PERMEABILITY OF EPIDERMIS 863 800 600 400 200 IO 20 3,0 TIME [MINUTES] Figure 7. Watcr dcsorption curves for previously immersed samples of stratum corncure i i I i i i i i , i i , i 900 700 500 500 I00 PERCENT WATER Figure 8. Water desorption rate rs. amount of "bound water" in tissue. These corrcspond to the same samples as in Fig. 7

864 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Wet and Dry Thicknesses of Stratum Corneum Membranes Period of Hydration (hr) Dry Relative Weight Dry Weight? Thickness b "Bound Water," W (mg) (t•) f Hydrated Thickness (•) 0.5 1.0 4.0 5.0 8.0 14.0 18.0 25.0 45.0 57.0 88.5 103.5 126.5 0.56 O. 49 1.04 0 46 0 50 1 O1 0 48 0 49 0 54 0 56 0 78 0 57 0 54 9.46 8.28 17.60 7 77 8 45 17 10 8 11 8 28 9 13 9 46 13 20 9 63 9 13 1.11 1.20 1.32 2 69 2 73 2 39 3 34 4 34 4 33 5 13 5 22 5 30 5 62 13.2 12.1 27.3 20.1 22.1 40.2 18.8 31 6 34 8 41 7 59 1 43 8 43 7 • Samples of stratum corneum (A = 0.78 cm •) from various parts of the body were used. These are not of uniform thickness or lipid composition both these factors affect the kinetics of hydration. The relative weight, f, is the ratio of the weight of sorbed water to the dry weight of the tissue. b Dry thicknesses were measured by the "infrared cell-spacer technique," which measures the average distance between top and bottom surfaces. Since the tissue is very convoluted at the ultramicroscopic level, the measured values are too large by approximately 50%. Correct values for the dry thicknesses can be obtained from measurements of dry weight, sample area, and tissue density. seems essential to the structural integrity of the protein-lipid matrix which then can stabilize large amounts of water. DMSO and DMFA Dimethyl sul[oxide (DMSO) and dimethylformamide (DMFA) form strong hydrogen bonds with water and compete effectively for hydro- gen donor molecules. They are effective vehicles for increasing skin penetration when used in solvent concentrations greater than 80%. It is not unreasonable to suppose that these solvents are able to increase the permeability of stratum corneum because of a capacity to displace the "bound water" from the tissue and substitute for it a looser struc- ture. The pure solvents are in fact less associated than water and forces between these solvent molecules are smaller (12). Figure 9 shows the increased permeability of H20 (HTO) when applied from essentially pure DMSO. The curves are linear, at least over the first 6 hours, and a steady state diffusion is attained. This indi-