Coarse-grained simulations of diffusion controlled release of drugs from neutral nanogels: Effect of excluded volume interactions.

The primary goal of this work is to assess the effect of excluded volume interactions on the diffusion controlled release of drug molecules from a spherical, neutral, inert, and cross-linked device of nanometric size. To this end, coarse-grained simulations of the release process were performed. In this way, the inner structure and topology of the polymer network can be explicitly taken into account as well. Our in silico experiments reveal that the boundary condition of constant surface concentration is not appropriate for nanogels. In particular, the predictions based on the perfect sink condition clearly overestimate the fraction of drug released. In addition, these simulations provide values for the release exponent that depends on both the diameter of drug molecules and the number of drug molecules loaded in the matrix, which clearly contrasts with the classical prediction of a constant release exponent. Consequently, the widely used classification of drug release mechanisms based on this kinetic exponent must be extended to include new situations.