Woodpecker's head inspires shock absorbers

By Paul Marks

4 February 2011

Woodpecker vs shock absorber

When aircrash investigators of the future retrieve a flight recorder from the wreckage of a plane they may have the golden-fronted woodpecker, Melanerpes aurifons, to thank for the survival of the flight data. The reason? A shock absorber inspired by the bird's ability to withstand severe deceleration.

A woodpecker's head experiences decelerations of 1200g as it drums on a tree at up to 22 times per second. Humans are often left concussed if they experience 80 to 100g, so how the woodpecker avoids brain damage was unclear.

So Sang-Hee Yoon and Sungmin Park of the University of California, Berkeley, studied video and CT scans of the bird's head and neck and found that it has four structures that absorb mechanical shock.

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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.

The researchers then set out to find artificial analogues for all these factors so they could build a mechanical shock absorbing system to protect microelectronics that works in a similar way.

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 (see diagram).

To test their system, Yoon and Park placed it inside a bullet and used an airgun to fire it at an aluminium wall. They found their system protected the electronics ensconced within it against shocks of up to 60,000g. Today's flight recorders can withstand shocks of 1000g.

"We now know how to prevent the fracture of microdevices from mechanical shock," says Yoon. "An institute in Korea is now looking into some military applications for the technology."

As well as a possible role protecting flight recorder electronics, the shock absorber could also be used in "bunker-busting" bombs, as well as for protecting spacecraft from collisions with micrometeorites and space debris. It could also be used to protect electronics in cars.

"This study is a fascinating example of how nature develops highly advanced structures in combination to solve what at first seems to be an impossible challenge," says Kim Blackburn, an engineer at Cranfield University in the UK, which specialises in automotive impact studies.

"It may inform our thinking on regenerative dampers for vehicles, redirecting the energy into a form more easily recoverable than dumping it to heat," Blackburn adds. "Ultimately, we need to learn from the woodpecker to recover energy and not give the driver a headache."

Nick Fry, chief executive of Formula One team Mercedes GP Petronas based in Brackley, UK, says such ideas could feed into crash protection for drivers taking part in motorsport: "One big issue with Formula One is protecting the driver by getting them to decelerate in an accident situation in such a way that his internal organs and brain aren't turned to mush."

"We do that with clever design of composites, very sophisticated seatbelts and a head and neck restraint system," Fry says. "But this research might be something we can draw on in future – it could be very interesting."

Journal reference: Bioinspiration and Biomimetics, DOI: 10.1088/1748-3182/6/1/016003

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