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.
Highlights
One of the new avenues in material science is the development of nanocomposites with novel mechanical properties such as dynamic strain hardening [1]
The building blocks of the cortex are trichocyte keratins intermediate filaments (IFs) with a 7.5 nm diameter embedded in a sulfur-rich protein matrix and arranged in a paracrystalline hexagonal lattice [4]
The keratin IFs are roughly aligned with the macroscopic fiber axis and arranged into macrofibrils typically 300 nm in diameter [5] (Fig. 1)
Summary
One of the new avenues in material science is the development of nanocomposites with novel mechanical properties such as dynamic strain hardening [1]. One source of inspiration is Nature’s hierarchical materials such as woods, hooves, beaks and skin appendages. All these materials are nanocomposites, lightweight and exhibit non-linear mechanical properties that are still poorly understood from a structural standpoint. In particular mammalian appendages such as wool, hair and quills have been studied for more than a century These fibres are composed of three different regions, the cuticule, the cortex and the medulla [2]. The cortex is responsible for the bulk mechanical properties of these fibers [3] It is a hierarchical fibrous composite from the nanometer to the micrometer scale [2]. The macrofibrils are contained in spindle-shaped cortical cells, 100 mm long and several microns wide (Fig. 1, inset)
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