title: Nanoscale strain-hardening of keratin fibres.
abstract: Mammalian appendages such as hair, quill and wool have a unique structure composed of a cuticle, a cortex and a medulla. The cortex, responsible for the mechanical properties of the fibers, is an assemblage of spindle-shaped keratinized cells bound together by a lipid/protein sandwich called the cell membrane complex. Each cell is itself an assembly of macrofibrils around 300 nm in diameter that are paracrystalline arrays of keratin intermediate filaments embedded in a sulfur-rich protein matrix. Each macrofibril is also attached to its neighbors by a cell membrane complex. In this study, we combined atomic force microscopy based nano-indentation with peak-force imaging to study the nanomechanical properties of macrofibrils perpendicular to their axis. For indentation depths in the 200 to 500 nm range we observed a decrease of the dynamic elastic modulus at 1 Hz with increasing depth. This yielded an estimate of 1.6GPa for the lateral modulus at 1 Hz of porcupine quill's macrofibrils. Using the same data we also estimated the dynamic elastic modulus at 1 Hz of the cell membrane complex surrounding each macrofibril, i.e., 13GPa. A similar estimate was obtained independently through elastic maps of the macrofibrils surface obtained in peak-force mode at 1 kHz. Furthermore, the macrofibrillar texture of the cortical cells was clearly identified on the elasticity maps, with the boundaries between macrofibrils being 40-50% stiffer than the macrofibrils themselves. Elasticity maps after indentation also revealed a local increase in dynamic elastic modulus over time indicative of a relaxation induced strain hardening that could be explained in term of a α-helix to β-sheet transition within the macrofibrils.
authors: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada.
authors: Patrick
authors: P
authors: Fortier
authors: Sandy
authors: S
authors: Suei
authors: Laurent
authors: L
authors: Kreplak
mesh_terms: D055119:Elastic Modulus; D006197:Hair / Q000737:chemistry / Q000166:cytology; D006244:Hardness; D006801:Humans; D007633:Keratins / Q000737:chemistry*; D057054:Molecular Imaging; D036103:Nanotechnology; D013314:Stress, Mechanical
chemical_list: D007633:Keratins