j. Soc. Cosmet. Chem., 44, 197-209 (July/August 1993) Near-infrared spectroscopy: A new approach to the characterization of dry skin J. de RIGAL, M. J. LOSCH, R. BAZIN, C. CAMUS, C. STURELLE, V. DESCAMPS, and J. L. LEVP•QUE, Laboratoires de Recherche Fondamentale de l'Orgal, 1 avenue Eugene Schueller, 93600 Aulnay sous Bois, France (J. d. R., M.J. L., J. L. L. ), and Laboratoires de Recherche Appliquge de l'Orgal, 188 rue Paul Hochart, 94550 Chevilly Larue, France (R. B., C. C., C. S., V.D.). Received November 30, 1992. Presented at the 17th IFSCC Congress, Yokohama, October 13-16, 1992. Synopsis A few years ago, Professor A.M. Kligman posed the question "Is dry skin dry?" Since that time, our knowledge of dry skin has improved considerably, but it is still not clear whether or not the water content of dry skin is lower than that of normal skin. This is, however, a fundamental question that involves one of the most important problems in cosmetics. In the near infrared, water molecules show two clear absorption bands at 1450 and 1936 nm. The amplitudes are sufficiently high for them to be easily identifiable in the spectrum of the human skin in vivo. We have modified a commercial apparatus (Infra-Alyser 500 Bran and Luebbe) in order to obtain a system suited to spectroscopic analyses of all skin sites in vivo. Special optic fibers with a very low coefficient of attenuation and an integration sphere have been added to the original apparatus without modifying its capacities of function. In a first set of in vitro experiments, we examined the penetration of IR waves into the various skin layers. The results showed the role of the stratum corneum (SC) in the amplitude of the two water peaks and that the amplitude is proportional to the water content. The dry skin type known as winter xerosis is characterized by a marked alteration of the appearance and surface state of the SC. This led us to describe the clinical condition in terms of the following criteria: inflammation, roughness (smooth/rough), and presence of flakes and scales. We attributed a score of 0-4, and an overall score was then calculated. The water content of the skin was measured in the same area (external surface of the leg) in a number of subjects, using two biophysical methods--near-infrared spec- troscopy and an electrical conductance method that provides only a very indirect measurement of water content. Correlations between the biophysical measurements and the clinical scores showed clearly that the skin judged as being the dryest gave the lowest peak amplitudes in the near infrared. This correlation was much better than that obtained using the clinical scores and the conductance method, particularly for very dry skin. A more thorough analysis of the correlations between the clinical scores and the NIR parameters provides further information on this phenomenon. From the previous population, three groups of 19 subjects with very dry skin were further studied. The subjects in each group applied a moisturizing preparation to one leg, the other leg serving as a control. The 197

198 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS state of the skin was tested after four weeks of daily application by means of the near-infrared and conductance methods as well as by using the clinical scoring system. The results indicate that the water content of the skin and its conductance changed in the direction opposite to the macroscopic aspect. Near infrared spectroscopy emerged as the most sensitive method for evaluating the efficacy of the preparations. We present the overall results in terms of the clinical description of dry skin, together with the actual efficacy of the moisturizing preparation relative to the expert evaluation. INTRODUCTION Despite thorough descriptions of dry skin, its causes are still poorly understood. Several biophysical methods are now available to characterize the state of the skin in vivo (1), but although progress has been made in our understanding of it functional properties (2), little is known of its precise nature, particularly with regard to lipid content, the reported variations of which are only minor (3,4). The fundamental question is whether or not dry skin is dehydrated. The lack of informative data is due to the fact that the numerous noninvasive methods used to described skin condition and to determine the efficacy of cosmetic preparations are based on measurements of physical parameters that are themselves dependent on the presence of water, lipids, and other components. In addition, the variation of the physical parameters studied is rarely directly related to differences in water content. In other words, these methods can provide adequate answers to the question "How much?" but not to the question "Why?" Although water is the main determinant of the overall physical properties of the stratum corneum, other components can play an important role. This is particularly the case of lipids, which modify the relationship between light diffusion, reflection, and transmis- sion, and which also influence the surface properties of the skin in addition, they can give a smooth or rough feeling according to their nature. Cosmetics are complex compositions of numerous ingredients, but all are composed of water and oil. Most cosmetic products improve the smoothness of the skin and its overall condition, but we are not yet in a position to describe how they do so. Infrared spectroscopy (5) can, in principle, provide valuable information on the water content of the skin. However, the infrared spectrum of water is wide and poorly defined. The amide I and II bands, the amplitudes of which are modified by the presence of water, are only indirect markers. In practice, measurements are made via a prism within which the rays undergo total reflection and show only poor penetration (less than one micron) into the tissue studied. Furthermore, the measurements take a considerable time, and the occlusive effect of the prism on the skin influences the water content. The near-infrared spectrum (1100-2500 nm) appears to be more practical, since the water bands are precise. Two types of apparatus--an infrared sensor and a data analy- ser-are required in addition, the radiation re-emitted by the skin cannot be deter- mined precisely because of the nature of the applicator (6). To overcome these difficulties, we have totally modified the optical elements of an integrating sphere of a near-infrared spectrophotometer (Infra-Alyser 500) in such a way as to record spectra at any skin site in a routine manner. We have conducted a study of the true nature of winter xerosis and the efficacy of various treatments, comparing the results with those provided by a simple and widely used electrical conductance method. In addition, the skin was characterized by trained experts in terms of several clinical