Percutaneous absorption: transport in the dermis

Results are presented for the percutaneous absorption of 3 different esters of nicotinic acid from circular patches; the values of the diffusion coefficients and triggering concentrations for visible erythema are in good agreement with previous work. For methyl nicotinate the radial spread of the erythema has been followed as a function of time; the results are shown to fit a radial diffusion equation. However, the value of the diffusion coefficient is found to be a thousand times larger than the expected value for molecular diffusion. A model is presented in which the radial transport is caused by uptake in the capillaries. Reasonable values are found for the rate constants describing uptake and for the geometry of the capillaries. This geometry is shown to be close to the optimum for efficient transport to and from the dermis.