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).

DECREASE IN HAIR VOLUME WITH AGE 491 Young's Modulus vs Diameter 2 0 ........4 X 1.5 ♦ 1 Cl.) 0.5 F - test : p=0.033 0 40 50 60 70 80 90 Diameter ( µ. m) Figure 7. Relationship between diameter and Young's modulus (2). Japanese females (age: 26-39, N = 21) and Caucasian females (age: 20-40, N = 35): ♦. Japanese females (age: 45-51, N = 17): e. Changing properties of hair with aging. Decrease in diameter and intrinsic structural changes. GPa, measured at 24°C, 62% RH by the bending method with the equilibrium fiber technique) (8a,8b), but rather close to that reported by Sogabe et al. (ca. 11 to 12 GPa, measured by a dynamic bending elasticity method) for Japanese hair (4). If the number or the thickness of cuticles is the same, regardless of the hair thickness, the ratio and contribution of cuticle are bigger when the hair diameter is smaller. Sogabe et al. (4) reported that the Young's modulus of cuticle is approximately four times bigger than that of the cortex. Taking this information, and the previously mentioned result on hair thickness (that Japanese hair is ca. 20 µm thicker than Caucasian), Japanese hair can be said to be less influenced by the cuticle, and so the mean Young's modulus is smaller. No such difference in Young's modulus was observed between the two ethnic groups, however, when the hair fibers from the panelists of the same age range were compared, as shown in Figure 5. The reason is not yet clear and only some possibilities can be put forward now. One of these may be that the original hypothesis above, about the number and/or thickness of cuticle, is not correct. Another possibility is that the difference in the cross-sectional shape between the two ethnic groups causes this phenomenon. As is