Millions of tons of food are thrown away each year because
the 'best before' date has passed. But this date is always a cautious estimate,
which means a lot of still-edible food is thrown away. Wouldn't it be handy if
the packaging could 'test' whether the contents are still safe to eat?
Researchers at Eindhoven University of Technology,
Universitá di Catania, CEA-Liten and STMicroelectronics have invented a circuit
that makes this possible: a plastic analogue–digital converter. This
development brings plastic sensor circuits costing less than one Euro cent
within reach. Beyond food, these ultra-low-cost plastic circuits have numerous
potential uses, including, pharmaceuticals.
The invention was presented last week at the ISSCC in San
Francisco, the world's most important conference on solid-state circuits.
Consumers and businesses in developed countries 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. Much of this wastage 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
Less than one cent
To fight food waste, producers could include an electronic
sensor circuit in their packaging to monitor the acidity level of the food, for
example. 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
(TU/e): "In principle that's all already possible, using standard silicon
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, which makes it cheaper to use. And it makes sensor circuits
costing less than one eurocent achievable.
The very first
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", says Isabelle Chartier, Printed
Electronics Business developer at CEA-Liten. The ISSCC rated the papers on
these inventions as highlights of the conference.
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 digitize 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," says Cantatore. 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.
Making this development was no easy task. The electrical characteristics
of 'ordinary transistors' are highly predictable, while those of plastic
transistors vary greatly. "All plastic transistors behave differently in
the low-cost production processes at low temperatures," explains
Cantatore. "That makes it much more difficult to use them in devices. You
need complex mathematical models to be able to predict their behaviour
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.