Nanotextiles for Protection

The latest advances in fiber technology should warm the hearts, hands and feet of those working outdoors.

scientist with aerogel

Aerogel is an incredibly light solid composed predominantly of air and is therefore a highly efficient thermal insulator when used in nanotechnology applications. NASA says it feels like hard styrofoam.

The venerable "Old Farmer's Almanac" — on store shelves now — is predicting a colder-than-normal 2009-10 winter, which, depending on where in the northern hemisphere you're reading this, might feel underway already. "Almanac" Editor Janice Stillman made the prognosis sound much more mellifluous, saying, "[I]ndeed, as the days lengthen, the cold will strengthen, and we're looking for more wet than white before Christmas, and after that, come January — a snowstorm; February — a snowstorm; wellbelow normal temps, and snow right up through April, which I know will make a lot of folks happy that go skiing."

Those who will spend the season working outdoors may regard this frosty forecast less felicitously. For workers in industries such as agriculture, commercial fishing, construction, mining, transportation, and utilities, severe cold, ice, and snow can hurt. The hazards of toiling in such conditions are many and well-documented. Slips and falls are perennial problems, as are the various types of cold stress — hypothermia, frostbite, trench foot, and chilblains. Add high wind speeds to an otherwise wet or icy mix, and the dangers are exacerbated. Th ose faced with working in such shivery situations must plan ahead by dressing appropriately. As OSHA notes, when you're working outdoors in winter, "your clothing makes all the difference."

Fibers for Health

Fortunately for workers — and, for that matter, anyone who willingly or by necessity will be exposed to wintry weather for any extended amount of time — today's technology has enabled clothing to make even more of a difference. Developments in fibers and coatings at the nano level have led to increasingly more protective materials for apparel manufacturers' use in recent years, and more developments are on the way.

Researchers at Cornell University, for example, are working with nanoporous membranes for apparel that simultaneously repels moisture while breathing sufficiently to allow perspiration to escape. "Anything that will keep you dry will keep you warmer," says Dr. Kay Obendorf, senior associate dean of research and professor of fiber science and apparel design at the school. "Your whole objective is to keep the textile assembly dry, because heat transfer through water is much more rapid than through air."

In 1978, Gore-Tex® fabrics were among the first generation of materials to accomplish some of this wind-blocking, water-repelling, breathable protection. But whereas Gore-Tex makes use of fluorocarbon- based microporous membranes bundled between breathable, high-performance fabrics, researchers now are working with nanofibers and nanoporous materials that make either the membrane or the top of the textile assembly repellent to water, allowing even less wetting of the textile in cold-weather environments, upping the protective ante.

"With today's membranes made of nanosized fibers, we can build on the things that Gore did," Obendorf says. "Nanotechnology allows us to do more engineering and get better characterization or control of the process. By changing the pore size, you can also make the membrane sensitive to the environment so that the pores get smaller or close whenever it's cold to increase the retention of heat."

Such environmentally responsive, "smart" materials, also called "phase-change fabrics," rely on the nanomolecular bonding of specific polymers with other materials used in the fabric. Different polymers cause the material to be sensitive to different environmental triggers. In medical protective clothing, researchers are trying to get the pores to be sensitive to moisture so they close when confronted by bloodborne pathogens, helping to reduce contamination. In the realm of drug delivery, the polymers are usually pH sensitive. In cold-weather clothing, they are thermally sensitive, expanding or shrinking with the temperature.

Another strategy for keeping apparel dry is positioning the polymers on nanocomposite materials in "lotus leaf " arrangements, which Obendorf says not only promote water repellency but also create a certain surface roughness and give the material selfcleaning properties. The bumpy nature of the lotus layout prevents water sheeting and naturally repels dust and other contaminants.

The combination of surface chemistry and the regularity of the surface roughness results in "superhydrophobic" materials. "Whereas earlier materials were using the surface chemistry for the repellency and then using the smaller pores of the microporous membrane to achieve hydrophobicity, we're now getting down to the nanoscale of pores," Obendorf says. "A nanoscale pore will prevent the flow-through of viruses, as well, so it's very helpful to have the small pores." Much of the science surrounding superhydrophobic materials is still in development, especially pertaining to apparel, but Obendorf says it has already "gone beyond just being a clever thing to do in the lab."

Building a Better Air Trap

Along with wind and water repellency, the name of the game in cold-protective gear is insulation. For much of history, the go-to material for people trying to stay warm in the cold has been wool. The reason wool is such a time-honored insulator is that even when it is spun or woven, as much as 60 to 80 percent of wool cloth can be air, and in insulation, air is what you want—dead air, that is — trapped air that will not move around freely and that is small enough that natural convection currents do not arise in it. Such air when next to the skin is heated by the body and in return provides a layer of warmth. The key to getting this dead air space with conventional clothing is to wear a number of layers.

Aside from its itchiness, one of the downsides to wool is that it can absorb a lot of water and get heavy in the process. Wool releases water slowly, with a minimum chilling effect, but any water on the skin can lead to heat loss. Water absorbency is also the main problem with using down feathers, another age-old insulator that is otherwise very effective because they, too, provide excellent dead air space when they're kept dry. When they get wet, though, they tend to clump, losing their dead air space and letting the cold come through. Still, using down in conjunction with hydrophobic or superhydrophobic materials makes for an effective Old World/New World insulative combination.

