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Nature points the way to tougher ceramics.

Taking a lesson from seashells and teeth, materials scientists have figured out how to make ceramics tougher and more versatile. One of their new "organo-ceramics" shows promise as artificial bone, they report.

Ceramics have enormous potential as high-tech materials: They resist wear and tea, withstand very high temperatures and do not easily corrode or disintegrate. One even holds the record as the highest-temperature superconductor developed so far.

But ceramics are difficult to process into mechanical parts, and their brittleness can make those parts reliable, notes Samuel I. Stupp of the University of Illinois in Urbana-Champaign. Materials scientists add polymers to minimize such drawbacks, but even these composites fall short for some applications, he says.

Since 1983, Stupp has worked to mimic natural composites such as teeth and shells. Nature's organoceramics contain only small amounts of organi polymers, yet they are quite tough and perfectly shaped for their functions, he says. At the Fourth Chemical Congress of North America, held in New York City last week, Stupp and graduate student Phillip B. Messersmith described successful attempts to strengthen ceramics with various organic polymers.

"The secret lies in the way the organic component is dispersed," Stupp says. By mixing tiny amounts of the long organic molecules into solutions saturated with inorganic crystals, and then precipitating out the resulting organoceramic, the researchers discovered they could make the polymers thread their way among very small ceramic particles. The resulting materials "trap squashed polymers between ceramic, so there's a very intimate dispersion," Stupp explains. These composites should fracture less readily than other ceramics and should be easier to manufacture into useful devices, he says.

Most of these synthesized composites consist of layers of ceramic with organic polymers sporadically jammed between the layers, which the polymers push apart slightly, says Stupp.

But with one of the polymers they used, the researchers found that the ceramic components formed crystals with channels running through them. They think the polymers fit into these channels. "We expect this to be a much more compact material, with a higher mechanical strength," Stupp says.

A new organoceramic made with calcium phosphate seems to work well as artificial bone, he adds. In experiments with dogs, Stupp and his collaborators replaced small pieces of bone with this material and monitored the animals for up to six months. "We've found excellent adhesion between the organoceramic and the surrounding bone," he reports.

The body seems to accept the foreign material as natural, Stupp says, noting that the implants did not activate the canine immune system. He adds that certain bone-eating cells eroded away some of the organoceramic, making room for natural bone to grow into it.

Stupp hopes to tailor ceramics for specialized microelectronic devices by adding organic polymers that conduct electricity or process light. He also envisions replacing the usual polymers with drugs to create organoceramics that slowly release medication within the body. Moreover, he says, "it's not out of the question to follow the same approach with superconducting ceramics" to make them more amenable to processing.
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Author:Pennisi, Elizabeth
Publication:Science News
Date:Sep 7, 1991
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