CRACKING OF HUMAN HAIR CUTICLES 153 (8) H. Ito, T. Miyamoto, and H. Inagaki, States of water sorbed on wool as studied by differential scanning calorimetry, Text. Res. J., 57, 66-72 (1987). (9) A.R. Haly and J. W. Snaith, Specific heat studies of various wool-water systems, Biopolymers, 6, 1355-1377 (1968). (10) M. Gamez-Garcia, Morphological Changes in Human Hair Cuticles upon the Simultaneous Action of Cyclical Mechanical and Thermal Stresses: Their Relevance to Grooming Practices, presented at the Annual Conference of the Society of Cosmetic Chemists, New York, December 1997. (11) M. Gamez-Garcia, Cuticle de-cementation and cuticle buckling produced by circumferential com- pression stresses on the cuticular envelope of human hair (in preparation). (12) M. Gamez-Garcia, Plastic yielding and fracture of human hair cuticles by cyclical torsion stresses (in preparation). (13) Hydrolyzed wheat polysiloxane copolymer, a Croda product with the name of Crodasone W. (14) E. I. Valko and G. Barnett, A study of the swelling of hair in mixed aqueous solvents,J. Soc. Cosmet. Chem., 3, 108-117 (1952). (15) L. A. Holt, Wool treatments from non-swelling solvents, Proc. 7th Int. Wool Text. Res. Conf Tokyo, Vol. IV, 1985. (16) J. H. Bradbury and J. D. Leeder, Sorption of liquids by wool. Part I. Determination of the sorbate content by a new technique, J. Appl. Polym. Sci., 7, 533 (1963). (17) W. A. Nash, Strength of Materials, Schaum's Outline Series (McGraw-Hill, New York, Chap. 3), pp. 40-53 also see F. Ziegler, Mechanics of Solids and Fluids (Springer-Verlag, New York, 1981), pp. 88-91. (18) C. B. Bucknail, "Deformation Mechanics in Glassy Polymers," in Toughened Polymers, C. B. Bucknail, Ed. (Applied Science Publishers Ltd, London, 1977), p. 136. (19) N.J. Abbott, S.C. Temin, and Chungi Park, The stress-strain behavior of wool in various swelling media, Text. Res. J., 38(10), 1026-1039 (1968). (20) J. D. Leeder, The cell membrane complex and its influence on the properties of the wool fibre, Wool Science Review 63 (Pub. Int. Wool Secretariat Dev. Center, October 1986). (21) P. Alexander and R. F. Hudson, in Wool, Its Chemistry and Physics, P. Alexander and R. F. Hudson, Eds. (Reinhold New York, 1954), pp. 218-219.

j. Cosmet. Sci., 49, 155-163 (May/June 1998) Urea analysis of extracts from stratum corneum and the role of urea-supplemented cosmetics D. HJ•NTSCHEL, G. SAUERMANN, H. STEINHART, U. HOPPE, and j. ENNEN, Paul Gerson Unna Skin Research Centre, Beiersdorf AG, UnnastraJ3e 48, 20245 Hamburg (D. H., G. S., U. H., J. E.), and Department of Food Chemistry and Biochemistry, University of Hamburg, Grindelalle 117, 20146 Hamburg (H. S.), Germany. Accepted for publication May 15, 1998. Synopsis This investigation was undertaken to quantify the amount of urea in extracts from stratum corneum of normal skin in comparison to extracts from skin after cleansing, or from skin after a prolonged topical application of urea-supplemented emulsions. We measured a dramatic decrease in the amount of extractable urea from stratum corneum after skin cleansing. This loss of urea can be partially compensated by a cleansing formula supplemented with urea. On the other hand, a skin care emulsion with urea supplementation increases significantly the amount of urea that can be solubilized from stratum corneum. From these results we conclude that the urea content of stratum corneum varies in a wide range, limited at the lower end by a reduced status that can be observed when skin cleansing had been performed, and at the higher end by an increased level that can be obtained after prolonged application of urea-containing emulsions. These findings might have important implications for therapeutic compensation of urea deficiency in pathological skin diseases and also for cosmetic compensation for a lack of water-retaining substances in dry skin. INTRODUCTION Urea is one of the most important soluble substances of the stratum corneum. In recent years this substance has become more and more important in dermatological therapy and cosmetics. Many diseases have been described that are characterized by a deficiency of urea, such as atopic dermatitis or clinical dry skin (1). The urea content of normal skin is nearly 1% (2). It contributes in a significant manner to the hydration of the stratum corneum. Besides amino acids, lactate, and other substances, urea contributes approxi- mately 3-7% to the natural moisturizing factor (NMF) (3). The NMF appears to be responsible for the hydradon status of stratum corneum. Otherwise urea is known for its keratolytic and pruritus-easing properties (4), and it is a very potent humectant in moisturizing creams (5,6). Its sources in the epidermis are sweat (7) and the decompo- sition of arginine by arginase during the process of keratinization (8). The high relevance of urea prompted us to look for a rapid method for determination of the urea content of stratum corneum. Here we describe a noninvasive method of deter- 155