Since ancient times, people have known of the germ-fighting qualities of silver. Dead bodies were wrapped in silver cloth to ward off bad odors. Milk stored in silver vessels didn't spoil as quickly. Now, silver is showing up as a bacteria- and odor-fighting material in a range of contemporary consumer products, from sports socks to washing machines.
Specialty retailer Sharper Image recently introduced a line of plastic food containers infused with silver nanoparticles that are intended to keep food fresher. The boxes, priced at $69.95 for a set of 12, have drawn positive reviews at Amazon.com., which sells Sharper Image products -- including one owner enthusing about strawberries staying fresh for 14 days.
In March, South Korea's Samsung Electronics launched a new washer in the U.S. that uses silver ions to sanitize laundry. Plank, a small Boston company that sells Yoga accessories, recently introduced Cor, a soap with silver as the main active ingredient. The company says its supplier is also developing silver-imbued shampoo and toothpaste.
About three years ago, consumer products incorporating silver as an antimicrobial ingredient -- some made using nanotechnology to bond materials at a molecular level -- took off in Asia. Now some observers believe they are poised to become big in the U.S.
"Silver nanoparticles may very well become the next 'it' product, much like antibacterial soaps that took the consumer sector by storm a decade ago," says Marlene Bourne, president of Scottsdale, Ariz.-based Bourne Research, which specializes in emerging technologies.
The products are typically expensive (the Cor soap, for example, costs $115 per bar). But because they use only tiny amounts of the precious metal, the surge in the price of silver over the past year hasn't had much impact on the products.
But the proliferation of silver-containing products is raising some concerns among state and federal environmental regulators because silver is highly toxic to aquatic life. (It isn't toxic to humans except in large quantities that aren't at issue when it comes to these consumer products.)
"The whole contaminant issue is starting to explode in our face and we need to look and study it further," says a spokeswoman for the National Association of Clean Water Agencies, a nonprofit association of wastewater and drinking water plants in Washington.
In a February letter, the organization asked the Environmental Protection Agency to review consumer products containing silver ions and to consider registering them as pesticides. The group's letter to the agency cited the silver ions that will be discharged into sewer systems when clothing is washed in Samsung's new machine.
The EPA's office of pesticide programs says it is studying the matter. A Samsung manager in the U.S. referred questions to the electronics giant's headquarters in Seoul. Officials there couldn't be reached.
Another broad question is whether resistant strains of bacteria could emerge if the market were flooded with silver nanoparticles.
Silver, in the form of a metal or as dissolved ions, fights microorganisms by interfering with processes such as how how they breathe and reproduce. Tests show that silver ions kill microorganisms ranging from harmful strains of e. coli that cause food-borne diseases to the staphylococcus bacteria responsible for serious infections.
The metal becomes more active against microbes when it's made into small particles because they can cover more surface area when they come into direct contact with bacteria, says Andrew Maynard, a physicist and chief scientific adviser to the Project on Emerging Nanotechnologies at the Woodrow Wilson International Center for Scholars in Washington.
In the new Samsung washer, a grapefruit-size device alongside the drum uses electrical currents to nano-shave two silver plates each time the washer is turned on (the silver should last 10 years or 3,000 cycles before needing replacement). The resulting silver ions are injected during the wash cycle. The company says this process removes or kills 99.9 percent of odor-causing bacteria. It also touts the the SilverCare Technology machines, priced at about $1,200 at Lowe's and Best Buy stores, as energy-efficient because they sanitize clothing without using hot water.
Samsung is also developing refrigerators using similar technology, a spokeswoman said. In Asia and Europe, South Korea's LG Electronics and the German unit of Daewoo already offer silver-lined refrigerators and vacuum cleaners.
Antimicrobial silver is also increasingly popular in athletic and outdoor clothing. Many apparel makers, including Adidas and Polartec, have licensed a silver-coated nylon fiber known as X-Static from Noble Biomaterials Inc. Brooks Sports introduced a line last fall touting the silver fibers' ability to promote thermal regulation as well as odor protection. The line of shirts, caps and socks, called HVAC, short for heating, ventilation and air conditioning, is supposed to keep athletes comfortable in different temperatures by taking advantage of silver's natural energy conduction qualities.
As more clothing companies are using silver yarns, threads and fibers, competition is brewing among manufacturers that make them. NanoHorizons Inc., founded four years ago by two graduate students and their faculty adviser at Penn State University in State College, Pa., uses a proprietary engineering process to disperse silver nanoparticles uniformly through pellets, powders and liquids it makes for textile applications. This helps prevent silver from leeching away in repeated washings, says Dan Hayes, co-founder and director of operations.
Performance apparel maker ARC Outdoors Inc. has incorporated NanoHorizons's SmartSilver in a brand of odor-eliminating underwear, gloves, stocking caps and other items. The company also says it is selling SmartSilver antimicrobial socks to soldiers in U.S. military stores around the world. ARC's E47 brand is available at Wal-Mart stores as well as outdoor specialty retailers Bass Pro Shops and Cabelas, which both cater to hunters and fishermen. (E47 is a reference to silver -- No. 47 on the periodic table of elements.)
ARC said it is also exploring embedding silver nanoparticles in hospital products from surgical scrubs to bed sheets.