Seashells Yield Tough Secrets
Scientists tap them for hints on building stronger cars, tanks
LA JOLLA, CALIF.
To tourists wandering through the La Jolla Cave and Shell Shop here, the abalone shells on display make eye-catching souvenirs.
To engineers in a lab at the University of California at San Diego campus up the hill, however, these shells and those of another mollusk known as conch, are spurring thoughts of lightweight materials that could toughen everything from tank armor to auto bodies.
By analyzing the shells' structure at the tiniest scales, researchers hope to uncover the secrets of their strength and duplicate them in a new generation of composite materials.
These efforts are part of a quest by researchers worldwide to replicate nature's materials - from the abalone's armor to the flexible, high-strength fibers of a spider's web.
Known as biomimicry, the field is attracting an increasing amount of interest not only for the qualities of the materials themselves, but for their manufacturing conditions. The best recipes for composite materials that humans have devised so far require large amounts of energy to combine, shape, and bake them. Nature's materials, formed through biological processes, assemble themselves from the simplest of basic chemicals and at more moderate temperatures than the ones needed to form man-made composite materials.
Marc Meyers, professor of materials science and director of the Institute for Mechanics and Materials at UC San Diego, is captivated by his subjects, the rock-hard "mobile homes" for abalone and conch. "These shells are ingenious," he says. "Their toughness is 10 times greater than that of the calcium carbonate they're made of."
The reason, he explains, lies in the two mollusks' skills as biological bricklayers.
Under an ordinary microscope, a cross-section of an abalone shell appears to consist of layers of calcium carbonate roughly 0.2 millimeters thick. Increase the magnification, however, and each of those layers is made up of yet more layers, each about a half a micron thick.
These layers are made of rows of tiny calcium-carbonate "bricks" set end-to-end and held in place by an organic glycoprotein glue. Like bricks in a garden wall, the layers are offset so that each brick rests atop an end-to-end joint between two other bricks.
The conch shell is even more elaborate, with the rows of tiny bricks arranged in a herringbone pattern.
When something strikes the shell, like a rock wielded by a hungry sea otter, a crack may occur in a straight line through a few layers of the tiny bricks. Eventually, however, the impact is dissipated by the layers of organic glue binding the bricks. The crack may continue, but it does so from a different location along the glue layer. It is also smaller than the initial fracture.
The process continues until all the impact's energy is absorbed and the fracturing stops. The shell remains intact because the fracture could not work its way through the layers in a straight line.
A sturdy shield
Although the structure of the abalone shell is well known, Dr. Meyers's team may be the first to have uncovered the conch's intricate pattern.
"I've never seen a [research] paper that describes this," Meyers says as he displays a photo of the unique structure.
Indeed, some research teams have begun the attempt to duplicate the abalone's shell structure. A team led by Mehmet Sarikaya, associate professor of materials science and engineering at the University of Washington in Seattle, used tapes made from a ceramic that combines carbon, aluminum, and boron to form layers 10 microns thick. The resulting "layered" composite was 40 percent stronger than the raw material itself.
Yet the layers were still bulky by molluskian standards. He and his team held that the strength could be improved by reducing the thickness of the layers even more.
Yet a good deal remains to be learned about how important the various components of the shell are to its performance as a shield against predators.
To this end, Meyers and a team of young colleagues, including his son, are using a high-tech air gun to fire steel pellets at small pieces of abalone shells. The group is interested in finding out how significant the glue's role is dissipating the energy of an impact.
"Prior research has shown that wet shells are stronger than dry ones," says Meyers's son, also named Marc. "We're trying to find out what the glue is doing."
The composition of the glue is well-known, he continues, "but under water, the protein may get denatured, so that the shell is held together by the adhesive effect of the water and the protein glue."
The shells are yielding their secrets only slowly.
"It will take many years to come up with [synthetic] materials" that mimic the abalone or conch designs, says the senior Meyers.
After all, he notes, "nature does this in a different way and over millions of years of evolution."