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Undamaged hair consists of a cortex (the core) surrounded by eight to 10 layers of cuticle cells (the sheath). The cortex consists of spindle-shaped cells held together by cell membrane complexes (CMCs). The tensile strength of hair depends mainly on the cortex that forms the major component of the fiber, with little effect from the cuticle. Some actives in cosmetic formulations are capable of diffusing into the fiber cortex and affect the fiber properties significantly. Compounds that are not capable of diffusing into the fiber penetrate the cuticular sheath and deposit only on the surface of the fiber.
Since cuticle cells have little effect on the tensile mechanical properties of hair, tensile mechanical properties cannot distinguish between the surface and the bulk effects. Torsional properties, on the other hand, are more likely to distinguish between the surface and the bulk effects because in torsional deformation the strain is at the maximum at the surface and decreases towards the center of the fiber.
This is only an excerpt of the full article that appeared in Cosmetics & Toiletries, but you can purchase the full-text version.