The Relationship Between Stratum Corneum Diffusion Coefficients and Temperature for Halogenated Hydrocarbons.

Abstract : The relationship between diffusion coefficients and exposure temperature was evaluated in isolated stratum corneum (SC). Due to the difficulty in measuring a diffusion coefficient directly, the movement of chemicals into and across the dermal barrier is often described as a function (the permeability constant) of the diffusion coefficient. The permeability constant also depends on the skin: chemical partition coefficient and the path length of diffusion. In the idealized expression of diffusion given in the Stokes-Einstein equation, diffusion is described in terms of kinetic energy and the viscous resistance of diffusion. If this relationship holds for the diffusion of halogenated chemicals in isolated stratum corneum then the observed diffusion coefficients will be dependent on the temperature of the exposure system. Rat or human stratum corneum was removed from whole thickness skin trypsin treatment and stored in a dessicator jar. Approximately 5 Mg of SC was placed into the weighing pan of a thermogravimetric analyzer and exposed to vapor or halogenated hydrocarbons. The increase in tissue weight over time for temperatures ranging from 27-40 deg C were evaluated. Diffusion coefficients were estimated by fitting the pre-equilibrium data with a non-steady state diffusion equation. The observed diffusion coefficients were dependent on the temperature of the exposure, indicating the importance of careful temperature control in dermal absorption studies using halogenated hydrocarbons.