| The mother-of-pearl growth surface of abalone shell (top) is colored because of how light refracts as it strikes tiny terraces of calcium carbonate. UCSD engineering researchers showed that the terraced, Christmas treelike surface of abalone shell has evenly spaced nucleation sites from which stacks of hexagonal "tiles" of calcium carbonate begin to grow. The top and bottom surfaces of each layer of tiles are separated by a protein adhesive, but the adhesive does not bind the edges of tiles to adjoining tiles. Under stress, tiles of calcium carbonate can slide, absorbing energy. Because of this microstructure, the abalone shell can absorb a great deal of energy without failing. |
Apparently, engineering researchers at the University of California, San Diego do. They are using the shell of the red abalone, a seaweed eating snail, as a guide for developing bullet-stopping armor. The colorful oval shell of red abalone is highly prized as a source of nacre, or motherof-pearl, used in jewelry. But the UCSD researchers are most impressed by the shell's ability to absorb heavy blows without breaking.
Red abalone creates its helmetlike home with 95% calcium carbonate "tiles" and 5% protein adhesive. Calcium carbonate, or chalk, is ordinarily weak and brittle, but research has shown that the mollusk creates a highly ordered bricklike tiled structure that is the toughest arrangement theoretically possible.
Of course, abalone shell can't stop an AK47 bullet. But it looks more promising than laminates and other materials, which have been disappointing as armor. Researchers figure they have exhausted conventional possibilities, so they are turning to biology-inspired, or biomimetic, structures. Biomimetic researchers interested in tough materials have discovered that 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.
Specifically, abalone shell at the nanoscale is made of thousands of layers of calcium carbonate "tiles," about 10 micrometers across and 0.5 micrometer 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.
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 let the layers 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 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. In contrast, the whole structure is weakened when a conventional laminate material breaks.
The abalone shell investigation is one category of biomimetic projects at UCSD. Researchers there are also analyzing the strong, but extremely lightweight bill of the Toco Toucan, a bird that squashes fruit and berries with its banana-shaped bill, and new drug synthesis techniques that duplicate those of microorganisms.