Olfaction / 嗅覚

Olfaction is one of the five senses of living organisms and is mainly caused by the stimulation of olfactory nerve cells by volatile molecules. The human nose contains a total of 5 million olfactory neurons of 396 different types. The human nose is also sensitive to extremely small amounts of malodorous substances, such as ppt (parts per trillion), but the mechanism by which such high sensitivity is achieved is still largely unknown.

There are hundreds of thousands of molecules that indicate odor and they are mixed together in arbitrary concentrations to form a single odor/smell. Moreover, they fluctuate continuously in time and space. Therefore, in order to accurately detect and discriminate such complex mixture of molecules, extremely high performance in terms of sensitivity, selectivity, and response is required, and at present, no olfactory sensor that can surpass the human sense of smell in overall performance has been realized.

On the other hand, there are some components that are difficult for humans to detect but easy for sensors to detect. A simple example is water (humidity). When it comes to water detection, olfactory sensors can achieve significantly higher performance than the nose. Thus, odor detection depends on the combination of the molecular species to be measured and the part that detects it (olfactory nerve cells in the case of the nose, or the sensitive part in the case of a sensor). Therefore, by creating a sensor specialized for a particular measurement target, it is possible to address a specific application.