Most gas sensors, such as the carbon monoxide detectors found in many homes, monitor for the presence of a single volatile compound. Electronic noses, or e-noses, combine several non-specific gas sensors, which together produce distinct response patterns in response to different vapors. Researchers believe the human nose also employs such pattern detection.
Already used for industrial applications, such as monitoring food freshness, quality control in the chemical industry, and odor control in wastewater management, e-nose technology has been inching into other arenas over the last few years, medicine and defense among them.
Recently, a group from the Cleveland Clinic Foundation in Ohio and Smiths Detection in Pasadena, Calif., trained a 32-sensor e-nose to recognize lung cancer by having 14 people with the disease, and 45 people without it, blow into balloons, the contents of which were then suctioned into the device. "The whole thing is the size of a desktop phone, as opposed to a mass spectrometer, and can be taken to the patient," says Raed Dweik, director of the pulmonary vascular program at the Cleveland Clinic Foundation. The device is also simple to operate, he adds. "Almost on the spot it will tell you that it's cancer or not cancer."
The e-nose software identified a pattern of sensor responses that distinguished the two groups, and then scored 76 test subjects, including 14 with lung cancer and many with other lung ailments, against that pattern.1 The system made the correct diagnosis in 10 of 14 cancer patients, and in 57 of 62 control subjects – a success rate comparable to that of other diagnostic methods, says Dweik. William Hanson, a University of Pennsylvania anesthesiologist who works on e-noses for detecting pneumonia in ventilated ICU patients, calls the results promising and says they warrant further study.
Smiths offers a commercial e-nose (the Cyranose 320), which is about the size of a "walkie-talkie," for $8,000. The company's sensors consist of a nonconducting organic polymer infused with nano-sized particles of metal or carbon, which make the polymer electrically conductive. When exposed to volatile compounds, the polymer swells and the conducting particles spread out, making it harder for electricity to flow between them and thus increasing the material's electrical resistance. Varying the composition of the particles and polymer tunes the material for different compounds.
Optimizing sensitivity is usually straightforward if chemists know what compounds they're looking for, says Steven Sunshine, president of Smiths Detection-Pasadena. In defense applications, "the big push is to become more and more sensitive for earlier warning," he says. "It's difficult to ever be good enough." Constructing a highly sensitive, badge-sized detector is complicated by interference from car exhaust and other vapors, he notes, and the sensitivity limits of the electronics themselves. Yet, such devices, projected to cost under $1,000 and be about the size of a PDA, could be ready in the next year, he adds.
Osmetech (formerly AromaScan) takes a slightly different approach. The company uses electrically conducting polymers such as polypyrrole that change their electrical resistance in response to certain gaseous compounds.
Osmetech has two US Food and Drug Administration (FDA)-approved applications for its e-nose technology: detecting urinary tract infections and bacterial vaginitis. Both sensor arrays consist of four different polymer types repeated 12 times. One major challenge was humidity, which affects the resistance in this type of sensor. "Usually there's water present in a lot of the stuff you want to sample," says Paul Travers, chief scientist at Osmetech. To get around this problem, the system incubates samples at a fixed temperature higher than the dew point, and keeps the evolved gas warm as it flows to the sensor array.
Jonathan Zenilman, an infectious disease specialist at the Johns Hopkins University School of Medicine, evaluated Osmetech's prototype e-nose for detecting vaginal acetic acid as a marker of bacterial vaginitis. One study compared it to the standard tests – staining and symptom-based criteria; another measured the disappearance of acetic acid after treatment. Zenilman says the e-nose worked great in both tests and was relatively easy to use, but he pointed out that it was complicated to set up. "I think the technology was quite innovative," he notes.
Although not as precise as gas chromatography or mass spectrometry, e-noses offer a more nuanced type of detection that has plausible applications in medicine and defense, says Peter Adrian, senior industry analyst for the technical insights division of Frost & Sullivan in Palo Alto, Calif. But applications will depend on the sensors' precision, he adds, and FDA approval can take time.
Nor is that the final hurdle. Even with two FDA approvals, Travers says Osmetech needs partners to fund commercial development of its technology.