Universities and companies are putting considerable effort into improving the understanding of how the human nose works and on simulating olfaction electronically. Because the five senses are so important to our interpretation of the world around us, we can expect to see continued progress over the next decade in enhancing and extending our sensory capabilities through new chemical and electronic technologies.
The human nose uses a complex system of interconnected receptors and neurons, which conduct signals to the brain for interpretation. The signals from these nerves travel directly to the limbic system in the brain. When an aroma is sensed, the molecules from the vapor interact with numerous receptors, causing them to send a signal to the brain. The pattern of signals is recognized and interpreted by the brain based on prior training.
An electronic nose works in a similar fashion. An array of chemical sensors is used to mimic the range of protein receptors in the human nose. Each of these sensors is made to respond differently to different odors. The signals from these sensors are connected via electronics to a microprocessor or computer. The electronics replaces the neurons in the olfactory limbic system and the computer or microprocessor acts as the brain. The signals that get sent to the computer can be interpreted using special pattern recognition software.
For example, we do not have a specific Swiss Cheese receptor in our nose but when we smell Swiss Cheese, numerous receptors in the nose are stimulated. This in turn causes numerous limbic system olfactory neurons to fire, sending a complex pattern or electrical signature to the brain. If we have smelled Swiss Cheese previously and stored its specific electrical signature, we recognize the aroma that our nose has sensed as Swiss Cheese. The first time we encounter a smell, we must be taught what this "fingerprint" is so that we can recognize it in the future. The human nose is clearly more sensitive to some smells than to others. While this may in part be evolutionary in nature, it is also possible to learn to smell certain substances.
At Cyrano Sciences, Inc. we are commercializing an electronic nose technology invented at the California Institute of Technology. This technology involves an array of sensors composed of polymers that are filled with conductive particles. When these sensors come in contact with a vapor, the polymer expands changing the resistance of the composite. This change in resistance is transmitted to a computer and the pattern derived from the sensor array is used to determine the type, quantity or quality of the odor that was sensed. This type of information is useful in a wide range of industries including the chemical, automotive, medical, petroleum, food, and fragrances.
One specific application of interest is non-invasive medical diagnostics. Prior to the invention of sophisticated medical equipment, smell was a common diagnostic tool (e.g. Diabetes Mellitus literally means "sweet urine"). While smell is still used as a secondary diagnostic tool in some illnesses, the lack of a reliable and objective analytical tool has prevented its wide spread usage in modern medicine. A dependable electronic nose may allow for a resurgence in the use of smell in the non-invasive diagnosis of disease.
Another possible application of the electronic nose is in areas where humans or dogs are currently used to identify smells. Dogs are commonly used to detect explosives or contraband agricultural goods in airports. Cyrano Sciences and the California Institute of Technology are jointly investigating the use of the electronic nose for land mine detection. Others are working on similar systems.
Deodorants, soaps, perfumes and wines are all consumer products developed with the help of human sensory panels. Replacing or augmenting the human approach with an objective electronic nose is currently under investigation. This tool would not only be useful in the development of new products but also for quality control in the manufacturing environment. Many manufacturing floors are equipped with automated visual inspection.
An inexpensive and reliable nose-on-a-chip may allow for automated aroma inspection during the manufacture of many products. Indeed, they may do for smell what the closed circuit television camera has done for vision. If interest continues at its current rate, smelling devices will soon become ubiquitous in our society.