Smell sensors are often called by "electronic noses". “In most electronic noses, each sensor is sensitive to all volatile molecules but each in their specific way. However, in bio-electronic noses, receptor proteins which respond to specific odor molecules are used.” "Electronic sniffers are also being tested for their ability to detect disease, by sensing blood glucose levels from one’s breath and sniffing out gut disease from samples of poop. "
"E-noses come in a variety of architectures, but most rely exclusively on chemical sensors, such as metal oxides or conducting polymers. " "For sniffing the smells: Odor samples are pushed across these sensors by small air pumps that can be programmed to take up puffs of air in a pattern."
One of the challenges that electronic noses are facing is that the accuracy. Many researchers have raised the accuracy to about 95%, but it is still very hard to meet 100% as I am assuming that will involve a tremendous set of data