Under stress, the tiles of calcium carbonate that compose the shell of the abalone can slide, absorbing energy. Because of this microstructure, abalone shells can absorb a great deal of energy without failing. The mollusk can repair these imperfections. Researchers at the University of California, San Diego, are using abalone as a guide in developing a new generation of bullet-stopping armor. (Date of Image: 2005) [Image 3 of 8 related images. See Next Image .]
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Researchers at the University of California, San Diego (UCSD), are using abalone as a guide in developing a new generation of bullet-stopping armor. Marc A. Meyers, a professor in UCSD's Jacobs School of Engineering, and engineering graduate student Albert Lin, are studying science mimicking nature, or biomimetic, projects. Abalone shells can absorb a heavy blow without breaking and for this reason,Meyers believes that studying the steps taken by abalone to create their shells may help materials scientists develop similarly lightweight and effective body armor for soldiers, police and others.
Biomimetic researchers 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. For example, at the nanoscale, abalone shell is made up 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.
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 are assembled determines their strength. The tiles abutting each other in each layer are not glued on their sides, they're 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. The adhesive properties of the protein glue, together with the size and shape of the calcium carbonate tiles, explains 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 plan to complete their analysis of the abalone shell and generate a mathematical description that can be used by others to construct body armor based on the abalone. In addition to the abalone research, Meyers is working on other biomimetic projects including a detailed engineering analysis of the strong but extremely lightweight bill of the toco toucan, a Central and South American bird that squashes fruit and berries with its banana-shaped bill. [This research supported by the National Science Foundation.]
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