IMEC

ELECTRONIC circuits that stretch like elastic bands or expand like balloons sound not only fanciful but also rather impractical. Such circuits are usually made from rigid materials and are certainly not expected to bend and stretch like new-year dieters in a gym during the first week of January. Yet a little flexibility could be useful, especially in those devices that need to remain in contact with their user's skin: bandages, body implants and health monitors, for example. Skin-like electronics would also be valuable for robots.

IMEC, a Belgian research group, has recently made a start on this idea by building tiny electric wires that can stretch to twice their length without breaking. IMEC produced its stretchy wires by encapsulating tracks of gold that look like meandering rivers within an elastic silicone film. The arrangement works like a flat spring: when it is stretched the spring opens up without the wires breaking. And just in case a break does happen, neighbouring tracks are arranged in sets of four that are cross-connected at regular intervals so that the current can switch to an unbroken track.

IMEC's stretchy circuits can be built with the standard techniques used to make printed-circuit boards. To demonstrate that this arrangement works, the firm has made a prototype stretchable digital watch. It is not completely there; its active (and rigid) electronics are contained in islands within the stretchy silicone material. Nevertheless Jan Vanfleteren, who is leading the project, hopes that future versions will be made fully flexible by using an ultra-thin chip-packaging technology that IMEC is developing.

IMEC's work is part of a collaborative project, called STELLA (Stretchable Electronics for Large Area Applications), which numbers Philips, a Dutch electronics giant, and Urgo, a French producer of sticking plasters and bandages, among its participants. But this is not the only group interested in the field.

Nokia, for example, is collaborating with Stéphanie Lacour, of the University of Cambridge's Nanoscience Centre, to develop a way of depositing gold films onto elastomers (elastic polymers), so that such films become stretchy as well as bendable. Normally, a gold film will fracture if stretched by as little as 1-2% of its original length. Dr Lacour's stretchy metal film, however, contains an array of tiny Y-shaped cracks. When it is pulled, the cracks open up and the film remains intact. If silicon chips a few hundred microns across were embedded in the elastomer as well, the film should provide a reliable way of wiring them together to produce a useful electronic device.

Such “stretchable electronic skin”, as Nokia dubs it, is intended, initially, to provide a touch-sensitive way of talking to machines. If it were turned into a screen, stabbing your finger harder on an icon on that screen could make something happen faster.

Tapani Ryhänen, Nokia's head of strategic research, suggests that stretchable devices, worn on the body, could be used to detect and transmit information about the wearer, such as to measure emotional states. And what about an entire body suit made from flexible electronics and sensors? That would be the ultimate Nintendo Wii game controller.