Materials science targets foot odors and microbes

Silver fibers and other examples of nano­technology are increasingly turning up in shoes, socks, and orthotic topcovers in an effort to control odors, reduce the risk of infection, and improve foot health by making these devices moisture wicking, breathable, antifungal, and/or antibacterial.

By Shalmali Pal

The idea of nanotechnology is more likely to conjure up images of semiconductors, solar panels, and circuit boards than sweaty feet, athlete’s foot, onychomycosis, and foot odors. But nanotechnology, or the ability to see and to control individual atoms and molecules, has become more prevalent in devices that claim to help manage all of these foot conditions and more.

More than 1600 products are currently manufactured with nanocomponents, and apparel, including footwear, ranks high on that list, according to the Consumer Products Inventory.¹ Nanotechnology turns up in shoes, socks, and orthotic topcovers, and can help make these devices moisture wicking, breathable, antifungal, antibacterial—and, of course, odor busting.

“If we can keep the foot in a healthy environment, we’ll minimize the chance for something to go seriously wrong with the foot. Regardless of whether a person has a particular disease, a general goal of everyone would be to keep the feet cool and dry,” said Robert P. Thompson, CPed, executive director of the Institute for Preventive Foot Health (IPFH).

Nanotechnology 101

How does nanotechnology work? It fulfills the “less is more” dictum, explained Michael Meador, PhD, director of the National Nanotechnology Coordination Office, which oversees the National Nanotechnology Initiative (NNI), in Washington DC.

“Once you get down to a scale on the order of one-hundred nanometers or less—one billionth of a meter; a single strand of human hair is about eighty-thousand nanometers wide—conventional physics tend to break down and you give rise to new phenomena,” Meador explained. “The push over the past fifteen-plus years in the NNI has been to understand the mechanisms behind these phenomena. Then, we want to figure out how can we control and exploit those mechanisms to create new materials or devices.”

Two of the most common ways to “control” these phenomena are nanoparticles and nanofibers, both of which are used in footwear. Nanoparticles have a very high surface area and a high aspect ratio—or the length/diameter of the nanoparticle—that makes objects very reactive.

“These two properties mean that, if you add nanoparticles to another material, you can improve the durability of that material and increase the wear resistance,” Meador said.

Nanoparticles are often deposited, sprayed, or used as a coating on another material, such as the topcover for an orthosis; they can also be applied to a fiber or blended into the raw material used to make fibers.

Nanofibers, on the other hand, are “built in” to a textile or fabric. The most common way to produce nanofibers is through a process called electrospinning.

“It basically involves taking a solution of a plastic in a certain carrier solvent. The solution is then passed through a needle that’s charged with a high electric field,” Meador said. “What happens is that you build up charge in this droplet of plastic solution to the point where the large droplet breaks up into very small droplets. These droplets can then be incorporated into the surface of a nonwoven fabric.”

In footwear, either nanofibers or nanoparticles are engineered into a material to achieve breathability or moisture wicking.

The data seems to bear out that these products get the job done, at least in the case of diminishing bacteria in the foot environment. A 2013 study² done at the Shoe Design and Development Centre in Chennai, India, tested insoles/footbeds with porous viscoelastic polyurethane sheets, which were modified using hydrophilic polymers, coated with silver sulfadiazine as an antibacterial agent, and placed in athletic shoes that were treated with bacteria. The hydrophilic polymers took in the moisture from foot sweat and released the silver; the researchers documented interaction between the silver and bacteria in the foot sweat, such as Pseudomonas and Staphylococci, both of which have been linked to diabetic foot infections.^2,3

Another study looked at how copper oxide fared as an anti­microbial and antifungal agent for battling tinea pedis. The copper-oxide impregnated fibers were woven into socks, which were worn by 56 patients with diagnosed tinea pedis. In the nine-day average follow-up, all 56 showed improvement or resolution of erythema, fissuring, vesicular eruptions, scaling, and burning and itching. The results held firm at around 40 days of follow-up for all of the same symptoms.⁴

Less is known about moisture-wicking materials and foot health. In a general sense, wicking fabrics, which are often a blend of synthetic and natural materials treated with nanoparticles, pull perspiration away from the skin and push it through the material’s surface, where it evaporates.

