HAIR MECHANICAL ANISOTROPY 311 to the arrangement of alpha-keratins in the intermediate plate of the nails (24). Figure 5 illustrates these details. It is worth mentioning that such an arrangement of exocuticle rods, combined with those of the KIFs in the cortex, provides the fi ber with a greater mechanical stability and may explain why external mechanical or chemical insults to the fi ber lead to breaking of the scales horizontally rather than longitudinally (25). At this stage, we assume the proposed rods to exist in the cuticle layers as non-regular intermediate fi laments, and the architecture of the layers as sketched in Figure 6. CONCLUSIONS Using AFM nanoindentation of hair fi ber cross and longitudinal sections, we have evalu- ated for the fi rst time, the mechanical anisotropy of a keratin fi ber. Voigt and Reuss mod- els were successfully used for calculating the behavior of the fi ber as an ideal composite material made of rods (KIFs) and matrix, and the experimental results of virgin hair fi bers matched fairly well to the theory. Figure 4. Evolution of index of anisotropy, IA, of (A) cortex and (B) exo- and endocuticle of the hair calcu- lated from values of axial and transverse elastic moduli measured at increasing relative humidity, RH, and shown in Figure 3A and B. The dotted lines are the plots of the theoretical models. The horizontal line at one is the ‘isotropic line’. Figure 5. Sketch of the presumed structure of corte x and cuticle as suggested by the evolution of index of anisotropy, IA, of the subcomponents with Relative Humidity, RH, as in Figure 4.
JOURNAL OF COSMETIC SCIENCE 312 Applying the models further to the cuticle components (exo- and endocuticle) and using the “index of anisotropy,” IA, introduced by equation (8), the experimental results indi- cated that, for high values of relative humidity, when the cuticle layers are swollen enough, a probable composite structure of exocuticle, masked so far by the heavy cross- linking of the environment, manifests. The rods reinforcing the exocuticle composite structure appear to be oriented perpendicularly to the fi ber-growing axis, in what we consider to be an attempt by nature to ensure a balanced design for a fi ber able to with- stand various directional stresses. REFERENCES (1) C. Popescu and H. Hoeck er, Hair—the most sophisticated biological composite material, Chem. Soc. Rev., 36, 1282–1291 (2007). (2) C. Popescu and H. Hoeck er, Cytomechanics of hair: basics of the mechanical stability, Int. Rev. Cell Mol. Biol., 277, 137–156 (2009). (3) J. W. S. Hearle, A crit ical review of the structural mechanics of wool and hair fi bres, Int. J. Biol. Macro- mol., 27, 123–138 (2000). (4) M. Feughelman, Natural protein fi bres, J. Appl. Polym. Sci., 83, 489–507 (2002). (5) H. Bogaty, Torsional pr operties of hair in relation to permanent waving and setting, J. Soc. Cosmet. Chem., 18(10), 575–590 (1967). (6) L. J. Wolfram and L. Al brecht, Torsional behavior of human hair, J. Soc. Cosmet. Chem., 36(1), 87–99 (1985). (7) G. Binnig and C. F. Qua te, Atomic force microscope, Phys. Rev. Lett., 56(9), 930–933 (1986). Figure 6. Sketch of the imagined architecture of exo cuticle layer.
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