24 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS kV 412001 5.0 k • Figure 9. Typical damage to fine wool fiber surface due to extension. a: Fiber extended --25%, magni- fication x3000. b: Fiber extended --25%, magnification x 10,000. It appears that movement of the scales relative to each other--although surprisingly low levels of relative scale displacement in comparison to the strain level are observed alleviates the stress buildup during extension, similar to the situation in human hair

STRESS RELEASE IN HAIR CUTICLE 25 c Figure 10. Typical damage to coarse wool fiber surface due to extension. a: Fiber extended --30%, magnification x 1500. b: Fiber extended --30%, magnification x 13,000. c: Fiber extended, --30%, magnification x 8000. fibers, but without any lifting of the surface scale. The lack of scale lifting may be due to the relatively larger thickness and, with it, rigidity of the cuticle cells in wool fibers. Furthermore, the scale cells on wool fibers are highly curved around this lower-diameter fiber. In fact, for very fine wool fibers, single-cuticle cells may envelop the whole fiber (11). This high curvature gives the cuticle cells additional rigidity and thus may prevent the scale lifting observed for the much less curved cuticle cells in human hair fibers. The need for further work regarding the cuticular response to keratin fiber extension is clearly indicated. SUMMARY We have established that surface scale edge lifting occurs during extension of hair fibers under ambient conditions. We used microfluorometry to quantify this loss of adhesion by measuring the extension at which various scale-lifting phenomena occur. We think that scale lifting is due to the development of shear forces during the movement of individual scale cells relative to each other. Failure occurs in the endocuticular domain, and the scale edge begins to lift. The onset of scale lifting occurs at lower extension levels in the tip end of the fiber, where previous grooming damage has been experienced. When the fiber is released and immersed in water, the fiber length and mechanical properties are largely recovered. However, this is not true for the cuticular layer, i.e.,