The understanding on the mechanical behavior of α-keratin broadens our knowledge in biological materials science. In this study, the hierarchical organization is studied from the intermediate filament to the structural levels. The effects of strain rate, relative humidity, and temperature are evaluated.
Human hair exhibits a high tensile strength, which is significantly dependent on strain rate and humidity. The strain-rate sensitivity is comparable to that of other keratinous materials. One distinguishing feature, the unwinding of the α-helices and the possible transformation to β-sheet structure of keratin under tension, is analytically evaluated and incorporated into a constitutive equation. The contributions of elastic and plastic strains on reloading are evaluated and correlated to structural changes.
The dynamic mechanical response over a range of frequencies and temperatures is analyzed. The α-keratin fibers behave more elastically at higher frequencies while they become more viscous at higher temperatures. The stress relaxation behavior of α-keratin fibers is established and fit to a constitutive relaxation equation based on the Maxwell-Wiechert model. The two relaxation constants are connected to two hierarchical levels of relaxation: the amorphous matrix-intermediate filaments interfaces, for the short term, and the cellular components for the long term. Results of creep testing provide important knowledge on the uncoiling and phase transformation of the α-helical structure as hair is uniaxially stretched. As the hairs are chemically treated, they exhibit a similar strain-rate sensitivity of ~0.05, which is attributed to the intermediate filaments. As a result, the strain-rate sensitivity of human hair is reduced by half, while that of horse hair remains unchanged. FTIR data confirms that the human hair is more sensitive to the -S-S- cleavage, resulting in an increase of cysteic acid. Therefore, the disulfide bonds in the matrix are experimentally identified as one source of the strain-rate sensitivity and viscoelasticity in α-keratin fibers.
In addition to human and horse hair that comprise the primary goal of this investigation, boar, giraffe, elephant, and bear hair were tested in tension, to establish the mechanisms of deformation and failure. In spite of differences in strength attributed to the condition of the hair, no significant changes were observed.