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INTRODUCTION
Cosmetics and skin care products are assuming an increasingly important role within dermatology as the sophistication of raw materials and the resulting formulations provide for the development of products that impact the functioning of the skin. It was traditionally thought that the stratum corneum was a non-living, biologically inert layer of the skin upon which skin care products and cosmetics impinge. It was also thought that the stratum corneum was a complete barrier unaffected by the application of non-prescription topical agents. An improved understanding of stratum corneum function, primarily obtained through the use of non-invasive bioengineering techniques, has demonstrated that the stratum corneum can be dramatically influenced by non-prescription topical agents, and that these agents can indeed penetrate the stratum corneum and influence skin function.
Recognition of the biologic importance of the stratum corneum to skin health led to the development of a new category of skin care product, referred to as the cosmeceutical. A cosmeceutical is a scientifically designed, useful product intended for external application to the human body that has desirable aesthetic effects and meets rigid chemical, physical, and medical standards. Even though many cosmeceuticals currently exist for use on the skin, hair and nails, there is no regulatory description that acknowledges the current scientific sophistication of these formulations. Rather, a cosmetic is defined by the US Federal Food, Drug and Cosmetic Act, written in 1938, as โarticles intended to be rubbed, poured, sprinkled, or sprayed on, introduced into, or otherwise applied to the human body or any part thereof for cleansing, beautifying, promoting attractiveness, or altering the appearanceโ, without affecting structure or function.
Cosmeceuticals are now a reality due to the ever-expanding dermatologic knowledge base, which has rendered obsolete the prior outdated clear distinction between cosmetics and drugs. It is somewhat bewildering to realize that water is a cosmeceutical given that water profoundly influences the structure and function of the viable epidermis. Many substances can function as cosmeceuticals if entry into and through the stratum corneum is facilitated via the use of penetration enhancers, such as propylene glycol, isopropyl myristate, or pyrrolidone derivatives. Propylene glycol profoundly alters the barrier characteristics of the stratum corneum, while isopropyl myristate is capable of penetrating into the lipid bilayer of cell membranes. Pyrrolidone derivatives interact with both keratin and stratum corneum lipids to drive substances into the skin. These enhancers are commonly found in cosmetic formulations; however, improved understanding of skin physiology allowed recognition of their profound effects on the skin, which were previously unknown.