As the olfactory modality gains a well-deserved importance and understanding among the human senses, there are numerous attempts to mimic performance of the sense of smell using man-made machine olfaction. One of the important problems in the machine olfaction field is the availability of miniature, bio-inspired gas/odor sensors capable of working in conditions similar to those for olfactory receptors. One of the emerging technologies with enormous potential for odor sensing is the spherical surface acoustic wave—ball-SAW—sensors. The chapter introduces the ball-SAW technology and presents the developments made in the field by describing methods of fabricating the chemically interactive membranes onto the ball-SAW devices, properties of the obtained sensors, and their practical implementation. A subsection is devoted to the perspectives of the gas/odor sensing using the ball-SAW sensors.
TopIntroduction
Biological olfaction is a powerful “instrument” enabling reception and processing of chemical stimuli. Thought to be one of the earliest senses developed in course of evolution, the sense of smell is common among virtually all eukaryotes—from insects to humans (Firestein, 2001). The human sense of smell has for long time seemed less important than the senses of vision and audition. Advances in studying and understanding biological/human olfaction make for a gradual increase of the sense’s significance (Buck & Axel, 1991). From the technological standpoint, the growing importance of the sense of smell can be measured in a number of initiatives to incorporate olfactory modality into everyday technology such as odor reproduction and/or presentation techniques.
An indispensable prerequisite for development of the above-mentioned techniques for odor reproduction and/or presentation is presence of a technique to measure odors. To date, detection and quantification of odors has been commonly made using techniques of odorimetry and olfactometry (Steinhart, et al., 2000). However, serious limitations of the techniques called for studies and development of the machine capable of detecting and quantifying odors—an “electronic nose” or “artificial olfaction” systems (Persaud & Dodd, 1982; Nakamoto, et al., 1994). The parallel between the biological and artificial olfaction systems is shown schematically in Figure 1. In simplistic brevity, when we smell an aroma or odor, the air containing mixture of volatile compounds passes through nasal cavity and comes in contact with olfactory receptors. Signals of those receptors are then processed in the specialized part of the brain called olfactory cortex. As the odor sensing systems are intended to mimic performance of the biological olfaction, the artificial olfaction needs to be composed of the two described, general stages—(1) reception and (2) processing of the received signals.
Figure 1. Analogy between biological and artificial olfaction systems
In this chapter our main interest will be in the reception stage—the odor sensors. In humans, the olfactory epithelium—an odor reception field in the nose—hosts 107 – 108 receptors on an area as small as 5 cm2 (Pearce, 1997). Mimicking the biological olfaction must therefore mean miniaturization of the sensors. For this reason we will pay special attention to the novel odor-sensing technology of the spherical Surface Acoustic Wave (SAW) devices—so called ball-SAW devices. Firstly, we will shortly review various technologies applied in the field of odor sensors. Then, we will focus more on the SAW sensors, explaining basics of the planar devices and differences between the planar and spherical SAW devices. In the main part of the chapter, we will introduce techniques used so far for fabrication of the ball-SAW odor sensors, results of the experimental evaluation of the fabricated ball-SAW sensors, and example application of the ball-SAW odor sensors. Finally we will discuss perspectives of the research on ball-SAW odor sensors.
TopOdor Sensors: Various Technologies, One Goal
The first stage of the odor reception in the biological system occurs at the olfactory epithelium, where the odorant molecules interact with the Olfactory Receptors (ORs). In the artificial olfaction systems, the interaction takes place at the odor sensors. Importantly, the gas sensors used in the artificial olfaction need to be non-specific, i.e. they respond rather to group of odorants than to a single analyte. This is based on the concept of electronic noses inspired by the biological olfactory system that uses large number of non-specific olfactory receptors. Although the receptors themselves posses remarkably high sensitivity toward odorants, their responses are not analyte-specific. Signals of those receptors are run through several processing stages leading to an output that not only makes sense of odors/odorants but does so on a remarkable level of sensitivity. For this reason, mimicking performance of the biological olfaction sets a very strict requirement for the sensing elements to be used in its artificial counterpart (i.e. detection limits, conditions of operation, flexibility in engineering the chemically interactive materials, size, etc.).