Effect of ethylene-vinyl acetate concentration on ethylcellulose-walled microcapsules: preparation and release kinetics of theophylline microcapsules.

The effect of concentration of ethylene-vinyl acetate (EVA) copolymer, used as a coacervation-inducing agent, on the preparation of ethylcellulose microcapsules was studied with theophylline as the core material. The influence of EVA concentration on the micromeritic properties of the microcapsules and their drug release behaviour were investigated. Particle size distribution of the microcapsules obtained was dependent on the amount of EVA copolymer. As the EVA concentration increased the quantity of larger particles was reduced and that of the smaller particles was increased. Thus EVA might be used as a protective colloid to prevent aggregation of the microcapsules. The porosity of the microcapsules decreased with respect to EVA concentration, but the wall thickness of the microcapsules showed a corresponding increase. Zero-order release kinetics, from the resulting microcapsules in the initial dissolution phase was obtained. The apparent zero-order release rate in the initial steady-state decreased with the increase of EVA concentration, but T50 increased. The higher concentration of EVA causing a thick, compact wall lead to an effective prolongation of drug release.