Shell shocked

Researchers at the University of California, San Diego are using the shell of a seaweed-eating snail as a guide in the development of a new generation of bullet-proof armour.

Engineering researchers at the University of California, San Diego are using the shell of a seaweed-eating snail as a guide in the development of a new generation of bullet-proof armour.

The shell of the red abalone is more often used as a source of nacre (mother-of-pearl), but the UCSD researchers are most impressed by the shell’s ability to absorb heavy blows without breaking.

In a paper published in the January 15 issue of Materials Science and Engineering, Marc A. Meyers, a professor in UCSD’s Jacobs School of Engineering, and engineering graduate student Albert Lin explain for the first time the steps taken by the abalone to produce a helmet-like home made with 95% calcium carbonate and 5% protein adhesive.

Meyers and Lin have demonstrated that a highly ordered brick-like tiled structure created by the mollusc is the toughest arrangement of tiles theoretically possible.

Meyers said a key to the strength of the shell is the 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.

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, said Meyers.

‘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,’ he said.

Meyers and Lin closely measured shell growth by coaxing abalone grown in a laboratory aquarium at UCSD’s Scripps Institution of Oceanography.

They gently pushed back a section of the mantle from the shell of individual abalones, glued 15 millimetre glass slides to the shell, and later withdrew the slides at various time intervals and examined the growth of a 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 neighbouring tile. Photographed from above by a microscope, the growth surface of the shells has a Christmas-tree appearance because abalones add layers of tile faster than each layer is filled in.

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 armour based on the abalone.