Many of us have played with whirligigs as kids, but now these playthings made of buttons and twine are getting a new life as medical lab tools for the developing world. Bioengineers at Stanford University have developed a blood centrifuge based on the children's toy that costs only 20 US cents in materials to build, yet can compete with commercial lab centrifuges costing thousands of dollars.

The poorer regions of the world are also hotbeds of diseases like malaria, tuberculosis, HIV, and sleeping sickness. To combat these scourges, doctors use centrifuges to process blood samples. Spinning at thousands to hundreds of thousands of RPMs, the bench-top lab machines generate thousands of G's that cause the various components in blood to separate out into layers like cream from milk in a cream separator. Combined with chemical dyes and microscopes, they are a powerful weapon in combating disease by helping to isolate and identify microbes for diagnosis.

The problem is that such machines can cost hundreds or thousands of dollars, and all but the most primitive ones need electricity to work that often isn't available in remote regions, which resulted in one case in Africa of a very expensive centrifuge being used as a doorstop.

"There are more than a billion people around the world who have no infrastructure, no roads, no electricity," says Manu Prakash, an assistant professor of bioengineering at Stanford. "I realized that if we wanted to solve a critical problem like malaria diagnosis, we needed to design a human-powered centrifuge that costs less than a cup of coffee."

According to Stanford, Prakash and post-doctoral fellow Saad Bhamla came up with the "paperfuge" while looking at toys like tops and yo-yos for inspiration. Noticing how the disc of a whirligig spins when the cords on either side are pulled, they decided to make a slow motion video of one, only to discover that it rotated at 10,000 to 15,000 RPM.

The pair started developing prototypes using a blood capillary tube mounted on a paper disc, but they went beyond simple tinkering as they recruited three undergraduate engineering students from MIT and Stanford to create mathematical models of how the whirligig could change a pulling motion into a rotary motion. Looking at variables like disc size, string elasticity, and pulling force, they combined this with equations from the physics of supercoiling DNA to gain a better understanding of the whirligig's mechanism.

The result was a centrifuge made of 20 cents of paper, twine, and plastic that could spin at 125,000 RPM, generate 30,000 G's, and process samples in 1.5 minutes.

"To the best of my knowledge, it's the fastest spinning object driven by human power," says Prakash. "From a technical spec point of view, we can match centrifuges that cost from US$1,000 to US$5,000."

According to the team, the paperfuge can help identify malaria in the field within 15 minutes by spinning the sample in tubes coated with acridine orange dye. After this, the dyed parasites became readily visible under a microscope. Bhamla and Prakash are now working on paperfuge field validation trials for malaria diagnostics with PIVOT and Institut Pasteur in Madagascar.

The results of the research were published in Nature Biomedical Engineering.

The video below discusses how the paperfuge works. And for another example of an inexpensive centrifuge, check out one that was made from a salad spinner.

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