244 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.5 1.0 0.5 EFFECT OF DIMETHYLACETAMIDE ON TRANS-EPIDERMAL WATER FLEXOR ASPECT OF MID-FOREARM NORMAL ANHIDROTIC SKIN THREE SUBJECTS A 8 12 16 20 TIME AFTER EXPOSURE (HOUI•) Figure Effect of dimethylacetamide on transepidermal water loss. RFFECT OF DMSO, DMF AND DMA ON WATER LOSS EXPRESSED AS P•RCENTAOR INCREASES OVER PRE-TREATMENT READINGS. DMF Figure 8 12 16 20 TIME AFTER EXPOSURE HOURS Effect on DMSO, DMF and DMA on water loss expressed as percentage increases over pre-treatment readings.

EXPERIMENTAL STUDIES ON PERCUTANEOUS ABSORPTION 245 TETRAHYDROFURYL ALCOHOL DIMETtIYL ACETAMIDE PROPYLENE GLYCOL (25% IN WATER' (25% CI•iAM ' (5qo IN 16 HOUI• OCCLUSION) [ 1• HOUI• OCCLUSION) 16 HOURS OCCLUSION) BEFOI• BEFORE 3 llOURS AFFER 3 HOL'• AFTER 0'9 1 •'9 0,3 0,3 0-2 • 0.1 •0, I INFLUENCE OF STEROID SOLVENTS ON !RANSEPIDERMAI, DIFFUSION Figure 5 Influence of 3 vehicles on transepidermal water diffusion. therefore have had approximately 5% of unhydrated DMA) caused doub- ling of TEWL in two and minor but upward changes in two more of five subjects. In seven experiments propylene glycol produced minor but constant depression of the water barrier in the same way. DISCUSSION If the planning of penetrant-vehicle combinations for topical therapy is ever to be put on a more rational basis than at present, it will require the characterisation of such combinations in terms of a profile consisting of a number of parameters expressing solubilities of penerrant and vehicle in each other and in stratum corneum, vehicle/corneum partition co- efficients, diffusion coefficient of penetrant within the vehicle, etc. Within this profile, occlusivity and chemical effects on the stratum corneum of vehicle and penetrant will be important.