Friday, February 27, 2009

Flexible Screens Get Touchy-Feely

The first bendable, touch-screen display flexes will be used by the military.

By Duncan Graham-Rowe

Touch and feel: Bendable, touch-sensitive screens could lead to a new generation of more rugged and easy to use portable displays. Credit: Flexible Display Center

Researchers have developed the first computer display that is both flexible and touch sensitive. They say that the breakthrough could lead to more practical and easier-to-use portable devices.

Over the past few years, there has been a drive to develop displays that more closely mimic the properties of paper.

E Ink, based in Cambridge, MA, already supplies displays that are easy to read in direct sunlight and require little power for both the Amazon Kindle and the Sony Reader, compared to LCDs and plasma screens. E Ink's technology uses a layer of microcapsules filled with submicrometer black and white particles to create a low-power, reflective screen.

Ultimately, though, the goal is to make displays that are not only flexible, but that also respond to touch. The first flexible electronic-paper product, the Readius, is due to launch later this year. This electronic reader features a roll-out E Ink display made by Polymer Vision, based in the Netherlands.

Sri Peruvemba, VP of marketing at E Ink, says that adding touch sensing to this kind of display presents a whole new set of challenges. There are a number of ways to make screens touch sensitive, he says, but most are designed to work with a rigid screen.

Resistive touch screens--such as those used in the Nintendo DS games console--rely on points of contact being made between two separate conducting layers. Flexing these layers can lead to false inputs, says Jann Kaminski, display engineer at the Flexible Display Center(FDC), at Arizona State University, which codeveloped the new display with E Ink. "For resistive, you have an air gap that has to be maintained," he says.

Similarly, capacitive touch screens--for instance, the one used in the iPhone--rely on conductive transparent films made out of indium tin oxide (ITO), a material that doesn't like to be flexed. "It is a brittle, ceramic-like material," says Kaminski.

Touch screens that detect changes in light or vibrations across a screen as it is touched fare no better when flexed because the signals used become distorted, says Kaminski.

The only approach left, he says, is inductive touch-screen technology, although this is not without its challenges. Inductive screens typically use magnetized styluses to induce a field in a sensing layer at the back of the display. This kind of layer is not inherently sensitive to flexing, and some are already commercially available. But most flexible displays feature a very thin stainless-steel backplane that allows the display to flex while maintaining enough rigidity to protect it from damage. And these metallic backplanes act as a barrier to the electromagnetic fields that make inductive touch screens work.

To get around this, the FDC team uses an alternative backplane material: a thin-film plastic material made by DuPont called Teonex polyethylene napthalate (PEN). This material is already widely used in thin-film transistor manufacturing. It provides support for the display while allowing the inductive touch layer to work, says Kaminski.

Peruvemba adds that the approach doesn't degrade the quality of the image because the sensing is carried out behind the display. This is particularly important because E Ink products rely on the reflection of ambient light rather than on an energy-sapping backlight.

Prototypes have undergone rigorous testing (watch a video of the testing), and the first place that the displays are likely to be used is in the military, says Shawn O'Rourke, director of engineering at the FDC. The military interest comes from the need for portable displays that do not shatter, he says. Most displays are built on a glass backplane, so there is a real need to have something more robust. "They want thin, lightweight displays that are rugged and low powered," says O'Rourke.