Observation of swelling process and diffusion front position during swelling in hydroxypropyl methyl cellulose (HPMC) matrices containing a soluble drug.

The behavior of gel layer thickness in swellable hydroxypropyl methyl cellulose matrices loaded with increasing amounts of soluble and colored drug and exhibiting swelling, diffusion and erosion fronts, was studied using a colorimetric technique. The effect of the drug loading on the front position in the gel layer, in particular, on the presence of a diffusion front and its movement, was investigated. In addition, the swelling, diffusion and erosion front positions at different releasing times were measured and a theoretical analysis of the overall process was provided. It was found that the diffusion front was visible in systems with more than 30% drug, due to the presence of an undissolved drug layer. The physical analysis of such systems clearly showed the importance of drug solubility and loading in the observation of the diffusion front.