A whirligig is so simple, even a kindergartener can make one. But this ancient toy—a pinwheellike device whose circular motion is powered by two twisting strings—may soon transform medicine in the developing world, thanks to an inexpensive new version that can separate blood as quickly as some commercial centrifuges. The “paperfuge,” as it is called, consists of little more than paper, string, and glue, and it costs about 20 cents to produce—versus hundreds to thousands for traditional centrifuges. If the new device makes it past regulatory hurdles, engineers say, the paperfuge could prove a portable and cheap tool for diagnosing anemia and infections such as HIV and malaria in places where resources are scarce.
Centrifuges, rapidly spinning machines that separate blood and other materials, are a mainstay in modern medical laboratories. But because they are expensive—more than $6000 for some top-of-the-line versions—doctors in developing regions of the world don’t always have easy access to the devices. Even when labs have centrifuges, they may not have the electricity to keep them running. To get around this problem, scientists have come up with all sorts of human-powered alternatives, including simple machines built from egg beaters and salad spinners. But the speeds of these devices have never come close to those of commercial centrifuges, which can spin upward of 100,000 revolutions per minute.
So a team of researchers, led by bioengineer Manu Prakash of Stanford University of Palo Alto, California, decided to take a systematic approach. They collected more than 10 spinning toys, from tops to yo-yos to gyroscopic wrist exercisers, and they used high-speed cameras to clock their speeds. None made the cut, even after they hired a circus performer to help explore the toys’ full potential. “We amassed a graveyard of spinning toys,” says Prakash, who has previously garnered attention for an inexpensive paper microscope he dubbed the Foldscope.
But one toy stood out: the whirligig, which Prakash had known in his childhood in India as the “button on a string.” Also called the buzzer, the bullroarer, and the zumbado, the 5000-year-old toy has been found from China to Israel to Venezuela. But ironically, no one knew how it worked, mathematically or physically. Prakash’s team set out to change that using computational modeling. They discovered, among other things, that twisting the strings into tightly clumped coils was key to storing energy that allowed for rapid spinning with minimal power. “The way the strings wind and unwind, they exploit a really interesting principle of supercoiling,” Prakash says. “These supercoils … let it go far beyond its geometrical limits.”
Over the course of about 6 months, the team optimized its toy for rotational speed, creating a prototype that worked by pulling back and forth on sticks attached by strings to a paper disk that holds tiny tubes of blood—the repeated winding and unwinding of the strings spins the blood. Clocking the process with a high-speed camera showed just how quickly: up to 125,000 revolutions per minute—faster than many commercial centrifuges. Similar to high-tech centrifuges, the paperfuge was able to separate plasma from blood samples in less than 1.5 minutes, the team reports today in Nature Biomedical Engineering.
“This is pretty promising in terms of its results,” says Muhammad Zaman, a biomedical engineer at Boston University who was not involved in the study. “I was really impressed both because it had a pretty real grounding in strong theory and strong simulation … and also because of the broad applications.” With a few minor tweaks planned for the device, the team envisions even more applications than simple plasma separation. In a pilot test, for example, users isolated malaria parasites from blood for detection under a microscope with 15 minutes of spinning, a time frame comparable to commercial centrifuges.
Still, Zaman says the paperfuge’s real test will come in the field. Regulatory, social, and cultural barriers can render new technology useless without ongoing investment from scientists and medical workers, he says. “People might not think this little paper thing can do something as good an instrument that costs [thousands of] dollars,” Zaman says. “It requires a trust in the system.”
Prakash already has plans to gain that trust. He recently signed on with a nonprofit health care organization called Pivot in Boston to test the paperfuge in a region of rural Madagascar, with full-scale trials to start in March. “I would guess that 90% of labs in Madagascar don’t have a working centrifuge,” says Pivot co-CEO Matthew Bonds, himself a health economist at Harvard University. “If it works this could be a game changer.”