Electronic Noses

The Biomedical Sensors Laboratory has a considerable track record in electronic noses (or e-noses). Warwick developed the world's first commercial electronic nose in collaboration with Bass Plc and Neotronic Technology Plc in the early 1990s. Since these developments, there are now around 20 different electronic nose companies selling approximately 200 machines a year, alongside numerous other research institutions throughout the world continuing to advance the technology.

What is an 'Electronic Nose'?

Artificial olfaction has been a subject of research since the early 1980s, but it wasn’t until 1994 that the term e-nose was formally introduced. Unlike traditional chemical analysis, which aims to identify individual compounds in a complex odour (coffee, for instance, contains over a thousand volatile molecules), e-noses focus on classifying the overall aroma profile. They achieve this using arrays of chemical sensors with overlapping sensitivities, each responding to broad classes of compounds, such as ketones or aldehydes. The combined responses of the sensor array create a distinctive “chemical fingerprint” for each odour, which can then be analysed using multivariate statistical methods or neural networks to achieve reliable classification. Today, e-noses are routinely used in applications ranging from medical diagnostics to environmental monitoring to food quality control, offering a rapid, non-invasive approach to odour detection.

Even with the success of many e-nose systems, their performance in terms of odour sensitivity and selectivity still lags behind that of the human olfactory system. It is generally believed that this is due to the lower complexity of e-noses when compared to their biological counterparts. For example the human olfactory system contains some 100 million olfactory receptors with approximately 350 different types of receptor binding proteins, distributed along the olfactory epithelium (the lining of the nose and part of the olfactory mucosa). In contrast e-nose systems typically have 32 or less chemical sensors in a basic chamber. The figure below gives a comparison of the biological and artificial olfactory systems.

The University of Warwick has developed a number of different e-nose systems, both on their own and in collaboration with a number of industrial partners. The range of applications e-nose technology has been applied to is considerable, ranging from food quality, perfumes, environmental heath, security and medical diagnostics.

WOLF E-Nose

As part of its activities, the Biomedical Sensors Laboratory is developing a new generation of electronic noses and gas analysers that combine both in-house and commercial gas sensor technologies. The Warwick OLFaction (WOLF) e-noses are the latest in this line of innovation. Importantly, as these systems are designed and built in-house, they offer a high degree of flexibility and can be tailored to meet the needs for specific applications. In addition to our in-house platforms, we have also developed a wide range of commercial systems. At present, there are four distinct series of E-noses, with selected examples shown below.

WOLF Series 1 Analyser

The series 1 analyser are small, portable units fitted with the latest in miniature gas sensors. Their purpose is geared towards environmental applications, and are therefore fitted with additional sensors to give an indication of the total air quality around the user. These sensors include light (including UV), sound, temperature, humidity, total volatile organic compounds (VOCs), toxic inorganic gases, and carbon dioxide. The units are battery powered, and can last up to three days on a full charge. The units communicate through an Android app to a mobile phone or portable tablet, to give a real time reading of the environment around the wearer.

The first generation unit was designed to be a similar size and shape to a smart watch. It included an integrated microcontroller, sensors and a Bluetooth BLE chip for communication with the user. An overview of the system is shown in the figure below:

The second-generation unit incorporated a carbon dioxide sensor along with an integrated microcontroller and Bluetooth module. Although slightly larger than the first version, it was designed to be worn on a belt or attached to a bag for convenient use in everyday settings. Like its predecessor, the device transmitted information to a mobile device, but it also recorded location data, enabling the creation of detailed maps of environmental quality. An example of this unit is shown in the figure below.

The third-generation Series 1 unit is currently under development. It will integrate a wider range of sensors and feature a convenient clip-on design, providing improved functionality for the wearer.

WOLF Series 2 Analyser

The Series 2 analysers are designed primarily for indoor and environmental monitoring applications. While they can operate on battery power, they are mainly intended for plug-in use, enabling long-term monitoring over periods of weeks or even months. Compared to the Series 1 units, they incorporate a broader range of sensors, including higher-specification devices and particle sensors that cannot be accommodated in the smaller Series 1 design.

These analysers have been deployed across a wide variety of environmental and agricultural applications. The first-generation Series 2 unit, shown in the photo below, features a robust metal enclosure and an OLED display to provide a real-time indication of sensor readings. Communication is enabled via a long-range wireless module, with an SD card included for situations where radio frequency communication is not possible. In addition, the system can be linked to a dedicated website, allowing online access to data from multiple deployed units.

he second-generation unit was built for use in rougher environments and equipped with protective rubber boots. It was developed specifically for transport applications, where the analyser can be placed inside a container or similar enclosure to monitor environmental conditions during transit. The device is battery powered, with a lifespan of several months on a single charge. An example of this unit is shown in the photo below:

WOLF Series 3 Analyser

The WOLF 3.1 and 3.2 analysers are compact, portable systems that can be taken on-site, while still being mains powered. The WOLF 3.1 features a gas chromatography (GC)–metal oxide detection system, in which pre-separation occurs in the GC column, followed by chemical analysis using an array of metal oxide sensors. It includes a fully integrated fluidic system, a touchscreen interface, and a real-time display for monitoring results, featured in the image below:

The WOLF 3.2 is currently under development and will use optical gas sensor technology to detect breath-based biomarkers of inflammation.

WOLF Series 4 Analyser

The WOLF Series 4 is a fully integrated electronic nose, featuring:

Desktop electronic nose design
Array of electrochemical and optical gas sensors
Fully integrated control and drive electronics
Environmental control hardware
Integrated flow control
Custom software interface developed in National Instruments LabVIEW
Intuitive graphical interface for ease of use
Data export to common software analysis platforms

This instrument is already being used in both medical and agricultural applications.