A thermoregulating fabric called Klimeo uses nanotechnology-developed microcapsules grafted to the fibers of Australian merino wool. Th ese capsules are designed to change with the temperature, becoming a solid material in the cold and a liquid when it's warmer.

While nano-free wool and down are still very much alive in the insulated apparel market, newer pile and fleece fabrics have come along with similar insulative capacities and have long been established as competitors. Th ese usually hold less water and dry more quickly than wool but often do not offer the same wind resistance. Other synthetic fibers are also used, and most are made to approximate the efficient provision of dead air space you get with down without its accompanying water-absorbing properties.

Newer still are the so-called "superthin" fabrics such as, among others, Thinsulate ®, Primaloft ®, and Microloft ®, which operate on the principle that by making the synthetic fibers thinner, down to the micorporous level, you can increase the amount of dead air space. The theory is, the more air you can trap, the more insulation you get. Lately, capitalizing on that theory, nanoscientists have taken things a step further, working with a substance called aerogel, which has been recognized by Guinness World Records as the solid with the lowest density on the planet.

Composed of 95 to 98 percent air, aerogel also has the lowest thermal conductivity of any solid and is therefore a highly efficient insulator. Because of its extreme light weight and the tendency of its "dust" to migrate, it must be encapsulated — or sandwiched — between inner and outer layers of material in much the same way down feathers need to be. A difference with the aerogel is that the materials used to encapsulate it must contain the aerogel dust particles, which are small. Nanoparticles of the stuff are thus infused into the fabric. The minute layer inside is essentially air. The tiny fraction of it that is not air is typically an amorphous, noncrystalline silica.

Although aerogel was invented in the 1930s, it has been only with the advent of nanotechnology that its uses in coldweather apparel have been broached. NASA, which used aerogel insulation to protect the electronics of the Sojourner rover on its 1997 mission to Mars and again on other rovers on the 2004 mission to the red planet, is said to be developing a spacesuit using aerogel. The agency's Jet Propulsion Laboratory says that because the substance is 1,000 times less dense than glass, it has earned the nickname "solid blue smoke."

To a large extent, nanoscientists are still in the process of understanding and developing the substance. Boston-based Cabot Corp. began producing Nanogel®, the trade name for its family of silica aerogels, in 2003 at its plant in Frankfurt, Germany. Aspen Aerogels, based in Northborough, Mass., introduced its version of the substance as suitable insulation for a range of applications including outdoor apparel, marketing it as "extreme insulation for extreme environments." Design firms today are taking the aerogels Cabot and Aspen produce and trying to develop them into practical outdoor wear for work or sport.

One of those companies, Memphisbased PolarWrap LLC, introduced an aerogel-infused insole insert three years ago and now has an aerogel-infused glove in production, scheduled for testing this year. PolarWrap President Bruce McCormick, who is also the company's head designer, said he also has an aerogel boot in development with a non-removable insole that is "beyond the testing phase" and simply waiting for the idea to catch on. "Th ere's a cost factor," he says. "The boot manufacturers have to decide if there is a big enough market for someone to pay extra for that extra performance. It's not a 5 or 10 or 15 percent improvement in thermal performance. We're talking 400 percent improvement."

Nano Everywhere

The presence of aerogel and a plethora of other nanoparticles in protective apparel will, in all likelihood, soon be commonplace. According to the journal Nature Nanotechnology, some $1.6 billion is scheduled to be invested in nano-related research in 2010. As the technology becomes not only increasingly accepted but sought out, and the manufacturing processes become refined, streamlined, and commercialized, materials such as nanoinsulation will be as ubiquitous as wool and feathers.

Protective fabrics and other products that have appeared since nanotechnology became all the rave in just the past decade have already achieved a sort of everyday, matter-of-fact quality, even though just two decades ago, they would have been seen mostly as matters of science fiction.

Dr. Karl Jacob, a professor at the Georgia Tech School of Polymer, Textile & Fiber Engineering, specializes in using nano applications for ballistic-resistant fabrics. He notes existing nanofibers can block toxins, kill bacteria on the surface, and still maintain sufficient breathability. Ridding a fabric of static, often less a hazard than a wintertime nuisance, is "pretty easy to do," he says, just by engineering the material with carbon nanotubes. Similarly, concocting "self-cleaning" fabrics is just a matter of attaching titanium dioxide molecules to fibers that then excite when exposed to some sort of radiation, such as daylight. Engineering the bacteria-killing properties on a fabric, which, he notes, makes for a useful material to have in wintertime or anytime, is likewise as "easy" as attaching chlorine-containing polymer molecules called halamides to textile fi- bers at the nano level. Forming such a permanent chemical bond between two dissimilar objects — combining nanomaterials with micro- or macrofibers — is at the heart of the science.

Although he speaks casually of such advancements, Jacob is enthusiastic about the technology and its potential for scientifi c breakthroughs. "The science of it is exploding," he says. "Th ere is a lot of money put toward it now, and a lot of people are working in different areas, with particle sizes that are relatively new. At Fiber Society meetings, for example, you'll see that probably about 80 percent of the papers are on nanofibers, particularly from electrospinning. Ten years ago, you wouldn't have seen even one or two. So the research and developments are moving fast."

Th at is good news for outdoor workers, in the next few months and in winters to come.

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