The detonation of a single pound of explosives hidden aboard an airliner flying over Lockerbie, Scotland, in 1988 was, for many, a turning point in understanding how vulnerable the public is to the actions of terrorists. The bombing of Pan Am Flight 103 and the deaths of its 259 passengers (and 11 people on the ground in Lockerbie) set off a behind-the-scenes, government-funded race to find better ways to detect explosives. That race acquired additional urgency after the September 11 attacks, and it became frantic when IEDs--improvised explosive devices--started killing U.S. soldiers in Afghanistan and Iraq. Now, 20 years after the Lockerbie bombing, Aimée Rose is playing a key role in creating and commercializing ultrasensitive detectors that help to protect us against explosives.
Largely because of Rose's work as a scientist, engineer, and research manager, new types of portable chemical "sniffers" are now widely used to detect trace amounts of explosives in the air. These sensitive instruments are already detecting land mines, IEDs, liquid explosives in sealed containers, and even people who have been in contact with explosives. "You can pull aside as many passengers in security lines as you want, but if you don't have the ability to detect explosives on them, it won't do much good," says Susan Martonosi, an operations researcher at Harvey Mudd College in Claremont, CA, who studies homeland security. "That has long been a weakness in the system."
Rose's chemical sniffers are part of a growing effort to develop explosives detectors that go beyond x-ray scanners, the large instruments commonly found in airports. Whereas x-ray scanners look for the characteristic shapes of bombs and can easily be fooled by tricks such as embedding explosives in electronic devices, the new kinds of detectors find explosives by picking out their distinctive chemical composition. But it's a complex problem, because those chemical signatures are diverse and often extremely faint. (Trained dogs that sniff out the vapor given off by explosives are still the most reliable and sensitive bomb detectors, but they're in short supply.) Airports have largely relied on ion-mobility spectrometers that examine the chemicals in either swabs from luggage or puffs of air blown at passengers in sealed chambers. Swabs can easily miss a well-hidden explosive, though, and analyses of air samples are frequently thrown off by dirt, dust, and other contaminants. Rose's technology, on the other hand, is the first explosives detector that matches the sensitivity of dogs. What's more, it's handheld and easy to use, and it's the only device capable of detecting the hidden liquid explosives that have become a serious security concern in the past few years.
Rose was in college when she first tackled the problem of detecting chemicals and toxic materials. "I wanted an opportunity to put something in people's hands that could affect their lives and maybe make them safer," she says. She was planning to do her graduate work in materials science at Harvard, until she was awakened at six one morning by a phone call from a stranger who spoke in long, enthusiastic rushes. "It was as if he couldn't get the words out fast enough," she recalls. "I was half asleep and very irritated." After a few minutes she was able to make out that the man was a chemistry professor who had just set up shop at MIT, complete with a new grant from the U.S. Defense Advanced Research Projects Agency to develop a chemical sensor capable of detecting land mines. He had seen her application to MIT, an application that everyone else in the department had ignored; would she consider visiting?
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She did visit--after all, she was going be in town anyway--and it didn't take long for the professor, Timothy Swager, to persuade her to sign on as a graduate student in his lab. For one thing, land-mine detection seemed like exactly the sort of project she had been looking for. "The poorest countries in the world are littered with land mines, and that hurts some of the most needy people on the planet," she says. "It's a very motivating cause."
When Rose joined Swager's lab, he was synthesizing polymers that fluoresce when exposed to certain wavelengths of light. If a certain type of smaller molecule--say, one found in an explosive such as TNT--binds with one of these polymer chains at any location, the whole polymer stops glowing. The sudden loss of fluorescence is measurable even if just a single target molecule has bound to the chain, so the polymer can serve as the heart of an extremely sensitive detector. "The polymer acts like a string of Christmas tree lights, where if you knock one out, the whole string shuts down," Rose explains. "The fact that you have a much larger molecule responding means you get a much larger signal, which means you have much greater sensitivities." Tweaking the composition of a polymer could enable it to detect different sorts of molecules. For Swager, Rose, and the rest of the team, the goal was to get the polymers to detect the tiny amount of vaporized explosive drifting in the air immediately above a buried land mine.
