|
Posted by --! on February 15, 2006 at 16:50:37:
Abalone Armor: Toughest Stuff Theoretically Possible Centuries of warfare have seen body armor develop from cow hides to Kevlar. Now
scientists are using lab experiments and mathematics to discover a stronger
bullet-proof solution in the beautiful, helmet homes that seaweed-eating abalones
make for themselves. Abalones create a highly ordered brick-like tiled structure for their shells that
is the toughest arrangement of tiles theoretically possible, says Marc A. Meyers of
the University of California, San Diego (UCSD). The tiles are comprised of calcium
carbonate, or chalk, sandwiches coated top and bottom with a thin protein. "The laminate structure of abalone shell has stimulated our group to develop a new
synthetic material using this lowly mollusk as a guide," Meyers said.
Abalone shell cannot stop an AK47 bullet, but careful examination of the steps
taken by abalone to make their shells may help Meyers and other materials
scientists develop lightweight and effective body armor for soldiers, police, spies
and others.
In the past 20 years, engineers and scientists have turned more and more to nature
for design inspiration. The field is called biomimetics.
The idea is that nature has evolved designs for maximum achievement with minimum
effort. One example is the leak-proof packaging provided by an apple and its skin.
Biomimetics is nothing new. The Wright Brothers drew their aviation plans from the
wings of birds. More recently, the development of Velcro came from a man’s
observation of the efficiency of burrs in sticking to a dog’s fur.
In the area of tough stuff, biomimetics scientists also recently have studied bird
bills, deer antlers and animal tendons.
"We have turned to nature because millions of years of evolution and natural
selection have given rise in many animals to some very sturdy materials with
surprising mechanical properties," Meyers said. In experiments with abalone shells, Meyers and his graduate student Albert Lin
discovered that they are made of irregular stacks of chalk tiles one-one hundredth
the thickness of human hair (a total of 0.5 micrometers). The bending of light
through these stacks produces the luster of mother of pearl.
In terms of strength, a positive charge on the protein coating binds to a negative
charge on the top and bottom surfaces of the hexagonally-shaped chalk tiles. This
"glue" is strong enough to hold layers of tiles firmly together, but weak enough to
permit the layers to slide apart, absorbing the energy of a heavy blow in the
process. "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," Meyers said. "On the contrary, when a conventional laminate breaks, the
whole structure is weakened." Lin and Meyers measured the growth of abalone shells grown in a laboratory aquarium
at UCSD’s Scripps Institution of Oceanography. They pushed back a section of the
body wall lining the individual abalone shells, glued 15-millimeter glass slides to
them, and later withdrew slides at various time intervals to examine the growth of
"flat pearl" under a microscope. They found that the abalone’s soft body wall, or mantle, near the shell initiates
chalk precipitation every 10 micrometers, at which points the extremely thin tiles
start to form and slowly expand outward into the shape of hexagons, eventually
abutting neighbor tiles. Photographed from above by a microscope, the shell
surfaces resemble a fir-tree because abalones add layers of tile faster than each
layer is filled in. The team’s goal is to generate a mathematical description of shell growth that can
be used by industry to construct body armor based on the abalone’s ingenious
design.
| 
