In the formation of a coated
controlled release preparation with functional coat layers,
hydroxypropyl-methylcellulose was used to form a diffusion layer which swelled immediately upon wetting.
Eudragit RS30D was used to form the outer retention layer. The
rupture of pellet coat occurred when the
Eudragit RS30D was unable to withstand the expansion in volume due to the influx of water and swelling of the
hydroxypropylmethylcellulose diffusion layer. The
sucrose core was able to contribute an osmotic effect. The hydrostatic pressure built up within the pellet can cause the pellet coat to
rupture.
Sodium chloride deposited in the diffusion coat was able to delay the bursting of the pellet coat. This was due to the competition for the imbibed water between
sodium chloride and
hydroxypropylmethylcellulose. The
rupture of the pellet coat did not result in a total failure of the controlled
drug delivery mechanism. Similar drug release rates were obtained irrespective whether there was a
puncture in the pellet coat or not. Pressure built-up in the region away from the
puncture pushed the core material towards the point of
puncture and sealed the
puncture point. In addition, the swelling of
polymer around the point of
rupture ensured continuity in the
drug diffusion barrier.