This homemade toy saves lives.

Nature shows how one of the simplest toys out there – a loop of string with a spinning bit of paper in the middle – can be used as a life-saving diagnostic device:

Growing up in India, Manu Prakash entertained himself with a bottle cap that spun around on two strings that he tugged with his fingers. As a physical biologist at Stanford University in California, he is now transforming that simple toy, called a whirligig, into a cheap tool to help diagnose diseases such as malaria.

Prakash started this project, the results of which are published on 10 January in Nature Biomedical Engineering, after a research trip to Uganda in 2013. While visiting health-care clinics, he noticed that most lacked a working centrifuge — or the electricity to power one — and could not separate blood samples to perform basic disease diagnostics.

“One clinic used its broken centrifuge as a doorstop,” says Prakash, a 2016 MacArthur ‘genius grant’ winner who has also invented a foldable paper microscope. “When we got back from Africa we asked ourselves, ‘Can we do centrifugation with no electricity, using only human power?’”

Video footage revealed that the toy’s strings not only twist around each other as they wind and unwind, but also form coils similar to structures found in DNA. Solving the equations that describe the forces behind that coiling revealed the specs for an ideal whirligig — from the size of its disc to the thickness of its strings — capable of spinning a million times per minute.

Human hands cannot spin the toy fast enough to hit that theoretical limit. But a new design made in the lab achieved 125,000 r.p.m. — submitted to Guinness World Records last year as the fastest device to rotate under human power.

After optimizing the whirligig, Prakash and his team then mounted plastic tubes for holding blood samples onto their paper device. Their final prototype, dubbed the paperfuge, can reach 20,000 r.p.m., separating plasma from blood in 1.5 minutes, and malaria parasites in 15 minutes.

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