244 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2b. TEM of bilayer structures of palmitic acid membrane at high magnification. lOO • 80 - 60 40 • 20 PA-SLM's filter alone 0 4 8 time (h) Figure 3. Water loss through PLM membranes as compared to untreated filter 12

ARTIFICIAL MEMBRANES 245 Table I Lipid Components of Different Synthetic Lipid Membranes Composition (weight percent) Cholesteryl Membrane type Cerebrosides Ceramides Cholesterol sulfate Fatty acid Palmitic (standard) -- 38 28 17 17 Stearic -- 38 28 17 17 Carnauba -- 38 28 17 17 Cerebroside/carnauba 5 40 25 18 12 Each membrane is referred to by its fatty acid component. All quantities are expressed as weight percent. to covalent linkages between corneocytes and their lipid envelopes. These lipid enve- lopes, in turn, are interdigitated with the lipid bilayers in the stratum corneum, giving rise to a remarkably cohesive unit. Indeed, the increase in partitioning of hydrophobic compounds into delipidized stratum corneum reported by Surber et al. (24) has been explained in terms of regions within the stratum corneum that are exposed upon delipidization and that were inaccessible to the drugs in intact stratum corneum. The implication of strong lipid-corneocyte interactions within the stratum corneum also 60 50 40 30 20 10 ß progesterone o estradiol o cortisol ß sucrose 0 1000 I 2000 time, minutes Figure 4. Typical permeation profiles of various markers across artificial model membranes.