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Structural and Functional Design Strategies of Biological Keratinous Materials

  • Author(s): Wang, Bin
  • Advisor(s): Meyers, Marc
  • et al.
Abstract

Keratinous materials are omnipresent, encompassing terrestrial, aerial and aquatic territories; they form diverse epidermal appendages and serve various interesting functions, triggering the curiosity of humans and inspiring the inventions of novel structures. Among these, pangolin scales and the feather shaft are systematically and analytically studied, answering the questions how the scales function as an armor and the shaft fulfills flight, which contribute to advancing the current knowledge of natural keratinous materials and providing valuable insights in developing new bioinspired designs.

The pangolin is the only known mammal that has keratinous scales covering the main body. Arboreal and ground pangolin scales show an overlapping pattern in a hexagonal arrangement to provide multi-layered coverage. Scales show three regions in the solid interior that features flattened cells forming crossed lamellae, rarely seen in most keratinous materials. Two dimensional x-ray diffraction reveals the presence of α-helices and possible β-sheet, and the microfibrils are crossed in a range of directions. Scales show an interlocking interface between lamellae, which results from the suture-like cell membrane complex. Scales are transverse isotropic and show a strain-rate dependence, which favor the function in resisting external forces from multi-directions.

The feather shaft, a naturally refined lightweight and stiff flight material, is distinguished in having a changing shape factor: circular at the calamus but square towards the distal rachis; this produces a tailored flexural stiffness along the shaft length fulfilling the local stress requirements. The cortex has a complex fibrous hierarchical structure, both making the shaft longitudinally strong, dorsal-ventrally stiff and torsionally rigid, yet capable of desirable deflection and twisting. Filling the cortex with foamy medulla introduces a synergistic strengthening and toughening mechanisms. Flexure of the shaft reveals decreasing flexural properties towards the distal end; nevertheless, the density-normalized flexural stiffness almost remains the same, and the specific flexural strength increases by 48% at the distal shaft. Knowledge from the structure design of the feather shaft have potentials in developing aircraft materials and biomedical scaffolds.

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