A New Helical Crossed-Fibre Structure of β-Keratin in Flight Feathers and Its Biomechanical Implications

Abstract

The feather aerofoil is unequalled in nature. It is comprised of a central rachis, serial paired branches or barbs, from which arise further branches, the barbules. Barbs and barbules arise from the significantly thinner lateral walls (the epicortex) of the rachis and barbs respectively, as opposed to the thicker dorsal and ventral walls (the cortex). We hypothesized a microstructural design of the epicortex that would resist the vertical or shearing stresses. The microstructures of the cortex and epicortex of the rachis and barbs were investigated in several bird species by microbe-assisted selective disassembly and conventional methods via scanning electron microscopy. We report, preeminent of the finds, a novel system of crossed fibres (ranging from ∼100–800 nm in diameter), oppositely oriented in alternate layers of the epicortex in the rachis and barbs. It represents the first cross-fibre microstructure, not only for the feather but in keratin per se. The cortex of the barbs is comprised of syncitial barbule cells, definitive structural units shown in the rachidial cortex in a related study. The structural connection between the cortex of the rachis and barbs appears uninterrupted. A new model on feather microstructure incorporating the findings here and in the related study is presented. The helical fibre system found in the integument of a diverse range of invertebrates and vertebrates has been implicated in profound functional strategies, perhaps none more so potentially than in the aerofoil microstructure of the feather here, which is central to one of the marvels of nature, bird flight.

Molecular evidence of keratin and melanosomes in feathers of the Early Cretaceous bird Eoconfuciusornis

“…This thin layer displays fibrous bundles in arrays that form regular angles ( Fig. 1 G and I), a feature previously reported in the epicortex of G. gallus feathers (27) and also observed in Fig. 1 C and D.…”

Light Like a Feather: A Fibrous Natural Composite with a Shape Changing from Round to Square

“…During bending, the dorsal and ventral cortex provide stiffness, while the lateral walls allow the shaft to flex with desirable strain under loading, thus delaying the onset of buckling and failure 18. Besides, the crossed‐fibers structure may be a key in limiting damage from barbs.…”

“…However, the detailed fibrous structure of the whole shaft is unexpectedly under‐documented. An increase of axially aligned keratin molecules toward the tip within the feather rachis was reported 17; circumferential, axial fibers, and crossed‐fibers were observed by selectively degrading the matrix proteins 18. A few studies used nanoindentation to obtain the local modulus and hardness of feathers by indenting in limited or unspecified locations 2, 19, 20…”

Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration

“…The lateral walls of cortex in the rachis and barbs consist of oppositely oriented in alternate layers of crossed-fibers (about 100-800 nm in diameter), shown in Fig. 51 (the arrows indicate the boundary between the cortex and sidewall of cortex) [240]. Transmission electron micrographs of feathers show the fine filament-matrix structure (Fig.…”

“…Scanning electron micrograph of the sidewall of feather cortex (epicortex) showing the crossed-fiber structure (indicated by two red crossing lines) from a domestic chicken (Gallus gallus). Arrows show the boundary between the cortex and sidewall of cortex [240]. Young's modulus and geometry along the length of feather rachis were identified [120,233,244].…”