AN EMPIRICAL CONCEPT OF E-NOSES
AN EMPIRICAL CONCEPT OF E-NOSES
Dept. of Informatics
Alluri Institue of Management Sciences
Electronic/artificial noses are being developed as systems for the automated detection and classification of odors, vapors, and gases. An electronic nose is generally composed of a chemical sensing system (e.g., sensor array or spectrometer) and a pattern recognition system (e.g., artificial neural network). We are developing electronic noses for the automated identification of volatile chemicals for environmental and medical applications. In this paper, we briefly describe an electronic nose, show some results from a prototype electronic nose, and discuss applications of electronic noses in the environmental, medical, and food industries.
An electronic nose (e-nose) is a device that identifies the specific components of an odor and analyzes its chemical makeup to identify it. An electronic nose consists of a mechanism for chemical detection, such as an array of electronic sensors, and a mechanism for pattern recognition, such as a neural network . Electronic noses have been around for several years but have typically been large and expensive. Current research is focused on making the devices smaller, less expensive, and more sensitive. The smallest version, a nose-on-a-chip is a single computer chip containing both the sensors and the processing components.
An odor is composed of molecules, each of which has a specific size and shape. Each of these molecules has a correspondingly sized and shaped receptor in the human nose. When a specific receptor receives a molecule, it sends a signal to the brain and the brain identifies the smell associated with that particular molecule. Electronic noses based on the biological model work in a similar manner, albeit substituting sensors for the receptors, and transmitting the signal to a program for processing, rather than to the brain. Electronic noses are one example of a growing research area called biomimetics, or biomimicry, which involves human-made applications patterned on natural phenomena.
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.
Keywords: enose, artificial nose, network, environment, sensing
The two main components of an electronic nose are the sensing system and the automated pattern recognition system. The sensing system can be an array of several different sensing elements (e.g., chemical sensors), where each element measures a different property of the sensed chemical, or it can be a single sensing device (e.g., spectrometer) that produces an array of measurements for each chemical, or it can be a combination. Each chemical vapor presented to the sensor array produces a signature or pattern characteristic of the vapor. By presenting many different chemicals to the sensor array, a database of signatures is built up. This database of labeled signatures is used to train the pattern recognition system. The goal of this training process is to configure the recognition system to produce unique classifications of each chemical so that an automated identification can be implemented.
The quantity and complexity of the data collected by sensors array can make conventional chemical analysis of data in an automated fashion difficult. One approach to chemical vapor identification is to build an array of sensors, where each sensor in the array is designed to respond to a specific chemical. With this approach, the number of unique sensors must be at least as great as the number of chemicals being monitored. It is both expensive and difficult to build highly selective chemical sensors.
Artificial neural networks (ANNs), which have been used to analyze complex data and to recognize patterns, are showing promising results in chemical vapor recognition. When an ANN is combined with a sensor array, the number of detectable chemicals is generally greater than the number of sensors . Also, less selective sensors which are generally less expensive can be used with this approach. Once the ANN is trained for chemical vapor recognition, operation consists of propagating the sensor data through the network. Since this is simply a series of vector-matrix multiplications, unknown chemicals can be rapidly identified in the field.
Electronic noses that incorporate ANNs have been demonstrated in various applications. Some of these applications will be discussed later in the paper. Many ANN configurations and training algorithms have been used to build electronic noses including back propagation-trained, feed-forward networks; fuzzy ARTmaps; KohonenÕs self-organizing maps (SOMs); learning vector quantizers (LVQs); Hamming networks; Boltzmann machines; and Hopfield networks. Figure 1 illustrates the basic schematic of an electronic nose.
Figure 1: Schematic Diagram of an electronic nose
Electronic Noses for Environmental Monitoring
Enormous amounts of hazardous waste (nuclear, chemical, and mixed wastes) were generated by more than 40 years of weaponsÕ production in the U.S. Department of EnergyÕs weaponsÕ complex. The Pacific Northwest National Laboratory is exploring the technologies required to perform environmental restoration and waste management in a cost effective manner. This effort includes the development of portable, inexpensive systems capable of real-time identification of contaminants in the field. Electronic noses fit this category.
