Some of today's high-tech engineering materials have much in common with the porcupine quill.
Along with wood, cork, sponge and bone, the porcupine quill is an example of one of nature's finest inventions -- the cellular solid. These assemblies of hollow chambers packed together to form a mass have some practical properties that humankind has been putting to good use for centuries. Today, these natural materials are leading the way to a new class of high-tech materials that feature the same cellular structure.
Lorna Gibson, the Matoula S. Salapatas Professor of Materials Science and Engineering and a leading expert in cellular materials, said humans can learn a great deal from nature's skillful engineering. "Nature has designed some remarkably efficient materials, even by the standards of modern engineering," she said. "Studying and emulating those materials can only make us better engineers."
Gibson said nature designs materials in response to the functional needs of animals and plants. "The porcupine wants his quills to remain rigid when a predator approaches -- he can't have them bending or buckling under pressure," she said. "But the quills also have to be lightweight enough to carry around on his back all day." For an engineer like Professor Gibson, the resulting structure -- a thin cylindrical shell with a foamy core -- is a true marvel of nature.
Professor Gibson and her students study both natural and man-made cellular materials to find out how their structure affects their mechanical behavior, particularly for structural applications like aircraft, boats and skis. For instance, aircraft flooring is often made of a sandwich panel where two thin layers of carbon fiber composite surround a cellular honeycomb core.
"Similar sandwich structures occur in nature," Professor Gibson said. "For example, the human skull is composed of two dense layers of bone with a layer of porous bone between them. The result is a thick, stiff skull that's much lighter than a corresponding solid structure would be."
PRACTICAL USE
The practical uses of cellular materials, both natural and man-made, go far beyond their utility as strong, lightweight structural components. Cork, for instance, was used in pre-Biblical times for fishing floats and shoe soles and is still the best material known for sealing wine bottles.
"Cork has tiny pleats in its cell walls that act like an accordion," Professor Gibson explained. "If you press on the top of a wine cork, it doesn't bulge out at the sides. It's a special property that makes it easy to get cork inserted into the neck of a bottle."
As engineers have created man-made cellular materials, they've discovered ways to use them that are unprecedented in nature. For instance, foamed polystyrene makes a great insulating coffee cup, and foamed polyethylene makes an excellent packaging material.
"We can make foam out of almost anything now -- metal, ceramic, and even glass," Professor Gibson said. "The structure is very similar to a natural foam." High-tech applications of man-made cellular materials include filtration systems for molten metals, insulation for shuttle rocket boosters, auto exhaust catalysts, and breathable yet waterproof athletic fabric.
Automobile manufacturers are even experimenting with a foam-filled car chassis that weighs a fraction of the traditional solid steel frame. "A tubular steel chassis filled with aluminum foam would save fuel by reducing the weight of the car," Professor Gibson said. "The impact-absorbing properties of the foam core would also increase crash-worthiness, which has always been a problem with lightweight vehicles."
UNEXPECTED INSIGHTS
Professor Gibson's work with cellular bone has also yielded some unexpected insights into how osteoporosis increases the risk of fracture in the porous bone of the hip and spine. Porous bone, which resembles a hardened foam, is made up of a network of tiny bone structures. With a loss in density, these structures first thin and then some of them disappear completely, leaving a larger cell size within the bone.
"We find that losing some of these tiny structures creates a much greater loss of strength and risk of fracture than simply having everything thin uniformly," Professor Gibson said. "If we knew the structure would start disappearing at a particular level of bone density, this might influence when to start drug therapy for these patients."
Professor Gibson pointed out how osteoporosis illustrates the natural engineering process that created cellular materials. "Natural materials evolve in response to the loads, or weights, that they experience," she said. "If your bones don't experience weight-bearing activities, they will begin to thin. That's why weight-bearing exercise is good for people with osteoporosis. The bone experiences a greater load, so it grows and gets denser."
Professor Gibson's office at MIT is a treasure trove of natural artifacts, including a cork pencil holder, assorted bones, and boxes of porcupine and hedgehog quills. One of the joys of her work, she said, is the opportunity to study such a wide variety of materials. She is fascinated by natural materials, she said, because they have so many lessons to teach us.
This article originally appeared in the Winter 1998 issue of MIT Spectrum.
A version of this article appeared in MIT Tech Talk on April 1, 1998.
