One of the pleasures of walking through a wood is hearing the distant drumming of woodpeckers. We know they are searching for food, but few of us grasp the extraordinary nature of their achievement. Drumming rates of about 20 impacts per second are normal, with decelerations of 1200 g, and the drumming sessions may be repeated 500-600 times per day. By contrast, humans can lose consciousness when experiencing 4-6 g and are left concussed with a single deceleration of about 100 g. The authors of a recent analysis of the woodpecker's shock-absorbing mechanism describes it as "advanced" and "special". By looking at video material of drumming and CT scans of the bird's head and neck, they found four structures that absorb mechanical shock:
"These are its hard-but-elastic beak; a sinewy, springy tongue-supporting structure that extends behind the skull called the hyoid; an area of spongy bone in its skull; and the way the skull and cerebrospinal fluid interact to suppress vibration." (source)
Golden-fronted Woodpecker (source here)
Informed by these findings, the research sought to mimic these characteristics and construct a system that could protect micromachined devices from high-g impacts.
"To mimic the beak's deformation resistance, they use a cylindrical metal enclosure. The hyoid's ability to distribute mechanical loads is mimicked by a layer of rubber within that cylinder, and the skull/cerebrospinal fluid by an aluminium layer. The spongy bone's vibration resistance is mimicked by closely packed 1-millimetre-diameter glass spheres, in which the fragile circuit sits."
To test out their shock-absorbing material, they used a 60 mm air gun capable of generating scenarios of 60,000 g. For comparison, a hard resin shock absorbing system was used (representing current state-of-the-art technology). They fired micromachined devices and checked them for damage. They found that the hard resin system protected up to 40,000 g but 26.4% were damaged at 60,000 g. By contrast:
"In the bio-inspired shock absorbing system, almost all the micromachined devices survived at a high-g mechanical excitation of 60,000 g. This is because high-frequency mechanical excitations corresponding to the resonance frequencies of the micromachined devices are absorbed by the bio-inspired shock-absorbing system and even the transmitted mechanical excitations are detoured around the micromachined devices."
The researchers are understandably pleased with their new shock-absorbing system, and already the work is creating interest - with many diverse application areas (see Marks). Of particular interest here is the way woodpecker drumming has stimulated this research and has provided the conceptual model for developing the biomimetic system. The authors refer to the conventional Darwinian framework for understanding design in nature:
"Nature causes some traits that aid survival and reproduction to become commoner, and makes other traits that hinder them to become rarer; all creatures in nature are believed to be perfectly equipped with biological features over successive generations through natural selection."
If this is the much-vaunted role of Darwinism underpinning biology, then it is not impressive. When the designs make the organism "perfectly equipped", Darwinism is the explanation; when the designs are 'imperfect' and 'cobbled together', Darwinism is the explanation. Whatever the evidence, Darwinism has the answer! Yet, when the power of natural selection to select characters is studied, it does not appear very effective at all. Whether it is finch beaks or peppered moths, the classic proofs of the power of natural selection do not take us very far. The suggestion that natural selection acting on successive generations of woodpeckers is a convincing explanation of all the adaptations necessary for the birds to engage in drumming must be challenged. What we have here is a complex and sophisticated system of interrelated traits. Natural selection does not begin to address the assembly of such an exquisite design. The only way we know such systems can be assembled is, like the researchers' new shock-absorbing system, by intelligent design.
A mechanical analysis of woodpecker drumming and its application to shock-absorbing systems
Sang-Hee Yoon and Sungmin Park
Bioinspiration & Biomimetics, 6(1), 2011, 016003 | doi: 10.1088/1748-3182/6/1/016003
Abstract: A woodpecker is known to drum the hard woody surface of a tree at a rate of 18 to 22 times per second with a deceleration of 1200 g, yet with no sign of blackout or brain damage. As a model in nature, a woodpecker is studied to find clues to develop a shock-absorbing system for micromachined devices. Its advanced shock-absorbing mechanism, which cannot be explained merely by allometric scaling, is analyzed in terms of endoskeletal structures. In this analysis, the head structures (beak, hyoid, spongy bone, and skull bone with cerebrospinal fluid) of the golden-fronted woodpecker, Melanerpes aurifrons, are explored with x-ray computed tomography images, and their shock-absorbing mechanism is analyzed with a mechanical vibration model and an empirical method. Based on these analyses, a new shock-absorbing system is designed to protect commercial micromachined devices from unwanted high-g and high-frequency mechanical excitations. The new shock-absorbing system consists of close-packed microglasses within two metal enclosures and a viscoelastic layer fastened by steel bolts, which are biologically inspired from a spongy bone contained within a skull bone encompassed with the hyoid of a woodpecker. In the experimental characterizations using a 60 mm smoothbore air-gun, this bio-inspired shock-absorbing system shows a failure rate of 0.7% for the commercial micromachined devices at 60 000 g, whereas a conventional hard-resin method yields a failure rate of 26.4%, thus verifying remarkable improvement in the g-force tolerance of the commercial micromachined devices
Marks, P. Woodpecker's head inspires shock absorbers, New Scientist (4 February 2011)
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