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Feather Design Is Better than Thought

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Man-made designs often get simpler the deeper you look. Once you get inside a steel girder or wallboard, for instance, the material looks basically homogeneous. Biological materials, by contrast, “are complex composites that are hierarchically structured and multifunctional,” notes Theagarten Lingham-Soliar in a paper on Nature Scientific Reports.

We all know that feathers have an elegant shape for flight and insulation: they are self-healing, aerodynamic, and lightweight yet strong. But when this materials scientist from Nelson Mandela Metropolitan University in South Africa inspected bird feathers with a scanning electron microscope (SEM), he found wonderful things — all the way down to the molecular level.

First, some terminology. The main shaft of a feather is called a rachis (rey-kis), which tapers from base to tip like a long, narrow cone. Branching off the rachis are barbs. Branching off the barbs are barbules with tiny hooks that zip the barbules together, forming the familiar feather surface. These are demonstrated in Illustra Media’s film Flight: The Genius of Birds. (See a short video clip about feathers at the film site). If you examine the rachis in cross-section, you will see some medullary pith in the center, surrounded by a stiff cortex of tough fibers made of the protein keratin.

Keratin is a very unique protein. Lingham-Soliar describes how the specific molecular arrangement of keratin makes it ideal for feathers:

Feathers of flying birds are subjected to extraordinary aerodynamic forces during flight. They are made of a remarkably hard material, keratin. During the first half of the last century pioneering X-ray studies indicated that the conformation of the polypeptide chain in the hard keratins of birds and reptiles is based on the β-pleated-sheet (β-form) rather than the coiled-coil α-helix (α-form) found in mammalian keratins. Early use of transmission electron microscopy (TEM) on chicken and seagull feather rachises showed that β-keratin was composed of a framework of fine microfibrils approximately 30 angstroms (Å) in diameter and that these long protein filaments were surrounded by an amorphous protein matrix, each filament possessing a helical structure with four repeating units per turn.

The cortical fibers of keratin along the rachis are called syncytial barbule fibers, or SBFs for short. “SBFs form long, continuous filaments of β-keratin, the majority of which are tightly assembled parallel to the longitudinal axis of the rachis,” Lingham-Soliar says — but not all of them. Therein lays a tale. What he found about those SBFs solves two design problems for the bird, and may inspire a new generation of structural engineers. Here’s the problem:

Perhaps one of the most intriguing questions in bird flight involves how toughness or a high work of fracture is achieved in the cortex of the rachis and barbs. Put another way, what are the material conditions that would help prevent (or delay) the feather from splitting or cracking down its length or across its hoop (circumference) during the stresses of flight. That question was intuitively first raised over 38 years ago by the notable aeronautical engineer, John Gordon although he declared it was a mystery at the time of writing. However, recent research on the cortical SBF structure of the rachis and barbs has allowed new light to be shed on the problem. But, as so often happens as we find new answers we also find more questions. I consider one such question here, which is also closely tied to bird flight. [Emphasis added.]

Lingham-Soliar could not understand how these SBFs could prevent catastrophic fractures in the feather. If the fibers were all parallel to the long axis, they would have to continuously terminate as the rachis tapers down toward the tip. This would open up thousands of fracture zones where small stresses could exacerbate the fractures, “analogous to the scissor-snip a tailor makes before tearing a piece of fabric,” leading to catastrophic failure of the feather.

In the 1920s, Griffith showed that according to thermodynamic principles the magnitude of the stress concentration at a crack tip is dependent on the crack length (L) i.e. that the strain energy released in the area around the crack length (L2, proportional to the crack length) is available for propagating the crack. From this principle we see that there is a dangerous potential of numerous self-perpetuating cracks in the feather cortex. How then have birds in the 150 million years of their evolution been able to respond to this threat? The present hypothesis is that there must be a means to eliminate this potentially catastrophic condition in a structure critical to bird flight — i.e., crucially a structural mechanism to avoid an inherent condition of notches or cracks in the feather rachidial cortices. To this end the feather cortices and comprising SBFs are investigated.

