Haute Couture from the Experimental Physics Lab

The breakthrough came with the arrival of technology for building up a sufficiently large electrical field in polymer foams. This enabled thin-film transistors to switch in reaction to pressure. Possible applications arising from this project funded by the Austrian Science Fund FWF include ultra-thin microphones, pressure sensors for replacement skin, and interactive clothing.

Concepts such as flat and ultra-thin are the latest big thing in the electronics industry, as can be seen from the flatscreens all around us. Applications of this type are made possible by means of thin-film transistors (TFT). Pressure sensitive foils have also been around for some time. Known as ferroelectrets, these are electrically charged polymer foams that generate an electrical signal in reaction to pressure. It has not been possible in the past to use this signal to switch thin-film transistors. However, a joint Austrian and American team has recently achieved precisely this – a breakthrough in the development of ultra-thin, pressure-sensitive switches that have a range of potential applications as a result of their sensitivity and low production costs.

ELECTRO-SANDWICH

“The key factor is the correct coating of the components,” explains project manager Prof. Siegfried Bauer from the Institute of Experimental Physics at the Johannes Kepler University in Linz. “We applied a propylene foam over a TFT on a polyimide base. These are the type of TFTs we know from flatscreens.” The polymer propylene foam is the actual sensor. When pressed, the differently charged sides of the individual cavities in the foam converge and produce an electrical signal. Prof. Bauer explains: “The great thing about this combination is that the transistor switches only temporarily. If the pressure on the propylene layer decreases, the transistor reverts to its original state. Previously similar experiments only created permanent switching of the transistor. The transistor did not revert to its original state. That is naturally not ideal for a pressure sensor. It would still generate a signal even if the pressure were released.”

FUNCTIONAL RESEARCH

The practical benefits of the work conducted by the team made up of Prof. Bauer and his colleagues at Princeton University in the U.S. stem from two facts. First the pressure sensitivity is high and exists at different pressure intensities, and second the materials used are cheap.

Prof. Bauer explains: “The pressure sensitivity of the sensor in our measurements ranged from just a few pascals to one megapascal. This is a difference of six orders of magnitude. A voltage of up to 100 V was measured, which is more than enough to switch the transistors. In fact, our calculations showed that the voltages could reach up to 340 V, but these could not be measured directly due to the capacities in the measuring apparatus.” This sensitivity means that the technology could be used as a microphone, for example. This is because a volume of 100 dB corresponds to a pressure of only 2 pascals. Prof. Bauer’s team has in fact been able to demonstrate a linear relationship between the air pressure and the voltage produced using a prototype of an ultra-thin microphone.

The favorable production costs of the materials used is a further reason suggesting that the new development from this FWF project will find practical application. For example, the propylene used for the polymer foams is now being employed both in the home and in the packaging and automotive industries – even without any use being made so far of its property as a ferroelectret. The prices of TFTs are also constantly falling and, if these two components are placed on a flexible substrate, there is very little standing in the way of them being used as a pressure sensor in artificial skin or as a textile. Fashionista beware: Designed by FWF on a catwalk near you.

Original publication: Flexible ferroelectret field-effect transistor for large-area sensor skins and microphones. Graz et al., Applied Physics Letters 89, 073501 (2006)