This model, about the size of a paperback book, shows how the ENose currently under development is expected to look.

Can you tell Coke from Pepsi? Heineken from Budweiser? With your nose? Well, JPL's Electronic Nose can!

Of much more importance to astronauts, however, will be the ENose's ability to monitor the air in their spacecraft and alert them if it becomes contaminated with a toxic substance.

How it works

The Electronic Nose is modeled on the human nose in much the same way that the video camera is modeled on the human eye.

Our eyes contain sensing cells called "cones." There are three kinds: red, green, and blue-violet. Different wavelengths of light stimulate different combinations of cones, each cone to varying degrees, depending on the light's color and intensity. Our brains interpret the mix as any of thousands of different hues. Video cameras perform the same function with red, green, and blue electronic sensors performing the function of the cones.

Our noses similarly contain sensing cells - about a thousand of them - that respond in different combinations to the chemicals that come their way. Our brains interpret the patterns of stimulated cells as odors.

ENose's 32 sensors, consisting of 16 different polymers

JPL's Electronic Nose uses an array of 32 films (two each of 16 different polymers) to simulate the human nose's sensor cells. The films are insulators, but are impregnated with carbon particles to enable them to conduct electricity.

Each film absorbs, to a greater or lesser degree, certain classes of chemical compounds - alcohols or ketones, for example - when a waft of air brings them into contact. Depending on the kind and amount of the compound it absorbs, the film swells or shrinks by a characteristic amount. Swelling drives the carbon particles apart, reducing the ability of the film to conduct electricity (or put another way, increasing its resistance). Shrinking draws the carbon particles closer together, making it easier for electric current to flow across the film (decreasing its resistance).

A computer program reads the pattern of resistance changes across the array, compares it to the patterns stored in its memory from laboratory testing, and identifies the chemical that has been "smelled." If the chemical is on the computer's watch list of hazardous substances and is in a concentration deemed to be dangerous, the ENose can sound an alarm to notify the crew.

The Nose knows

The ENose has a much wider dynamic range than the human nose. It can detect chemicals in concentrations so small that people could not smell them, or so large that they would overwhelm our noses (or a mass spectrometer!). It can also detect chemicals that people find "odorless." And it doesn't suffer from "odor fatigue," the human tendency to grow accustomed and insensitive to smells that start small and increase gradually.

The Polymers

The ENose's 16 sensing polymers are off-the-shelf, selected from among thousands available on the market. During the early stages of the project, its developers used the "Edisonian Method," namely trial-and-error, to find the best-suited films. Now, they're creating computer models to calculate, based on chemical and physical principles, what the response will be between an analyte (the chemical to be analyzed) and a particular polymer. They can analyze the suitability of many more polymers much more quickly this way than by actually trying them out.

Sixteen polymers allow for more than 65-thousand possible combinations, each of which could signal a different chemical. But the ENose is currently capable of identifying 15 substances, and the goal is 24. The difficulty is in training the nose to recognize what it sniffs.

Training the ENose

At 7" x 4" x 4", this ENose was about twice as big as the one now being developed, and needed an external computer.

Training the ENose is a matter of developing computer algorithms for the targeted chemical compounds. The ENose is exposed repeatedly to varying concentrations of known compounds in varied order, and the resulting patterns of resistance changes across the array of polymer films are turned into a computer program. After successful training, the ENose recognizes chemicals by comparing the patterns of their "smells" to those recorded in its data bank.

Innovations

Various electronic noses have been used by industry for a number of years, but the one being developed at JPL is smaller, lighter, and less power-hungry than any of its predecessors. It's also the only one able to identify the components of mixtures of compounds - up to three at a time - and to determine the concentrations of the substances it smells.

The ENose's speed has also been considerably improved. While the polymers respond in fractions of a second, the computer processing of the data they provide takes much longer. In the current version of the ENose, programming advances have reduced analysis time from 10-15 minutes to 2-5 minutes.

Some of the delay in processing time is deliberate - to see whether a response changes over several tens of seconds, as it would if there were a leak, or to determine whether a response is genuine or caused by a drift in sensor resistance. Nevertheless, further time reductions are expected as better, faster computers are developed.

Getting Small

In JPL's current design, a self-contained ENose, including its computer, is about the size of a paperback book: 7 1/2" long by 3" wide by 2" high, down from a device about twice the volume. It may get as small as a deck or two of playing cards.

In case even smaller size is desired, JPL is also exploring the option of a system of golf ball-size sensor heads connected to a central computer - possibly via the WARP wireless communications system that is also under development at JPL, or a sensor web.

Nose in Space

The ENose flew as an experiment aboard Space Shuttle Discovery with John Glenn in 1998, providing the first continuous monitoring of an occupied spacecraft. It performed its scheduled tests successfully, but since there were no outbreaks of pollution during the mission (fortunately for the astronauts) the ENose didn't get an opportunity to show how it could help astronauts deal with unexpected problems.

Goals

The ENose's developers intend to train their instrument to recognize the 24 chemical compounds on NASA's 24-hour SMAC list - that is, chemicals that would be hazardous if they were breathed at a particular concentration for 24 hours or more. This makes the ENose a good monitor for events such as leaks and spills.

They are also working toward giving the ENose the ability to classify substances it hasn't been trained to name, using the computer model they are developing to select polymers for the sensing array. It will ultimately be able to learn what the Space Shuttle or other environment smells like under normal circumstances, and sound an alarm if it detects changes that may be dangerous.

One of its primary objectives will be to give early warning of incipient fires or smolders - especially electrical fires - by detecting the chemicals they emit during their earliest stages, long before they would be noticed by a common household smoke alarm.

A candle flame in microgravity

Fire behaves differently without the influence of gravity, and different chemicals are released before a flame ignites. The characteristics of fire in space are still being studied.

Spinoffs

Back on Earth, the ENose might one day be put to uses as diverse as sniffing for explosives at an airport or for unexploded land mines, monitoring the atmosphere for pollutants, protecting the environments of submarines and other enclosed places, sensing the ripeness of plants for harvesting, warning of spills in chemical plants, even sniffing out tumors and other diseases in a doctor's office.

To learn more about JPL's ENose, go to http://enose.jpl.nasa.gov/.