MORPHOLOGY OF ASIAN AND CAUCASIAN HAIR 335 Table II Classification of damage Sample name Roughness Deposits Delamination Asian Al A2 A3 A4 AS A6 + + A7 A8 A9 Caucasian Cl C2 + + C3 + C4 +* cs + + + C6 +* + + C7 + cs + + * Only slight difference found. Figure 9. Particulates obtained from extended and sonicated hair. a: From Asian hair. b: From Caucasian hair. Therefore, we can assume that they come from the inner side of the cuticle cells and are probably endocuticle. The deposits found at the cuticular edges of extended hair may also be considered to be a part of these substances. MEASUREMENT OF CUTICULAR ELASTICITY Figure 10 shows the results of nanoindentation measurements of cuticle cross sections. It was shown that the endocuticle of Asian hair was harder than that of Caucasian hair. The exocuticle of Asian hair was similar to or only slightly harder than that of Caucasian hair. The difference in elasticity between exocuticle and endocuticle was larger for Caucasian hair than for Asian hair, as indicated by the ratios of exo/endo (for Asian hair: 1.26 for Caucasian hair: 1.41). This means that the hardness of a whole Caucasian cuticle cell is relatively uneven. From these results, it was found that there is a difference, not

336 JOURNAL OF COSMETIC SCIENCE 800 exo/endo= 1.26 exo/endo= 1.41 700 600 500 endo exo endo exo ·u ·.;::: 400 300 200 100 0 Asian Caucasian Figure 10. Elasticity of exocuricle and endocuticle measured by the nanoindentation method. only in configuration but also in the hardness of the cuticular components, between Asian and Caucasian hair. Thus, the following hypothesis is suggested (Figure 11): Once Asian hair receives extension stress, the cohesion between cuticle cells (cell membrane complex), rather than the cuticle cells themselves, absorbs the stress and the cuticles lift up while keeping their original shape. When further stress is applied, Asian hair cuticles are broken at the cracks generated in the extension and fail as large fragments. In contrast, the relatively fragile Caucasian cuticles fall into small pieces when they receive such stress, especially with the splits generated in the fragile endocuticle or boundary between exocuticle and endocuticle. This failure is confined to the removal of small fragments even if stress is furthered. lift up complex Caucasian hair Figure 11. Assumed damage mechanism.