Release and permeation kinetics of caffeine from bioadhesive transdermal films.

The aim of this work was to investigate, in vitro, the kinetics of release and permeation of caffeine, chosen as model drug, from bioadhesive transdermal films. These films are not self-adhesive but become adhesive when applied to wet skin. Permeation experiments were performed from films with different drug loadings using rabbit ear skin as barrier. In order to characterize the release kinetics of caffeine from the film, a polyethylene membrane, impregnated with isopropyl myristate was employed. The data obtained in the present work suggest that caffeine release from transdermal bioadhesive films was controlled either by the permeability characteristics of the skin or by the film itself, depending on drug loading. When drug loading is low (ie, caffeine is dissolved in the polymers constituting the film), the control resides in the skin. When caffeine loading exceeds its solubility in the film, the permeation profile is not linear, but shows a sort of burst effect in the early times of permeation, probably owing to the presence of solid drug and/or to a certain degree of "conserved supersaturation" in the solid phase.