Researchers have tested a proof-of-concept device that allows people who have lost their sense of smell to detect certain odors. Instead of using the usual smell pathway, where nasal cells send signals along olfactory nerves to the brain, the technology relies on another nerve in the nose that senses other stimuli, such as the heat of wasabi or the coolness of mint.

Zara Patel, a rhinologist at Stanford Medicine who was not involved in the study published in Science Advances, says that while it is an interesting study, the device does not restore the sense of smell but activates a different sensory system. She and others caution that it is still unclear how beneficial this approach might be for people with anosmia.

Humans have around 400 different olfactory receptors, which allow the detection of a vast number of odors. People can lose some or all of their sense of smell for various reasons, including head injuries and viral infections such as COVID-19. Long-term anosmia can significantly reduce quality of life and increase the risk of mental health issues, according to Halina Stanley, a research scientist at CNRS and co-author of the study.

Previous research in 2018 showed that electrodes placed in the sinuses near the olfactory bulb, the brain region that processes odor signals, could induce the perception of certain smells, like onions or fruits. Scientists are now exploring implants to directly stimulate the olfactory bulb, similar to cochlear implants for hearing. However, such approaches are complex, invasive, and not yet ready for clinical use.

The current study took a different approach. The trigeminal nerve, which runs throughout the face, including the nasal cavity, detects temperature and irritants. Certain chemicals in food, such as capsaicin in chili, trigger this nerve and produce sensations of heat, cooling, or burning. The researchers aimed to use this nerve to help people with smell loss detect and differentiate odors.

To achieve this, Moustafa Bensafi, Stanley, and colleagues designed a system with a chemical sensor developed by Aryballe to detect specific odors and convert each into a unique pattern of electrical signals. These signals were delivered via electrodes placed inside the nose.

In initial experiments, nine participants—five with normal smell and four with olfactory disorders—were tested with the device to see if they could perceive the electrical signals. Participants described the sensations as slightly unpleasant and irritating. The researchers also tested whether participants could distinguish between four different odors. Scented sticks were held near the odor sensor, which encoded the odors into electrical signals. For example, the smell of lilac was converted into two pulses separated by 400 milliseconds, while raspberry was represented by four pulses spaced 100 milliseconds apart.

Some participants could tell the signals apart but could not identify the corresponding smells. Others had difficulty with the task. After simplifying the system, the researchers found that participants could reliably distinguish between two odors in some experiments. Stanley suggested that with training, people could learn to associate specific electrical patterns with real-world scents.

Eric Holbrook, an otolaryngologist at Massachusetts Eye and Ear and Harvard Medical School, said the technology could have practical applications. A miniaturized version might alert people with anosmia to hazardous odors, such as natural gas, and a wearable in-nose sensor could be more convenient than a handheld device.

However, the technology is unlikely to recreate the overall experience of smell. Even with training, it cannot encode the vast range of human odors or trigger the emotions and memories linked to smells. Stanley said the team is exploring ways to better encode odors digitally and make electrical stimulation more pleasant.

The researchers also face technical challenges, particularly with the odor sensor itself. Unlike a microphone that can record any sound, current odor sensors typically detect only a few smells. They also need to function accurately in different temperatures, humidity levels, and intensities. According to Stanley, the chemistry involved is complex.