Image: Mark Hooper

Several hundred years ago, village doctors in rural China diagnosed diabetes by the characteristically sweet smell of a patient's breath. Today hospitals use a battery of blood tests and laboratory analyses to make that same diagnosis, but doctors may soon be sniffing their patients' breath again. This time the doctors will have electronic noses small and cheap enough to carry in their pockets.

This e-nose will be the culmination of decades of work at countless laboratories, where researchers have sought to create a tiny, cheap, automatic sniffer that would let wine bottles monitor the aging of their contents, allow meat packages to flag spoilage, and enable mailboxes to check for bombs. Imagine barroom coasters that double as Breathalyzers, bumper stickers that monitor car emissions. Until now, it's been just so much sci-fi.

E-nose technology has quietly advanced during the past two decades. Commercial models equipped with sensor arrays came to market in the mid-1990s, and today they're used to distinguish wines, analyze food flavors, and sort lumber. Benchtop systems are also used in the pharmaceutical, food, cosmetics, and packaging industries, while smaller, portable units are used to monitor air quality.

But these noses cost in the range of US $5000 to $100 000. A coming convergence between e-nose technology and advances in printed electronics will finally bring the price down—way down. Within a decade we'll see e-noses that cost tens of dollars and appear in smart packaging for high-end items like pharmaceuticals or as part of intelligent or inter­active ­appliances—­picture a refrigerator that knows when milk has gone bad. Prices could easily drop to under a dollar by 2020.

The secret? Conducting polymers. Developers of both electronic noses and printed electronics are exploiting these materials, which can be sensitive to the chemicals that make up odors and are also capable of producing electrical signals. E-nose developers are concentrating on honing the ­sensing properties of conducting polymers, while the printed-­electronics people are investigating ways of using these materials to fabricate ultralow-cost electronics. Combining the fruits of these two separate efforts will finally bring e‑noses into our supermarkets, homes, and daily life.

The human nose is an astounding organ, with millions of odor sensors of hundreds of different types. They let an average adult detect 10 000 ­different odors, which are usually a complex mixture of vapors, or what chemists call volatile organic compounds. Some arise from chemical concentrations in air down in the parts-per-trillion range. A normally functioning person can tell the difference between fresh milk and milk that's gone bad or walk into a house and notice that it's a pie that's baking, not a turkey, merely by sniffing.

Volatile organic compounds shape the aroma and taste of most foods and can act as keen indicators of freshness and quality. But a fresh-cut orange, say, or a piece of Swiss cheese may release hundreds of these chemicals. As far back as the 1950s, researchers built sensors that could detect and quantify the sprawling assortment of chemical components of an odor. But these sensors were difficult to design and had limited use. Even today, most of these basic chemical sensors operate on a lock-and-key strategy, in which a targeted sensing mechanism picks out one specific kind of molecule from the dozens or more in an odor.