Cosmet. Sci., 49, 183-184 (May/June 1998) Book Review SKIN BARRIER, by Hans Schaefer and Thomas E. Redelmeier, 324 pages, Karger, Basel, 1996 (hard cover). This book is a comprehensive and up- to-date review of the field of percutaneous absorption, its relation to skin structure, and applications to product development and toxicology. The organization is logical and convenient. Drawing upon a wide range of authoritative sources, the current state of knowledge is summarized and, in some cases, interpreted. Researchers will find this volume a useful summary of the state of the art as well as a valuable key to the recent primary literature. The book is divided into six chapters, which begin with a description of the bio- logical underpinning of the skin's resis- tance to diffusion, go on to the relation between chemical structure and penetra- tion, penetration measurement and predic- tion, formulation and biological factors, and conclude with recommended measure- ment approaches. Each chapter begins with a brief introductory statement and ends with a summary of the main points. The references are collected in a single sec- tion at the end of the book. Many dia- grams and graphs illustrate the text. The first chapter, "Structure and Dy- namics of the Skin Barrier," reviews struc- ture and biochemistry, emphasizing the dynamic nature of the skin. The second chapter zeroes in on the makeup of the horny layer, examining in particular lipid composition and structure, as permeation is believed to occur chiefly via the inter- cellular lipid. The "brick-and-mortar" model is used as a basis for discussion. Various techniques used to obtain infor- mation on physical properties of the bar- rier membrane are described, along with the salient results. The next chapter describes the role of per- meant chemistry in percutaneous absorption and shows that there is a correlation with studies utilizing model membranes, such as liposomes. The evidence for the existence of functional pores in stratum corneum is presented. Some of the permeant variables discussed are octanol-water partition coef- ficients, ionization, and molecular weight. Methodology is the focus of the fourth chapter. The most relevant measurements, in vivo studies in humans, are discussed first. For various reasons it is often neces- sary to rely on in vivo data from animal studies or in vitro experiments utilizing ex- cised skin. Recommended protocols and the strengths and limitations of various techniques are discussed. In vitro penetra- tion experiments rely on the fact that the stratum corneum is the skin's primary bar- rier to penetration. They are the source of most of our current knowledge of the effect of vehicles and environmental variables on skin penetration. Next is a chapter on formulation and biological factors affecting percutaneous absorption. Practical matters affecting de- livery into the skin, such as evaporation and the in situ formation of supersaturated solutions, are included. The uses and drawbacks involved in penetration en- hancement by chemical agents are dis- 183

184 JOURNAL OF COSMETIC SCIENCE cussed. Certain types of diseased skin are more permeable than normal, intact skin. Other variables include hydration, tem- perature, contact time, and anatomic site. The final chapter teaches when and how to apply percutaneous absorption measurements to meet four major objec- tives: transdermal development, formula- tion of dermatologicals, safety assessment of cosmetics, and risk assessment following dermal exposure to hazardous environ- mental or occupational chemicals. Trans- dermal products target the circulation, and so standard techniques for monitoring bioavailability, i.e., blood level profiles, are utilized. In the case of dermatologic preparations, blood concentrations are usually below detection limits. In vitro comparisons utilizing excised human skin or a relevant animal substitute are useful in formulation but are not yet accepted as a bioavailability standard by regulatory agencies. Systemic toxicity following skin exposure is a function of the inherent tox- icity of the compound in question and its absorption efficiency. For recognized tox- ins, the biological implications are known the percutaneous absorption must be esti- mated or measured. This book is highly recommended for scientists working in the areas of develop- ment of topical products and toxicology following topical exposure. Those involved in regulatory affairs related to dermato- logicals and cosmetics may also benefit.- Joel L. Zatz--Rutgers University