WORLD SCIENCE

"Long before it's in the papers"
December 19, 2005

Sept. 25, 2005
By Eve Downing/Massachusetts Institute of Technology
and World Science staff

Many people love pearls for their soft, delicate beauty. But some engineers admire mother-of-pearl—the substance of which pearls are made—for its brute strength. Evolution has designed it over ages to protect soft-bodied creatures such as sea snails.

A mussel shell showing the nacre or mother-of-pearl layer. (Courtesy U.S. National Oceanic and Atmospheric Administration)

Now, researchers are trying to steal nature’s secret by investigating the material’s structure at scales not much bigger than the width of an atom. They’re hoping to use what they learn to develop better armor for soldiers, airplanes and cars. 

There’s nothing simple about mother-of-pearl. When nature designs a material for toughness, she designs a structure whose intricacy becomes remarkable under the gaze of the most powerful microscopes.

Christine Ortiz and colleagues at the Massachusetts Institute of Technology in Cambridge, Mass., studied these patterns and describe them in a paper that they say will appear in an upcoming issue of the Journal of Materials Research.

Mother-of-pearl, also called nacre, forms the inner layer of shells of mollusks, shelled ocean invertebrates such as snails and clams. 

Ortiz and colleagues studied nacre at the nanoscale, a size scale where lengths are measured in nanometers, or millionths of a millimeter. One nanometer is the width of several average-sized atoms. Researchers have studied the material before, but only down to scales about 1,000 times bigger, Ortiz said. 

The nanoscale is where the material’s structure and properties set the foundation for its overall behavior, Ortiz said, and its complexity at this level is “quite amazing.” 

Nacre is composed of two relatively weak materials: 95 percent calcium carbonate, a brittle ceramic, and 5 percent a more flexible material made of chains of molecules, called a biopolymer. 

These materials are organized into a “brick-and-mortar” structure with millions of ceramic plates, each a few thousand nanometers in size, stacked on top of each other like rolls of coins. Each layer of plates is glued together by thin layers of the biopolymer.

Nacre’s toughness results from the way the materials are combined at different size scales, Ortiz said. “Even though the calcium carbonate is very weak and brittle on its own, one can get enormous increases in toughness” this way, said Ortiz. 

Replacing the weak building blocks of nacre with stronger materials—in a similar design—has the potential to yield much tougher composites for use in armor systems or structural applications like automobile panels or plane wings, she added.

The team imaged the tiny plates cut from the nacre of Trochus niloticus, a sea snail, using a powerful instrument called an atomic force microscope. They found that each plate was divided like a pie into separate sectors, with cylindrical beams running through the thickness of the plates, and a fine surface of bumps, called nanoasperities, which were further organized into groups. Biopolymer molecules crossed over and bound to the array of bumps. 

The researchers then used a diamond-probe tip a few hundred nanometers in size to push into a plate while measuring the resistance it offered. “I was surprised to find that the tablets were both extremely stiff and strong at these length scales,” said doctoral student Benjamin Bruet, a member of Ortiz’s team. 

The team is now studying the forces that glue the plates and biopolymer together, as well as the properties of a single biopolymer molecule, they said. Ortiz’s is also doing similar work with other natural materials, such as bone and cartilage.

“Nature uses nanoscale structural design principles to produce materials with superior mechanical properties,” said Ortiz. “In many aspects, human engineers have yet to achieve the same skill. However, as nanotechnology methods advance, the creation of artificial nacre—and other kinds of high-performance armors—is becoming a more and more realistic goal.”

* * *

WORLD SCIENCE

WORLD SCIENCE