Clever Coating: New polymer may prolong life of medical implants

Lacing the protective polymer coatings on medical implants with copper could reduce the clot-inducing tendency of these devices and may keep them functioning for months or even years.

The new coating material staves off clotting because its copper ions catalyze the production of nitric oxide in the blood, says Mark Meyerhoff of the University of Michigan in Ann Arbor. In turn, nitric oxide relaxes blood vessels, increases blood flow, and prevents clot-forming platelets from attaching to implant surfaces.

“Whenever any foreign body comes into contact with your blood, there’s a propensity for clots to form,” says Larry Keefer of the National Cancer Institute in Frederick, Md. “Platelets often see these artificial implants as damage, so they stick to them.”

Researchers have long recognized that nitric oxide helps the cardiovascular and nervous systems remain in good working order. Meyerhoff and other chemists have spent the past several years developing medical polymers that release nitric oxide directly to prevent clot formation on implants (SN: 1/5/02, p. 13: Available to subscribers at Getting Out the Thorn). Because the polymer coatings are extremely thin, they can carry only a limited supply of nitric oxide.

Nitric oxide–releasing polymers may work fine for short-term applications, says Meyerhoff, but for such jobs as glucose sensing for diabetes control, “you may want the sensor implanted for a year,” he says.

Meyerhoff and his colleagues decided to make a polymer that, instead of directly supplying the body with nitric oxide, would stimulate local production of the therapeutic chemical. To do that, the Michigan researchers turned to copper ions, which they knew catalyze the breakdown of blood chemicals called nitrosothiols, thereby releasing nitric oxide.

The researchers synthesized molecular complexes, each harboring a single copper ion, and incorporated them into the polymer.

To test their material, the Michigan team soaked films of the copper-doped polymer in fresh blood taken from sheep. The polymer readily broke down the blood’s nitrosothiols into nitric oxide. What’s more, after 3 days, the films retained their catalytic activity, says Meyerhoff.

Preliminary animal experiments also look promising. The researchers coated sham catheters with the new material and inserted them into the blood vessels of pigs. When the researchers removed the implants the next day, they didn’t find a single clot on the coated catheters. In contrast, almost half the uncoated catheters that the researchers inserted into pigs became covered in clots, says Meyerhoff, who presented his findings on March 15 at the American Chemical Society meeting in San Diego.

If longer studies validate the technology, just about any medical implant that contacts blood could become safer and longer lasting, says Mark Schoenfisch of the University of North Carolina at Chapel Hill. For example, coronary stents, intended to hold open a blood vessel after angioplasty surgery, eventually can provoke reclogging, so the coatings may prove particularly valuable there.

Says Schoenfisch: “That would be a dynamite future direction.”