Abstract
The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall input-output (I/O) relationships. Up to this point, our accounts of the systems go along similar lines. The next processing steps differ considerably: whereas in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers, were little studied. Only recently has there been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little-connected fields.
Highlights
We thereby assume that odorants reach their olfactory receptors/sensors wherever the latter are situated in the olfactory epithelium (OE) or the artificial olfactory systems, with time differences that are negligible compared with dynamics of receptor neuron/sensor responses
Our understanding of how biological olfactory system (OS) detect odorants and process olfactory information and how the brain makes sense of this information has considerably increased during the last decades
Artificial solutions not relying on biological OS can be employed for the identification of molecules not perceived by biological systems, e.g., in medical diagnostics, where they can detect pathologies from the measurement of pathognomonic molecules [67], noxious gases such as carbon monoxide (1160), or explosives (1161)
Summary
The sense of smell is of crucial importance for almost all animal species. Its origins lie in the detection and perception of chemical molecules (odorants) in the environment and probably reach back to chemotaxis in unicellular organisms. Recently have there been some attempts to develop artificial OS that mimic their biological counterparts and include receptors (sensors) of biological origin, mostly proteins extracted from biological OS such as olfactory receptors and olfactory binding proteins (OBPs) [16,17,18] Most of these artificial OS designed at the interface between biology and engineering are technically still immature, but some of these very challenging approaches are promising and aiming at sensitivities and selectivities close to their biological counterparts. Possibly disposable, devices able to detect and differentiate between many molecular structures have since ever been the bottleneck of artificial electronic noses, and that is presumably the reason why biological detectors such as olfactory receptors or OBPs have periodically been tried as electronic nose sensors, it has always been obvious that both the handling and the incorporation of biological material in artificial systems, i.e., the coupling of or the readout from such sensors to an electronic interface, are far from being trivial and may even remain elusive and infeasible. This way, colleagues from either side should get a detailed overview of the complementary fields and understand where the parallels between them can be found and exploited
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