This 20-Cent Paper Centrifuge Uses The Physics Of A Child’s Toy To Lower Lab Costs

Rapidly spinning blood is an important step in diagnosing diseases, but the machines that do it are expensive. This paper unit offers an alternative.

Manu Prakash was visiting a health clinic in Uganda when he noticed a centrifuge–an expensive piece of lab equipment–propping the door open. “They were using it as a doorstop, because of course it had stopped working,” says the Stanford engineer.

The tool, which rapidly spins biological samples, is used in the diagnosis of malaria, HIV, anemia, and other diseases. But centrifuges cost thousands of dollars–often too much to replace if one breaks–and require electricity, meaning that the poorest clinics can’t use them at all. At the clinic in Uganda, a sign on the wall listed 15 tests; without the centrifuge, 12 of those tests could no longer happen.

Prakash, who is known for creating the Foldscope, a cheap, origami-inspired paper microscope, decided to design an alternative when he returned to his Stanford lab.

“The constraints that we put were it had to be human-powered; it had to be ultra-low-cost, but not compromise on the performance,” he says. “It had to be equal in performance to something that you buy for $1,000 or $5,000.”

The device is inspired by a whirligig, a simple toy that has existed since at least 3000 B.C. “I spend a lot of time and money at toy stores,” says Prakash. His team began by exploring all types of spinning toys. “They’re simplistic in their form factor, but in the end, they hide physical properties that are very powerful.”

Initially, they tested yo-yos. A visiting scientist at the lab also happened to be a circus artist, and taught Prakash the ideal way to throw a yo-yo; with perfect form, they realized that it was possible to reach speeds of 4000 rpm, which was fast enough to separate blood. But the yo-yo required too much skill to easily train health workers, so the team moved on to other toys, like spinning tops.

A post-doc student in the lab, Saad Bhamla, was the first to test a simple whirligig, which clocked in at 10,000 rpm on the first try. “That got us very excited,” says Prakash.

They came up with a mathematical equation that explains how the toy works, and used that to design variations that worked even faster. The maximum speed is now 125,000 rpm, far faster than a conventional centrifuge.

As the device spins, the centrifugal force pushes heavier materials, such as red blood cells, away from plasma. If it spins longer, it separates another layer of white blood cells and platelets. Technicians can then look at the sample under a microscope to clearly diagnose disease–and treat people who might not otherwise have received an accurate diagnosis.

The first prototypes are made from polymer-coated paper, the same material that Prakash used to make his paper microscope. The Paperfuge could also be made from different polymers, allowing complex geometries to be built into the design.

The team is starting to test the device in the field, beginning with clinical trials in Madagascar that will use the Paperfuge to screen for malaria, sleeping sickness, anemia, and other diseases. They plan to test the device in extreme conditions. “Picture doing diagnostics under a tree,” Prakash says.

The paper centrifuge can be used along with the paper microscope, in a new ecosystem of ultra-low-cost scientific tools that Prakash calls “frugal science.”

He plans to continue to look for solutions in unexpected places. “This is what I find sort of remarkable: how many mechanisms and things are out there under our noses, both that should be understood better, but might be solutions to some of the tricky challenges that we face as society,” he says.

About the author

Adele Peters is a staff writer at Co.Exist who focuses on sustainable design. Previously, she worked with GOOD, BioLite, and the Sustainable Products and Solutions program at UC Berkeley.



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