Since the first description of an artificial system by Persaud & Dodd in 1982 ( 5), advances in sensor design have increased the number of different platforms that have been reported, and new data-mining techniques have streamlined data extraction and improved odorant classification. The entire field of artificial olfaction is, at its core, centered on devices that monitor the responses from a collection of broadly cross-reactive sensors and use pattern recognition algorithms to connect these multidimensional signals with a particular odor. Viewed in its entirety, mammalian olfaction is an incredibly complex, yet extremely sensitive and efficient system it is not at all surprising that researchers have labored for many years trying to mimic its capabilities in artificial systems. These patterns are relayed to higher level processing centers in the brain where they are perceived as a particular odor and stored in the memory of the organism. The response of the system to odor presentation clearly has a dynamic response component. All the cells containing each receptor type converge on the olfactory bulb, increasing the signal-to-noise ratio through signal summing. The system is therefore limited not by the number of different sensor types that it contains, but rather by the number of distinct patterns it can generate. Integrating the responses over all the activated receptor types, however, can provide an incredibly complex, multidimensional response pattern that encodes the identity of the odorant. Olfactory receptors (ORs), the membrane proteins responsible for odorant binding and signal initiation, at first glance appear to respond in a highly nonspecific manner. Although mammalian genetics codes for approximately 1000 different olfactory receptor types ( 3, 4), the olfactory system is able to detect many more odorants than the number of receptors by utilizing each receptor in a cross-reactive manner ( 4). Unlike the specific receptor-ligand interactions that characterize virtually all other biological sensing systems, the olfactory system is based on cross-reactive receptors in which a single receptor type can respond to many different odors ( 1, 2). Second, partitioning of molecules within the confines of the nasal cavity into the mucus layer likely causes a separation of the various components in a mixture. First, sampling by a gated “sniffing process” prepares the system for receipt of an odor. The power of the mammalian olfactory system is due to the many unique design elements that are not present in other biological sensing systems. The dynamic range of the olfactory system enables detection of concentrations ranging from saturated vapor to, in some cases, the low parts-per-billion range. Mammalian olfactory systems can detect thousands of different pure odors, as well as complex mixtures, within seconds following exposure.
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