A paper-and-string whirligig that costs 20 cents to make could change the game and help end malaria and other worldwide epidemics.
Every two minutes, a child under five somewhere in the world dies of malaria. More than 150,000 children were infected with HIV last year. More than one million kids became ill with tuberculosis last year, and 170,000 died. These and other statistics underscore the disproportionate impact of global epidemics on the planet’s most vulnerable inhabitants: our kids. How fitting that a novel twist on a child’s plaything could dramatically reduce the tragic toll of these and other infections.
Global health experts’ heads are spinning over the "Paperfuge," a superfast, super-simple diagnostic tool based on the “whirligig,” the familiar rotating button-and-string toy kids have been crafting for centuries. With 20 cents of twine, paper, wood, and plastic, Stanford engineers have put a new spin on the whirligig. Their innovation has the potential to bring a standard laboratory technology in industrialized nations within reach of health care workers in the areas hardest hit by global scourges like malaria, HIV, TB, and hepatitis.
Spinning speeds up to 125,000 rpm – perhaps the fastest-spinning human-powered device ever built – the Paperfuge is a DIY alternative to a laboratory centrifuge. Co-developer Manu Prakash, a Stanford assistant professor of bioengineering and 2016 recipient of a John D. and Catherine T. MacArthur Foundation “genius grant,” said it can match the technical specifications of a commercial centrifuge costing $1,000 to $5,000. With small amounts of commonplace materials, the device calls forth some complex engineering principles to create a centrifugal force of 30,000g, enough to process a blood sample for diagnostic analysis in only 1.5 minutes. “This is a tool that requires no electricity, no infrastructure, you can carry them around in your pockets, for a price point of twenty cents,” he says.
A Preventable Burden
The Paperfuge was inspired by a toy, but it’s not designed for fun and games. It’s one small step in the effort to turn the tables on preventable and treatable infections that threaten at least a third of the world’s population. In addition to the “big three” most lethal infections (malaria, HIV, and TB), there are scores of other known or emerging pathogens that wreak havoc in regions too poor, too remote, or too underdeveloped to provide basic medical care. Even in areas where modern vaccines and treatments are obtainable, too many infected individuals spread disease without knowing they are infected. In the case of HIV, for instance, the World Health Organization’s Global Health Sector Strategy on HIV, 2016-2021 reported that at least 17 million of the 37 million people worldwide living with HIV in 2014 did not know their disease status, and 22 million were not taking advantage of antiretroviral therapies (ARTs) that could keep their infection in check and prevent its transmission to others. Statistics like that are why early diagnosis is one of the best weapons against epidemics, and, officials say, why local clinics need simpler ways to screen and test their patients with the resources at their fingertips.
Laboratory centrifuges are ubiquitous in modern clinical laboratories. They serve many scientific purposes but in disease diagnosis, they are used to separate the individual components of a patient’s blood sample prior to testing. Whole blood samples are positioned in the device’s circular rotating chamber and are spun at high speed while centrifugal force partitions the blood based on component density. Heavier red blood cells end up on the bottom of the tube and plasma rises to the top, leaving microbes of interest clustered in between. This helps the diagnostician obtain a quality sample and definitive test results. But like any modern lab instrument, a centrifuge is costly, requires a power source, takes up precious workspace, and presumes the presence of trained operators. For hard-hit clinics without those advantages, a traditional electric centrifuge is essentially a high-tech doorstop, a sight Prakash has literally witnessed first-hand while visiting a rural Ugandan clinic.
A New Spin
That experience validated what Prakash already knew about the need to replace basic-yet-vital clinical tools with simpler, off-the-grid ideas that work anywhere. He has applied frugal design philosophy in inventions like a fold-up paper microscope and a programmable chemical assay device for field work.
Recognizing the benefit of a way to duplicate the effects of a centrifuge without electricity, Prakash and his postdoctoral researcher, Saad Bahmla, turned to the toybox for inspiration. They began a serious study of tops, yo-yos, and whirligigs to understand the mechanics behind the rotational forces at work. Then, after some time fooling around with a particularly fast button-and-string whirligig, Bahmla decided to measure its spin rate with a high-speed camera, finding to his surprise that it was turning at 10,000 to 15,000 rpm. “I couldn’t believe my eyes.”
A few weeks later, after trading the button for a paper disc, he created a proof-of-concept prototype that successfully separated a blood sample. From there, the team set out to understand and model how and why it worked. They created a computer simulation to study how the human-powered linear forces on the string translate into the rotational forces created by the oscillating paper disc. They factored in mathematics explaining the physical coiling of DNA strands to model key physical and mechanical variables: string length and elasticity, dimensions of the spinning disc, the level of hand force applied, and so forth. Their findings resulted in a new design that spun 100 times faster than earlier prototypes and was, with modifications, suitable for field testing. His team is currently working with public health agencies in Madagascar on a validation trial for a Paperfuge-based diagnostic test for malaria.
In its optimized form, the basic Paperfuge comprises a length of stretchy fiber threaded through the center of two circular paper discs and secured on either end by wooden pull-handles. Thin plastic capillaries containing whole blood samples are sandwiched between the two paper discs, which are fastened together with Velcro tabs. The user creates the spinning motion by pulling the handles in opposite directions, alternately tightening and relaxing tension. The coiling and uncoiling of the string causes the conjoined discs to spin, oscillating between forward and reverse rotation as tension on the string builds and ebbs. After about two minutes, the blood components have separated within the capillaries. Additional steps can further separate out and test for pathogens such as malaria.
The Stanford team is not the first to take aim at the centrifuge problem. The World Health Organization Color Scale test, as well as other low-cost point-of-care commercial approaches work in some clinics but require either more operator expertise or scarce consumable products that some facilities cannot access. Commonplace people-powered machines like the salad spinner have also been modified to solve the same problem. Thus far, no one has come up with a device rivaling the Paperfuge for cramming so much engineering theory and performance into something so deceptively simplistic.
Michael MacRae is an independent writer.
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