(1) Background: The objective of this work was to elucidate the hair microstructure which correlates with the stiffness of human hair fibers. (2) Methods: Bending moduli of hair fibers were evaluated for the hair samples from 156 Japanese female subjects. Hair transverse sections were dual-stained with fluorescent dyes which can stain para- and ortho-like cortical cells separately, and observed under a fluorescence light microscope. Atomic force microscopy nanoindentation measurements were performed to examine the modulus inside macrofibrils. (3) Results: The difference in bending moduli between the maximum and the minimum values was more than double. The hair of high bending modulus was rich in para-like cortical cells and the bending modulus significantly correlated with the fraction of para-like cortical cells to the whole cortex. On the other hand, the elastic moduli inside macrofibrils were almost same for the para- and ortho-like cortical cells. (4) Conclusions: Hair bending modulus depends on the fractions of the constitutional cortical cell types. The contribution of the intermacrofibrillar materials, which differed in their morphologies and amounts of para- and ortho-like cortical cells, is plausible as a cause of the difference in the modulus of the cortical cell types.
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