Disulfide Bonds in the Outer Layer of Keratin Fibers Confer Higher Mechanical Rigidity:  Correlative Nano-Indentation and Elasticity Measurement with an AFM

Abstract

Nanomechanical properties of biological fibers are governed by the morphological features and chemically heterogeneous constituent subunits. However, very little experimental data exist for nanoscale correlation between heterogeneous subunits and their mechanical properties. We have used keratin-rich wool fibers as a model of composite biological fibers; a wool fiber is a simple two component cylindrical system consisting of a core cellular component surrounded by an outer cell layer and their ultrastructure and chemical composition are well-characterized. The core is 16-40 micrometer in diameter and rich in axially aligned keratin microfibrils. Outer cells have multiple laminar layers, 60-600 nm thick and distinctly rich in disulfide bonds. We used an atomic force microscope (AFM) to examine the nanomechanical properties of various structural components using complementary techniques of force-volume imaging and nano-indentation. AFM images of transverse sections of fibers were obtained in ambient environment, and the mechanical properties of several identified regions were examined. The outer cell layer showed a significantly higher mechanical stiffness than the internal cellular core region. Chemical reduction of disulfide bonds eliminated such dichotomy of mechanical strengths, indicating that the higher rigidity of the outer layer is attributed primarily to the presence of extensive disulfide bonding in the exo-cuticle. This is the first detailed correlative study of nano-indentation and regional elasticity measurements in composite biological systems, including mammalian biological fibers.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…It has also been used to investigate the influence of film thickness on the apparent elastic modulus using a wedge shaped gelatin film sample [1185], to correlate distribution of elastic modulus over an agar gel surface to the network structure of agar fibers [1186], to measure the dependence of compressive modulus of poly(styrene-co-vinyl-benzyl-(trimethyl)-ammonium chloride) (VBTA) microspheres on the VBTA content [270], and to characterize the mechanical stiffness of the inner and outer layer of wool fibers [1187].…”

Thermal, chemical and morphological properties of carbon fibres derived from chemically pre-treated wool fibres

“…However, the improvement in the carbon yield achieved for the polystyrene sulphonate graed wool was only a marginal, although polyvinyl sulphonate is reported to improve the thermal stability of polyurethane foam. 24 During heating treatment, lignin and tannic acid both probably formed crosslink between carboxyl groups of amino acids of wool keratin thorough condensation reactions, which improved the thermal stability and reduced their degradation. In the case of Hercosett resin and polystyrene sulphonate-treatment, those polymers only formed a thin layer on the surface of wool bres and therefore showed limited effect on thermal stability.…”

“…23,24 In spite of the high carbon content of wool bre, there is scant information in the literature on the conversion of wool bre into carbon bre. Chen et al produced activated carbon powder via pyrolysis of wool bre at 280 C under a ow of nitrogen.…”

Effect of Oxidation Stress on the Mechanical Property of Human Hair

“…10). In addition, from nano-indentation studies [9], [10], it is reported that Young's modulus of cuticle is larger than that of cortex, which is also the factor that the contribution of the cuticle is larger.…”