DECREASE IN HAIR VOLUME WITH AGE 489 I Ellipticity of Diameter (a)- Japanese - I 12 ,--___!:============------, 10 ----------------------- - --------------------- 2 --------------------- 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 Ellipticity of Diameter 14 EllipticityofDiameter (b)- Japanese vs. Caucasian - I 12 ------------ --------------------- ■ Japanese ________ D Caucasian 2 0 L-L.......JL.......JIIJ.a.lJIUi.RliallJJJRJIUJ.JRJJal 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 Ellipticity of Diameter Figure 4. Hair cross section and fiber ellipticity. Left: Japanese females (age: 26-51, N = 52). Right: Japanese females (age: 26-51, N = 52) and Caucasian females (age: 20--40, N = 35). The ovality of diameter shows narrower distribution than that of diameter. The level of Japanese ovality of diameter is about 0.1 s larger. 0.19 _,,,,-..,. 0.17 0.15 co 0.13 00 00 0.11 0.09 co 0.07 •""'"'4 0.05 co 0.03 0.01 2 - -- --- - - - - -- -- - ---- - - - - - ----- - - ---------- --♦- -♦ ♦ Japanese D Caucasian 3 4 5 6 7 Cross Sectional Area ( µ m2 x 103) 8 Figure 5. Relationship between the cross-sectional area of hair and the bending stress from Japanese females (age: 26-39, N = 21) and Caucasian females (age: 20--40, N = 35). The bending stress decreases according to the reduction in the cross-sectional area. Both bending stresses fit the same line.

490 JOURNAL OF COSMETIC SCIENCE BENDING ELASTICITY The relation between the bending stress {M/(1/p)} and the cross-sectional area of the hair samples from relatively young panelists is shown in Figure 5. Figure 5 shows that all the Japanese and Caucasian hairs fit on a curved line according to equations 1 and 2. This result means that the bending stress is mostly determined by the size of the cross­ sectional area of the fibers, regardless of the ethnic origin. The bending Young's moduli calculated from the result shown in Figure 5 are plotted against the minor hair diameter in Figure 6. Different symbols are used for the different ethnic groups (age range: 20s and 30s). The symbol ( ♦) is used for Japanese panelists and the symbol (D) is used for Caucasian panelists. The Young's moduli of Japanese and Caucasian hair were not dependent on the hair diameter and had a constant value of 10 GPa. This value is relatively higher than those reported in the literature by Rebenfeld et al. (2.08 GPa, measured at 20°C in an aqueous buffer) (12) and by Scott and Robbins (3.79 2 1.5 1 Cl) 0.5 0 40 Young's Modulus vs. Diameter ♦ Japanese □ Caucasian □ 50 60 70 Diameter ( J1 m) 80 90 Figure 6. Relationship between diameter and Young's modulus (1). Japanese females (age: 26-39, N = 21) and Caucasian females (age: 20-40, N = 35). Regardless of diameter, the Young's modulus of hair is constant (about 10 GPa).