J. Soc. Cosmetic Chemists, 18, 149-160 (Mar. 4, 1967) Nature of Cosmetic Films on the Skin O.K. JACOBI, Ph.D.* A Contribution from the Kolmar Research Center, GMBH, Wiesbaden, Germany Synopsis--A synthetic copy of the natural •noisturizing factor present in human skin has been formulated on the basis of amino acid analysis. It is shown that this synthetic material has penetrating and moisturizing properties similar to those of the natural product. The porosity for water vapor of human sebum is attributed to its content of branched-chain com- pounds. It is shown in in vivo experiments that the addition of branched-chain aliphatic compounds enhances the penetration of water vapor through oily films applied to the skin. INTRODUCTION In the last two decades the question of moisture control of the human skin or, to be more specific, of the stratum corneum of the epidermis has been studied extensively by a number of researchers (1-6). A major result of this work was the finding that a combination of chemical compounds in the stratum corneum of the epidermis is responsi- ble for the moisture control of this layer of the skin. This combination of compounds is generally known as "natural moisturizing factor" (NMF). The mechanism by which the NMF is formed by the skin is still unknown. Under normal conditions the skin produces enough NMF to insure the right amount of moisture for the stratum corneum. There are, however, conditions in which not enough or an incomplete NMF combination is produced by the skin, e.g., in psoriasis (7). Ex- tensive washing of the skin or even long contact with plain water leaches * Hauptstrasse 20, 62 Wiesbaden-Igstadt, Germany. 149

150 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the NMF from the stratum corneum and leaves the skin in a dry, scaly condition (8). One of the tasks of cosmetic preparations is to keep the skin in a nor- mal water-balance condition. Moistuddzers have been tried for this purpose, but, as has been shown by several authors, the hygroscopic mateddais used in cosmetics (like glycerine, glycol, or certain sugar alcohols) are not able to fulfill this task (9-12). On the other hand, attempts have been made, based on the analysis of NMF published in the literature, to reproduce parts of the NMF. Apparently these attempts have not been satisfactory because it has been recommended repeatedly to use occlusive films on the skin to keep the moisture in the skin and to regulate the moisture requirements of the epidermis in this way (13-15). The skin exhibits the physiological phenomenon of the so-called insensible perspiration which is the invisible water vapor loss from the skin into the environment. In addition, a gas exchange through the skin takes place. Carbon dioxide is constantly rdeased through the skin, and to a certain extent oxygen is absorbed. The oxygen uptake is part of the skin cell's metabolism. These combined functions, namely insensible perspiration, the carbon dioxide output, and the oxygen uptake are called "skin-breathing." These functions will automatically be suppressed by the application of an ocdusive film on the skin surface. Therefore, this procedure of regulating the moisture content of the epidermis inter- feres with normal physiological functions of the skin, which certainly should not be the function of cosmetic treatments. In this connection, the question whether the sebum, which is the natural protective wax film on the skin surface, is an occlusive film or not is of utmost interest. As will be shown below, the sebum film is not ocdusive but, on the contrary, is porous. Similar results have been found in the case of certain surface waxes and fats of animals. Con- sequently, it seems that the function of sebum is not prevention of moisture loss. As a consequence, it seems that moisturizing of the epidermis has to be done differently, possibly in the way the skin does it. At the same time it appears necessary to formulate cosmetic prepara- tions which leave porous films on the skin surface. The porosity of these films should be similar to that of human sebum. This research group, which has worked on the NMF problem for many years, has repeated the analysis of the NMF of human skin. Based on this latest analysis, an NMF has successfully been synthesized. The efficacy of this synthetic NMF has been studied with respect to the moisture regulation of the skin. In cooperative research by Weitzel