A KINETIC-MODEL FOR ION-TRANSPORT ACROSS SKIN

A mathematical model for ion transport across a multilaminate barrier is presented which satisfactorily describes the exponentially nonlinear and slightly asymmetric current-voltage properties of skin subjected to an applied DC potential. This is accomplished using rate equations, analogous to the Butler-Volmer theory of electrode reaction rates, in which voltage-dependent potential energy barriers govern ion transport. In skin, the physical location of these barriers may be lipid lamellae in the stratum corneum and/or the walls of the epithelial cells lining skin appendages. Model parameters for the number of barriers, n, the symmetry of the barriers, alpha, and rate constants for sodium and chloride ion, K(Na) and K(Cl), are calculated based on the current-voltage characteristic of excised human skin immersed in saline solutions and the sodium ion transference number for this tissue obtained during constant current ion-tophoresis. The ratio K(Na)/n is found to be equal, within experimental error, to the sodium ion permeability coefficient, P(Na), obtained by measuring the passive diffusion of Na-22(+) through the same skin samples, as predicted by the theory. Using the parameter values developed in this paper in conjunction with the kinetic model, it should be possible to obtain a prediction of iontophoretic delivery rates for other ions based on the passive diffusion of these ions through skin.