WATER AND THE SKIN 205 permeability is measured in terms of evaporative loss from an epidermal sur- face covering the aqueous phase. Most methods depend upon the use of two chambers, in one of which is placed a known quantity of the substance under study, separated from a second chamber by a piece of whole skin or separated epidermis. The amount of the substance recovered from the second chamber is measured (10, 60, 98-100). Barrier renewal is a complex process. After loss of the stratum eorneum, a temporary barrier is formed by rapid conversion of granular cells into para- keratotie cells. As the parakeratotie layer thickens water loss gradually creases. The temporary barrier persists until the regenerating epidermis be- comes capable of forming normally keratinized eelIs (gl,101). The reforma- tion of the barrier to increased sensible water loss is very rapid. Three days after the injury, the water vapor loss of the skin has been restored to such an extent that the difference between the water vapor loss of the damaged and normal skin has become very small. Total regeneration requires about two weeks (95, 101-103). There is limited knowledge of the composition of the barrier. However, it can be equated with that of the eorneum. The main cellular components are proteins, ]ipids, and water combined into an ordered structure. The eelIs are bound into a layered membrane which is ca. 15/• thick when dry but increases to ca. 48/• when fully hydrated (104). When hydrated, the stratum eorneum contains approximately 75% water, g0% protein, and 5% lipid. The sebaceous secretion, sebum, forms an irregular film on the surface of the skin, which has been estimated as averaging 0.4-4/• thick (73). Sebum does not contribute to any significant effect to the epidermal barrier. liemov- ing the sebum by mild swabbing with acetone or ether has no effect on the rate of water loss (105) and adding up to 10 times the normal amount (30 thick) has only a slight effect (106). The sebum apparently does not form a functionally complete lipid layer over the skin, leaving gaps which allow ac- cess to the underlying stratum eorneum (107). It is nonoeelusive and rela- tively porous due to the presence of branched-chain aliphatie compounds (87). However, the oil does help retain the water on the surface of the skin. Frequent washing with soaps and detergents, which may dissolve the oil, can cause loss of the water-holding capacity of the epidermis. While the sur- face lipids offer little resistance to the passage of compounds, studies of re- moval of lipids from the cutaneous surface (•, 7, 16, 69, 88, 109) indicate that lipids participate in epidermal water function. Onken and Moyer (7) sho•ved that barrier function is restored when extracted lipids are returned to the skin. This suggests some possibility of marked variations in biological membrane permeability, dependent largely on the specific nature or distribu- t/on of the lipid contained in the cell membrane. Swcency and Downing (88 applied a number of solvents to the epidermal side of hairless mice and hu- man skin. All of the solvents increased the rate of diffusion of tritiated water,

206 JOUBNAL OF THE SOCIETY OF COSMETIC CHEMISTS but there was no correlation between the type or amount of lipid extracted and the degree of alteration of the water barrier function. There are a number of agents that increase the permeability of the skin by reducing the epidermal barrier to water transpiration. All of these agents must have one common property, the ability to damage or alter the nature of the stratum eorneum in such a way that its diffusiona] resistance is reduced (6). For example, the activation energies for water through epidermal mem- branes, pretreated with a mixture of chloroform-methanol (2:1) to delipidize the membrane, range from 6.08-6.5 keal/mole (29). These values are much lower than the activation energy of approximately 15 keal/mole (20) for water penetrating normal epidermis and undoubtedly reflect substantial mem- brane damage. Solvents with both polar and lipophilie properties apparently have the feet not only of removing skin lipids but also of breaking down the complex structure of the barrier, thus making it defective and possibly subject to entry by other chemical agents which do not penetrate intact skin. Substances such as dimethylsulfoxide (DMSO), dimethylformamide (DMF), and dimethyl- aeetamide (DMA) all have the ability to produce a striking but reversible suppression ooe barrier resistance. They are all strongly hygroseopie and it is likely that the presence of these substances in the stratum eorneum increases the hydration of the tissue and therefore its permeability. DMSO has a per- meability constant of 300 /zem/min (110) as compared to a value of 45 /zero/rain for water through excised human skin (73). DMSO appears to alter the complex interrelationship of water, lipids, pro- tein, and mueopolysaeeharides regulating epidermal water barrier function (111). This may explain, in part, the sometimes observed variable role of DMSO. Swelling may induce the formation of channels within the matrix of the stratum eorneum which either favor the passage of varied compounds (112) or lower the diffusional resistance of the stratum eorneum. DMSO can extract soluble components from the stratum eorneum, suggesting ultrastruc- tural modifleations consistent with an increase in permeability. There are possible reversible eonflguration changes in skin protein structure brought about by substitution of integral water molecules by DMSO, with resultant swelling (113). A sharp increase was found in the flow rate of tritiated water through hairless mouse skin if the DMSO solutions used to pretreat the skin contained in excess of 70% DMSO. Low concentrations appear to produce little change in the skin barrier, whercas marked effects are observed in solu- tions containing 50% DMSO, or more (114). Polar, hydrogen-bonding solvents like water and DMSO are capable of re- acting with the bulk of the tissue (the protein) and not just the minor lipid component. When applied in large concentration, they become incorporated into the tissue and constitute a large percentage of the membrane substance. The membrane expansion is determined by the extent of their incorporation