US scientists have made microfluidic devices using only paper, a pen and sunlight. The cheap and simple equipment could be used in the developing world for diagnosing disease and monitoring water.

George Whitesides and colleagues from Harvard University, Cambridge, call their new method FLASH (Fast Lithographic Activation of SHeets). Using a special type of paper, they made patterned microfluidic devices using only an ink-jet printer, UV lamp and hot plate. They demonstrated that a pen and sunlight can be used instead if printers, lamps or hot plates are unavailable.

Whitesides says the FLASH paper can be prepared in bulk in advance and stored for more than six months. It is made from a piece of paper impregnated with photoresist (a light-sensitive chemical) and sandwiched between a transparency film and black paper. When a microfluidic device is needed, a pattern is printed on to the transparency film with an ink-jet printer, photocopier or pen. The paper is then exposed to UV light or sunlight, to polymerise the photoresist where it is not covered by ink, before the transparency film and black backing paper are removed. Finally the paper is baked - although this step is not needed if sunlight is used instead of UV light - and rinsed to remove the unpolymerised photoresist. Making the device takes less than 30 minutes, says Whitesides.

'I like the simplicity of the method and the ingenuity to impregnate an entire sheet of paper with photoresist to create barriers,' says Abraham "Abe" Lee, an expert on microfluidics from the University of California at Irvine, US. 'It triggers the "why didn't I think of this?" thought that so many great ideas do,' he adds.

The Harvard team have shown that a variety of different papers of different sizes can be patterned with channels as small as 200 micrometres wide. They say the patterns can be reproduced quickly using a photocopier and the cost of materials per device is as low as one to three US cents, depending on its size.

Freya Mearns