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Therapeutic Plastics

New polymers repel germs, battle cancer and repair nerves
aging



J. W. Stewart
tube
Image: CHRISTINE SCHMIDT

INDIVIDUAL NERVE CELLS shown above grow on an electricity-conducting polymer--the first such material used in this way.

If the latest annual meeting of the American Chemical Society gives any indication, Mrs. Robinson's set is still right: Plastics hold great promise for the future--particularly in medicine. Indeed, some of the most interesting papers presented at the conference, which brought about 20,000 scientists to San Francisco last week, described innovative new polymers, the generic chemical term, that could help thwart ovarian cancer and infections, and regenerate damaged nerves.

Take, for example, the work of Charles Carraher and Deborah Seigmann-Louda of Florida Atlantic University in Boca Raton. They revealed findings on March 28 that certain plastics can dramatically boost the effectiveness antibacterial drugs for ovarian cancer. They embedded cephalexin--marketed as Keflexa and Keftaba--in a polymer containing tin and tested it against two cell lines, both obtained from ovarian cancer patients who had not responded to conventional chemotherapy.

What Carraher and Seigmann-Louda found--but are as yet unable to explain--was that their drug-polymer mix dramatically inhibited the growth of both lines; in one, cell division was stalled by 97 percent and in the other, 80 percent. They do know that the inclusion of metal seems crucial, and are now testing cephalexin with polymers containing arsenic, alimony and bismuth. "Nothing is a miracle cure," Carraher warns, but this polymer-cephalexin elixir may be a "candidate as a last-defense cancer drug."

Another therapeutically promising plastic comes from Christine Schmidt's lab at the University of Texas in Austin. She has developed an electricity-conducting polymer that--in collaboration with a sugar molecule found in blood vessels and most tissues--can stimulate new growth in damaged peripheral nerves. Schmidt is just starting to work with rodents, but if her findings pan out, it would be clinically significant: Currently the only hope for repairing severely damaged nerves is by transplanting others from elsewhere in the body.

Schmidt's remedy, presented on March 29, works as follows: She brigdes gaps in a damaged nerve with hollow tubes made from the plastic and sugar. Once in place, the sugar slowly breaks down, yielding angiogenetic byproducts--substances that encourage the growth of blood vessels. These new vessels help the nerve begin to regrow within the tube, which degrades over a period of two to six weeks. Hers is not the first synthetic material tested in this way, but it is the the first polymer that conducts electricity, which, Schmidt says "has a beneficial effect on the nerves."

tube
Image: CHRISTINE SCHMIDT

PLASTIC TUBE that conducts electricity helps bridge gaps in damaged peripheral nerves. Sugars in the plastic stimulate new growth and the tube eventually dissolves.

Yet another recent power imparted to some plastics is the ability to kill pathogens on contact. These so-called antimicrobial plastics are made by mixing polymers with special, time-released disinfectants. They have found numerous applications in toothbrushes, mattress pads and children's toys, but have a serious disadvantage: They slowly loose their effectiveness over time as the disinfectants dissapate.

Now they face competition from the first antimicrobial rubber, desribed by Shelby Davis Worley of Auburn University at the ACS meeting on March 27. Worley's creation works in an entirely new way and, as a result, its germ-fighting powers are renewable. In addition, the rubber kills not only bacteria, but viruses and fungi as well. Potential applications range from such medical supplies as gloves, aprons and catheters to consumer goods, including food containers and babies' bottles, nipples and pacifiers; also, condoms made from this new rubber could more readily prevent the spread of sexually transmitted diseases.

Worley synthesized the material by introducing N-halamine groups into polystyrene molecules, polymers common in a variety of rubbers. The N-halamines contain receptors that bind chlorine atoms, which kill pathogens on contact. Although the chlorine atoms in the rubber are slowly used up, they can be replaced by simply putting the rubber into bleach. The N-halamine receptors refill themselves with fresh chlorine from the bleach. The more N-halamine groups you add to a rubber's structure, Worley notes, the stronger its antimicrobial kick.

In all, it is this last technology that is nearest to market. Worley has applied for a patent on antimicrobial rubber and Halosource Corporation of Seattle has plans of creating plastics, clothing and rubbers containing N-halamines. Many other new therapeutic plastics--such as Carraher's anticancer polymer and Schmidt's nerve healing compound--have a long way to go. Still, it seems clear that they too, in one form or another, will eventually come of age. Watch this space.

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