When a polymer film is exposed to a vapor, some of the vapor partitions into the film, causing it to swell. The electronic nose detects this as an increase in the electrical resistance of the film, a value that can be quantified for each film.

(20 detector array creates pattern identifying
the vapor and concentration)

An array of sensors that respond individually to vapors can produce a unique pattern for a given vapor mixture. Pattern recognition done on the output signals from the electronic nose can classify, identify and even quantify, the odor being investigated. The researchers claim that this response is very similar to the way that our own olfactory sense produces diagnostic patters and then sends them to the brain.

The Caltech nose has been demonstrated to detect odors in an ordinary room background; it can direct robotics to turn toward the source of the odor. Signals can be read in real-time or near real-time, because the swelling of the polymer begins immediately after exposure to the vapor.

Even better, the Caltech nose can be used over and over (just like your nose). The films return to their initial, unswollen state after the vapor source is removed; this reversibility has been demonstrated for tens of thousands of exposures.

My first exposure to the electronic nose idea was, of course, the mechanical hound of Ray Bradbury's classic Fahrenheit 451 The mechanical hound could also turn and move in the direction of an odor - very quickly, it turns out, and in real-time.

The mechanical Hound slept but did not sleep, lived but did not live in its gently humming, gently vibrating, softly illuminated kennel back in a dark corner of the fire house...

Nights when things got dull, which was every night, the men slid down the brass poles, and set the ticking combinations of the olfactory system of the hound and let loose rats in the fire house areaway. Three seconds later the game was done, the rat caught half across the areaway, gripped in gentle paws while a four-inch hollow steel needle plunged down from the proboscis of the hound...
(Read more about Bradbury's mechanical hound)

Use these links to get a snootful of additional olfactory electronics:

• SPOT-NOSED Nanobiosensors For Doctors
  The SPOT-NOSED (Single Protein Nanobiosensor Grid Array) project will ultimately result in "electronic noses" to help doctors detect and identify problems by odor.
• RI-MAN Face Tracking, Electronic Nose Robot Update
  RI-MAN, a five-foot tall humanoid robot, is the latest in a series of prototype robots intended to assist with one of Japan's most pressing problems; the care of its older citizens.
• Sniffer Robot With Infotaxis Algorithms On The Hunt
  Is it possible for a robot to follow a scent? Massimo Vergassola and his colleagues at the Pasteur Institute in Paris, France, think so.