杏吧原创

TIRED of staring at the same four walls? Cream is so 1990s. Would a lick of paint help? Don鈥檛 reach for the brush just yet. With electronic wallpaper, your chameleon-like walls would change to suit the mood or occasion. Flick a switch and watch as cream wallpaper transforms itself into fashionable terracotta. Flick it again and the room changes to brilliant green. You鈥檒l never need to lift a paintbrush again.

For indecisive home improvers who are forever changing their rooms, high-tech decorations like these are on their way. Last year, Magnus Berggren of Link枚ping University in Sweden and his team printed cheap, electronic displays on paper, paving the way for moving images, changing colours and text on everything from wallpaper to milk cartons and advertising billboards.

杏吧原创s have been trying to modernise paper for decades. Most research so far has concentrated on replacing paper with other materials, such as plastic and glass. Companies such as Gyricon Media in Silicon Valley and E-Ink in Cambridge, Massachusetts, already offer relatively cheap paper substitutes made of plastic for personal organisers and advertisements. But these replacements miss one simple point: people like paper.

Paper has survived the test of time because you can write and draw anything you like on it, fold it up and put it in your pocket. And unlike an electronic screen, you can read a newspaper from any angle in bright sunlight or in a dimly lit room. Paper also feels good and above all it is cheap to make in huge quantities. Every year the global paper industry churns out nearly 320 million tonnes of the stuff.

Paper does have one major disadvantage though: once text or an image is printed, you can鈥檛 change it. It鈥檚 a weakness that Gyricon and E-Ink are exploiting with their electronic 鈥減aper鈥�. But what if you could transform normal, run-of-the-mill paper into electronic displays?

That鈥檚 exactly what Berggren and his colleagues at Link枚ping University and the Advanced Centre for Research in Electronics and Optics in Norrk枚ping, Sweden, are doing. Four years ago, they asked paper makers if they鈥檇 like more from their newspaper and packaging by adding electronic circuits, sensors and displays, also made from paper. The response was an overwhelming 鈥測es鈥�, and last year the researchers unveiled an electronic display printed on standard paper (Advanced Materials, vol 14, p 1460). Eventually they hope to develop the technology to convert any piece of paper into a fully functioning display.

Their current display is an 鈥渁ctive matrix鈥�, similar to the thin-film transistor (TFT) screens found on laptops. Each pixel on a laptop screen is made of a liquid crystal display cell connected to a transistor, which controls the voltage across the cell, its chemical properties and hence its colour. Most screens, including TFTs, rely on expensive silicon electronics. But Berggren鈥檚 team makes both the transistors and display cells by printing semiconducting polymers onto paper (see 鈥淧aper pixels鈥�).

The display exploits the unusual electrochemical properties of poly(3, 4-ethylenedioxythiophene) or PEDOT. Pixels made from this transparent polymer turn blue when you apply a voltage. Reverse the voltage and the pixels become clear again. The chemical reactions behind this transformation are 鈥渂i-stable鈥�. This means that to flip the polymers from one state to the other you only need to apply a voltage when you want to update the information.

With just 40 pixels, each slightly larger than a postage stamp, Berggren鈥檚 display hardly competes with TFT screens. But it鈥檚 proof that paper electronic displays do work. The real application of the technology, Berggren stresses, is huge, low-resolution displays. His team is already demonstrating a 鈥渟even-segment鈥� display, similar to a digital clock.

And they have bigger plans. They鈥檙e aiming to create poster-sized displays for shops, flashing cereal boxes and packaging for toys, and within three to five years, even changing text in magazines. Although the technology is still in its infancy, companies are already taking notice, including the advertising giant JCDecaux, which owns over 600,000 billboards in 40 countries.

There are still some hurdles to overcome. At present, each pixel takes around five seconds to update. As the speed of the chemical reaction is inversely proportional to the cell area, a pixel as large as a poster or a strip of wallpaper would take minutes or even hours to change hue. This shouldn鈥檛 be a problem for advertisements that are updated overnight, but it might limit other uses. 鈥淲e鈥檒l never be able to compete with video rates,鈥� says Berggren. His colleague Peter Andersson is testing individual pixels just 1 millimetre across that change colour in less than a second, but he has yet to use them in a display.

Another difficulty is that the colour is no match for the bright hues of computer screens. Berggren admits that commercial printers describe his display as 鈥渢errible鈥�. To compete, the team will need polymers that produce magenta, cyan and yellow, the primary colours used in printing. Andersson reckons he has found a complex polymer that can turn yellow, but he won鈥檛 say what it is.

More pressing than pretty colours is a power supply. At the moment the power comes from batteries connected to the paper with crocodile clips. This obviously won鈥檛 work for magazines or cereal packets. Because the displays operate at such low voltages, the team is thinking about using radio waves to transmit power over less than a metre, or attaching flexible, printable batteries. For example, Power Paper, a company based in Einat, Israel, prints batteries directly on paper. Chemical inks act as the anode and cathode and can produce an electric current up to 1.5 volts 鈥� enough to run the paper displays.

Whatever the power source, there are two good reasons why electronic paper might just take off. Firstly, if the printing is done using industrial reel-to-reel processes it will cost only a couple of dollars per square metre. Secondly, you can throw it in the recycling bin with everything else when you鈥檙e finished with it.

Back at home, your livid green walls still don鈥檛 look right. Overwhelmed by choice, with a rainbow of colours available at the flick of a switch, a return to cream seems the only sensible option. Like our inability to give up paper after some two thousand years, old habits die hard.