]. Soc. Cosmet. Chem., 25, 253-262 (May 1974) Skin Moisturizers. II. The Effects of Cosmetic Ingredients on Human Stratum Corneum MARTIN M. RIEGER, Ph.D., and DONALD E. DEEM, M.S.* Synopsis-The four parameters described in Part I of this study, i.e., elastic modulus, re- laxation function, water absorption, and water vapor transmission, have been used to study the effect of typical COSMETIC INGREDIENTS on human STRATUM CORNEUM. The elastic modulus and the stress relaxation modulus are useful measures of the ability of various cosmetic materials to alter the VISCOELASTIC BEHAVIOR of stratum corneum. It has been demonstrated that typical cosmetic HUMECTANTS increase the rate of transepidermal water loss in vitro, and an attempt is made to explain this phenom- enon. INTRODUCTION A variety of humectants and of occlusive lipids has been used for many years to improve human skin and to protect it against damage, even though the principles underlying these approaches to skin treatment were not clearly understood. Blank showed in 1952 how important water is to the well-being of human skin and, in effect, established a rational basis for these time honored methods for skin conditioning ( 1 ). In his very thoughtful review, Chudzikow- ski comments on the undesirability of highly occlusive barriers because they have a tendency to lead to edema he a]so rejects the traditiona] treatment of dry skin with humectants because they will absorb moisture not only from the air but also from the skin (2). It is apparent, therefore, that the cosmetic benefits of either treatment are debatable and that basic knowledge on how skin moisturizers work is lacking. In the first part of this study, in vitro methods for determining the response of human stratum corneum to moisture were developed. It is the purpose of this second part to describe the influence of common cosmetic moisturizers on the properties of stratum comeurn. *Warner-Lambert Research Institute, 170 Tabor Rd., Morris Plains, N.J. 07950. 253

9.54 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ExrEmMENT•m The techniques are those described in Part I (3) with minor modifications if required. Generally, strips of stratum corneum (with the attached tabs) were immersed for one hour into the test material or solution. Thereafter the strip was floated onto a stainless steel wire mesh, drained, and carefully blot- ted. The strip was then air-dried and placed into the appropriate relative hum'•dity chamber for at least 24 hours before testing. Sometimes, the test substance was applied to the stratum eorneum with a cotton swab. Any ex- cess was carefully blotted off before the stratum eorneum was conditioned. For the determination of water sorption by humectants, 50% aqueous solu- tions of the humeetants were applied to small preweighed pieces of Gelman Type A glass filter paper. After conditioning at 0% RH, the weight of humee- tant was determined and the weight gain at a given RH was based on this "dry" weight. The glass filter paper was shown to absorb less than 1% of water at 90% RH, and this sorption was generally neglected. RESULTS AND DISCUSSION Mechanical Properties Many cosmetic materials are reported to soften skin and to act as emol- lients. It was expected, therefore, that these materials would exert some in- fluence on the mechanical properties of stratum corneum. Of particular inter- est is glycerol which is widely used by cosmetic formulatots as a humectant and skin moisturizer. It is generally recognized (2) that glycerol is hygroscop- ic and will absorb water until it reaches equilibrium with the ambient RH. Glycerol has been employed for many years to "heal" chapped skin, and this effect is attributed to its ability to hold water in contact with the skin. Values of the modulus AxE obtained at 22øC on strips of stratum cor- neum immersed in mixtures of glycerol and water are summarized in the curve shown in Fig. 1. The experimentally obtained values of AxE are nor- realized by dividing them by AxE of the same strip of stratum comeurn immersed in distilled water to yield the plotted value of E,. The results indi- cate that glycerol does not increase the elasticity of stratum corneum by itself but actually stiffens the stratum corneum at concentrations above about 30%, possibly by removing stratum corneum-hydrating water. These results are in accord with Blank's findings that water is the only plasticizer of horny material (4). It was considered possible that the interaction of the stratum corneum/ glycerol/water system under investigation was temperature sensitive. In order to study this aspect, the effect of temperature on the value of AxE was first determined by extending strips of stratum corneum, taken from the same specimen, under water maintained at various temperatures between 2 ø and