I. Soc. Cosmet. Chem., 24, 197-212 (March 2, 1973) Water and the Skin BERNARD IDSON, Ph.D.* Presented in part April 5, 1972, be[ore the New York Chapter, Cli[ton, N.J. Synopsis-Problems of dry SKIN are those of inadequate hydration of the stratum comeurn. WATER is continually diffusing outward from sweat glands and transepidermal diffusion as long as the relative hmnidity remains-less than 100%. HYDRATION dra- matically increases the permeability of the skin. Yet, even the hydrated stratum comeurn is an effective BARRIER against water loss. Diseased skin is more permeable to water loss. The whole stratum corncure is probably the principal skin barrier to water. Agents that increase the permeability of the skin, such as DMSO or surfactants, damage or alter the nature of the stratum corneum reducing the diffusional resistance. Diffusion through the stratum corncure is passive, influenced chiefly by the water vapor pressure gradient. Much of the water-binding capacity of the corncure is due to the presence of hygroscopic water-soluble substances, which appear to be protected by lipid material which must be removed with solvents before they can be extracted by water. The osmotic properties of the horny layer suggest there is a semipermeable membrance system which prevents the water-soluble substances from being washed out when the comeurn is immersed in water but does not prevent the hygroscopic substances from holding water in humid atmospheres. The membrance system may be the CORNEUM CELL WALL. INTRODUCTION Water plays a unique role in the dry skin condition. The lack of adequate water in the upper layer of the skin, the stratum comeurn, results in dry and chapped skin (1-3). A level of between 10 and 20% water is needed to main- tain normal softness and pliability of the skin (1, 2). The normal supple ap- pearance of skin is due in large part to the capacity of the horny layer to bind water. The skin presents a formidable "barrier" to water loss and this is prob- ably due to the whole stratum corneum. When the relative humidity is low, the amount of water diffusing into the stratum comeurn from the underlying tissues is insufficient to replace the water lost to the atmosphere and chapped skin results (4, 5). The water de- ficiency in the cornified epithelium can only be remedied by restoring the normal water content (2, 6). Fats and other oily materials do not aid this dry •Hoffrnann-La Roche Inc., Nutley, N.J. 07110. 197

198 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS skin condition unless they are sufficiently occlusive to reduce the water loss into the air and permit the buildup of moisture in the skin (6, 7). The prob- lems of dry skin are those of adequate transport of xvater into the horny layer and prevention of gross water loss. Dry stratum corncure presents a high resistance to penetration of water and xvater-soluble substances. Hydration dramatically increases the permeability of the skin ( 1, 2, 6-42), as well as affecting its mechanical properties ( 1, 2, 13, 43-48). This review is concerned with the nature of the hydration of the skin, water binding, and the "barrier" zone. Hx•mt•r•o• The state of hydration of the stratum comeurn is dependent on the rate at which water reaches the comeurn from the layers beneath it, the rate at which water leaves the skin surface by evaporation, and the ability of the cor- ncure to hold moisture (49). The rate at which water reaches the skin surface is governed by the supply of water from the eccrine sxveat glands and moisture obtained by transepi- dermal transport. Secretion from sweat glands consists mainly of water and electrolytes, and would therefore be expected to influence the water content of the stratum corncure by inducing skin hydration. It is estimated that a total transepidermal water loss of about 85-170 ml/day occurs from an average adult under average conditions. In such conditions, at rest and without visible sxveating, sweat secretion amounts to about 300-500 ml/day, a substantial proportion of which is from the palms and soles. Transepidermal or upward diffusion of water yields smaller amounts of xvater since it is limited by the loxv water vapor permeability of the stratum comeurn itself (50). Hydration of the skin varies with changes in the relative humidity or moisture saturation of the air and not the absolute amount of moisture in the atmosphere (5, 13). The degree of hydration is never constant as the variable environmental rela- tive humidity alters the amount of xvater reaching the surface of the skin. Hence, water is constantly diffusing from the lower cells of the epidermis. As long as the ambient relative hmnidity remains less than 100%, water is con- tinuously transferred outward. Under normal conditions, therefore, a gradient in xvater concentration exists xvithin the stratum corncure. The activation energy for the diffusion of water through fully hydrated stratum corneum is 13-16 kcal/mole (14, 20). Through dry tissue it is most likely larger since hydration sxvells and softens the tissue. Hydration increases the thickness of the corncure several fold, as it changes h'om a tissue which normally contains 5-15% •vater to one which may contain as much as 50% water (10). Upon immersion in water, swelling of the cor- ncure starts in a few minutes and continues up to 3 days. The tissue can ab- sorb up to 5-6 times its weight when fully hydrated and this •vater is strongly bmmd within the intracellular keratin (19, 5l, 52).