Stanford's Paperfuge turned a button-on-string toy into a 20-cent centrifuge

In 2013 Stanford bioengineer Manu Prakash visited remote clinics in Uganda and saw a centrifuge used as a doorstop in Tororo; broken equipment was common. Centrifuges separate plasma from blood, a step in diagnosing malaria and HIV, but need electricity and cost thousands of dollars. Earlier hand-powered attempts, an egg beater at Harvard in 2008 and a salad spinner at Rice in 2011, reached only 1,200 and 600 rpm.

Prakash's team wanted a far bigger jump and turned to toys. Tops did not spin long enough, and yo-yos took too much skill. In early 2016 postdoc Saad Bhamla strung a button on a thread, a whirligig from his childhood in India, and a high-speed camera showed it spinning at 10,000 to 15,000 rpm.

The team then modeled the toy and found that supercoiling of the string stores energy and accelerates the disc. They tuned string length and disc size and settled on waterproof synthetic paper, the material used in the Foldscope. The result, published in Nature Biomedical Engineering, reaches up to 125,000 rpm.

Treat an ancient spinning toy as a physics problem: model the whirligig, then optimize it into a lab instrument.

Whirligig motion spins one way, stops and reverses, and the article says this gives the same plasma-separation result as a lab spin-up and spin-down.

Supercoiling of the string stores extra energy, which lets the disc reach much higher speeds.

Modeling the whole design space of the toy let the team optimize it rather than just copy it.

Synthetic paper was waterproof, strong, and already on hand from the Foldscope work.

When a lab device is too costly or fragile for the field, look for a cheap toy that already does the same physics and optimize that.

WIRED reported in January 2017 that the Paperfuge separates plasma from blood in 90 seconds, against up to two minutes for a StatSpin MP centrifuge that tops out at 15,800 rpm. It weighs about 2 grams and costs under 25 cents in materials. Prakash and Bhamla then tested it in Madagascar with local health workers. A malaria technician there said she had waited years for such a tool, as moving centrifuges to villages had meant Jeeps and generators.