Biomedical Engineering Reference
In-Depth Information
1999; Dickinson et al., 1998; Nagle, 1998; www.cyranosciences.com). The technology is now at
a level where electronic noses are commercially available, and they have been applied to environ-
mental monitoring and quality control in such fields as food processing. Generally, an electronic
nose is an array of weakly specific chemical sensors, controlled and analyzed electronically,
mimicking the action of the mammalian nose by recognizing patterns of response to vapors. Unlike
most existing chemical sensors, which are designed to detect specific chemical compounds,
the sensors in an electronic nose are not specific to any one compound, but have overlapping
responses. Gases and gas mixtures can be identified by the pattern of the responses of the sensors
in the array.
Chemical sensors are made from several different materials that act by several different
mechanisms. Conducting polymers such as polyanilines or polypyrroles can be used as the basis
for a conductometric sensor, where change at the sensor is read as change in resistance. The ability
of conducting polymers to detect a wide variety of compounds can be extended by mixing other
polymers with the conductor (Freund and Lewis, 1995). An electronic nose that uses polymers as
the basis of the chemical sensors is under development at JPL for such applications as event
monitoring on the International Space Station. The polymer-based sensors used in the JPL ENose
were developed at Caltech (Lonergan et al., 1996). They are insulating polymers, which have been
loaded with a conductive material such as carbon black. A thin film of the polymer or conductor
composite absorbs vapor molecules into the matrix, and the matrix changes shape and the relative
orientation of the conductive particles. This change results in a change in resistance, which is used
to form the pattern of response. The magnitude of the response can be related to the concentration of
vapor, and mixtures of a few compounds can be deconvoluted. The library of compound patterns
that the ENose contains depends on the particular space in which it is used and the hazards of that
space. New compounds can be added to the library as the device is exposed to them. ENoses in
different spaces can be equipped with different polymers in the array, and therefore, a different
library. The polymers for an array are selected by molecular structure of the polymer and the target
compounds for that array.
1.7.8 Sense of Taste and Artificial Tongue
The sense of taste is another chemical analyzer in biology; it examines dissolved molecules and
ions and it uses clusters of receptor cells in the taste buds (Craven and Gardner, 1996). Each taste
bud has a pore that opens out onto the surface of the tongue enabling molecules and ions taken into
the mouth to reach into the receptor cells. Generally, there are five primary taste sensations
including: salty, sour, sweet, bitter, and umami. A single taste bud contains 50 to 100 taste cells
representing all five taste sensations. Each taste cell has receptors on its apical surface and these are
transmembrane proteins that bind to the molecules and ions that give rise to the five taste sensations.
Several receptor cells are connected through a synapse to a sensory neuron and from there to the
back of the brain, where each sensory neuron responds best to one of the taste sensations (http://
users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Taste.html).
Similar to the electronic nose, researchers explored the development of an electronic tongue that
mimics the biological sensory capability (Vlasov and Legin, 1998; Krantz-Ruckler et al., 2001;
http://csrg.ch.pw.edu.pl/prepapers/pciosek/etong.html). Generally, the electronic tongue is an auto-
matic system for analysis and recognition (classification) of liquids using nonspecific sensors
arrays, data acquisition elements, and analytical tools. The result of E-tongue tests can be the
identification of the sample, an estimation of its concentration or its characteristic properties. Using
this technology allows overcoming the limitations of human sensing including individual variabil-
ity, inability to conduct online monitoring, subjectivity, adaptation, infections, harmful exposure
to hazardous compounds, and effect of mental state. The artificial taste sensors that mimic the
olfactory system consist of various types of sensors including potentiometric sensors, conductivity
measurements, voltamperommetry, and optical sensors. Various techniques and methods can be
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