Swager, now the head of the chemistry department at MIT, says Rose's contributions were critical to the team's success in using the polymers in ultrasensitive detectors. First, he says, Rose added to the researchers' theoretical understanding of how the polymers respond to light. Then she figured out how to employ that insight to develop polymers that fluoresce more brightly and are thus more easily monitored in a working device.
Still, progress with the polymers came slowly. "We were trying to take what a chemist does with a lot of equipment and boiling beakers on a bench, and make it so it could be embedded in a handheld device that's small, robust, and sensitive," says Rose. But in 1999 the lab successfully tested early prototypes in mock land-mine fields. The potential applications would soon expand greatly in number and urgency. "I had been thinking only about making a contribution to land-mine safety, not the safety of U.S. soldiers and everyone who travels by plane," Rose says. "We weren't expecting September 11; we weren't expecting Afghanistan or Iraq; we weren't expecting terrorists to carry liquid explosives in bottles on planes."
While still a graduate student at MIT, Rose had begun to collaborate with researchers at a startup called Nomadics, which had licensed the polymer technology and had government funding to develop explosives detectors for the U.S. military. Rose completed her PhD in 2003, and in 2004, she joined Nomadics as a research scientist, helping to commercialize the technology. (ICx Technologies acquired Nomadics in 2005.) After years working on basic scientific questions, she now faced an entirely new challenge: how to use the promising advances in chemistry to make a practical working device. The polymer worked well enough in liquid in a test tube, but for use in an actual detector, it would have to be deposited as a thin film and still fluoresce brightly enough for any interruption in light output to be reliably detected.
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The first product to result from the efforts of Rose and her colleagues was a handheld military explosives detector called Fido XT. Unlike conventional detectors, Fido XT can detect a few trillionths of a gram of explosive in the air; it rarely gives out a false positive signal; and it resets almost instantly (some detectors require hours after a hit). Has the device saved lives? "There are definitely success stories," says Rose. "But I can't talk about them." The military isn't keen to let the world evaluate its bomb-sniffing capabilities.
Rose can say that the handheld detectors are frequently deployed at security checkpoints and during patrols in Iraq and Afghanistan. Because of its extreme sensitivity, Fido XT has been particularly useful in catching bomb makers themselves: telltale explosives residues often cling to their clothes and skin. By employing the technology at traffic stops, in public places, and when checking homes in neighborhoods suspected of harboring terrorists, the military hopes to identify and arrest those who have been preparing bombs. "Terrorist cells have a lot of people willing to go out and blow themselves up or plant a bomb, but they only have one or two people who are expert at making bombs," Rose says. "If you can take that person out of the chain, you can prevent hundreds of bombings."
After the release of Fido XT, Rose and ICx set their sights on airport security. Unfortunately, by 2006 the threat had changed. Terrorist plots broken up in the United Kingdom suggested that would-be airline bombers were turning to liquid explosives invisible to existing detectors--including the polymers in the company's device. But Rose's efforts to develop a thin film for the Fido XT provided another payoff: the same method proved useful in making thin films of other polymers, including those capable of detecting the new explosive threat. As a result, the company was able to rush out a new product. The Fido PaxPoint handheld detector is now deployed at airports, where it can instantly pick up molecules of the liquids wafting out of even hidden, sealed containers. "We had been looking at those substances for a few years, and when the threat came to the front burner we were able to move from first prototype to a working airport device in less than a year," Rose says. The ability to tweak the polymers to detect new types of explosives is one of the technology's biggest advantages, she adds.
That's a good thing, because terrorists are likely to keep changing the game, notes Harvey Mudd's Martonosi. "Just as our ability to detect explosives is evolving, the ability to create new explosives is evolving," she says. "It's a moving target." --David H. Freedman