Environmental applications of electronic noses include analysis of fuel mixtures , detection of oil leaks , testing ground water for odors, and identification of household odors . Potential applications include identification of toxic wastes, air quality monitoring, and monitoring factory emissions.
Electronic Noses for Medicine
Because the sense of smell is an important sense to the physician, an electronic nose has applicability as a diagnostic tool. An electronic nose can examine odors from the body (e.g., breath, wounds, body fluids, etc.) and identify possible problems. Odors in the breath can be indicative of gastrointestinal problems, sinus problems, infections, diabetes, and liver problems. Infected wounds and tissues emit distinctive odors that can be detected by an electronic nose. Odors coming from body fluids can indicate liver and bladder problems. Currently, an electronic nose for examining wound infections is being tested at South Manchester University Hospital.
A more futuristic application of electronic noses has been recently proposed for telesurgery. While the inclusion of visual, aural, and tactile senses into telepresent systems is widespread, the sense of smell has been largely ignored. An electronic nose will potentially be a key component in an olfactory input to telepresent virtual reality systems including telesurgery. The electronic nose would identify odors in the remote surgical environment. These identified odors would then be electronically transmitted to another site where an odor generation system would recreate them.
Electronic Noses for the Food Industry
Currently, the biggest market for electronic noses is the food industry. Applications of electronic noses in the food industry include quality assessment in food production, inspection of food quality by odor, control of food cooking processes, inspection of fish, monitoring the fermentation process, checking rancidity of mayonnaise, verifying if orange juice is natural, monitoring food and beverage odors , grading whiskey, inspection of beverage containers, checking plastic wrap for containment of onion odor, and automated flavor control to name a few. In some instances electronic noses can be used to augment or replace panels of human experts. In other cases, electronic noses can be used to reduce the amount of analytical chemistry that is performed in food production especially when qualitative results will do.
Electronic nose for evaluation of tea flavour(designed by cdac kolkata)
Electronic Nose is a smart instrument that is designed to detect and discriminate among complex odours using an array of sensors. The array of sensors consists of a number of broadly tuned (non-specific) sensors that are treated with a variety of odour-sensitive biological or chemical materials. An odour stimulus generates a characteristic fingerprint from this array of sensors. Patterns or fingerprints from known odours are used to construct a database and train a pattern recognition system so that unknown odours can subsequently be classified and/or identified.
An electronic nose system primarily consists of four functional blocks, viz., Odour Handling and Delivery System, Sensors and Interface Electronics, Signal Processing and Intelligent Pattern Analysis and Recognition. The array of sensors is exposed to volatile odour vapour through suitable odour handling and delivery system that ensures constant exposure rate to each of the sensors. The response signals of sensor array are conditioned and processed through suitable circuitry and fed to an intelligent pattern recognition engine for classification, analysis and declaration.
The most complicated parts of electronic olfaction process are odour capture and associated sensor technology. Any sensor that responds reversibly to chemicals in gas or vapour phase, has the potential to be participate in an array of sensor in an electronic nose. For black manufactured tea, an array of Metal Oxide Semiconductor (MoS) sensors have been used for assessment of volatiles.
E-Nose for Assessment of Optimum
As soon as tea leave cells are ruptured in the CTC or Rolling process, the process of fermentation starts. Limiting the reactions and chemical transformations during fermentation process to an optimum limit is vital for producing superior quality tea. It is, thus, critical that the leaf be allowed to ferment only up to the desired limit so that the complex series of chemical changes within the leaf are accomplished optimally. Conventionally, length of fermentation is subjectively estimated by human senses of smell and vision. Human experts can sense conversion of grassy smell to floral smell of inprocess leaves after fermentation. But, such odour emanation travels through cycles of so called “First Nose” and “Second Nose” etc. A colorimetric approach also is also used at times where fermentation completion time is determined based on colour.
FOR EVALUATION OF TEA FLAVOUR
A specially designed Electronic Nose has been successfully used to monitor volatile emission pattern in fermentation process over passage of time. Through prolonged experimentation with various clones, fermentation processes and climatic variations, it has been established that smell changes during the process may be reliably detected repeatedly by Electronic Nose. Even the smell peaks during so called “First Nose” and “Second Nose” may also be clearly detected with this new smart instrument.
