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electrical properties in response to pressure and changes shape when an electrical current is applied.The interaction between these behav- iors provides feedback that adjusts the force the material exerts ac- cording to the resistance it encounters, as we humans do.

Sensitive artificial touch is an engineering challenge because it requires many sensors that are densely distributed over an area; syn- thetic smell and taste are difficult to implement because of the sheer variety of what they sample. Nevertheless, concerns about security and crime are motivating researchers to develop artificial smell. A sensitive nose, natural or artificial, can detect explosives, buried land mines, and smoke from fires, as well as hidden drugs. Although the sense of smell is not fully understood, we know that humans identify smells by means of about a thousand special proteins in the nose, each of which reacts to a particular group of molecules, typically of an organic substance. Most odors do not come from just one chemical element or compound. When we recognize a smell as coffeeor vanilla,we are identifying a set of molecules that has activated a particular pattern of proteins, which means we can recognize many millions of odors.

An artificial nose, therefore, must first react to specific chemicals, and then register the different compounds in a given odor. Moreover, to become a useful digital technique, it must change chemical reac- tions into electronic impulses. The Cyrano 320, an electronic nose made by Cyrano Sciences of Pasadena, California, uses a small chip with 32 receptors. Each receptor consists of a specific polymer mixed with some carbon black, a form of carbon that conducts electricity. When exposed to a vapor, each polymer expands by an amount de- termined by the molecules making up the vapor. This expansion changes the electrical resistance of each polymer and hence of the entire chip, producing a composite fingerprint reflecting all the mol- ecules the chip has detected.Although 32 receptors is not many com- pared to the thousand proteins in the human nose, it is still enough to identify a lot of odors.

An artificial tongue can operate in a similar way, because all the flavors we experience, from ice cream to sushi, arise when our taste buds respond to a basic palette: the traditional bitter, sour, sweet, and

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