For 20 cents, it is possible to make a centrifuge out of cardboard, string, and velcro. A peer-reviewed paper has demonstrated that this handmade contraption can swiftly separate plasma from blood and be used to diagnose malaria where medical facilities are not available.

Centrifuges work by creating artificial gravity, causing mixtures to separate based on density. This capacity is used for everything from testing an astronaut's tolerance for extreme forces to enriching uranium for nuclear power or nuclear weapons. The major use, however, is in medical laboratories, where they can quickly separate blood and other bodily fluids into component parts for diagnosis or transfusion.

Biomedical centrifuges are not the most expensive equipment around, but they can still be outside the price range of healthcare clinics in the poorest parts of the world, and their absence costs lives. Dr Manu Prakash of Stanford University wondered if it was possible to make something cheaper.

Inspired by the whirligigs children have made for at least 5,000 years, Prakash put together his own version with a pouch to hold a capillary tube with the blood sample in it. With the disk made out of stiff card, the fishing line made of strings, and the handles made of wood or PVC, the whole thing can be put together almost anywhere. Slow-motion observations of how this “paperfuge” works helped Prakash refine it so the separation could occur much more quickly than existing low-cost models.

Powered by nothing more than a winding and unwinding of two strings, the smallest versions of Prakash's device can be spun to 125,000 revolutions per minute by hand, at which speed plasma can be separated from blood in under two minutes. The force experienced by the blood in these conditions is 30,000 times that of gravity. Fifteen minutes is enough to create what is known as the buffy coat, the layer of centrifuged blood containing most of the white blood cells and platelets, which can be used for malaria diagnosis.

Larger models spin more slowly, but one with a radius of 5 centimeters (2 inches) can achieve 10,000 times the force of gravity, doing the same job slightly more slowly.

In Nature Biomedical Engineering, Prakash has demonstrated the viability and effectiveness of the paperfuge. He explains how more advanced models can be made using 3D printers, and has submitted an application to the Guinness Book of World Records for the fastest recorded rotational speed of a human-powered device.

The paper also speculates about the potential for “lab-on-a-chip” devices that can diagnose a wide range of diseases without the need for electricity or other technology that may be inaccessible in remote locations.