E-Nose for Finished Tea Classification:
Flavour and Aroma are important quality attributes of finished tea. Human experts – called “Tea Tasters” – conventionally determine tea quality. Tea tasters usually assign scores to samples of tea under evaluation in a scale of 1 to 10 depending on the flavour, aroma and appearance of the sample. Electronic Nose is a unique tool that is capable of sensing the volatile compounds of the
tea sample and reliably predicts Tea Taster like scores with a high degree of accuracy.
Neural Network based Soft Computing Techniques are used to tune near accurate co-relation smell print of multi-sensor array with that of Tea Tasters’ scores. The software framework has been designed with adequate flexibility and openness so that tea planters themselves may train the system with their own system of scoring so that the instrument will, then on, reliably predict such smell print scores.In addition, encouraging results have also been obtained during the preliminary experimentations with Withering Process to expect Electronic Nose also to be a useful instrument for determining the optimum Withering time.A user-friendly interactive software has been carefully designed which has got features like programmable sequence control, dynamic fermentation profile display, data logging, alarm annunciation, data archival, etc.The Tea Industry may adopt such easy-to-operate though hi-tech smart instrument.
Nanotechnology electronic noses-Next Gen E Noses
(Nanowerk Spotlight) The concept of e-noses – electronic devices which mimic the olfactory systems of mammals and insects – is very intriguing to researchers involved in building better, cheaper and smaller sensor devices. 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. Nevertheless, 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. For example, 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. 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 require 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. 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. Last year we reported on a research effort Towards The Nanoscopic Electronic Nose. Scientists involved in this effort now report a second-generation, far more advanced e-nose system based on metal oxide nanowires.
“Despite encouraging demonstrations of an array of individual metal oxide nanowires, there still exists a technological gap between the laboratory demonstrations and a practical e-nose micro device suitable for up-to-date large-scale microfabrication and capable of operating in real-world environments” Dr. Andrei Kolmakov explains to Nanowerk. “Hence, our aim was to bridge this gap and demonstrate the excellent performance of a practical device made by combining ‘bottom-up’ fabricated SnO2 nanowires/nanobelts as sensing elements with a ‘top-down’ technology of the state-of-the-art multi electrode KAMINA platform.”
Kolmakov, an assistant professor in the physics department at Southern Illinois University at Carbondale points out that this work is an example of successful truly international collaboration between his group, Dr. Victor Sysoev (electronic noses specialist, Saratov State University, Russia) and group of Dr. Joachim Goschnick (developers of the commercial KAMINA e-nose system Forschungszentrum Karlsruhe, Germany).
“Basically, we took a very robust and successful KAMINA (KArlsruhe Micro NAse; pdf download German/English datasheet, 128 KB) electronic nose platform and instead of using the traditional thin-film sensing element we implemented completely new morphology of the sensing layer, which in our case, is composed out of the layer of tin oxide nanowires” says Kolmakov.
Artistic comparison of the major steps in functioning of the human (left) and artificial olfactory systems (right). However, the e-nose based on nanowire mats is yet too primitive even in comparison to the simplest of insects’ olfaction. (Image: Dr. Kolmakov, Southern Illinois University at Carbondale)
He explains that their device shows a certain degree of analogy with neurons: The randomly distributed nanowires contact each other and form multiple percolation paths for signal transmission. The resistance of these percolating nanowires is a very sensitive function of the gas environment. Due to a stochastic difference of the percolating pattern between every two electrodes (the space between each couple of electrodes serves as a separate sensor), the sensor array of multi-electrodes produces a different response pattern to differing analytes. Similar to our brain, the processor attached to the e-nose conditions and analyzes the electrical signals coming from the sensor array and, using pattern recognition techniques, produces the image of the ‘odor’.
This is a far more advanced system than what Kolmakov and his collaborators demonstrated last year. In this new device, the scientists decided to benefit from the stochastic nature of the percolating nanowire network.
