19 January 2005

Bullet proof abalone

Police officers and soldiers walking in the line of fire may one day want to stop and thank the abalone. That is because engineering researchers at the University of California, San Diego (UCSD) are using the shell of the seaweed-eating snail in an effort to develop a new generation of bullet-stopping armor.

The colorful oval shell of the red abalone is highly prized as a source of nacre, or mother-of-pearl, jewelry, but the researchers found another non-cosmetic use—the shell has an incredible ability to absorb heavy blows without breaking.

The abalone produces a helmet-like home made with 95% calcium carbonate "tiles" and 5% protein adhesive, said Marc A. Meyers, a professor in UCSD's Jacobs School of Engineering, and engineering graduate student Albert Lin.

The highly ordered brick-like tiled structure created by the mollusk is the toughest arrangement of tiles theoretically possible, the researchers said.

An abalone shell can't stop an AK47 bullet. However, laminates of aluminum and other materials have been disappointing as armors. Meyers said a careful examination of the steps taken by abalone to make their shells may help materials scientists develop similarly lightweight and effective body armor.

"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," Meyers said. "The laminate structure of abalone shell has stimulated our group to development of a new synthetic material using this lowly mollusk as a guide."

Mollusk shells, bird bills, deer antler, animal tendon, and other biocomposite materials have recurring building plans that yield a hierarchy of structures from the molecular level to the macro scale, biomimetic researchers found. For example, at the nanoscale, an abalone shell consists of thousands of layers of calcium carbonate "tiles," about 10 micrometers across and 0.5 micrometers thick, or about one-one hundredth the thickness of a strand of human hair. The irregular stacks of thin tiles refract light to yield the characteristic luster of mother of pearl.

Meyers said a key to the strength of the shell is a positively charged protein adhesive that binds to the negatively charged top and bottom surfaces of the calcium carbonate tiles. The glue is strong enough to hold layers of tiles firmly together, but weak enough to permit the layers to slip apart, absorbing the energy of a heavy blow in the process. Abalones quickly fill in fissures within their shells that form due to impacts, and they also deposit "growth bands" of organic material during seasonal lulls in shell growth. The growth bands further strengthen the shells.

The precise way that building blocks of shells assemble determines their strength, and many of those details have been unknown. "Contrary to what others have thought, the tiles abutting each other in each layer are not glued on their sides, rather they are only glued on the top and bottom, which is why adjacent tiles can separate from one another and slide when a strong force is applied," Meyers said. "The adhesive properties of the protein glue, together with the size and shape of the calcium carbonate tiles, explain how the shell interior gives a little without breaking. On the contrary, when a conventional laminate material breaks, the whole structure is weakened."

Meyers and Lin closely measured shell growth by coaxing abalone grown in a laboratory aquarium at UCSD's Scripps Institution of Oceanography. They pushed back a section of the mantle from the shell of individual abalones, glued 15 millimeter glass slides to the shell, and later withdrew the slides at various time intervals and examined the growth of "flat pearl" with a transmission electron microscope.

The flat pearl samples revealed that about every 10 micrometers, the abalone mantle initiated calcium carbonate precipitation. At those points, tiles began to form, growing 0.5 micrometer thick and slowly outward and assuming a hexagonal shape as individual tiles in each layer gradually grew to abut a neighboring tile.

For related information, go to www.isa.org/manufacturing_automation.