TAKE a cardboard disc and punch two holes in it, on either side of its centre. Thread a piece of string through each hole. Now, pull on each end of the strings and the disc will spin frenetically in one direction as the strings wind around each other, and then in the other, as they unwind.

Versions of this children’s whirligig have been found in archaeological digs across the world, from the Indus Valley to the Americas, with the oldest dating back to 3,300BC. Now Manu Prakash and his colleagues at Stanford University have, with a few nifty modifications, turned the toy into a cheap, lightweight medical centrifuge. They report their work this week in Nature Biomedical Engineering.

What goes around...

Centrifuges’ many uses include the separation of medical samples (of blood, urine, sputum and stool) for analysis. Tests to spot HIV, malaria and tuberculosis, in particular, require samples to be spun to clear them of cellular debris. Commercial centrifuges, however, are heavy and require power to run. That makes them impractical for general use by health-care workers in poor countries, who may need to carry out diagnostic tests in the field without access to electricity. They also cost hundreds—often thousands—of dollars.

Dr Prakash’s device, which he calls a “paperfuge”, costs 20 cents and weighs just two grams. The standard version (pictured) consists of two cardboard discs, each 10cm across. One of the discs has two 4cm-long pieces of drinking straw glued to it, along opposing radii. These straws, which have had their outer ends sealed with glue, act as receptacles for small tubes that contain the blood to be centrifuged.

Once the straws have been loaded, the two discs are attached face to face with Velcro, sandwiching the tubes between them. For string, Dr Prakash uses lengths of fishing line, tied at each end around wooden or plastic handles that the spinner holds.

The result, which spins at over 300 revolutions per second (rps) and generates a centrifugal force 10,000 times that of gravity, is able to separate blood into corpuscles and plasma in less than two minutes. This is a rate comparable to that of electrical centrifuges. Spinning samples for longer (about 15 minutes is ideal, though that is a lot of effort for a single spinner) can even separate red corpuscles, which may be infected by malarial parasites, from white ones, which cannot be so infected. The team is now trying the system out for real, to find out what works best, by conducting blood tests for malaria in Madagascar.

Once samples have been separated, they still need to be analysed. Fortunately, the paperfuge is not the first cheap laboratory instrument Dr Prakash has invented. In 2014 he unveiled the “foldscope”, a microscope made from a sheet of paper and a small spherical lens. The foldscope goes on sale this year, but his laboratory has already distributed more than 50,000 of them to people in 135 countries, courtesy of a charitable donation that paid for them. He plans to ship a million more by the end of 2017. Putting this together with a paperfuge means it is now possible to separate biological samples and analyse them under a microscope using equipment that costs less than a couple of dollars.