In developed countries, the average person throws away about one hundred kilos of food every year, partly because the expiration date on the packaging has come and gone. This waste is a problem for economic as well as environmental reasons. The main issue is that it’s hard to determine when food goes bad. The freshness of refrigerated products, for example, depends on the exact storage temperature, which is why producers of food are forced to make a fairly conservative estimate as far as the expiration date goes.

One of the ways to prevent food waste is by equipping packaging with an electronic food sensor that monitors the food’s acidity. A scanner can read the sensor to see if your steak is decomposing or whether your chips are still crispy or not. Technically, this idea is already feasible, says Eugenio Cantatore, associate professor at Mixed-signal Microelectronics. “All you need is a sensor and some basic electronics to read out the sensor. Standard silicon chips would be fine for that, except they’re too expensive.”

According to Cantatore, a simple silicon chip costs ten eurocents easily. While that’s not a lot when used in a computer or smartphone, it’s a substantial amount for a bag of chips worth less than a euro. “We’re working on electronic chips in which the semiconductor isn’t made of silicon, but of organic material. Plastic electronics, so to speak. These chips can be put into plastic packaging right away.” If dissolved in water, the plastic semiconductor can even be printed onto all kinds of flexible surfaces such as paper and wrapping plastic, not unlike ink. And that makes plastic electronics cheaper than silicon electronics – sensor circuits at less than a cent are realistic - and they’re suitable for uses where conventional electronics just won’t do.

The fact plastic chips are flexible makes them unique, the Italian explains. “Current silicon chips are produced at high temperature, up to over a thousand degrees for crystalline silicon computer chips. Even the production of amorphous and polycrystalline silicon, which is used in monitors for example, requires a temperature of six hundred degrees. Because of these high temperatures, a substrate is needed: solid bedding that won’t melt. For computer chips that’s silicon, and for monitors it’s glass. Our semiconducting materials can be processed at temperatures below two hundred degrees. That means they can be applied to plastics like polyethylene naphthalate, a cheap and flexible plastic related to PET that many bottles are made of.”

Of course, there are downsides to working with plastic electronics as well: there’s a reason silicon is still the standard. The basic components of electric circuits are transistors. And plastic transistors work according to a different principle than silicon specimens, says Cantatore. “They are accumulation transistors and that has a major influence on their characteristics.”

The electric characteristics of ‘ordinary transistors’ are very predictable, because all transistors in a single circuit are practically identical. “Plastic transistors, however, vary widely”, Cantatore explains. “The disadvantage of a cheap production process at low temperatures is that all plastic transistors differ. And that makes it much harder to create circuits. After all, you have to take into account the variability of the electric characteristics of the transistors.  It requires complex mathematical models to predict the behavior of a circuit consisting of plastic electronics properly. We are experts in the field of designing circuits, but we also know about the physics of the transistors, and about mathematical models.”
With all that knowledge on board, Cantatore and his colleagues at Mixed-signal Microelectronics (MsM) have managed to build two different plastic ADCs (analog-to-digital converters) as a result of two different collaborations. Both results were heralded as one of the highlights of the authoritative electronics conference ISSCC in San Francisco earlier this week.

With an ADC, analog signals (the measuring signal of a sensor, for example) are converted into digital data. Until now, there were only two plastic ADCs. “We’ve added two new ones to the collection, doubling the world production”, the Italian researchers laughs. “One of the circuits, made within the European collaborative project COSMIC, is the very first printed ADC ever. On top of that, we’re using p-type as well as n-type transistors in this specific ADC, making it much easier to create all kinds of functionalities.” The researchers used screen printing to ‘stencil’ the circuits into the plastic.
With the development of plastic ADCs, implementation in the food and pharmaceutical industry is suddenly within reach. The fact is a sensor circuit consists of four components: the sensor itself, an amplifier, an ADC that digitalizes the signal, and a radio that enables distance reading. There are already plastic sensors measuring pressure, temperature, light intensity and certain chemicals. And even amplifiers for radio stations like RFID are readily available in plastic. ADC then, is the last link in the chain.

The characteristics of the plastic ADC are not very mind-blowing yet: its maximum resolution is seven bits and speeds come in at a few Hz. Plastic electronics are much slower than silicon ones anyway. It’s not just the components that are larger -several micrometers at least- but the signal speed is slower by a factor of a thousand as well.

A smartphone with plastic electronics won’t be happening, then. But for a sensor that monitors temperature for several weeks, or sends out a warning signal as soon as the steak starts going bad, size doesn’t matter. The same applies for a related use: plastic solar cells. Progress is being made in this field as well, thanks in part to the group of TU/e professor René Janssen. It seems plastic electronics and a sustainable future can go hand in hand, after all.

In the picture: the analog-to-digital converter (ADC). In this model, the conducting connections are made of gold and printed silver paste (a polymer in which drops of silver have been dissolved). Resistors are shown in black. The inset pictures the ADC by the COSMIC project. The ADCs from the ORICIS project are visible in the background. Left and right PhD students Sahel Abdinia and Daniele Raiteri. In the center: Eugenio Cantatore.