Physicists from Austria and the U.S. have built ultra-thin pressure sensors which can be woven into sensitive textiles. In "Haute couture from the experimental physics lab," they say that their new thin-film transistors (TFTs) can switch back and forth in reaction to pressure. To achieve this effect, they coupled TFTs with ferroelectrets, which are thin films of polymer foams. After demonstrating pressure sensors for replacement skin or ultra-thin microphones, they are now ready for interactive clothing. For example, these sensitive textiles could be used in hospitals to know if patients have left their beds. But haute couture is still far away...
In fact, I've already mentioned the integration of electronics and fabrics several times. You can take a look at The Physics of Haute Couture or Sensitive Textiles for example.
Neither TFTs nor pressure sensitive foils are new. These polymer foams are also known as ferroelectrets. For a good definition of ferroelectrets, you might want to read this article at Wikipedia or this concise definition (Credit: Taylor & Francis Online Journals).
Ferroelectrets are thin films of polymer foams, exhibiting piezoelectric properties after electrical charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air.
Now that we all know what ferroelectrets are, we can read the explanations of Prof. Siegfried Bauer from the Institute of Experimental Physics at the Johannes Kepler University in Linz.
"The key factor is the correct coating of the components," explains Bauer. "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."
Below is a "sketch of the transducer arrangement for the investigation of the ferroelectret field effect. The ferroelectret film is mechanically and electrically interfaced with the amorphous Si field-effect transistor via a thin dielectric coupling layer." (Credit: American Institute of Physics)
Below is a picture of such a ferroelectret field-effect transistor on a flexible film (Credit: Ingrid Graz, for the Austrian Science Fund). Here is a link to a larger version (3.65 MB) of this photo.
For more information about these ferroelectrets, the research work has been published by Applied Physics Letters under the title "Flexible ferroelectret field-effect transistor for large-area sensor skins and microphones" (Volume 89, Issue 7, Article 073501, August 14, 2006). Here is a link to the abstract.
The authors made [ferroelectret field-effect] transistors by laminating cellular polypropylene films and amorphous silicon thin-film transistors on polyimide substrates. They show that these ferrroelectret field-effect transistors respond in a static capacitive or dynamic piezoelectric mode. A touch sensor, a pressure-activated switch, and a microphone are demonstrated. The structure can be scaled up to large-area flexible transducer arrays, such as roll-up steerable compliant sensor skin.
You also can read the full paper (PDF format, 3 pages, 383 KB), from which the top image has been extracted.
Even if it seems certain that electronics will be more present in our clothes, it looks like the author of this press release is dreaming. Here is the punch line: "Fashionista beware: Designed by FWF on a catwalk near you."
Sources: Austrian Science Fund FWF press release, September 28, 2006; and various websites
You'll find related stories by following the links below.
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