Engineers at UCSD working to replicate structure of mollusk shell to make new armor

By: RUTH MARVIN WEBSTER - For the North County Times | Saturday, February 5, 2005 10:32 PM PST ∞

The Mayans ground up abalone shells to use as dentures. Other ancient peoples traded the shells as currency. More recently, the nacre of the abalone has been shaped and polished to fashion lustrous jewelry. But now, the abalone shell may have a new use ---- helping engineers create a better, more resilient kind of bullet-stopping armor.

The body armor used to protect today's soldiers and law enforcement officers and made of laminates of aluminum and other materials has been relatively disappointing in its effectiveness against increasingly aggressive projectiles, said Marc A. Meyers, professor at UC San Diego's Jacobs School of Engineering.

"In our search for a new generation of armors, we have exhausted the conventional possibilities, so we've turned to biology-inspired, or biomimetic, structures," he said.

This month Meyers published a paper in Materials Science and Engineering demonstrating that the ordered tile structure of the abalone shell is the toughest arrangement of tiles theoretically possible.

And while an abalone shell cannot stop a bullet from an AK-47, the team plans to generate a mathematical description of the structure that can be used by others to create similarly effective body armor using new synthetic materials ---- rather than the weak and brittle calcium carbonate (aka chalk) ---- used by the mollusk.

"The abalone takes a really weak and abundant material like calcium carbonate, or chalk, and over millions of years, it has developed this amazingly hard shell," said Albert Lin, an engineering graduate student working with Meyers. "We are trying to recreate this self-organization. We are looking at the geometric organization of the tiles to see how materials self-assemble and what makes the final material so tough."

Meyers and his team believe the key to the shell's strength is the assembly of the calcium carbonate tiles. "The mollusk is a bricklayer," Meyers said "and quite a smart one at that."

Thin hexagonal tiles ---- each about one-half micrometer thick and 10 micrometers across ---- are stuck together with an organic, protein glue. At the nanoscale, the shell is composed of thousands of layers of these tiles with an adhesive on the top and bottom of each tile. This allows the layers to slip apart and absorb the energy of a heavy blow unlike a conventional laminate material which breaks.

Meyers and Lin grow abalone in a laboratory aquarium at Scripps Institution of Oceanography and measure its growth using a transmission electron microscope. Photographed from above by the microscope, the growth surface of the shells has a Christmas-tree appearance with thousands of layers of tiles. The irregular stacks of the thin tiles refract light and give off the characteristic luster of mother-of-pearl.

Meyers' investigation into the structural properties of the abalone is one of many biomimetics ---- or science-mimicking-nature ---- projects at the Jacobs School of Engineering. Meyers is also analyzing the strong, but extremely lightweight bill of the Toco Toucan, a Central and South American bird that breaks fruit and berries in its banana-shaped bill.

"The lessons are there in nature," Meyers said. "It is up to us first to understand how it is done and then how we can mimic it."

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