Aroma therapy

Two UK universities have joined forces to develop ‘artificial mucus’ made of a polymer mix to enhance the sensing capabilities of electronic noses.

Electronic noses, which reproduce human senses with sensor arrays and pattern recognition systems, have been used for years in factories to detect problems such as tainted food supplies.

The technology has undergone constant development, but the device’s basic components have always consisted of sample, delivery, detection and computing systems.

For an e-nose to give a reading, the delivery system will inject a sample into the detector. Consisting of a sensor set, this is the reactive part of the instrument and when it comes in contact with volatile compounds, the sensors will experience a change of electrical properties.

From this, a specific response is recorded by the electronic interface, which will transform the signal into a digital value. The computing system will then work to combine the responses of all the sensors and interpret the information to provide results and representations that can be easily interpreted.

A natural nose, on the other hand, uses more than 100 million specialised receptors which act together in complex ways to identify and tell apart the molecules they encounter. Electronic noses use the same method but often have less than 50 sensors, which means that they discern a much smaller range of smells than ours.



Human processes

That is why the University of Warwick and Leicester University have joined forces on a three-year project to develop an electronic nose that uses more human-like processes to distinguish scents. The researchers did this by placing a 10-micron-thick layer of a polymer normally used to separate gases on the sensors in an effort to mimic nasal mucus.

Warwick’s Prof Julian Gardner — one of the pioneers of the e-nose almost 20 years ago — said his team took inspiration from recent medical studies that highlighted the importance of nasal mucus in the ability to smell. According to the research, said Gardner, it dissolves scents and separates out different odour molecules so they arrive at the receptors at different speeds and times. Humans then evaluate the differences in time taken to reach different receptors to pick apart a diverse range of smells.

‘In a traditional electronic nose all the different chemical compounds in the smell reach the sensors at the same time and therefore the signals get mixed up,’ he said. ‘Whereas with this retentive coating we slow down some chemicals relative to the others and change the order that they hit the sensors.’

The coating is able to accomplish this by a series of microfluidic passages fabricated on the sensor head. ‘We had to create a microfluidic channel that would make the odour goes through the channel and comes out at the right speed,’ said Gardner. ‘It was a systems approach rather than a sensor project.’

The actual type of polymer coating used for the sensor will differ with each application. ‘It is important to choose retentive coatings that retain the sort of chemicals you want to detect,’ he said. ‘So if you want to detect polar compounds such as ethanol or non-polar ones like toluene or xylene, different coatings must be chosen.’

Gardner said the researchers have already proved that by using mucus the electronic nose is able to differentiate between the smells of simple things like banana or peppermint, which had previously been challenging smells for the device. ‘For that we chose a parylene C coating, which should retain non-polar compounds,’ he said, adding that this coating has proved to be beneficial. ‘With our tests on separating simple odours we were able to show that using this mucus we could get better results than not using it.’

While the new electronic nose sensors are ready to be applied to areas such as the food industry, the researchers would like to improve their device for use in healthcare.

‘With infections like MRSA and MSSA, we’ve had some success using a traditional electronic nose, but we couldn’t in all cases tell the difference between the two,’ he said.



Advanced algorithms

The greater hope for researchers, said Gardner, is to develop electronic noses further so that their capabilities might closer resemble the human sense of smell.

‘What we’re doing is quantitative rather than qualitative and the problem with the human nose is that it is subjective,’ he said, referring to how someone determines a good or bad smell.

By developing more advanced algorithms for the nose’s hardware, however, Gardner thinks human-like abilities can be achieved. We still have a little way to go, but I feel that probably in two or three years we will be pretty much approaching the capability of the human nose.’