Studies of ostensibly moisture-wicking fabrics have reached conflicting conclusions.^5,6 And lower extremity experts also have given mixed reviews to these nanotechnology devices.

For instance, the American Podiatric Medical Association suggests wearing “wicking socks made of natural or acrylic fiber blends that draw the moisture away from your feet instead of trapping it. Some synthetic blends are designed to wick moisture away from the skin and work best to keep the feet dry.”⁷ But an article in Orthopaedic Knowledge Online Journal was more tepid in its assessment, noting that “for hyperhidrosis, ventilated footwear, insoles, and topical aluminum chloride hexahydrate have been used with variable success. Malodor…usually responds to antimicrobial agents.”⁸

Nanotech in practice

The lower extremity practitioners who spoke with LER: Foot Health also offered mixed reviews on these technologies.

The IPFH’s Thompson, for one, is a fan.

“If we can keep the foot in a healthy environment, we’ll minimize the chance for something to go seriously wrong with the foot. Regardless of whether a person has a particular disease, a general goal of everyone would be to keep the feet cool and dry,” Thompson said. (Disclosure: IPFH has a sponsor company that uses nanotechnology-based antimicrobial materials in some of its products.)

Although Thompson has retired from practice, he said one of his preferred devices when he actively saw patients was a spongy, silver-embedded topcover for orthoses.

“It was an all-purpose device in that it absorbed moisture, absorbed odors, and was antimicrobial,” he explained.

But Thompson didn’t stop there; he said he found nanotech devices were most successful when used as part of a system, such as a pair of moisture-wicking socks along with a silver-embedded orthotic topcover.

“My goal was to try to take away any opportunity for the foot to be in distress. By keeping the bacteria count down, we took away the opportunity for athlete’s foot or toenail fungus,” he said. “By adding the moisture-wicking socks, we minimized the environment where the foot was going to get damp and wet, which is what you’d see with a one-hundred percent cotton sock.”

Of course, one of the primary questions Thompson said he heard from patients was “Does this product cost more?” And, not surprisingly, the answer was “yes.” In today’s market, a pair of moisture-wicking socks can cost $10, which could buy a 10-pack of simple all-cotton socks.

George Trachtenberg, DPM, a podiatrist based in Vestal, NY, said he takes time to educate patients that nanotech devices are an investment in their overall foot health.

“It’s a matter of explaining to the patient what the benefits are,” Trachtenberg said. “There may be more dollars spent upfront on [nanotech footwear], but in the long run, there’s a value to these products.”

And the price point on nanotech products should come down as the technology gains more market traction, Meador suggested.

“Silver is a precious metal, but there isn’t a lot of it used in a pair of socks,” he said. “As manufacturers scale up the production of these fabrics, I think you’ll see a corresponding drop in price, and then consumers should see a benefit in lower cost in the end products.”

Trachtenberg said a good portion of his practice is dedicated to laser-based procedures to treat toenail fungus, which is what attracted him to nanotech devices to control the environment around the foot. Anecdotally, Trachtenberg said, patients who undergo treatment for onychomycosis, but don’t use a nanotech-based system for foot health maintenance, are at a much higher risk of recurrence than those who do, even if they are diligent about changing out standard shoes and socks.

His preference is for moisture-wicking socks embedded with copper, bamboo, or charcoal nanofibers.^9,10

Like Thompson, Trachtenberg has an affinity for system-based nanotech utilization. He said he views the shoe as a container, so “the antimicrobial top cover on the orthoses is only going to address one aspect of that container. If you add wicking socks, you’re controlling more of the environment in that container.”

He recommends taking it one step further by using sanitizing products, such as nanosilver sprays and ultraviolet light-based sanitizing products.

