Electronic nose
An electronic nose is a device intended to detect odors or flavors.
Over the last decades, "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 1982, research has been conducted to develop technologies, commonly referred to as electronic noses, that could detect and recognize odors and flavors. The stages of the recognition process are similar to human olfaction and are performed for identification, comparison, quantification and other applications, including data storage and retrieval. 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.
Other techniques to analyze odors
In all industries, odor assessment is usually performed by human sensory analysis, by chemosensors, or by gas chromatography. The latter technique gives information about volatile organic compounds but the correlation between analytical results and mean odor perception is not direct due to potential interactions between several odorous components.In the Wasp Hound odor detector, the mechanical element is a video camera and the biological element is five parasitic wasps who have been conditioned to swarm in response to the presence of a specific chemical.
History
Scientist Alexander Graham Bell popularized the notion that it was difficult to measure a smell, and in 1914 said the following:In the decades since Bell made this observation, no such science of odor materialised, and it was not until the 1950s and beyond that any real progress was made.
Working principle
The electronic nose was developed in order to mimic human olfaction that functions as a non-separative mechanism: i.e. an odor / flavor is perceived as a global fingerprint.Essentially the instrument consists of head space sampling, sensor array, and pattern recognition modules, to generate signal pattern that are used for characterizing odors.
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 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.
In most electronic noses, each sensor is sensitive to all volatile molecules but each in their specific way. However, in bio-electronic noses, receptor proteins which respond to specific odor molecules are used. 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.
Bio-electronic noses use olfactory receptors - proteins cloned from biological organisms, e.g. humans, that bind to specific odor molecules. One group has developed a bio-electronic nose that mimics the signaling systems used by the human nose to perceive odors at a very high sensitivity: femtomolar concentrations.
The more commonly used sensors for electronic noses include
- metal–oxide–semiconductor devices - a transistor used for amplifying or switching electronic signals. This works on the principle that molecules entering the sensor area will be charged either positively or negatively, which should have a direct effect on the electric field inside the MOSFET. Thus, introducing each additional charged particle will directly affect the transistor in a unique way, producing a change in the MOSFET signal that can then be interpreted by pattern recognition computer systems. So essentially each detectable molecule will have its own unique signal for a computer system to interpret.
- conducting polymers - organic polymers that conduct electricity.
- polymer composites - similar in use to conducting polymers but formulated of non-conducting polymers with the addition of conducting material such as carbon black.
- quartz crystal microbalance - a way of measuring mass per unit area by measuring the change in frequency of a quartz crystal resonator. This can be stored in a database and used for future reference.
- surface acoustic wave - a class of microelectromechanical systems which rely on the modulation of surface acoustic waves to sense a physical phenomenon.
In recent years, other types of electronic noses have been developed that utilize mass spectrometry or ultra-fast gas chromatography as a detection system.
The computing system works to combine the responses of all of the sensors, which represents the input for the data treatment. This part of the instrument performs global fingerprint analysis and provides results and representations that can be easily interpreted. Moreover, the electronic nose results can be correlated to those obtained from other techniques. Many of the data interpretation systems are used for the analysis of results. These systems include artificial neural network, fuzzy logic, pattern recognition modules, etc. Artificial intelligence, included artificial neural network, is a key technique for the environmental odour management.
Performing an analysis
As a first step, an electronic nose needs to be trained with qualified samples so as to build a database of reference. Then the instrument can recognize new samples by comparing a volatile compound's fingerprint to those contained in its database. Thus they can perform qualitative or quantitative analysis. This however may also provide a problem as many odors are made up of multiple different molecules, which may be wrongly interpreted by the device as it will register them as different compounds, resulting in incorrect or inaccurate results depending on the primary function of a nose. The example of e-nose dataset is also available. This dataset can be used as a reference for e-nose signal processing, notably for meat quality studies. The two main objectives of this dataset are multiclass beef classification and microbial population prediction by regression.Applications
Electronic nose instruments are used by research and development laboratories, quality control laboratories and process & production departments for various purposes:In quality control laboratories
- Conformity of raw materials, intermediate and final products
- Batch to batch consistency
- Detection of contamination, spoilage, adulteration
- Origin or vendor selection
- Monitoring of storage conditions
- Meat quality monitoring.
In process and production departments
- Managing raw material variability
- Comparison with a reference product
- Measurement and comparison of the effects of manufacturing process on products
- Following-up cleaning in place process efficiency
- Scale-up monitoring
- Cleaning in place monitoring.
Possible and future applications in the fields of health and security
- The detection of dangerous and harmful bacteria, such as software that has been specifically developed to recognise the smell of the MRSA. It is also able to recognise methicillinsusceptible S. aureus among many other substances. It has been theorised that if carefully placed in hospital ventilation systems, it could detect and therefore prevent contamination of other patients or equipment by many highly contagious pathogens.
- The detection of lung cancer or other medical conditions by detecting the VOC's that indicate the medical condition.
- The detection of viral and bacterial infections in COPD Exacerbations
- The quality control of food products as it could be conveniently placed in food packaging to clearly indicate when food has started to rot or used in the field to detect bacterial or insect contamination.
- Nasal implants could warn of the presence of natural gas, for those who had anosmia or a weak sense of smell.
- The Brain Mapping Foundation used the electronic nose to detect brain cancer cells.
Possible and future applications in the field of crime prevention and security
- The ability of the electronic nose to detect odorless smells makes it ideal for use in the police force, such as the ability to detect bomb odors despite other airborne odors capable of confusing police dogs. However this is unlikely in the near term as the cost of the electronic nose is quite high.
- It may also be used as a drug detection method in airports. Through careful placement of several or more electronic noses and effective computer systems, one could triangulate the location of drugs to within a few metres of their location in less than a few seconds.
In environmental monitoring
- For identification of volatile organic compounds in air, water and soil samples.
- For environmental protection.
Examples
BreathBase Solutionis an innovative Dutch medtech startup focusing on improving healthcare and reducing costs. Their BreathBase Solution comprises an eNose, a platform that transfers high-quality breath measurements to the generation and validation of diagnostic models, and a clinically validated breath database that serves as a reference for new patients. Their solution is able to discriminate between COPD patients that develop lung cancer within a year and those who don't. These results show that eNose assessment may detect early stages of lung cancer and may therefore be of value in the screening of patients with COPD.
They also used their solution to phenotype asthma and COPD patients. Phenotyping a combined sample of asthma and COPD patients using the eNose provided validated clusters that were not determined by diagnosis, but rather by clinical/inflammatory characteristics. The eNose identified systemic neutrophilia and/or eosinophilia in a dose-dependent manner.
Cyranose
The Cyranose 320 is a handheld "electronic nose" developed by Cyrano Sciences of Pasadena, California in 2000. Cyrano Sciences was founded in 1997, 9 years after the concept of an "electronic nose" based on using multiple semi-selective sensors combined with electronic computation was first proposed by Gardner and Bartlett. The Cyranose 320 is based on sensor research performed by Professor Nathan Lewis of the California Institute of Technology.
Applications researched using the Cyranose 320 include the detection of COPD, and other medical conditions as well as industrial applications generally related to quality control or contamination detection.
The Cyranose 320 is still being manufactured in the USA by Sensigent LLC, the successor company to Cyrano Sciences.