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J. Cosmet. Sci.) 58, 283-294 (July/August 2007) Formation of nanostructure on hair surface: Its characteristic optical properties and application to hair care products SHUNSUKE WATANABE, HIRAYUKI SATO, SATOSHI SHIBUICHI, MASAYUKI OKAMOTO, SHIGETO INOUE, and NAOKI SATOH, Tokyo Research Laboratories, Kao Corporation, Sumida, Tokyo, Japan 131-8501 (S. W., S.S., N.S.), Tochigi Research Laboratories, Kao Corporation, Haga, Tochigi, Japan 321-3497 (H.S.), and Wakayama Research Laboratories, Kao Corporation, Minato, Wakayama, japan 640-8580 (M.O., S.I.). Synopsis Uneven structures on hair fiber surface, such as lift up of cuticle or build up of hair spray ingredients, generally cause a diffuse reflection which results in a dull and unhealthy appearance. However, in the case of finer structure than wavelength of visible light, the optical properties change significantly. An application of the phenomenon to hair care products is reported in this paper. Formation of the fine structure on hair surface was achieved by only a shampoo and rinse-off conditioner system including amino-silicone. Chroma enhancement of hair and light introduction into hair fibers were observed simultaneously with formation of the fine structure on the hair surface. The light introduction phenomenon is understood in terms of "Effective Medium Approximation" (EMA). The simulation study based on EMA indicates that a very low refractive index surface is expected to be realized, which well explains the optical experimental results. When the shampoo and conditioner system developed to form the structure on fiber surface was applied to dyed hair, enhancement and long-lasting of vivid appearance was confirmed in spite of dye elution. INTRODUCTION Hair optical factors have been analyzed to understand beautiful and healthy appearance of hair. Especially, shine, color and texture based on optical properties of hair fiber have been focused (1,2). Cuticle has relatively higher refractive index (n = 1.55, 3) as an organic material which may lead to both relatively intense surface reflection. For healthy shine, cuticle smoothness is one of the most important factors to reduce diffuse reflection on hair surface. Furthermore, in the case of lighter hair including less amount of melanin granule, reflection light with colors of fiber from inside also becomes more emphasized. Therefore, the optical transparency of hair fiber is added in important factors in order to obtain beautiful appearance. In general, it is known that chemical treatments, excessive dryer heat, or residue by 283