Issue Date: April 9, 2012
Shielding Soldiers With Fabric
The threat of chemical and biological warfare agents being released during times of conflict requires today’s soldiers to have protective garments on hand. For men and women in uniform, that means carrying currently available protective gear—gloves, boots, and a special multilayered suit—during critical situations.
This outfit, called joint service lightweight integrated suit technology (JSLIST), often gets donned over the camouflaged uniform the soldier is already wearing. And although it’s called “lightweight,” the suit is made of three separate layers that seal out harmful compounds and airborne pathogens, but also seal in body heat and sweat.
“If you’re in Virginia in the middle of June, with 80% relative humidity, you can wear your JSLIST for about one hour before you become a heat casualty,” explained Tracee L. Harris when kicking off a symposium, held during last month’s American Chemical Society national meeting in San Diego, aimed in part at reinventing the JSLIST. Harris, a science and technology manager at the Defense Threat Reduction Agency (DTRA), was there to moderate the session, sponsored by the Division of Colloid & Surface Chemistry, as well as to put forth a challenge to the research community.
“The thermal burden of the JSLIST limits the duration of wear, affecting the ability of the soldier to complete missions in a chemical/biological warfare environment,” Harris said. “We want suits that have reduced thermal burden, but we’re also looking to broaden their protection.”
DTRA sees fabrics coated with metal nanoparticles and biocidal polymers as a way to accomplish those goals. The idea is that the textiles would kill pathogens and catalytically break down toxic chemicals. So the agency is funding research in that vein.
“The tack being used right now is to develop multifunctional materials and textiles,” Harris explained. The current JSLIST protects soldiers via layers of material, each with a single specific function: The top layer blocks moisture and battlefield contaminants; the middle layer transfers moisture away from the skin; and the bottom layer, which contains activated carbon beads, absorbs chemical and biological agents that make it past the first two layers.
Instead of having each layer carry out one function, as in the JSLIST, “we want to get all those functions into one layer,” Harris added. That combination in one textile should make the garment more breathable and lightweight.
Many of the researchers who talked during the nanoparticle-textile session at the meeting have received funding from DTRA in the past. In San Diego, they presented some of the fruits of that funding: early-stage studies to combine textiles with particles or polymers that have some of the capabilities DTRA desires for its future protective suits.
One of those capabilities is to kill airborne pathogens such as bacteria. David G. Whitten’s group at the University of New Mexico is developing biocidal polymers, in collaboration with Kirk S. Schanze’s group at the University of Florida, to address this threat. During the session, former Whitten postdoctoral fellow Thomas S. Corbitt presented the team’s efforts to combine these polymers with fibers and textile mats.
The polymers, phenylene ethynylene chains containing positively charged trimethylammonium groups, kill microorganisms in two ways. In the absence of light, the conjugated polyelectrolytes—in both long- and short-chain form—stick to the negatively charged membranes of bacteria. They then either penetrate and break apart the membrane or worm their way into the cell, interfering with its inner workings.
“So they kill in the dark,” Whitten told C&EN. “But they do it even more quickly in the light.” In that environment, the polymers absorb visible and ultraviolet light and react with oxygen to form cell-damaging species such as singlet oxygen. Because singlet oxygen is a short-lived species in water, Whitten thinks it is important to the polymers’ activity that they adsorb to the bacterial membrane. That way, when the singlet oxygen is generated, it is close enough to the cell to do damage.
Previously, Whitten and Schanze’s group grafted the polymers directly onto cotton fibers and demonstrated that the polyelectrolytes are still active against infectious bacteria such as Pseudomonas aeruginosa (ACS Appl. Mater. Interfaces, DOI: 10.1021/am200820a). During the ACS meeting, Corbitt discussed the team’s efforts to incorporate the polymers into electrospun fibers as well.
In electrospinning, a polymer is extruded through a high-voltage nozzle and swirled onto a grounded platform to produce porous nonwoven mats of nanoscale fibers. It shows great promise for producing lightweight filtration fabrics such as those desired by DTRA, Harris said.
Corbitt showed that he and the rest of the team are able to distribute their polymers throughout polycaprolactone fibers during electrospinning and that those fibers readily kill Escherichia coli cells.
Another team working to distribute a protective species onto electrospun fibers is led by Juan P. Hinestroza of Cornell University and Omar M. Yaghi of the University of California, Los Angeles. At the meeting, Frederick O. Ochanda, a postdoc in Hinestroza’s lab, summarized the group’s studies on attaching metal-organic framework compounds to fibers.
