A Silver Bullet For Infections?

APS Meeting News

There's more to silver than its pretty shine. For thousands of years, people have been using silver and silver ions for their bactericidal properties. The ancient Romans treated their water with silver coins, and the silver colloid medicine Argyrol dominated wound care in the early 20th century until the advent of antibiotics.

Now, with antibiotic resistance on the rise, scientists are turning to silver once again to ward off infections. Antibacterial silver coatings are showing up on everything from deodorizing insoles to mobile phones to wound dressings.

Silver and silver oxides control infection by killing bacteria and viruses at wound sites, explained Daniel M. Storey, chief technical officer at Nexxion in Longmont, Colo. Silver blocks infection by preventing electron transport in microbes and by impairing cell replication via interaction with DNA. Ionic silver also disrupts microbial structures and functions.

"To date," Storey said, "no pathogens have been able to survive contact with silver, and there have been no reports of allergic reaction in patients." That doesn't mean that silver doesn't have problems, he added. Silver oxide coatings tend to flake and peel away from surfaces. And releasing high levels of silver ions over extended periods of time can kill cells. The latter problem prompted one company to withdraw a sewn-in silver heart valve from the market a few years ago when doctors suspected it was preventing proper healing.

The scientists at Nexxion think they have found a way to overcome these problems. They've developed a low-temperature ionic plasma deposition process for applying silver and silver oxide thin-film coatings to substrates. Because the technique doesn't require elevated temperatures, the Nexxion researchers can infuse silver and silver oxide into the surfaces of plastic medical devices so that they won't slough off. The plastics, Storey said, release ions without cellular necrosis.

Most silver oxide coatings are Ag₂O, Storey explained. The Nexxion process lets them put down an AgO coating, which quickly dissociates into a silver ion and an oxygen radical when the coating contacts moist areas on the body. "We get a double whammy there," Storey remarked. "There's antimicrobial activity from both the silver and the oxygen. That makes it a lot more potent."

Storey said the company is now working on making coatings with nanoscale roughness for medical implants. This roughness mimics the surface of bone, he explained, and coaxes cells to attach to implant surfaces.

Nexxion has partnered with a handful of companies to create silver-coated medical devices, some of which are already being tested on animals. They hope to win Food & Drug Administration approval for a soft-tissue implant sometime this year, and they are beginning the approval process for a catheter coated with antimicrobial silver.