“From the point of view of the sensitivity and selectivity toward the gases we probed, this e-nose was showing the excellent results compared to existing macroscopic counterparts” says Kolmakov. “With this new methodology we are getting intriguing results, which we are still analyzing. For example, we learned that in order to discriminate between different analytes, our artificial nose does not require any temperature gradient (which is a basic principle of operation of the traditional KAMINA e-nose) and we can easily play with just a density of the percolating nanowires at the substrate to substantially change the gas-recognition properties of the array. In addition to that, there are plenty of other functionalization possibilities for our sensing elements to tune the sensor properties in a rational and easy way. That’s why we consider mimicking the net of neurons as one of the most promising paths in the development of next generation electronic noses.”
Kolmakov and his grup are currently testing the strategy to eliminate any heaters in the device, thus making it possible to operate a microchip at room temperature. This drastically reduces the power requirements
This work shows that gradient micro arrays with sensing elements based on metal oxide nanowire mats of different density appear to be a novel technologically simple and powerful approach for fabrication of robust, cost-effective, sensitive, and highly selective nose-like gas analytical devices.
This kind of nanosensor device opens a new direction in the development of extremely cheap (a few dollars, since there is no need for any expensive nanomanipulation and sophisticated top-down protocols) but yet ultra small and powerful electronic detectors which are able to recognize complex chemicals against a disturbing background of other gases and report if any dangerous thresholds have been approached.
There is a huge need for these sensors in the modern urban environment, in environmental applications, and for security. Kolmakov points out that such ‘intelligent’ sensor systems could be installed almost everywhere – cell-phones, car dashboards and exhaust systems, manufacturing plants, oil platforms, even in soldiers’ helmets etc. Moreover, the system is sensitive to ionizing radiation and can be a monitor for x-rays and nuclear contaminations.
New emerging technologies are continually providing means of improving e-noses and EAD capabilities through interfaces and combinations with classical analytical systems for rapid discrimination of individual chemical species within aroma mixtures. E-nose instruments are being developed that combine EAD sensors in tandem with analytical detectors such as with fast gas chromatography (FGC) . More complicated technologies such as optical gas sensor systems also may improve on traditional e-nose sensor arrays by providing analytical data of mixture constituents . These technologies will have the capability of producing recognizable high resolution visual images of specific vapor mixtures containing many different chemical species, but also quantifying concentrations and identifying all compounds present in the gas mixture. Similar capabilities for identifying components of solid and liquid mixtures may be possible with devices called electronic tongues. Several recent reviews provide summaries of electronic tongue technologies and discuss potential applications for food analyses.
The potential for future developments of innovative e-nose applications is enormous as researchers in many fields of scientific investigation and industrial development become more aware of the capabilities of the electronic nose. The current trend is toward the development of electronic noses for specific purposes or a fairly narrow range of applications. This strategy increases e-nose efficiency by minimizing the number of sensors needed for discriminations, reducing instrument costs, and allowing for greater portability through miniaturization. New potential discoveries in this relatively new sector of sensor technology will continue to expand as new products, machines, and industrial processes are developed. These discoveries will lead to the recognition of new ways to exploit the electronic nose to solve many new problems for the benefit of mankind.
- Electronic Nose for evaluation of tea flavourhttp://www.kolkatacdac.in/html/imgtea. PDF- November 2005.
- Electronic Noses : whatis.techtarget.com/definition
- http://www.nanowerk.com/spotlight/spotid=3331.php(The researchers published their results in the October 10, 2007 online edition of Nano Letters (“A Gradient Micro array Electronic Nose Based on Percolating SnO2 Nanowire Sensing Elements”).
- This paper was presented at Neural Network Applications Studies Workshop in the IEEE Northcon/Technical Applications Conference (TAC’95) in Portland, Oregon, USA on 12 October 1995.
- Electronic Noses Sniff Success Chang, J.B. Subramanian, V. This paper appears in: Spectrum, IEEE Publication Date: March 2008 Volume: 45, Issue: 3 On page(s): 50-56
- Intelligent electronic nose system for basal stem rot disease detectionVolume 66 Issue 2 (May 2009)- Elsevier Science Publishers B. V. Amsterdam, The Netherlands, The Netherlands –ACM Journal
About the Author
Dept. of Informatics
Alluri Institue of Management Sciences
A basic TRIAC 120V circuit