Controlling surface porosity and release from hydrogels using a colloidal particle coating

Abstract

Recent studies suggest that coating microcapsules by a shell composed of impenetrable colloidal particles (thereby forming ‘colloidosomes’) can be used to control surface porosity, and therefore, permeability. The voids between the particles in the coating define the size of the surface pores available for transport. However, to date, data demonstrating this selectivity has been largely qualitative. In this paper we examine, quantitatively, the effect of a surface coating (shell), composed of colloidal particles, on release from hydrogels. We find that the presence of a colloidal shell does indeed reduce the rate of transport of three model molecules: Aspirin, caffeine, and FITC-dextran with MW of ∼3000–5000. Contrary to expectation, however, we find that for all three molecules the reduction in transport rate is largely independent of the dimensions of the particles composing the shell, despite differences that range over three orders of magnitude. In the case of the small molecules, caffeine and aspirin, the colloidal shell reduces the effective diffusion coefficient by a factor of 3. In the case of dextran, the suppression in the release rate due to the colloidal shell was much larger. These results are explained using a simple diffusion model that accounts for the volume fraction and diameter of the colloidal particles in the shell, and the size of the diffusing molecules.