Assessment of dermal absorption by thermogravimetric analysis: Development of a diffusion model based on Fick's second law.

Recent work in our laboratory shows that it is possible to reproducibly measure the weight change over time of a piece of porcine skin exposed to a chemical vapor by thermogravimetric analysis (TGA), that is, using a microbalance maintained at constant temperature and humidity. Here, we develop a diffusion model by which the TGA skin weight curve can be interpreted. A TGA-specific analytical solution for Fick's second law of diffusion was derived. The solution contains two chemical-dependent parameters; a diffusion coefficient (D) and a skin/air partition coefficient (P). The resulting function was fitted to weight curves from 83 experiments with 4 chemicals; n-butyl acetate, methanol, 2-propanol, and toluene. In most experiments, a single weight function could not adequately describe the shape of the weight curve, therefore up to five parallel compartments, each with its parameter set Di and Pi, were tested. A two-compartment model was adequate for 58% of the experiments according to the Akaike Information Criterion. Dermal penetration of the chemicals was also measured with Franz diffusion cells. The diffusion coefficients obtained by the two methods agreed well.