Monitoring the diffusion of topically applied drugs through human and pig skin using fiber evanescent wave spectroscopy (FEWS)

The objective of this study was to examine if the diffusion process of topically applied drugs can reliably be monitored using FEWS in respect to timely distribution of the drug and chemical alterations of the drug during the diffusion process. In order to do this, recently excised human and pig skin was cut into slices of different thickness while also taking into account the different layers skin is composed of (e.g. Dermis, Stratum Corneum). These layers were first characterized spectroscopically and optically using a microscope before the drug itself was applied topically. The diffusion process was monitored by placing the sample on an ATR (attenuated total reflection) element. Time series from 1-4 hours were taken and the characteristic absorption bands of the drug were analyzed in the mid-infrared. By using an first order approach on Fick's diffusion equations (skin assumed to be homogeneous) we were able to fit these experimental values and to obtain diffusion constants, e.g for water at 3376 cm^-1 in the order of 10^-5 cm²/s, which compare well with previously published values. The results indicate that this technique can be applied to the prediction of transdermal drug delivery.