Electronic Mucosa: A Natural Successor to the Electronic Nose System?

Background

The sense of smell has always been seen as the least significant amongst the five major human senses (i.e. sight, touch, taste, hearing, and smell). Evolution created the sense of smell for several uses, such as a warning mechanism for avoiding predators, finding a suitable food source, and locating/choosing a mate for reproduction. However, as humans have evolved, the human olfactory system has degraded in function, probably due to its reduced importance for our survival and our reproductive process.

An electronic nose (or e-nose) is essentially an instrument that comprises an array of chemical sensors with partial specificity coupled with an appropriate pattern recognition system, and used to classify different simple and complex odours. There is no standard definition of what exactly we mean by an e-nose and it is often a topic of discussion amongst researchers. But the definition above is that defined by Gardner and Bartlett over ten years ago (Gardner & Bartlett, 1999) and covers the essential elements of a sensor-based electronic nose as opposed to one based upon, say, Field Asymmetric Ion Mobility Spectroscopy (FAIMS).

In the mammalian olfactory system, lungs are used to draw gases and Volatile Organic Compounds (VOCs) through our nostrils and transport them into the upper chamber or turbinate inside our nasal cavity and so across the olfactory epithelium. This epithelium consists of millions of olfactory neurons containing odour-sensitive receptor cells. In addition, the human epithelium has a mucous coating with cilia (see Figure 1) that acts as both a chemical headspace filter and pre-concentrator as the molecules move along its surface. The olfactory receptor neurons then convert the chemical responses to electronic nerve impulses, i.e. a spike train. The unique patterns of nerve impulses are propagated by neurons through a complex network of glomeruli nodes and mitral cells before going through the higher brain (hypothalamus) for interpretation. For further details see Pearce et al. (2003).