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NATURAL CHEM/BIO TAGS 12 An optimal system in the future would have sensor arrays with 50â100 unique sensor types. The array would be small enough to allow one-sniff coverage and would be cheap and easy to replace. It would have low power consumption, interface easily with processing electronics, and have enough processing power to implement fast, new pattern analysis algorithms. Detector replacement is necessary because, like natural olfactory cells, sensor arrays will eventually become poisoned with use. To use artificial nose technology for tracking, robots are being built that can follow a scent using search algorithms inspired by the pheromone-following strategy of a mothâi.e., moving back and forth perpendicular to the wind direction to pick up the scent, then moving upwind toward the source. MICROSENSOR DEVELOPMENT Steven Martin discussed the miniaturization of analytical devices, focusing on the MicroChemLab, a handheld gas chromatograph (GC) that can detect chemical warfare agents or other vapors of interest. Microfabricated components such as sample concentrators, analyte separators, and detectors offer small size, low power, low sample volumes, few (or no) reagents, and rapid analysis (e.g., 2 minutes for MicroChemLab). Disadvantages include high initial costs, less versatility than full-size instruments, less sensitivity, and lower resolution. MicroChemLab uses three microfabricated analysis stages: a sample preconcentrator, a micro gas chromatography stage, and a surface acoustic wave (SAW) detector to provide sensitive detection. The preconcentrator and the SAW detector have nanostructured surfaces. The chromatograph stage uses a thin-film sol-gel coating on the column surface that has tailored porosity to provide separation of analyte chemicals. A system can be designed with multiple parallel GC columns to confirm the analysis and reduce the false alarm rate. Various kinds of detectors (other than the standard SAW detector) are possible. In principle, it is possible to use the MicroChemLab as a reader for a signal encoded as a chemical mixture. The retention time window is used to form individual bits: An eluted peak within a given window is a 1; no peak is a 0. If detectors with different specificities are used, further information can be encoded. PANEL 3 DISCUSSION Initial discussion focused on the information requirements for electronic noses. These include training, pattern recognition algorithms, a database for matching patterns, and an algorithm to determine the statistics of matching. Gelperin mentioned an experiment in which an artificial nose was tested to see if it could identify grocery store produce by its odor. The rapidity of response is criticalâthe system must identify the odor within about 2 seconds. In addition to speed, reliability and effective background subtraction are key features. There are trade-offs to be made among these characteristics. Gelperin indicated that the prototype device had trouble distinguishing between some types of produce. A question was raised as to whether these artificial systems really mimic natural systems. For example, the eye does not raster over a patternâit processes certain recognition elements. Do we understand this preprocessing step for olfaction? Gelperin agreed, but felt that the Hopfield model is a good place to start. It was commented that one should focus not just on the size of the array but also on the manner in which the sensors are clustered and on the time derivative of their responses. Another question was raised about whether we understand the vomeronasal organ in mice, where neurons respond to specific pheromones. If we can design an artificial nose whose sensitivity is specific to a given compound, such as TNT, then our analysis is simplified.
NATURAL CHEM/BIO TAGS 13 Can we distinguish one human from another on the basis of scent? There is ongoing research to determine what molecules are different from person to person and what level of difference can be identified. Laboratory tests have been able to distinguish differences in the scents of fraternal twins but not identical twins. Quantitative studies on sensitivity and recognition are very recent. It was noted that natural systems (e.g., a dog's nose) use cascade amplification mechanisms to increase sensitivity; do we need similar amplification in artificial noses? Optically based sensor systems are already within a factor of 10 of the sensitivity of a dog's nose. We would like amplification, but we are getting close to the sensitivity of natural systems without it. Some researchers are looking at ion channel amplification as one mechanism for improving sensitivity.
NATURAL CHEM/BIO TAGS 14