Metal-organic frameworks (MOFs) are crystalline inorganic-organic hybrids with extremely high surface area, a property enabling their use as strong adsorbents for many different types of gaseous chemicals. DTRA is interested in these compounds’ ability to sequester chemical warfare agents and toxic industrial chemicals.
Ochanda demonstrated that MOFs can be combined with polyacrylonitrile fibers during electrospinning to make a MOF-fiber fabric mat. These composites can suck a significant amount of ammonia gas from the air to which they are exposed, the researchers found.
“But there are also advantages to growing MOFs on electrospun fibers,” Hinestroza told C&EN. For example, he explained, the MOF gets permanently attached to the fiber.
For this reason, the research team also synthesized MOFs directly on polyacryl-onitrile electrospun fibers. To do this, the scientists functionalized the fibers with hydroxylamine and then built the MOFs onto the resulting reactive groups with copper(II) nitrate and 1,3,5-benzenetricarboxylic acid.
When fabricated in this manner, Ochanda showed, the textiles can absorb approximately 50% of a dose of methyl parathion they are exposed to in about two hours. Research groups are using methyl parathion, an insecticide, to test protective fabrics because it resembles the much more toxic nerve agent VX.
Rather than simply sequester toxic species on a textile surface, some researchers are also trying to go a step further and break them down with catalytic particles. Craig L. Hill, a chemist at Emory University, works with polyoxometalates, which are charged nanoscale complexes capable of oxidizing molecules on their surfaces.
At the meeting, Hill demonstrated that because the transition-metal oxygen anion clusters are so heavily charged—the ones he works with often have a charge of minus 10—they readily stick to cotton fibers treated to have a positive charge. And although he and his group are working on other ways of immobilizing the polyoxometalates onto fabrics, he said: “electrostatic immobilization is hard to beat.” When attached in this way, the polyoxometalates survive washing cycles, clinging tightly to the cotton strands.
A host of polyoxometalates can oxidize target molecules such as sulfur mustard, commonly known as mustard gas. The clusters “have this wonderful ability to be fine-tuned to oxidize specific toxic targets,” Hill told C&EN. Even better, a large number of polyoxometalates can then be reoxidized in ambient air for reuse, making them true catalysts.
Hill recently joined forces with UCLA’s Yaghi to incorporate catalytic polyoxometalates into the pores of superadsorbent MOFs (J. Am. Chem. Soc., DOI: 10.1021/ja203695h). The collaborative team showed that the polyoxometalate [CuPW11O39]5– fits snugly into a copper-based MOF and stabilizes both components relative to their normal states. In addition, the MOF boosts the rate at which the polyoxometalate can oxidize the poisonous gas hydrogen sulfide in air.
Still other researchers are working toward DTRA’s vision by studying how to load large numbers of active particles onto fibers. Laura E. Lange, a graduate student in S. Kay Obendorf’s group at Cornell, talked during the ACS meeting about improving fiber morphology to expose more attached nanoparticles to the air.
The group found a winning combination by electrospinning magnesium oxide nanoparticles with a specific ratio of the polymers cellulose acetate and polyethylene oxide. When the scientists expose the freshly fabricated fiber mats to water, the polyethylene oxide dissolves to generate nanofibers with porous channels. These crevices help protect the MgO and expose a large portion of the particles’ surfaces at the same time.
Lange demonstrated that these fibers perform better than nonporous versions at adsorbing and breaking down the VX mimic methyl parathion.
These results and others highlight the progress being made at incorporating active polymers and nanoparticles into textiles, Harris told C&EN. But issues will undoubtedly arise as
“Combining functionalities is always a challenge because some functions may interfere with others,” Cornell’s Hinestroza said. “A proper compromise has to be achieved.”
Washability and production costs will also be obstacles along the path to these materials, Harris said. “At the end of the day, there are going to be many soldiers depending on this suit, and you’re going to have to make yards and yards of fabric,” she added.
Although these multifunctional textiles are a long way from being put onto the battlefield, Harris said, the fundamental research being done now on single-function textiles could have applications beyond the realm of defense.
“There are many potential markets for these materials,” Hinestroza said. They might eventually be applied to high-performance filters for industrial plant emissions, sterile garments for medical environments, or purification systems for planes and cars.
“Science for the warfighter,” Harris said, “ultimately trickles down to all of us.”
C&EN Covers The ACS National Meeting
Want the scoop on the ACS meeting in San Diego? Check out C&EN Picks, a series of videos that spotlight sessions selected by C&EN staff. Reporters also fan out across the meeting to bring you news coverage. Find it all collected at C&EN's meeting page, cenatacs.tumblr.com.
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