US scientists have turned a sheet of paper into a cheap, lightweight and foldable electronic display.

George Whitesides and colleagues at Harvard University, Cambridge, printed a message on to one side of the paper and patterned the other side with conductive wires. They then sprayed a thermochromic (changes colour with temperature) dye over the message. When they passed an electrical current through the wires, the heat generated changed the dye from coloured to transparent, revealing the message underneath. The colour change is reversible, says Whitesides, and after cooling, the device can be used more than 100 times again.

Point-of-care diagnostics is an important field, especially for developing countries, where access to medical and analytical resources is limited. Adam Siegel, who worked on the project, says the team wanted to avoid expensive, complex equipment. 'The goal was to make a simple display that will give very clear information in the language of the person using the device, or even in picture form,' he comments. The display is easier to make and cheaper than current electronic display technologies - it costs less than six pence per square metre - which makes it accessible and practical for single-use applications, such as medical tests. And because it is made from non-toxic components, it can be easily disposed of.

Picture messages allow complex instructions to be communicated to populations with high illiteracy or where multiple languages are used

Shuichi Takayama, an expert in the field of point-of-care diagnostics at the University of Michigan, Ann Arbor, US, thinks there is a lot of scope for this kind of technology. 'You can envision having a paper computer and a paper cell phone,' he says.

The team say they hope that this display will be used as part of larger, more complex systems. They envisage that a signal from an electrochemical sensor testing water purity, for example, could be converted into a current that causes the thermochromic display to reveal the purity result.

Anna Roffey

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