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Crystal-coated lipids promise new materials.

Nature is the ultimate material maker. Whether in the form of mollusks' shells or vertebrates' bones, nature can produce some of the lightest, strongest substances known.

Taking their cues from nature, Douglas D. Archibald and Stephen Mann of the University of Bath in England report in the July 29 NATURE their success in using tiny lipid tubules as templates for growing minute crystals. Their process mimics some aspects of bone formation, where crystals grow in an organic matrix, hardening the soft tissue.

Specifically, the straw-shaped microtubules, measuring some 120 nanometers in diameter, are made of the sugar-based lipid galactocerebroside. Incorporating a charged, sulfated version of the lipid on the tubules' surface and then soaking them in a solution containing iron causes small iron oxide crystals to grow on the outer surface. Scientists have already coated lipid tubules with metals, but only now have they done so with minerals.

By changing the conditions under which they make the lipids, the researchers can also alter the lipids' shapes, generating crystal-studded tubes, disks, and even fibrous webs. In fact, Archibald and Mann believe other mineral-containing fibers and organic-ceramic composites may be within reach.

The new materials have the added advantage of being exceptionally biocompatible, or friendly to living systems. Many new composites require toxic resins, high temperatures, and organic solvents, making them difficult to manufacture and leaving much toxic residue. In contrast, Archibald thinks scientists can produce these new crystal-coated tubules at low temperatures without corrosive solvents or toxic by-products, thus helping to promote a cleaner environment.

These materials also build their own structures out of simple, repeating patterns - a process called self-assembly that nature has mastered but scientists are just learning.

"Self-assembly occurs all the time in nature," says Archibald, now at the U.S. Naval Research Laboratory in Washington, D.C. "Whenever you grow a crystal, it's self-assembling. In the biological world, we see self-assembly when bones form in animals or sea creatures make shells. In both cases, crystal structures are building themselves on templates. The inorganic crystals are deposited out of aqueous solutions, giving the softer, organic materials some rigidity That's why shells and bones are so strong. It's the crystals embedded within the organic material."

Mastering self-assembly is a formidable task, but one promising the ability to make entirely new types of fabricated materials. "Once the mechanisms that govern the self-assembly of these organic-inorganic composites have been worked out, the possibility of designing new materials will really be with us," notes Mark E. Davis, a chemical engineer at the California Institute of Technology in Pasadena.

"Without self-assembly," Archibald adds, "bones and shells couldn't grow, This process is very important, and much of it is a great mystery, We're learning a lot about the biology of mineralization as we go along, about what gives these organic structures their special physical and chemical properties."
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Author:Lipkin, Richard
Publication:Science News
Date:Jul 31, 1993
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