PROTECTIVE BARRIERS FOR THE SKIN By Gsov, cF• E. Os}•o•, P•.D. and Ro•r j. GERP.^tYGHTV, Pu.D.* Presented November 3, 2/960, Seminar New England Chapter, Storrs, Conn. EvEP.¾ LIV•[•G OP-GA•ISM maintains its identity by means of a covering which serves the dual purpose of enclosing the organism and its functions within its structure and of separating the organism from its environment. In unicellular life, the covering is a rather simple cell wall in higher plants and animals, it becomes a very complex and highly special organ, itself composed of several subordinate parts that function triply at this level of biological evolution to protect the organism, to eliminate waste products, and in many animals to regulate the temperature of the body. As a protective membrane, the skin ought not to permit substances to pass through it to the underlying tissues the healthy, intact skin should prevent the absorption of the many natural elements of the environment, such as moisture, gases, fumes, dirt, and germs, as well as the synthetic products more recently introduced into everyday human activity: chemi- cals, detergents, pharmaceuticals and commercially useful adaptations of natural products. At the same time, it must be recognized that the skin is not a totally exclusive barrier it is not whole, rather, it is holey both excretory and secretory products pass through it (but from the inside out and not vice versa) (1). To facilitate the performance of the skin as a protective and as a tem- perature regulating organ, man has, since before the beginning of the historical era, made use of extraneous outer clothing, at first, animal skins, and later, leathers and textiles. These heavy, loose, extra layers of pro- tection encumber the operation of the anatomical members, particularly the small-muscle functioning of the hands and, while there have always been available cosmetic products with protective properties, there has developed in the last quarter century a concentrated search for agents that can be incorporated into preparations that will enhance the barrier function of the skin and that can be applied directly to it. It is the purpose of this paper to discuss in general terms these barrier agents and their formulation into cosmetic products. * Dept. of Pharmacy, University of Rhode Island, Kingston, R. I. 271

272 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Since the term barrier has within its present connotation a concept of obstruction that can be taken in several ways, it seems important at the outset to attach certain descriptive terms to it, which should help to clarify the original use of the word and to define the degrees of effective- ness being dealt with here. A universal barrier, if thought of as a per- fectly exclusive obstruction, does not exist (certainly the skin is not one). Rather, all barriers presently available are imperfectly exclusive that is, no known agent will repel every attack made on it by every element of the environment each one is susceptible of penetration to some degree by some substance or other. Most of these agents are nonselective in their exclusiveness, permitting a wide variety of materials to penetrate their defense mechanisms. Others of them, however, are very selective in their ability to repel some things and not others for example, sun- screens will permit the transmission of light rays within only a limited spectral band. In the pharmaceutical and cosmetic industries the diver- sities of the nonselective agents are used to advantage in the formulation of specific products, designed to accomplish a specific kind of protection against specific attacking substances. In the composition of such a product, the protection may arise from several sources. To begin with, barrier agents are usually thought of as additives certainly the search for new agents is focused on chemical entities that can be added to or carried in vehicular materials. At the same time, a great amount of protection can be derived from the vehicles themselves. (Pharmacists have for generations spoken of the lack of penetration of "epidermatically acting" ointment formulations, e.g., medicihals incorporated into hydrocarbon bases.) The great body of vehicular necessities including emulsified cream bases, might be considered to comprise a group of "pharmaceutical barriers." The special agents dispersed in the vehicles then, might constitute a group of "chemical barriers." Both groups, it can be reasonably argued, effect their protec- tion by setting up "mechanical barriers," mere refinements of the clothing layers of the past. The most obvious mechanical barriers would be relatively heavy, stable layers of unctuous masses, virtually impenetrable by reason of their ability to fill the pores of the skin, their thickness, their density, and the physical- chemical nature of their components, to practically the entire environ- ment. Equally obvious are the cosmetic disadvantages of such protection. The same mechanism of action, in a highly reduced, much less visible form, is effected by the film formers, which group includes perhaps the majority of the nonspecific, protective barrier materials. In the case of ' these agents, the factors of ultimate importance are the physical-chemical properties, chiefly the ability to maintain high interfacial tensions between the film and the attacking elements. True chemical action resides in those