I. Soc. Cosmet. Chem., 25, 239-252 (May 1974) Skin Moisturizers. I. Methods for Measuring Water Regain, Mechanical Properties, and Transepidermal Moisture Loss of Stratum Corneum MARTIN M. RIEGER, Ph.D., and DONALD E. DEEM, M.S.* Synopsis-Four parameters for assessing the interaction between human STRATUM COR- NEUM and WATER are described. Two methodologies, ELASTIC MODULUS and STRESS RELAXATION, for determining the MECHANICAL PROPERTIES of stratum corneum have been utilized. It is shown that both of these parameters depend on the moisture content of the stratum corneum, i.e., the ambient relative humidity. The rate of WATER VAPOR ABSORPTION by human stratum corncure, the third parameter exam- ined, is a function of the ambient relative htm•idity. Surprisingly, the equilibrium moisture content of stratum corneum at humidities below approximately 80% appears to be essen- tially the same for unextracted stratum corncure and for stratum corncure extracted with lipid solvents. The fourth parameter, the rate of WATER VAPOR TRANSMISSION through stratum corncure in vitro, is a linear ftmction of the ambient relative humidity and has been shown to be markedly affected by changes in temperature. INTHODUCTION Since Blank's observation in 1952 (1), that the water content of skin is re- sponsible for its softness, the properties of epidermis as a function of its mois- ture content and of the presence of "natural" and extraneous moisturizers have been studied extensively by cosmetic chemists and dermatologists. The subject of transepidermal moisture loss in vivo under various ambient conditions of humidity, temperature, and air flow has been studied by several investigators. Of particular interest is a paper by Grice et al., who indicated that the relationship between transepidermal water loss and ambient relative humidity in vivo is not linear (2). This is not in accord with the data by Wild- nauer et al., who suggested that the relationship between mechanical proper- ties and relative humidity is linear (3). *Warner-Lambert Research Institute, 170 Tabor Rd., Morris Plains, N.J. 07950. 239

240 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The relationship between the mechanical properties of stratum corneum and its moisture content has been studied by several techniques (4-6). Much of this work has been conducted on stratum corneum obtained from guinea pig foot pads or human callus. It was pointed out by Kligman that the stratum corneum of guinea pigs allows water to pass three times as fast as human abdominal stratum corneum (7). He also states that "the specialization of the horny layer of the palms and soles is so unique as to require separate status." These tissues contain lower quantities of the water-soluble substances present in abdominal skin and are more permeable to water data from callus will not "accurately apply to the membranous horny layer." A considerable lzody of work on the mechanical properties of human skin in vivo is not perti- nent because it deals primarily with the mechanical properties of the dermis which presumably is not touched by normal cosmetic treatments. Water vapor transmission experiments in vivo are made difficult by sweating unless this is repressed by vasoconstricting drugs. As a result, well-controlled experi- mentation in vivo is very difficult. Substrate variability and the noted complications of in vivo studies make it attractive to study mechanical and water-holding properties of human stratum comeum in vitro. This study has been designed to study and to compare tech- niques suitable for measuring the interaction of stratum corneum with water. Part II of this work deals with the effect of a variety of typical cosmetic in- gredients in order to establish a rationale for their use in skin treatment prep- arations. THEORETICAL TREATMENTS AND EXPERIMENTAL PROCEDURES Mechanical Properties Elastic Modulus When a viscoelastic strip of material of length 1 and cross sectional area A is subjected to a normal tensile force ()t), the stress (or) is -•, and the strain (e) is the fractional increase in length, A1/h Below the yield point, Young's modulus of elasticity (E) is the ratio of or/e, i.e., the slope of the load-elonga- tion curve. Since A of the stratum corneum is not constant and is very diffi- cuIt to measure at all points, it was decided to use the parameter AxE (eq 1) and assume that strips taken from the same specimen of stratum corneum have approximately the same cross-sectional area. It was, therefore, necessary to determine AxE of an untreated control for each specimen of stratum cor- neum in order to compare the effect of a given treatment. A x E --fl A1 (1}