What if you could change your cell (or mobile) phone's color to reflect your outfit, mood or environment? Not just the display alone, but the entire surface. That's the beauty of electronic skin: a chameleon-like, color-changing technology that could one day give us more control over our personal devices.

Philips Research has created an ultra low-power, lightweight, full-color technology that could potentially open up new worlds for personalizing electronic devices, for information delivery or even ambience creation. ÔElectronic skin' technology (or 'e-skin') is one of the first applications of this new concept.

Based on a variation of electrophoretic e-paper, these thin plastic Ôwrappings' can change the color of your cell or mobile phone, for example, when a call is coming in. You could also personalize your phone with the touch of a button, changing its color to match your outfit, mood or environment, even using different colors to tell you who's calling.

Instead of emitting light, e-skins reflect it. It uses electronically controlled 'ink' sandwiched between two plastic membranes. This solves one of the big challenges of making e-skins, adding bright colors with a paint-like appearance and a very low power consumption while keeping the skin light and thin.

Philips' e-skin technology is based on the company's previous work with electronic paper (or e-paper). While in e-paper, each pixel is controlled individually to display a page of text, the approach of e-skin technology is to control batches of pixels in larger display blocks, enabling colors, patterns and simple graphics, or messages in a less complicated and less expensive way. By design, the e-skin is a very energy-efficient system.

Since the particles in suspension carry a surface charge, their motion can be controlled using an electric field, a phenomenon known as electrophoresis. If you create a pixel with colored particles in a clear suspension, applying an electric field perpendicular to the surface makes the particles migrate to the top of the pixel, turning it dark. This is the basis of monochrome e-paper used in e-book readers like the Sony Reader and Amazon Kindle.

To go from monochrome to polychrome, Philips turned the electrophoresis idea on its head, or rather on its side. Instead of applying the field perpendicular to the surface, researchers apply it parallel to the surface. This causes the colored particles to spread across the pixel, again turning it dark.

When the pixel is reset, the colored particles Ôhide' behind a mask, so the pixel is completely transparent. A Ôgate' electrode has been built into each pixel, which gives control over how many colored particles spread across the pixel and hence the saturation or shade of each color.

A full-color e-skin could be created from two electrophoretic layers, each with two colors. With only the yellow and magenta inks showing, the pixel appears red.

Using this in-plane electrophoresis, different types of particles can be included in the same suspension. And because they are engineered to have different surface charges, they can be controlled independently. So you could have cyan, magenta, yellow and black particles, the ingredients to make any color of the rainbow, all within just one or two suspension layers.

The multilayers in action, mixing cyan, yellow and magenta.

Uniquely, this technology allows different colors of ink to be built into one layer with each color controlled separately. This means the layer can be transparent, the color of any one of the inks or even a mixture of multiple colors. Moreover, the saturation of each color can be separately and accurately controlled, so any shade can be produced.

Different colors of ink can be built into one layer with each color controlled separately.

Elise van den Hoven, professor of industrial design at the Eindhoven University of Technology in the Netherlands describes this technology as "information decoration: using responsive materials to provide information via changing colors and patterns".

"What really excites me about the Philips e-skin technology is that you could add it anywhere. It doesn't need a lot of power or batteries, and it really is a thin skin that doesn't add weight to an object," she adds. "When it's not Ôon', it can be completely camouflaged or even transparent so you can still have your own pattern underneath. There aren't many technologies that can do all that."

The technology can yield many shades of colors.

The first applications using the technology could be skins for small devices such as MP3 players or cell phones. However, the technology is easily scalable, and Kars-Michiel Lenssen, e-skin project leader at Philips Research, predicts that e-skins may be bringing color and atmosphere to much larger equipment sometime in the near future. "Just as Philips' Ambient Experience uses light and color to make hospital exam rooms much less intimidating, a large e-skin could extend the concept to the MRI or CT scanner itself, potentially making patients more at ease", he notes.

Wrapping the room

But why stop at equipment? Why not just 'wrap' a whole room? The technology is light enough and consumes so little power it could easily be integrated into wallpaper. In fact, you could even redecorate an entire room with just a flick of the switch for a special occasion or to match your mood. You could even combine e-skin wallpaper with advanced sensor technology to create lighting and environmental controls that Ômagically' appear when you want them, and then fade back into the wall.

In the retail sector, these atmosphere-creation applications could be adopted by using color-changing e-skin wallpaper to freshen up with each new season, to guide shoppers through the store or to create one-off displays to draw in customers. In that sense, reflective e-skins are the perfect complement to the emissive atmosphere-creation technologies that use LEDs and OLEDs to create colorful light. "You could use LEDs or OLEDs when you want a theatrical look and e-skins when you want something subtler and more natural-looking that uses less energy", Lenssen proposes.

Philips e-skin technology features a gradient of grey levels from a highly transparent optical state (left) to full black (right). This enables future applications like smart windows.

Ever noticed that buildings in hot countries tend to be white to reflect sunlight away during the heat of the day, while in cold countries they tend to be dark to absorb as much heat as possible? Imagine building e-skins into the roofs of buildings in countries with variable climates and adapting the e-skin color to the season, soaking up sunlight in the winter while reflecting it during the summer. And because e-skin and OLEDs can be transparent when desired, you could integrate both technologies into windows.

During the day you could have an ordinary window that turns as a blind if the sun is too bright, fine-tuning the saturation so you get just the right amount of light in but without additional heat, lessening the need for air conditioning. Then at night, the OLED could turn the window into an atmospheric lamp, with the e-skin on the outside blocking stray light to prevent light pollution.

Of course, these applications are all highly speculative but e-skin's potential for attractive, unobtrusive information delivery and personalized atmosphere creation is intriguing in its own right. Electronic skins could one day do much more than turn your phone blue.

Hans Driessen (hans.driessen@philips.com) is senior communications manager at Philips Research - www.research.philips.com

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