For the first time, Lingham-Soliar could see the answer. He had to look at the detailed microstructure of the SBFs on the order of millionths of a meter (micrometers) with SEM. What came to light was “a biomechanically ‘ingenious’ and novel architecture of the fibre organization” that solves the fracture problem and does something else, too: it distributes the stress load throughout the feather. This multi-functional “distinctive architecture of the SBFs” is bound to inspire engineers faced with the demands of designing lightweight yet strong materials that can absorb stress without failing.

Here’s the basic idea: instead of terminating along the rachis, some of the SBFs (fiber bundles) branch off into the barbs. As you travel down the tapering shaft, you see more SBFs branch out, thinning the cortex toward the tip. It’s an elegant solution. Here’s what the author says about it:

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.

It’s actually more sophisticated than it sounds. In addition to the branching, there’s a glue-like substance that holds adjacent SBFs together. The glue has properties that “improved the tensile stiffness of the material by improving lateral slippage.” The SBFs are also held together longitudinally by hooks and rings.

Think about the design problem as the feather emerges from the follicle during development. How do some of the SBFs know to bend out into a barb? What teaches these growing fibers to cross over the longitudinal access in successive waves and branch out left and right into the barbs, leaving enough material behind to continue building the cortex all the way to the tip? What concentrates the glue where it is needed, in the right amount? What tells the barbs to grow barbules with hooks and channels that fit just right? Building a machine that could do this by extrusion would seem like an engineer’s nightmare.

Lingham-Soliar examined the feathers of different birds — chickens, falcons, eagles, swans, geese, ibises, pheasants, macaws, and toucans — and found that all their feathers use this design principle. As an evolutionist, he assumes they all got it from a common ancestor. But he seems conflicted at the design facts visible under his microscope and the story that must be told, “perhaps the most controversial question, the origin and evolution of the feather.” Unfortunately, he sticks to the Darwin story:

It is clear that this extraordinary cortical microstructure of the feather has evolved and been perfected over the millions of years of bird evolution….

After describing what engineers learned from gluing nanotubes to increase tensile strength, he says:

But we also see that the architecture of SBFs in the feather is more sophisticated compared to the engineered tubules i.e., with respect to the dogbone shape (aided frequently by hooks), which functions in fibre pull-out… This must be a tribute to over 150 million years of feather evolution in response to extreme aerodynamic stresses involved in bird flight.

It’s tragic to watch scientists who have been trained to think Darwinly becoming enslaved to chance explanations for fantastic designs that are so good, engineers want to imitate them.

The field of biomimetics is relatively new but has been receiving increasing attention in recent years. Biological materials are complex composites that are hierarchically structured and multifunctional. Their mechanical properties are often outstanding, considering the weak constituents from which they are assembled. Structures in nature have evolved over millions of years (frequently hundreds of millions) and because of their multifunctionality are difficult to resolve or break down into simpler components that could help in engineered materials. Whereas current engineering designs benefit from simplicity, future ones might be more sophisticated with a much wider performance envelope and broader range of applications inspired by biology’s vastly different scales of architectural organization and robust multi-functionality. As biologists and evolutionists we hope to help untangle some of the complexities of natural materials by extensive investigations and to collaborate with materials scientists and engineers with the ultimate goal of mimicking them in synthetic systems.

Wouldn’t it be nice if someday soon the shackles of methodological naturalism were taken off, so that authors could freely talk about design in nature? Darwinian evolution and eons of time don’t contribute anything of value to this investigation. The author could have left out that “most controversial question” about feather origins and evolution entirely. He came close. “Their unique morphology, which includes nodes with hooks and rings, plays a major part in the design strategy of keeping the filaments locked together,” he said earlier.

We say, toss the evolutionary talk and leave it at that. This is a design paper that inspires design. It can inspire us all to appreciate even more “the genius of birds.” After learning about those SBFs, we will never look at a crow the same way again.

Photo: Eurasian eagle-owl, by Peter Trimming [CC BY 2.0], via Wikimedia Commons.