How nature turns weakness into strength.

By: Aimee Cunningham.

                                                                                            March 25, 2006 Vol. 169


his father some 40 years ago. They were in the forest near their home in a small town in Brazil, and Meyers stopped to rest. That’s when he noticed a toucan skull lying on the ground. The bird’s previously bright-yellow beak had faded, he recalls, but was otherwise intact. He picked it up. “The beak was so light, yet it was reasonably strong and stiff,” he says.

Another researcher likes to browse through shell shops for some of her study subjects. On one visit a few years ago, Joanna Aizenberg of Bell Laboratories in Murray Hill, N.J., came upon a type of deep-sea sponge that she had never seen before. “It was clearly, incredibly beautiful design-wise,” she says. After having studied its strength in detail, she adds , “I would now say it’s the most perfect design I have ever seen.

These materials scientists are not the first to be inspired by nature’s engineering skill. For decades, researchers have been marveling at seashell nacre, commonly called mother-of-pearl. But as engineers continue to seek stronger, lighter, more durable materials, they are increasingly looking to examples from nature. “We develop all of these wonderful synthetic materials—metals, polymers, ceramics, composites—but we are kind of running out of ideas,” says Meyers, a materials scientist at the University of California, San Diego.

Nature’s design secrets are particularly valuable because organisms, unlike most engineers, must make do with whatever materials are at hand. “In biological systems, resources are often limited,” says Aizenberg. For example, silica and chalk, two building materials in tough sponges and seashells, aren’t usually known for strength.

A recent crop of studies, including the first to describe the structures and mechanics of the glass sea sponge Euplectdlla ctspcigillum and the Toco toucan’s beak, exemplify how nature finds strength in unlikely places. But while such blueprints are illuminating, borrowing designs from nature to build structures from synthetic materials remains technically challenging.

Identifying the structure “is the easy part,” says Meyers. “More difficult is trying to reproduce it.”

GLASS HOUSE The strength of the sea sponge E. aspergillum impressed Aizenberg. It lives in the western Pacific Ocean as deep as 1,000 meters. The sponge consists of a thin layer of cells that coats an intricate silica, or glass, cylindrical skeleton roughly 20 centimeters long and a few cm in diameter.

“It’s almost 100 percent glass, but it’s very rigid,” Aizenberg says. “You have to really jump on top of this glass cylinder to introduce some cracking, and you still won’t break the whole structure.”

In the July 8, 2005 Science, Aizenberg and her colleagues in California and Germany described how the sponge’s design avoids the normal brittleness of glass. The first, visible level of design consists of vertical and horizontal beams that form the grid making up the cylinder’s walls. Every second square of that grid contains two diagonal beams, and every third set of diagonal beams is thick enough to stick out of the grid’s plane. This three-dimensional structure prevents the cylinder from being crushed when it’s squeezed, notes Aizenberg. Think of how much sturdier a soda can would be if its sides had ridges.

The researchers employed visible-light and scanning electron microscopy to dig further into the design. At the micrometer scale, they found that each beam consists of thinner cylinders cemented together by more glass. These parallel bundles of cylinders are stronger than each cylinder alone, says Aizenberg, because if one cylinder fails, its neighbors can take up the slack.

Furthermore, each thin cylinder consists of concentric rings, like tree rings, of glass glued together by an organic material. The rings are thicker toward the center of the cylinder: Out side rings are roughly 0.2 micrometer thick, while inner rings span about 1.5

This structural characteristic is what makes the sponge “almost unbreakable,” says Aizenberg.

A regular glass rod will crack easily, but in a layered glass rod, the incoming energy from a mechanical load dissipates into the glue between the layers. A crack in one of the thin, outer layers of the cylinder doesn’t travel very far before it reaches the organic glue, which diverts the crack from the next layer.

A final design detail is the glass wires that attach the sponge to the ocean floor. Anchor points are typically weak spots in structures, notes Aizenberg. Rather than thickening the point of attachment, the sponge employs flexibility, loosely incorporating additional thin cylinders into the vertical beams at the bottom of the sponge. This way, the sponge can swing freely, moving with whatever force it encounters says Aizenberg.

The mechanical principles integral to the sponge’s design diagonal ridges, bundled beams, and layered rods—can be found in structures that engineers build every day. But Aizenberg points out that engineers tend to use these principles separately Combining these design elements to create even stronger materials “is something that nature can still teach us,” she says

“This is probably the strongest glass structure that one can imagme,” she continues. “In a way, it’s a glass house at which you can throw stones.

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