Olfactory Sensors / 嗅覚センサ

Olfactory sensors (electronic nose, e-nose, etc.) are sensors and systems that aim to reproduce the sense of smell. The paper published in 1982 (Krishna Persaud & George Dodd, Nature 299, 352 (1982)) is known as the first report on olfactory sensors. More than 40 years have already passed since then, but unfortunately, no olfactory sensor has yet been realized that is continuously and effectively working to solve problems in the field.

Although there are many factors that make the realization of olfactory sensors difficult, the lack of basic performance such as sensitivity, selectivity, response time, and reproducibility are considered to be the major factors. For example, when measuring odors in the air, it is necessary to accurately measure multiple types of minute odor components at the ppm (parts per million), ppb (parts per billion), or even ppt (parts per trillion) level, while humidity and other parameters fluctuate greatly at the percentage level. This is equivalent to instantly detecting and recognizing several thousand types of fluctuations of 1 mm, 1 µm, or even 1 nm in the ocean, where waves of several meters are fluctuating. Furthermore, it must be reproducible at all times, unaffected by small changes in temperature or humidity (the human nose has been shown to be less sensitive to changes in temperature and humidity: T. Drews et al., European Archives of Oto-Rhino-Laryngology 278, 1465 (2021)).

While the ultimate goal of olfactory sensors is to completely replace and even surpass the performance of the nose, there are potential applications that can be addressed by current olfactory sensors. For example, a stationary device that has a mechanism to expose gases/odors/smells at a constant rate onto olfactory sensor elements while controlling temperature, humidity, etc., has been confirmed to outperform the human nose, even with current olfactory sensors. Although there are still a limited number of applications where such a rather large olfactory sensor device is acceptable, we believe that the ultimate olfactory sensor that surpasses the human or even dog nose will be realized in the near future by gradually refining each component of the olfactory sensor as we realize such applications one by one, contributing to world peace.

嗅覚センサ(ニオイセンサ、electronic nose, e-noseなど)とは、人間の嗅覚機能の再現を目指したセンサ、およびそのシステムのことを指します。1982年に発表された論文(Krishna Persaud & George Dodd, Nature 299, 352 (1982))が、嗅覚センサに関する最初の報告と言われています。それからすでに40年以上が経過していますが、継続的に現場での課題解決に有効に機能している嗅覚センサは、残念ながら未だに実現していません。

嗅覚センサの実現を難しくしている要因は多岐に渡りますが、やはり感度・選択性・応答速度・再現性といった基本的な性能が不足しているというのが大きな要因と考えられます。例えば、大気中でニオイを測定しようとする場合、湿度などがパーセントレベルで大きく揺らいでいる中で、ppm(100万分の1)やppb(10億分の1)といった微量なニオイ成分を、複数種類的確に測定する必要があります。これは言わば、数メートルの波が揺らいでいる海の中で、数種から数千種類の、1 mm、1 µm, さらには1 nm程度の揺らぎを、瞬時に捉えて認識することに相当します。さらに、それを少々の温度や湿度の変化に影響されること無く、いつでも再現性よく実施する必要があります(人間の鼻は、温度や湿度の変化にはあまり影響を受けないことが確認されています:T. Drews et al., European Archives of Oto-Rhino-Laryngology 278, 1465 (2021))。