j. Soc. Cosmet. Chem., 29,283-306 (May 1978) Noninvasive, rapid characterization of human skin chemistry in situ R. E. BAIER Calspan Corporation, Buffalo, NY 14221. Received May 26, 1977. Presented at Annual Meeting, Society of Cosmetic Chemists, December 1976, New York, New York. Synopsis The experimental yield of internal reflection spectroscopic and contact angle techniques applied to LIVING HUMAN SKIN is demonstrated for natural, cosmetically treated and wounded epidermis. The critical SUR- FACE tension and chemical composition of clean human skin surfaces is provided, along with spectral data bearing on the efficacy and quality of cosmetics, the depth profile of skin moisture, and the CHEMICAL NATURE ofexudates from epidermal wounds. INTRODUCTION Professional groups, individual research chemists, consumer safety advocates and legis- lative bodies are increasingly calling attention to the potential hazards, and unverified claims for quality and efficacy, of cosmetic and therapeutic products applied to skin surfaces. Despite the pressures generated, even modern industrial research labora- tories assessing the influence of various reagents on human skin continue to assume the end effects of frequent skin contact with certain chemical types without proof of the validity of such assumptions. For example, surfactant-containing liquids are assumed to damage human skin by excessive removal of skin lipids, deposition of the surface-active agents themselves (upon or immediately subjacent to the skin surface) and denatura- tion of the proteinaceous structures in the epidermal layers. These actions are sup- posed to lead to the abnormal features of human skin encompassed by clinical symptoms of roughness, scaliness, unsightly wrinkling, irritation and dermatitis. Since many of the functional and aesthetic qualities of human skin are attributed to the skin moisture balance, a continuing goal of most cosmetic "moisturizing" preparations is to limit or prevent transepidermal, moisture losses. Here, also, the in situ functions of the main ingredients of such preparations have not been convincingly demonstrated--merely assumed--to play a role in moisture retention, rather than simply lubricating and/or plasticizing the skin through ester imbibation. It is no longer necessary to accept without proof such assumptions in the development and certifica- tion of benefits for cosmetic or skin-healing products. Direct, noninvasive, relatively 283

284 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS inexpensive and widely applicable methods for assessing the fate and consequences of materials applied to human skin in situ are now available. Some of the situations prompting wider application of these methods are the following. Concerns have been expressed by the American Medical Association that traditional vegetable and animal cosmetic products are safe but inefficient, and new products derived from synthetic compounds are capable of inducing dermatitis more readily (!). These concerns can be addressed by direct experimental techniques. Hazel Bishop has cautioned that, although necessarily safeguarding their existence by "the bread and butter techniques available to them," manufacturers of cosmetics who wish to share in the continued growth of the cosmetic industry must perform additional serious scientific research "which holds the key to the prestige--and the profits--which go with the dramatic new scientific find" (2). The type of research introduced here fits one of Bishop's categories "more likely to pay-off": creation of a chemical product to which one can validly apply exciting, yet unexploited claims, attracting potential customers. Even if the motives of professional concern and enhancement of profit were not operating, the mounting pressures from consumer research organizations, supplying well documented arguments to receptive committees of federal legislators, force the increased attention of cosmetic and skin treatment producers to unambiguous proofs of their product safety and efficacy. In particular, the generally held belief that cosmetics stay on the skin, reflecting the im- penetrable nature of the stratum corneum surface, has been convincingly challenged by the recitation of research results showing very high systemic absorption of topically ap- plied chemicals the protection of cosmetic preparations from the scientific and regula- tory scrutiny that many other products have been subjected to will no longer be tolerated (3). Citing the routine exposure to skin-damaging strong soaps and de- tergents, one recent legislative brief also challenged the new generation of synthetic plastic resins (in skin-stripping agents and cosmetic "beauty masks") as further de- teriorating already permeable skin (3), suggesting these products be banned by the Food & Drug Administration. Techniques for judging epidermal retention of such ma- terials are described here. Natural polymeric extracts now incorporated in various cos- metic and wound-healing preparations, including proteoglycan preparations claimed to have both cosmetic and wound-healing properties (4), may also be more carefully studied in situ. The subject of epidermal wound healing has received more attention than has the subject of cosmetic improvement of human skin (5), but advances in the in situ evaluation of chemical influence on skin quality (6) and of measurement in vivo of the skin's moisture balance (7) suggest a growing willingness of the cosmetic industry to address the very real problems at hand. The main purpose of this report is to expand the use and acceptance of two additional in situ methods for noninvasive characterization of skin chemistry (before and after the application of cosmetic preparations, or in various states of damage) by demonstrating the actual experimental yield of these methods in both cosmetic testing and wound- healing contexts. METHODS AND MATERIALS All of the measurements reported here were carried out in a constant temperature-- constant humidity, clean room, free of dust and maintained at 2 IøC and 40% relative