Drug diffusion front movement is important in drug release control from swellable matrix tablets.

Swellable controlled release devices of buflomedil pyridoxalphosphate in hydroxypropyl methylcellulose were prepared, and their swelling and release behavior was investigated. The drug release as a function of time was investigated for various system parameters. Three distinct fronts were observed during the swelling and release processes, i.e., a swelling, a drug diffusion, and an erosion front. The drug diffusion front could be readily determined due to the drug's yellow color. The relative positions of the fronts and the drug release rate were studied as functions of the initial porosity and the molecular weight of the polymer carrier. It was shown that the drug diffusion front best describes the overall release behavior of the system. The fractional drug release was a strong function of the dissolved drug gel layer thickness, which separates the diffusion front from the erosion front. The effect of drug solubility was also investigated by altering the pH and the ionic strength of the dissolution medium. It was shown that as drug solubility increased, the undissolved drug gel layer thickness decreased, again showing the importance of the movement of the diffusion front in controlling the overall release.