The plastic analogue-to-digital converter (ADC) (credit TU Eindhoven/Bart van Overbeeke)

Plastic electronics packaging could reduce food waste

Researchers have invented a circuit which could be used in packaging to test whether food is still safe to eat.

The scientists at Eindhoven University of Technology, Universitá di Catania, CEA-Liten and STMicroelectronics, have developed a plastic analogue-digital convertor which could help to greatly reduce food waste worldwide.

Consumers and businesses in developed countries are estimated to throw away around 100 kilograms of food per person, mainly because the ‘best before’ date on the packaging has passed.

That waste is bad for consumers’ budgets and for the environment, and much of it results from the difficulty in estimating how long food will stay usable.

To minimise the risk of selling spoiled food to consumers, producers show a relatively short shelf life on their packaging.

The researchers say that producers could fight food waste by including plastic electronic circuits in food packaging to monitor for example the acidity level of the food.

The sensor circuit could be read with a scanner or with your mobile phone to show the freshness of your steak, or whether your frozen food was defrosted.

Researcher Eugenio Cantatore of Eindhoven University of Technology said: "In principle that’s all already possible, using standard silicon ICs.

"The only problem is they’re too expensive. They easily cost ten cents. And that cost is too much for a one euro bag of crisps.

"We’re now developing electronic devices that are made from plastic rather than silicon. The advantage is you can easily include these plastic sensors in plastic packaging."

The plastic semiconductor can even be printed on all kinds of flexible surfaces, making it cheaper to use, and makes sensor circuits costing less than one eurocent achievable.

The researchers have succeeded in making two different plastic ADCs (analogue-to-digital converters).

Each converts analogue signals, such as the output value measured by a sensor, into digital form.

One of these new devices is the very first printed ADC ever made.

"This paves the way toward large area sensors on plastic films in a cost-effective way through printing manufacturing approaches", said Isabelle Chartier, Printed Electronics Business developer at CEA-Liten.

The new plastic ADCs bring applications in the food and pharmaceuticals industries within reach.

A sensor circuit consists of four components: the sensor, an amplifier, an ADC to digitise the signal and a radio transmitter that sends the signal to a base station.

The plastic ADC has been the missing link; the other three components already exist.

"Now that we have all of the pieces, we need the integration," Cantatore said.

He expects that it will still take at least five years before we can expect to see the new devices on supermarket shelves.

Other potential applications are in pharmaceuticals, man-machine interfaces and in ambient intelligence systems in buildings or in transport.

The researchers said it was not easy to develop the ADC, as while the electrical characteristics of ‘ordinary transistors’ are highly predictable, those of plastic transistors vary greatly.

"All plastic transistors behave differently in the low-cost production processes at low temperatures," Cantatore said.

"That makes it much more difficult to use them in devices. You need complex mathematical models to be able to predict their behaviour accurately."

The printed ADC circuit offers a resolution of four bits, and has a speed of two hertz.

The circuits printed by CEA-Liten include more than 100 n- and p-type transistors and a resistance level on transparent plastic substrates.

The carrier mobility of the printed transistors are above the amorphous silicon widely used in the display industry.

The invention was presented last week at the ISSCC conference on solid-state circuits in San Francisco, which rated the papers on the invention as highlights of the event.

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