Odor Code Sensor and Odor Reproduction

Introduction

The methodology of artificial odor evaluation is mainly performed based on some assumed fundamental odorants in odor sensing systems, which are developed utilizing quartz-crystal odor microbalance, metal-oxide semiconductor, or conductive polymer sensor. However, there exists no obvious fundamental or elemental odor in olfaction; therefore, it cannot be said that this methodology is appropriate for the qualitative evaluation of general odor sensation.

Recent progressed research on biological olfaction reveals existence of hundreds number of odor receptors, receiving modality, and odor recognition mechanisms (Buck & Axel, 1991; Mori & Shepherd, 1994; Torinelli, et al., 2009). One receptor can recognize multiple odorants, and each odorant is recognized by multiple receptors in the olfactory system (Malnic, et al., 1999). This indicates that the olfactory system uses a combinatorial coding scheme. On the standpoint of molecular recognition, odor receptors recognize not whole chemical structures of odorants but their partial structures; i.e. odotope hypothesis (Araneda, et al., 2000; Malnic, et al., 1999). It can be said that a certain odorant activates several odor receptors and evokes combination of activated olfactory receptors; therefore, odor qualitative sensation depends on the combination of the molecular partial structures. Such combinatorial molecular information of odorants is “odor code” that can encode odor identity. Thus, receptors are able to recognize different features of molecules with flexible selectivity, and a particular odor compound may also consist of a number of these determinants that possess some of these features.

A first central nerve system connected to olfactory cells is an olfactory bulb, in which combinatorial coding information of activated olfactory receptors generates odor maps which are composed of activated glomeruli. The activated glomerulus points can be categorized into odor clusters, which respond to groups of odorants that has similar chemical properties (Matsumoto, 2010; Johnson, et al., 2010). Smell brought about natural stuffs forms a sparse pattern of odor cluster map (Lin, et al., 2006); therefore, odor information is not very complex compared with an image treated in biological sense of sight. Thus odor code can be expressed by the odor cluster map that has features of metric space, i.e. similar smell has similar spacial pattern of activated glomeruli in olfactory bulb space (Furudono, 2009). Consequently, similarity of odor qualitative sensation can be determined quantitatively through odor cluster maps.

The odor code is combinatorial information about odor molecules that consists of a substructure combination. Therefore, an odor sensor can be established based on the odor code concept; determinants of odor molecules, such as substructures of molecule, molecular size or functional group (Masunaga, et al., 2008; Izumi, et al., 2008) are target information to be detected by an odor sensing system that detects various volatile chemicals. In order to recognize molecular odor code information like the olfactory system, nanostructure on the sensor surface was developed to control the surface affinities that are attributed to molecular substructure (Masunaga, et al., 2005). Thus, design of nanostructures is important and necessary to detect a substructure of molecules, and such sensor technology may leads to an odor code sensor.

On the other hand, if the odor coding is successfully achieved, the code can be used to reproduce odor by elemental chemicals that represent each elemental odor code. The odor reproduction is very important aim also for sensor developments, because one of the most difficult problems to evaluate odor sensor ability is lack of reproducing way using developed odor sensor output. In other word, the problem of the odor sensor is lack of well-defined sensor output; no one can recognize the odor sensor output is proper or not. Odor code is based on distinct information of molecules; therefore, we can communicate sufficient information about odor using odor code. Consequently, odor sensing system and odor reproducing system can be developed on odor code information other than assumed fundamental odorants, and odor code concept can be a useful guiding principal for development of instruments treating odor (Figure 1).