Mechanocaloric effect of human hair.

Abstract

Although the uniaxial tensile deformation of human hair has been extensively studied, the temperature response under tensile loading has not been directly measured, limiting insight into the entropic and enthalpic contributions. To address this gap, we developed a tensile apparatus equipped with a thermocouple capable of detecting temperature variations with ±50 μK precision. The temperature change exhibits a characteristic N-shaped profile during extension up to fracture at λ≈1.6. This behavior can be interpreted using the two-phase model in which crystalline α-helical filaments are embedded in an amorphous matrix. In the Hookean region (λ=1-1.05), stress increases linearly with strain while the temperature rises by several tens of mK, indicating an exothermic response dominated by the amorphous matrix. Cyclic extension-contraction tests reveal reversible thermal responses together with gradual accumulation of residual strain. In the transformation region (λ=1.05-1.3), the temperature response reverses and exhibits a pronounced endothermic decrease, reflecting the α-to-β structural transition. In the post-transformation region (λ=1.3-1.6), the temperature increases again, consistent with the progressive development of β-sheet structures.