Microfluidic paper passes the litmus test

Harvard University professor George Whitesides is making very sophisticated litmus paper. Unlike the simple acid or alkali strips familiar from school, his pioneering work owes more to microfluidics than pH sensitive dyes. The development of cheap and simple paper diagnostics may now help revolutionise third-world medical testing.While everyone takes it for granted, paper is more than a surface for writing on. Its porosity can ensure that drops of liquid quickly spread out by capillary action or wicking. As the capillary effect provides a useful free pump, a sample drop may be moved across the paper into test wells, reacting there to produce visible colour changes. "The basic idea of making channels and directing fluids - that was clearly a good idea," says Whitesides.By harnessing a photoresist technique, he created polymer-lined channels to direct and manipulate minute quantities of fluids. Chromotography paper is soaked with a light-sensitive polymer photoresist that...

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