http://meyersgroup.ucsd.edu/papers/journals/Meyers%20326.pdf

Toucan and hornbill beaks: a comparative study.

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.

/pdf/a-new-helical-crossed-fibre-structure-of-b-keratin-in-flight-8s61ykf909.pdf

A new helical crossed-fibre structure of β-keratin in flight feathers and its biomechanical implications.

Millions of years of biological evolution have produced efficient materials and structures that are a source of inspiration to engineers. The paper reviews the overall design principles in the feather rachis and elaborates upon recent functional interpretations. It concentrates on recent findings that shed new light on feather microstructure and on how keratin fibres in a protein matrix are arranged in intricate ways to achieve specific combinations of stiffness and strength on the one hand and flexibility and elasticity on the other. This includes the syncitial barbule cells of the rachis and barb cortex, the crossed-fibre architecture of the epicortex (lateral walls of the cortex), and the foam-like structure of the medullary pith. Discussion of the biomechanics of feather microstructure uses engineering principles for a better understanding of the functional ramifications. Further research is proposed with respect to feather micro- and macrostructure in trying to expand our knowledge on bird flight, behaviour and ecology in different species. The discussion also considers the validity of a study purporting to use quantitative methods and engineering principles to show that the iconic fossil bird Archaeopteryx was incapable of flapping flight.

Feather structure, biomechanics and biomimetics: the incredible lightness of being

Seagull feather shaft: Correlation between structure and mechanical response

Feathers do not have to be especially strong but they do need to be stiff and at the same time resilient and to have a high work of fracture. Syncitial barbule fibres are the highest size-class of continuous filaments in the cortex of the rachis of the feather. However, the rachis can be treated as a generalized cone of rapidly diminishing volume. This means that hundreds of syncitial barbule fibres of the rachis may have to be terminated before reaching the tip – creating potentially thousands of inherently fatal crack-like defects. Here I report a new microstructural architecture of the feather cortex in which most syncitial barbule fibres deviate to the right and left edges of the feather rachis from far within its borders and extend into the barbs, side branches of the rachis, as continuous filaments. This novel morphology adds significantly to knowledge of β-keratin self-assembly in the feather and helps solve the potential problem of fatal crack-like defects in the rachidial cortex. Furthermore, this new complexity, consistent with biology’s robust multi-functionality, solves two biomechanical problems at a stroke. Feather barbs deeply ‘rooted’ within the rachis are also able to better withstand the aerodynamic forces to which they are subjected.

/pdf/microstructural-tissue-engineering-in-the-rachis-and-barbs-kk8atiqkj6.pdf

Microstructural tissue-engineering in the rachis and barbs of bird feathers.

Thin-walled, cylindrical structures are found extensively in both engineering and nature. Minimum weight design of such structures is essential in a variety of engineering applications, including space shuttle fuel tanks, aircraft fuselages, and offshore oil platforms. In nature, thin-walled cylindrical structures are often supported by a honeycombor foam-like cellular core, as for example, in plant stems, porcupine quills, or hedgehog spines. Previous studies have suggested that a compliant core increases the elastic buckling resistance of a cylindrical shell over that of a hollow cylinder of the same weight. We extend the linearelastic buckling theory by coupling basic plasticity theory to provide a more comprehensive analysis of isotropic, cylindrical shells with compliant cores. The minimum weight design of a thin-walled cylinder with a compliant core, of given radius and specified materials, subjected to a prescribed load in uniaxial compression or pure bending is examined. The analysis gives the values of the shell thickness, the core thickness, and the core density that minimize the weight of the structure for both loading scenarios. The weight optimization of the structure identifies the optimum ratio of the core modulus to the shell modulus. The design of natural, thin-walled structures with cellular cores is compared to the analytical optimal, and the deviation about the theoretical optimum is explored. The analysis also discusses the selection of materials in the design of the cylinders with compliant cores, identifying the most suitable material combinations. Finally, the challenges associated with achieving the optimal design in practice are discussed, and the potential for practical implementation is explored.

/pdf/biomimetics-extending-nature-s-design-ofthin-wall-shells-36eito2dj0.pdf

M. A. Dawson

Papers

Biomimetics: Extending Nature’s Design OfThin-wall Shells With Cellular Cores