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 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.
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. Most electronic noses use sensor-arrays that react to volatile compounds on contact: 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. Recorded data are then computed based on statistical models. The computing system works to combine the responses of all of the sensors, which represents the input for the data treatment. 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.
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. Due to their poor selectivity - i.e., all sensors can respond to a single volatile compound but in different magnitudes sensor arrays must be employed.
Like a human nose, the e-nose is amazingly versatile, yet it's much more sensitive. Electronic noses were originally used for quality control applications in the food, beverage and cosmetics industries. Current applications include detection of odors specific to diseases for medical diagnosis, and detection of pollutants and gas leaks for environmental protection. The e-nose is best suited for matching complex samples with subjective endpoints such as odor or flavor. The e-nose can match a set of sensor responses to a calibration set produced by the human taste panel or olfactory panel routinely used in food science. The e-nose is especially useful where consistent product quality has to be maintained over long periods of time, or where repeated exposure to a sample poses a health risk to the human olfactory panel. Although the e-nose is also effective for pure chemicals, conventional methods are often more practical. e-nose can monitor the air in space-ships, submarines, and factories to warn people very early if something should make the air unsafe to breathe. It can be used in processing food to identify the first signs that something is beginning to spoil. It can be used to monitor the quality of the air anyplace where pollutants and toxins may reach unsafe levels.
Scientists have developed an electronic nose that can help hospitals detect the superbug MRSA (methicillin-resistant Staphylococcus aureus). Ordinarily, tests for MRSA takes days to complete, making it difficult to manage outbreaks, but the e-nose can apparently detect the infection within minutes. In addition to detecting superbugs, the e-nose is also being used to look for early signs of pneumonia in intensive care patients. The e-nose is also being studied for its possible use in diagnosing other conditions such as lung cancer and liver and kidney diseases. It might be possible to detect chemicals and biological agents with the device. Doctors say the device could radically change and improve the way both conditions are diagnosed.
Electronic Nose can learn to recognize almost any compound or combination of compounds at even very low concentrations and can detect an electronic change of 1 part per million. Engineers are working on a stand-alone version of e-nose i.e, “everything is in one package”. This set up of e-nose will have polymer films, a pump to pull everything in the air through the device, computers to analyze data, the energy source. In future, the noses could simply be posted, like smoke detectors, at various points of application. With some modifications, an e-nose could be used to check for gas buildups in offshore oil rigs. Similarly, sanitation workers would benefit by knowing if any poisonous gases have collected down in the sewers. Sharp, the giant Japanese electronics company, is looking at incorporating the technology in its microwave ovens to allow the ovens to shut off automatically when they begin to detect chemicals associated with overcooked food. On the disadvantageous side, although the instrument performs well, one problem is that it is so sensitive it can detect small amounts of harmless odors.
The olfactory system of even the simplest insects is so complex that it is still impossible to reproduce it at the current level of technology. The biological receptors are regularly replaced during the life of mammals in a very reliable way so that the receptor array does not require to be recalibrated. Therefore, the performance of existing artificial electronic nose devices is much more dependent on the sensor’s aging and, especially, the sensor’s replacement and frequently requires a recalibration to account for change. Moreover, current electronic nose devices based on metal oxide semiconductors or conducting polymers that specifically identify gaseous odorants are typically large and expensive and thus not adequate for use in micro- or nano-arrays that could mimic the performance of the natural olfactory system. With present status of technology, e-nose(s) don't come cheap (approx. £60,000 ≈ Rs.60 lacs) and they're about the size of two desktop PCs. It is hoped with the development better, cheaper and smaller sensor devices in future, the size and cost of e-nose will come handy and its applications will be more versatile and common. Further, e-nose devices making use of alternatives technologies being developed are:
Single-stranded DNA can be used to identify explosives and other airborne compounds. Scientists have found a way to quickly identify which DNA sequences are ideal for detecting a particular odor and turn dried DNA into odor detectors. This new platform could be used to create a wide array of sensors using existing high-throughput molecular-biology equipment.
Nanotechnology is seen as a key in advancing e-nose devices to a level that will match the olfactory systems developed by nature. Nanowire chemiresistors are seen as critical elements in the future miniaturization of e-noses. It is now also believed that single crystal nanowires are most stable sensing elements what will result in extending of life-time of sensors and therefore the recalibration cycle. However, the e-nose based on nanowire mats is yet too primitive even in comparison to the simplest of insects' olfaction.
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