MOISTURIZATION EVALUATION BY IMPEDANCE 427 MAIN FINDINGS WITH IMPEDANCE METHODS The principal results obtained by the method of impedance in skin investigations and in the study of the moisturizing effects of cosmetic products are to be found mainly in the papers by Clar, Serban, and Tagami. The most investigated points are the following: Effect of moisturizers (urea, PcNa, glycerol, Na-lactate .... ) (5,8,13). Barrier regeneration after strippings (5,13) Skin diseases and therapies (5,13). Concerning the first point, Clar and Tagami only reported effects within a short period of time Serban, however, reported hydration effects lasting 8 hours and more after the application of a product. Some remarks can be made about these findings: 1. It is not the most hydrophilic material which gives the most important effects. For example, skin impedance is decreased to a greater extent by glycerol than PcNa though the former is less hydrophilic. A similar result has been outlined by measuring the SC elasticity variation (17). 2. Urea gives a negative effect one hour after application but a positive one 20 hours later (8). By measuring in vivo the SC elasticity after application of basic emulsions containing these different moisturizers, we remarked that their efficiency lasted for a shorter period of time than that quoted by Serban (18). These two remarks raise the problem of the equivalency between decrease of impedance, increase of hydration, and increase of plasticity. Concerning the two other investigated points (strippings and skin diseases), the authors focus on a particular characteristic of the horny layer feature which appears a few days after stripping: it cannot bind water and, as in the case of the skin disorders considered by Tagami (eczema, psoriasis), the loss of water is high, whereas recorded impedance is weak. This result, also outlined by Jacques (16), is of interest however, in all cases mentioned (stripping, eczema, psoriasis) the SC is lightly disturbed and shows a cracked surface. Therefore, we can suppose that a defective contact between the electrode and the surface of the skin accounts for the strong impedance found in these cases. CONCLUSION Among these methods, the techniques by Clar and by Serban using low frequencies and the high frequency method by Tagami are the only ones described in the literature in sufficient detail to permit a scientific discussion. The first two involve the entire SC, but the procedure is time consuming. Tagami's method and those derived from it are more convenient to handle however, they have the disadvantage of testing only the surface of the SC. To summarize, we can say that in vivo impedance measurements are characterized by the following restrictions. They are only relative because all the technical and theoretical difficulties mentioned above have not yet been completely solved. To be reproducible and accurate they need a close monitoring of the people tested and of their environment. They can give information on the variations of the water content of the SC, but one has to be cautious in interpreting the results in terms of efficiency. From this point of view, comparative measurements (impedance measurements--

428 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS elasticity measurements) on the same subject treated with a given product would be very valuable. However, we have to point out that some of these techniques are relatively easy to carry out in vivo on man. They allow a screening of the moisturizing efficiency of some raw materials and the optimization of cosmetic preparations. Finally, because the quality and effectiveness of a cosmetic product results from the combination of very different substances essential to the quality of the formula, we may consider that impedance methods, which measure, above all, water, provide a tool for the optimization of an essential property rather than a universal means of investigation of the total performance of a product. REFERENCES (1) E.J. Clar, C. P. Her and C. G. Sturelle, Essai d'interpr•tation de la dispersion di•lectrique basse fr•quence de la peau humaine, Seminaire INSERM, Lyon, 7/8, Dec. 1972, ed. INSERM, 127-138 (1973). (2) E. M. Grant, Protides of the biological fluid, Proc. 19th Colloquium Bruges (Pergamon Press, Oxford, 1971), p 347. (3) E.J. Clar, C. P. Her and C. G. Sturelle, Skin impedance and moisturization,J. Soc. Cosmet. Chem., 26, 337-353 (1975). (4) R. Edelberg, Electrical properties of the skin, in Biophysical Properties of the Skin, ed. H. R. Elden (Wiley-Interscience, New York, 1971). (5) E. J. Clar, M. Cambrai and C. Sturelle, Study of skin horny layer hydration and restoration by impedance measurement, Cosmet. & Toilet., 94, 33-40 (1982). (6) M. Cam brai, E.J. Clar, E. Grosshans and C. Altermatt, Skin impedance and phoreographic index in psoriasis, Arch. Derm. Res., 264, 197-211 (1979). (7) P. A. Isherwood, Physiology of the stratum corneum. II. Measurement of water content, Symposium Dermatologicum Participatione Internationali BRNO, Tchecoslovaquie, 1964. (8) G. P. Serban, S. M. Henry, V. F. Cotty and A.D. Marcus, In vivo evaluation of skin lotions by electrical capacitance. II. Evaluation of moisturized skin using an improved dry electrode, J. Soc. Cosmet. Chem., 32,421-435 (1981). (9) G. P. Serban, S. M. Henry, V. F. Cotty and A.D. Marcus, In vivo evaluation of skin lotions by electrical capacitance. I. The effect of several lotions on the progression of damage and healing after repeated insult with sodium lauryl sulfate,J. Soc. Cosmet. Chem., 32,407-419'(1981). (10) S. D. Campbell, K. K. Kraning, E.G. Schibli and S. T. Momii, Hydration characteristics and electrical resistivity of stratum corneum using a non-invasive four-point electrode method,J. Invest. Dermatol., 69, 290-295 (1977). (11) R.J. Jackson, B. D. Ridge and G. C. Forward, The measurement of the moisture content of the skin by a novel impedance technique, Proceedings of the 11th International IFSCC Congress, Venice 1980, 667-672. A. Winkler and H. H. Wagener, Weltere Experimentallefunde mit dem Dermatometer nach Bingmer, Arch. Derre. Res., 254, 287-292 (1975). H. Tagami, O. Masatashi, K. Iwhtsuki, Y. Kanamaru, M. Yamoda and B. Ichijo, Evaluation of the skin surface hydration in vivo by electrical measurements,J. Invest. Dermatol., 75, 500-507 (1980). H. Fellner-Feldegg, The measurements of dielectrics in the time domain, J. Phys. Chem., 73,616-623 (1969). E. J. Clar and C. Sturelle, Etude des constituants du derme par spectroscopie temporelle, Parrums Cosmet. Aromes, 4, 55-60 (1975). S. L.Jacques, H. Maibach and C. Sunkind, Water content in stratum corneum measured by a focused probe: Normal and psoriatic, Bioengineering and the Skin, 3, 2, 118 (1981). L. Rasseneur, J. de Rigal, and J. L. Leveque, Simultaneous determination of the static and dynamic modulus of elasticity of stratum corneum, Bioengineering Newsletters, 4, 1, 52-64 (1982). J. de Rigal andJ. L. Leveque, In vivo measurement of the stratum corneum elasticity, 4th International Symposium on Bioengineering and the Skin, Besanqon, France, September 1984. French patent no. 7310935 (March 1973). (12) (13) (14) (15) (16) (17) (18) (19)