Badges that change color or brightness upon detecting certain chemical compounds could one day warn people of hazards ranging from spoiled food to toxic nerve gas. According to a report in the current issue of the journal Science, researchers have developed wearable liquid crystal-based sensors that respond to parts-per-billion vapor concentrations within seconds.

Laboratories currently use techniques such as mass spectrometry to sniff out suspicious chemicals, but the equipment involved is bulky and expensive. In contrast, the new devices, engineered by Nicholas L. Abbott of the University of Wisconsin and Rahul R. Shah of the 3M corporation, are small enough to wear and would be relatively cheap to produce.

The sensor consists of an ultrathin gold film to which chemical receptors are attached. These receptors weakly anchor liquid crystal in a specific orientation along the film's corrugated surface. When the receptors are exposed to their target chemical, they bond more strongly to that chemical than to the liquid crystal. As a result, the chemical displaces the liquid crystal into a new orientationa shift that is visible to the naked eye. (Building multiple kinds of receptors into the sensor enables the detection of various chemicals.) When removed from the chemical, the sensor returns to its normal orientation.

So far, the researchers have shown the sensors to be capable of detecting chemicals associated with food spoilage and certain kinds of pesticides. But they note that the devices could find military applications as well.