Stretchability is needed in electronics if it is to be foldable, tightly conformal or following the form of something that changes in shape, like the human body. It is needed for uses from healthcare to toys and robots.
Tokyo University has put a rubbery sheet containing pressure sensors on top of a sheet of plastic transistors, which was then wrapped around the fingers of a robot's hand. To make an effective human skin the substrate needs to be able to stretch and Dr St´phanie Lacour, a researcher at the University of Cambridge, UK and the University of Princeton Professor Sigurd Wagner, found that gold deposited on silicone rubber could stretch by up to 100% without breaking the connection and 20% without increasing electrical resistance. Human skin stretches about 10%.
The Technical University of Berlin used wavy patterns of metal to let the material stretch but Lacour discovered that microcracks in the gold may not be the only material that can produce stretchy electronics. "If you can get this microcrack structure into any material then it should stretch in the same way," asserts Lacour. She is now working on a way to combine it with conventional electronics and sensors. Islands of a stiffer material on the silicone can carry micro chips and thin film transistors printed directly on the material.
"There will be a lot of applications in robotics and consumer products and a lot of people are interested in stretchable displays," she says. It will help mend broken nerves and as a sensory skin for prosthetic limbs she told IET Magazine. "With nerves in limbs we know these nerves regenerate, but they sometimes reconnect the wrong cells. We are looking at ways to re-route them by having an implant that lets us work out which nerves should be connected to each other." Stretchy, flexible substrate is needed to cope with the movement of the body: "We need to work with much softer materials than a silicon wafer."
Stretchable electronics could find many uses in medicine. Alain Bouffioux a researcher at NXP Semiconductors has helped the EU-funded Stella project developing stretchable technologies. He notes, "One potential application is in baby respiration monitoring to guard against sudden infant death syndrome. The baby's apparel could contain strain sensors to monitor how it breathes and then raise an alarm if it suddenly stops or becomes erratic. The "My Heart" project has electrodes built into a vest to provide electrocardiogram readings.
Work on Stella has focussed on putting conventional electronic components inside a silicon rubber bandage. Beyond that it will be possible to put transistors into the fibres of clothes and knit a circuit. Indeed Eleksen has put electrical wiring and keypads into textiles, to make jackets that carry the audio from an iPod in a special pocket to headphones attached to the collar, with a remote control printed on the sleeve.
Professor of Annalisa Bonfiglio of the University of Cagliari in Italy reckons the combination of electronics and apparel can go much further not least because of the stretchability of weaving. Her team has developed a doughnut-like organic transistor wrapped around conductive fibres and sealed inside an insulated coating. To wire up this "textile transistor", production equipment may knit or weave the fibres together and cut through the insulation at contact points to produce electrical circuits.
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