Before biomimetics, there was little interest in studying biological materials to gain inspiration for human invention. This is because researchers assumed that living things originated via “blind watchmaker” mechanisms. Since most researchers had discarded any thoughts of intelligent agency, it seemed only natural to think that living things would not help the human quest for improved or innovative designs. However, this stance has been almost completely replaced by a much more positive perspective. Somehow the “blind watchmaker” has morphed into an immensely skilled craftsman. Now it is realized that life forms display structures with design elegance, the watchmaker is considered to behave as though he is not only sighted, but also astoundingly intelligent.
“Biological materials such as the feather are hierarchically organised and intimately constructed such that the final design makes it difficult to separate the parts or their functions from the whole.” [. . .]
“Many biological systems have mechanical properties that are far beyond those that can be achieved using the same synthetic materials with present technologies. As Ashby et al. (1995) show, it is not so much the material properties of the components as their arrangement within the natural composites which give rise to the exceptional multifunctional properties.” (p.324)
The avian feather, mentioned in the above quote, is a case in point. Scholars have started by assuming simplicity and this has led to the under-estimation of hierarchical organization. Four years ago, Lingham-Soliar and co-authors reported work based on the hypothesis that the “mechanical structural organization of the rachis was hierarchical” and that this structure would be revealed if the unstructured matrix material surrounding the beta-keratin fibres could be removed. They managed to achieve matrix removal by a process of degradation using fungi. The emerging model of the structure has three zones, fully supporting the hypothesis (plus the foam-filled centre of the rachis, which acts as an energy absorber),
“These findings of Lingham-Soliar et al. (2010) refuted a previous contradictory claim that the beta-keratogenic tissue of the rachis and barbs was fully characterized ultrastructurally by histodifferentiation, i.e. the bulk of the rachis, calamus, and barb rami were comprised of typical, tile-like, stratified squamous epithelial tissues.” (p.325)
[. . .] “We now know, with the discovery of a cross-fibre system in the epicortex of the barbs and rachis, that the feather microfibre structure is far more complex than previously thought and highly anisotropic.” (p.333)
In the paper reviewed here, Lingham-Soliar summarises previous work before showing how these new insights into feather structure increase our understanding of functionality and performance. He refers to the feather as “a classic example of bioengineering”. Taking the fracture mechanics of feathers as his starting point, the discussion covers: (1) crack-stopping, (2) stiffness, ductility and buckling, (3) torsion and flexion, and (4) ductile tearing. These findings feed into the conclusions. A relevant excerpt is below.
“In the feather, the cross-fibre architecture may provide a key mechanism for preventing damage to the rachis and barbs. However, a rigid system risks being loaded with dangerously high forces during flight. In this context, it is noteworthy that the longitudinal fibre system of the cortex not only provides stiffness but, in contrast to the cross-fibre system of the epicortex, importantly, allows torsion, which would help to lower the critical bending moment needed to cause local buckling failure. At the core of this understanding is the presence of two distinctive fibre systems, that of the epicortex and of the cortex, which in given circumstances will inevitably function in synergy to promote ideal feather aerodynamics. The potential for future biomechanical discussions are clear.” (p.333)
Having heard (earlier this year) Gareth Dyke speak on aspects of his research, I was particularly interested in Lingham-Soliar’s critique of his view (co-authored with Robert Nudds, published in Science) that Archaeopteryx was incapable of flapping flight because its feathers could not cope with the stresses and strains that flapping imposes. Lingham-Soliar draws on his own biomechanical analysis and concludes that Nudds and Dyke’s approach is flawed:
“Unfortunately, the rhetoric involved in the claims by Nudds and Dyke that their method over others is “quantitative” rather than “subjective”, and that they are supported by the laws of physics, is contradicted by faulty science, feather structural knowledge and mechanical analyses. Nudds and Dyke’s calculations ignore a major component of the feather rachis, the foam core, and also make use of a formula that is inappropriate – the calculations have little quantitative basis with respect to real feather structure and lack statistical significance, given they are based on a bird sample number of 1.” (p.334)
There is a tension in this paper between the appeal to “millions of years of biological evolution [that] have produced efficient materials and structures” and the avian fossil record that these structures appeared abruptly and in a mature form. (This is one reason why feather evolution has been relocated to the theropod dinosaurs). Lingham-Soliar finds himself using teleological language to describe what he sees:
“Biological structures, e.g. the feather, have developed even more ‘cunning’ methods for increasing the “work of fracture”.” (page 328)
[Fibre scientists have developed materials to absorb and dissipate large amounts of energy before failure], “Yet, this was achieved in a natural material, beta-keratin in the feather of birds, specifically in the structure of the rachis – in both conditions, i.e. increased fibre bundle thickness and an interfacial polymer matrix or “glue” – remarkably some 150 million years earlier.” (page 331)
We can respectfully point out that the evidence-base for such statements is profoundly weak. Many textbooks make similar statements but do not get beyond Darwinism when discussing mechanism. But Darwinism is a theory built primarily on the philosophy of naturalism. The evidences supporting it relate to ecology, not to the origins of complexity. We can predict that while naturalism reigns supreme, these tensions will stay with us. What we would like to see is a culture of academic freedom for dissenting views, particularly avenues of research relating to intelligent design.
Feather structure, biomechanics and biomimetics: the incredible lightness of being
Journal of Ornithology, April 2014, Volume 155, Issue 2, 323-336 | DOI: 10.1007/s10336-013-1038-0
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.
Palmer, C. Response to Lingham-Soliar: Feather structure, biomechanics and biomimetics: the incredible lightness of being. Journal of Ornithology, online July 2014 | doi:10.1007/s10336-014-1090-4
Lingham-Soliar, T. Response to comments by C. Palmer on my paper, Feather structure, biomechanics and biomimetics: the incredible lightness of being. Journal of Ornithology, online June 2014 | DOI: 10.1007/s10336-014-1093-1
Nudds, R.L. & Dyke, G.J. Narrow Primary Feather Rachises in Confuciusornis and Archaeopteryx Suggest Poor Flight Ability, Science, 14 May 2010: Vol. 328, 887-889 | DOI: 10.1126/science.1188895