A Raman spectroscopy investigation of the resilience of hair cuticular scales under uniaxial stress.

Abstract

Human hair is a hierarchical composite, primarily composed of α-keratin, in which the cortex contributes to tensile strength through keratin intermediate filaments, while the cuticle ensures surface protection, chemical resistance, and water regulation due to its multilayered architecture. Although the tensile properties of the cortex have been extensively studied, the mechanical response of the cuticle under stress remains less understood. In this work, we investigate cuticular mechanics by correlating morphological deformation with molecular-level responses using Raman spectroscopy. A human hair was subjected to uniaxial strain up to 32%, and the spacing between cuticular scale edges was monitored. A 1:1 correlation was observed between cuticular strain and applied elongation in the range 3-26%, indicating that scale deformation follows the macroscopic fiber extension. Raman analysis revealed strain-induced alterations in disulfide (-C_α-CH₂-S-S-CH₂-C_α-) crosslinks, with portion of disulfide bridges undergoing a gauche-to-trans conformational change, predominantly within the sulfur-rich A-layer and exocuticle. Although a slight reduction in hydrogen-bond strength was observed, no α-helix to β-sheet transition was detected. The structural stability of the cuticle under stress is attributed to its high content of keratin-associated proteins and dense crosslinking via disulfide and isopeptide bonds. Furthermore, the experimental results concerning the relaxation under macroscopic strain are compatible with thinning of lipid layers and gliding of cuticular scales. These findings collectively demonstrate that the cuticular Cell Membrane Complex (CMC) facilitates scale gliding while preserving structural integrity, highlighting its critical role in hair mechanics and resistance to external stresses.