Digital Control of Smell

In our daily lives, we acquire information through all five senses—sight, hearing, smell, taste, and touch—to recognize the existence and state of our surrounding environment and objects. Of course, much of the information in our living environment is visual and auditory, and our reliance on them has hindered our sense of smell, making it the most atrophied of the five senses. In our daily lives, apart from eating, we rarely notice the role of our sense of smell. In fact, it is no exaggeration to say that scent media is not used at all in the world of ICT, which includes indispensable parts of our lives such as mobile phones, the internet, and computers.

One reason for this is that, unlike images and sounds, it is difficult to generate and control smells. There are no primary odors that can be mixed to synthesize any smell, like the three primary colors of light. When switching scents, the previous scent must be quickly deodorized, or they will mix. If you continue to smell the same scent, a phenomenon called adaptation occurs, and you will no longer perceive the scent within a few tens of seconds. For these reasons, most scent generators currently being developed are intended for room fragrancing, and there are none that can switch scents at high speed in sync with video.

In my laboratory, with support from the Ministry of Internal Affairs and Communications' Strategic Information and Communications R&D Promotion Programme (SCOPE), we are developing an olfactory display that generates scents in pulses with a duration of less than one millisecond. This device applies inkjet printer technology and can control fragrances in units of several picoliters, ejecting four types of scents at 127 different intensity levels.

Using this device, you can smell fascinating scents you have never experienced before. For example, it is impossible to identify individual fruits in a fruit basket in front of you by smelling them in a single breath unless you are highly trained. This is because scent molecules mix in the air, so what you perceive in one breath is a single, blended smell. Humans perceive smells for about one second after they start inhaling, but by using this olfactory display to eject two types of scents as fine pulses with a certain interval between them, it is possible to clearly recognize the two distinct scents within a single breath without them mixing.

Conversely, when the same scent is ejected twice, if you gradually shorten the interval between the two scent pulses, at a certain point you will perceive it as a single ejection. This is very similar to how in a vision test, the gap in the ring appears to close as it gets smaller. Currently, the sense of smell is rarely tested in health checkups. However, if a practical medical olfactory display is developed, the day may not be far off when you are told, "Your olfactory acuity is 1.5."

We also found that by combining the olfactory display with a respiration sensor and ejecting a scent pulse of about 100 milliseconds once per breath, it is possible to continue perceiving the same scent for several minutes without being significantly affected by adaptation. Furthermore, by dynamically controlling the ejection volume, it is possible to achieve scent fade-ins and fade-outs. This kind of scent direction is still an unexplored field, and it seems to hold many interesting possibilities.