Our Silver-Coated Future
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As nanotechnology explodes, and as federal agencies wrangle over whose responsibility it is to deal with an essentially unregulated industry, it's all the more crucial to take stock of the emerging field as soon as possible.
"This is not a technology we want to say no to out of hand," says Jennifer Sass, a senior scientist at the Natural Resources Defense Council (NRDC). "I think this is a technology that is potentially transformative, but we want to use it in a way to take advantage of that while reducing the risk."
Maynard sees this moment as a crossroads for nanotechnology. "What concerns me," he says, "is that if we're not smart about this we'll get something wrong, which would cause unnecessary damage to the environment or to people and would undermine the potential of all nanotechnology."
Nowhere is the tension between real and perceived risk -- not to mention the tension between the mundane and the transformative -- more apparent than with nanosilver. Nanosilver offers an important early test case for two reasons: It is now used in more consumer products than any other nanomaterial, and it is principally designed for use in products that come into direct contact with the human body. Since late 2005, the Project on Emerging Nanotechnologies has been compiling an inventory of products that contain nanomaterials. In the first two years the list more than doubled, to more than 500 products as of the summer of 2007. Of these, nearly 100 contain nanosilver, almost always because of its antimicrobial action.
In its ordinary form, silver is a metallic element with brilliant luster, great malleability, and the ability to conduct both heat and electricity. Its most common use is in photography, as silver halides and silver nitrate on photographic paper, and it has also long been used to make jewelry, coins, and tableware. Its luminosity has inspired songsmiths and poets; Emily Dickinson, for one, described the ocean as "an everywhere of silver."
Silver can also, in its regular form, kill bacteria, fungi, and other infectious microorganisms. (So can other metals, such as mercury and lead, but they, unlike silver, are almost as toxic to the human host as they are to microbes.) When silver is converted to nanosilver, this germ-killing quality is amplified, probably because of a change in surface-to-mass ratio. Silver's antimicrobial action is due to the release of positively charged silver ions on the surface, says Maynard, and "you get higher performance from the same mass of material" at the nanoscale. In addition, nanosilver can be incorporated into plastics, fabrics, and other consumer items more easily than can larger silver particles.
Nanosilver is added to socks and shoe liners to combat foot odor, to bandages to promote healing, to the insides of refrigerators and food storage containers to retard spoilage. It may soon be applied to artificial joints and other implants to reduce the risk of infection. Already there are nanosilver coatings or infusions in computer keyboards, computer mice, nail clippers, dog food bowls, spatulas, back support pillows, pay phones, air purifiers, handrails, ATM buttons -- anywhere one set of hands might come into indirect contact with another set.
"But do we really need to put antimicrobial coatings on a computer mouse?" asks Maynard. "I mean, how many infections are really transmitted by someone using a mouse that has germs on it?"
Just before the 2006 flu season, the government of Hong Kong put nanosilver coatings on the handrails and grab-poles in the city's subway system to help prevent the spread of avian flu. The uncertainty over the health consequences of such an action was driven home in photographs of workers applying the nanosilver. As they sprayed on the nanomaterial -- applied, remember, to protect the public's health -- they were covered head to toe in protective hazmat suits. Granted, nanosilver might be more toxic in aerosol form than it is after it dries, but that's the point -- no one knows for sure. The image of workers spraying stuff on handrails while wearing protective gear and facemasks was, to say the least, disconcerting -- and no one can say whether the nanosilver coating will remain intact once it dries, or whether it will be rubbed off and dispersed after contact with thousands of commuters' hands.
And if hazmat suits are required for workers to apply nanosilver to handrails, what is nanosilver doing in Theramed S.O.S. Sensitive toothpaste? Or in a baby bottle made by the Korean company Baby Dream? Or in another child care product from Korea, NANOVER Wet Wipes, which the manufacturer says are "soft like cotton, protect babies' frail skin"?
Silver has a long history of use in humans, and it has generally been found to be safe. In the days before antibiotics, silver was used as a curative; as long ago as the fourth century B.C., the Greek physician Hippocrates recommended as an ulcer treatment "the flowers of silver alone, in the finest powder."
Because of its germ-killing power, silver has taken on an almost mystical aura. According to folklore, silver repels vampires, and a silver bullet is the only way to kill a werewolf. Housewives in the early 1900s would drop a silver dollar into a bottle of milk, hoping to keep the milk fresh longer. And many doctors still routinely administer eyedrops of silver nitrate to newborns to prevent blindness that could result from an infant's exposure to gonorrhea, chlamydia, or other microorganisms living in the birth canal.
The alternative-medicine community has latched on to silver as an antimicrobial too. Silver is sold in health food stores as colloidal silver, a liquid mixture of silver and water. A suspiciously broad range of claims has been made for colloidal silver, from healing wounds to treating skin cancer. Similarly extravagant claims are being made for nanosilver, as on the Web site of one distributor, Spirit of Ma'at of Sedona, Arizona, which states that its nanosilver supplement "protects against colds, flu, and hundreds of diseases (even anthrax)." The only health risk known to be associated with such supplements is an unsightly (though benign) condition known as argyria, in which the skin is permanently stained blue. (A Libertarian candidate in last year's U.S. Senate race in Montana, Stan Jones, who started using a homemade colloidal silver concoction in 1999, was famous for his ashen blue-gray skin.)