[Determination of an intrinsic value of diffusion coefficient of active principle in polymeric matrix for the formulation of a controlled release system]. / Détermination d'une valeur intrinsèque du coefficient de diffusion d'un principe actif dans une matrice polymérique en vue de la formulation d'un système à libération contrôlée.

The study of diffusion in a polymeric system often poses difficult experimental problems, notably when the rate of release is determined in a liquid receptor phase which can interact with the system. In this publication, we propose two experimental methods to determine the diffusion coefficient by a gel-gel kinetic release study, in which the receptor is a same gel unloaded initially. The first method is based on the use of radioactive tracers and a multi-channel radioactivity counter to obtain, at different times, the concentration profiles in the diffusion medium. The theoretical model is given for a gel donor with the concentration C0 of diffusing molecules is below or above the solubility Cs. The second method is based on the measurement of the displacement of solubility front during the diffusion within the system in the case C0 > Cs. The theoretical model shows that this displacement is a function of square root of t. For illustration, two experimental determinations of testosterone diffusion coefficient on the one hand in a silisic acid gels and on the other hand in an acrylic acid gels are given.