 The
term 'electronic nose' has been coined for describing chemical sensing
instruments that use an array of non-specific sensors coupled with some
form of multivariate analysis or neural network to analyze the response
patterns. Comparing the functions of the electronic nose with the
biological smelling system it is easy to see the parallels and understand
why the term "electronic nose" came into being. However this does give
rise to the unfortunate conclusion that these machines can smell,
potentially as well as humans or dogs, which is not the case. Here we'd
like to explain biological smell in more detail.
The cross section of a head shows the route of volatile odorant
compounds into the olfactory area from the nose and the mouth. When you
sniff currents of air swirl up over the turbinate bones and to a sheet
called the olfactory epithelium. Our sense of taste is greatly influenced
by our sense of smell, as shown the odor from our food is also brought
into the olfactory epithelium. In an electronic nose it is not the act of
sniffing or masticating that brings the odor into contact with the sensors
but usually an electrical pump.
The olfactory epithelium is small, approx. 1 square centimeter per
nostril, and yet contains an estimated 50 million primary sensory cells in
humans. Each of the sensory cells has miniscule filaments that extend from
the surface of the epithelium into the watery mucus. Each filament
contains a protein that is the molecular receptor that interacts with the
incoming odorant molecules. Biological systems have thousands of
non-specific chemical receptors that can respond to almost any combination
of chemicals whereas electronic systems currently have tens of unique
sensors.
The exact nature of the interaction between receptor and odorant
molecule is still being studied, for latest research please consult our olfaction
links. It is known that not every receptor interacts with every
odorant molecule. When an interaction does occur a sequence of events is
initiated involving a rush of electrical activity in the brain. This is
shown in the image by the flashing olfactory bulb and then sequentially
the flashing brain. It is in the brain that the signals from the neurons
in the nose are processed, decoded and the interpretation occurs. In the
electronic nose the brain is the microprocessor programmed with algorithms
to decode the pattern of response from the sensors.
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