Engineering whiter bites.

For cost and longevity, it's hard to beat the silver-amalgam fillings that glitter through the smiles of baby boomers and their parents. But over the past two decades, dentists have tended to fill gum-line and other very visible cavities with tooth-colored composites made from ceramic particles embedded in an organic polymer known as BIS-GMA (for bisphenol-A, glycidyl dimethacrylate).

Most dentists have resisted using the more aesthetically pleasing composites for grinding surfaces--initially because the BIS-GMA materials did not hold up as well to wear. Even now, the presence of saliva on the tooth being filled reduces the composites' ability to adhere--and therefore to last--as long as silver-amalgam fillings.

Researchers at the University of North Carolina School of Dentistry in Chapel Hill and Virginia Tech's Center on High-Performance Polymeric Adhesives and Composites in Blacksburg are collaborating to make the composites less sensitive to saliva. Normally, BIS-GMA carries some hydroxyl (OH) subgroups that cause portions of the molecule to attract water. The researchers began by substituting methyl ([CH.sub.3]) groups -- which shun water--for the OH groups, explains UNC materials scientist Duane F. Taylor. In another version, they've started with the methyl-substituted molecule and incorporated some fluorine. "Fluorine makes Teflon water repellent," Taylor says. "We thought that by adding fluorine groups...we could also increase this molecule's water repellency."

Though both strategies worked, versions still on the drawing board may prove even better, he says. When will volunteers get to chew on the new composites? Perhaps within a year, Taylor suggests.