Diffusion modelling of percutaneous absorption kinetics: 4. Effects of a slow equilibration process within stratum corneum on absorption and desorption kinetics.

One of the main functions of the skin is to control the ingress and egress of water into and out of the body. The transport kinetics of water in the stratum corneum (SC), the dominant site of resistance in the skin, is normally described assuming a homogeneous membrane model. In the present work, the desorption of water from SC was studied and profiles obtained for amount desorbed versus time profiles that were more consistent with water transport occurring in a heterogeneous membrane. Analysis of the resulting profiles yields a model that is consistent with a slow equilibration/slow binding of water within SC as well as its permeation through the SC. Diffusion model solutions were used to derive the steady-state flux, lag time and mean desorption time for water in SC. The slow binding kinetics of water in the SC are limited and most pronounced in the early transient stages of transport and are not easily discerned using steady-state penetration studies. The practical importance of this work is in its use of desorption experiments to recognise and define the skin reservoir for water and other solutes as well as penetration parameters in defining their transdermal kinetics.