“A shoe will absorb up to eight to ten ounces of perspiration a day, and it takes twenty four hours for the shoe to dry out,” Trachtenberg said. “So you can change the shoes frequently, but you are still putting on a shoe that houses that bacteria, fungi, or viruses. If you spray the shoe with a nanosilver, and let it go from a mist to dry without wiping it out, that will kill a lot of those microbes.”

Expressing some caution about nanotech devices was Andrea Coda, PhD, a lecturer in podiatry at the University of Newcastle in Ourimbah, Australia. In his estimation, these products aren’t commonly used by private practice podiatrists in Australia, though he did deem that as “unfortunate, because I think this technology could be of benefit to podiatrists and their patients with specific podiatric issues.”

One reason these devices aren’t more popular in his community may be a perceived lack of evidence to back their widespread use, Coda said. Another consideration is whether the devices that are embedded with antibacterial metals—silver, copper, titanium—may cause an allergic reaction.

A 2010 review article¹¹ reported that silver compounds used for their antimicrobial properties in wound care can cause contact dermatitis, albeit infrequently. “It is possible that more cases have been missed simply because silver is not commonly considered to be an allergen,” the review authors noted.

While there have also been some reports of allergic reactions to silver sulphadiazine when used for burn therapy, or to copper found in dental products and birth control devices,^12-14 reports on footwear are less easy to come by.

Coda acknowledged that he’s not aware of any contraindications to the use of antimicrobial or antifungal devices in foot care patients, but practitioners should keep in mind the potential for allergic reactions “before we permanently fix an antimicrobial top layer with silver or copper in it to the orthoses.” He advocated asking patients or their caregivers to look for typical signs for allergies—rashes, swelling, redness, itchiness, sudden onset of pain—for at least a week after receiving a device.

Coda also pointed out that the devices can be tailored. In a patient with a recently healed ulcer under the first metatarsal head, for example, rather than using a full antibacterial topcover on that patient’s orthosis, he might just use the antibacterial material on the designated area to reduce the risk of any negative interactions on the rest of the foot.

But, if such a patient wanted to purchase moisture-wicking socks with an antibacterial agent from their local sporting goods store, Coda said, he wouldn’t necessarily bar them from doing that.

“Once a patient has been through the hassle of caring for a long-standing ulcer, then he should be encouraged to adopt any small measure to avoid recurrence and promote self-management,” he said.

Lasting effects

Adding nanoparticles to materials can improve their durability and increase the wear resistance without requiring any more care than traditional fabrics or apparel, Meador said.

“You don’t need to do anything special to any of the nanofabrics, above and beyond what you’d do with normal fabrics,” Meador said. “A lot of these nanofabrics are being using in sports apparel, so I think the idea is to make them ‘wash and wear’—very user friendly.”

Meador did note that, with washing over time, some of anti­microbial nanoparticles will be removed, but not enough to render the device inactive. Nanotech devices also shouldn’t require additional sterilization, he said.

The clinicians LER: Foot Health spoke with were a bit more tempered in their assessment of the devices’ durability.

Thompson cautioned that any orthotic topcover is bound to wear out with use, nanotech or otherwise. Depending on the durometer of the material, he estimated that a topcover could go from a quarter-inch thick to flat in around three months. However, he stressed that this wear will depend on how often the orthoses are used and how well they are cared for.

“These [nanotech] topcoats tended to wear and compress under the bony prominences of the feet, but it was no different than any other topping material in terms of the rate of compression,” Thompson said.

Trachtenberg noted that he has patients who have worn a pair of nanotech socks for up to a year with regular washing without a recurrence of their onychomycosis.

“So I tend to believe that the socks will maintain their [nanotech] properties,” he said. “But I also suspect that the socks will wear out—get holes, wear at the heels—and will need to be replaced before they experience a change in properties because of washing.”

Coda said he would still urge patients—whether they use nanotechnology-based devices or not—to follow the basics of foot care: cleaning, swapping out footwear regularly, drying between the toes, and checking for any obvious signs of a change in foot health.

Shalmali Pal is a freelance writer based in Tucson, AZ.