Diffusional release of a solute from a spherical reservoir into a finite external volume.

An exact solution has been obtained for the release kinetics of a solute from a spherical reservoir with the burst effect initial condition into a finite external volume. The exact solution is derived based on the time Laplace transform method. The results presented here indicate that as the external fluid volume increases, the cumulative release at any time and the releasable amount of the solute at infinite time increase. In addition, for a given external volume, as the polymeric coat thickness increases the fractional release at any time decreases. Experimentally, cumulative release profiles of theophylline microspheres coated with ethylene vinyl acetate copolymer into different external volumes agreed with the mathematical predictions.