J. Cosmet. Sci., 53, 89-100 (March/April 2002) Influence of internal structures of hair fiber on hair appearance. I. LiDht scatterinD from the porous structure of the medulla of human hair SHINOBU NAGASE, SATOSHI SHIBUICHI, KENICHI ANDO, EMIKO KARIYA, and NAOKI SATOH, Hair Care Research Laboratories, Kao Corporation, 1-3, B•/nka 2-chome, S•/mida-k•/, Tokyo 131-8501, Japan. Accepted for publication December 31, 2001. Based on a presentation at the 21st IFSCC Congress, Berlin, Germany, September 12, 2000. Synopsis In this study the influence of the medulla structure on hair appearance was examined. Hair with a porous medulla gave a whitish and lusterless appearance because of light scattering from the fiber center, whereas in the cases without pores, a clear and brilliant appearance was observed. The optical influences of the medulla pores were measured by a spectral goniophotometer, and obtained data were analyzed in terms of the CIE L*a*b* color system. Both contrasts in lightness and apparent color (chroma and hue) decreased in the hair with medulla pores, and the decreases in contrast caused a whitish and lusterless appearance. The distribution of the amount of medulla pores was investigated for Japanese females. The histogram was further analyzed by hair care behavior of individual panelists, and it was found that the pores in the medulla can be generated in a heat-drying process. INTRODUCTION Hair shine is one of the greatest concerns of consumers, and so many intensive studies have been carried out. Most of them are to elucidate the mechanism regarding the contrast of intensities, such as specular reflection versus diffusion scattering, in relation to the surface structure of the hair fiber. However, hair shine involves optical processes more than just the reflection from the outermost surfaces. Recently consumers have tended to color or lighten their original hair because they regard their hair as one of the vital means for expression of their style. Bleached or colored hair sometimes shows a whitish tone, giving a lusterless and dull impression, even when no cause can be found for this on the cuticle surface. Light penetrates deeply into such hair fibers. Thus the inner structure must play a more crucial role in hair shine. The authors found that hair shine is strongly affected by the medulla structure, and they report here new findings on the influence of the porous structure in the medulla upon hair shine. Furthermore, it was discovered that pore generation occurs in the medulla 89

90 JOURNAL OF COSMETIC SCIENCE through daily hair care processes. Medulla tissue has long been known as a porous structure located at the center of the hair fiber. Morphological studies have been carried out mainly from the viewpoint of forensic medicine (1,2), histology (3), and the mor- phological regulation related to the hair growth cycle by dermatological concerns (4). But chemical analysis of the medulla of human hair has not been fully investigated because of difficulties in analysis, such as the poor solubility of the medulla and difficulty in the isolation of the medulla. Rogers (5) studied the amino acid composition of the medulla of an African porcupine quill and reported a low cystine content in the medulla compared to that of the whole shaft. Mapping of the infrared absorption of the cross section of the human hair fiber was achieved by synchrotron radiation, showing that the lipid content is higher in the medulla than in the other parts (6). The morphological studies of the medulla have been carried out mainly from the viewpoint of medical jurisprudence (2,7). Generally, it has been recognized that the medulla is either com- pletely absent, continuous along the fiber axis, or discontinuous (3-4,8-10). However, any acquired transformation of the medulla structure by external stimuli has not been reported so far. The authors present here the results of analyses of the pore-generation process through heat. EXPERIMENTAL Observations of actual states of the medulla were carried out for Japanese (n = 120) women whose age variation was between 20 and 49 (average age of 30). The hair fibers used in this study were chemically untreated hair from Japanese women. The pore generation processes were confirmed with hair tresses whose medulla scarcely scatters light in the medulla's original state. Optical microscopic observations were carried out with a handy type microscope equipped with a pen-type flashlight (Peak Wide-stand-micro), a stereoscopic microscope (Nikon SMZ-10), and a digital microscope (KEYENCE VH-6100). The amount of medulla scattering light was evaluated with an optical microscopic view of the hair with a relatively smaller magnification, comparing these images with standard references. The angle between the incident light and the axis of sample hair fiber was usually fixed at 45 degrees, and the scattered light was observed from the vertical direction. This optical condition allows direct observation of scattering light, avoiding strong specular reflec- tion. The optical properties of a hair sample were measured by a spectral goniophotometer (Murakami Color Tech. Lab., GCMS-3). The incident angle was fixed at 45 degrees. The spectral reflectance of visible light (390-730 nm) was measured at each receiving angle from 0 to 80 degrees. Each spectral reflectance was converted to the lightness, chroma, and hue angle of the CIE L*a*b* color system under the condition of standard illumi- nation D65. A scanning electron microscope (SEM, Hitachi S-4000) was used to observe cross sections of the fibers. RESULTS AND DISCUSSION DIFFERENCE IN HAIR APPEARANCE ACCORDING TO MEDULLA STRUCTURE Figures la and lb show hair tresses evaluated by selected panelists as shiny and brilliant