Always fascinated with nature's superb engineering feats, scientists and engineers are always on the run to replicate/mimic the natural engineering. Human body's five sensory organs are also such great feats of natural engineering. With the growing dependency on electronic gadgets due their suitability, versatility, compatible cost and easy adoptability scientist are trying to develop an electronic nose (e-nose) which will be Hand-held model of e-nose

more impartial, versatile, and cost effective to diagnose any thing in daily life to extra ordinary situations. Over the last decade, electronic sensing or e-sensing technologies have undergone important developments from a technical and commercial point of view. The expression "electronic sensing" refers to the capability of reproducing human senses using sensor arrays and pattern recognition systems. Since 1992, research has been conducted to develop technologies, commonly referred to as electronic noses, that could detect and recognize odors and flavors.

Electronic noses are one example of a growing research area called biomimetics, or biomimicry, which involves human-made applications patterned on natural phenomena. A better understanding of the reception, signal transduction and odor recognition mechanisms for mammals, combined with achievements in material science, microelectronics and computer science has led to significant advances in this area. The stages of the recognition process are similar to human olfaction and are performant for identification, comparison, quantification and other applications. However, hedonic evaluation is a specificity of the human nose given that it is related to subjective opinions. These devices have undergone much development and are now used to fulfill industrial needs. There's even an Electronic Tongue, which identifies compounds in liquids.

Working Principle:

Electronic noses include three major parts: a sample delivery system, a detection system, a computing system. The sample delivery system enables the generation of the headspace (volatile compounds) of a sample, which is the fraction analyzed. The system then injects this headspace into the detection system of the electronic nose. The sample delivery system is essential to guarantee constant operating conditions. The detection system, which consists of a sensor set, is the "reactive" part of the instrument. When in contact with volatile compounds, the sensors react, which means they experience a change of electrical properties. Each sensor is sensititive to all volatile molecules but each in their specific way. The adsorption of volatile compounds on the sensor surface causes a physical change of the sensor. A specific response is recorded by the electronic interface transforming the signal into a digital value. Combining the signals from all the sensors gives a "smell-print" of the chemicals in the mixture that neural network software built into the e-nose can learn to recognize. The e-nose recognizes and identifies these patterns. This part of the instrument performs global fingerprint analysis and provides results and representations that can be easily interpreted.

Sensors:

At present, e-nose uses 32 tiny sensors, which together are about the size of the human nose. The type of sensors and how the sensors are made and used in the instrument is what really differentiates the instruments on the market today but, each manufacturer uses its own proprietary sensor technology. The more commonly used sensors include metal oxide semiconductors (MOS), conducting polymers (CP), quartz crystal microbalance, surface acoustic wave (SAW), and field effect transistors (MOSFET). In recent years, other types of electronic noses have been developed that utilize mass spectrometry or ultra fast gas chromatography as a detection system. Sampling and sensor array improvements will increase the sensitivity of detection. Metal oxide sensors demonstrate good sensitivity to organic vapors (ppm or even ppb detection limits) for a very broad range of chemical compounds. Sensors made of conducting polymer resins are generally used instead of metal oxides as they have an inherent charge or base resistance, and as volatile components are absorbed onto the surface of the conductive polymers, they change their base resistance.

-Dept of Physics, SLIET Longowal