug-device combination products that have been commercialized and demonstrated to be safe and efficacious in treating
coronary artery disease. They have been very effective in reducing the extent of neointimal
hyperplasia and therefore in preventing or minimizing the occurrence of in-
stent restenosis. In order to develop a successful DES, it is imperative that the coating be designed so as to deliver, after
stent implantation, a therapeutic dose of the
drug for the desired time duration at the site of the arterial blockage. Mathematical models are very valuable tools that can be used to study the effect of different coating parameters on
drug delivery and can therefore help in coating design. We have developed a bimodal lumped-parameter mass transport model to describe the release of the
drug everolimus from a biodurable
fluoropolymer-based DES coating. We assume that the dispersed
drug phase contributes to two discrete modes of
drug transport through the coating. These are the fast mode (mode I) which is the release of the
drug from a highly percolated structure of
drug phase within the
polymer, and the slow mode (mode II) which is the release of the
drug from a nonpercolated,
polymer-encapsulated phase of the
drug within the coating. The three coefficients in the governing equations describing the model, i.e. the two effective diffusivities corresponding to each of the two modes and the fraction of the
drug in one of the two modes, were determined by fitting with available DES release data. The predictive power of the model is demonstrated by comparing the release rate from different coating configurations (thickness and
drug to
polymer ratios) with experimental data. Also, it is demonstrated that if limited experimental data are available at early time points, the model can be used to predict drug release at subsequent time points.