paper centrifuge
The paper centrifuge. Blood samples are placed in sealed drinking straws and the 2 paper disks are attached together with Velcro to serve as a cover.

Rapid and accurate diagnosis of many diseases requires separating a blood sample into several parts, including plasma and red blood cells. This is typically done by spinning the sample at an extremely fast rate in a device called a centrifuge. The high centrifugal force causes the fluid to form layers based on the density of the components. Unfortunately, this key piece of medical equipment is typically expensive, bulky, and requires electricity to work, making it impractical in many global settings.

A team led by Dr. Manu Prakash at Stanford University set out to design a low-cost, portable, humanpowered centrifuge that could be used in places that lack resources such as electricity. The research was funded in part by an NIH Director’s New Innovator Award and NIH’s National Center for Advancing Translational Sciences (NCATS). Results were published online in Nature Biomedical Engineering on January 10, 2017.

The scientists were inspired by spinning toys invented before the industrial age, such as yo-yos and tops. They found that the whirligig—a toy in which a button or other circular disk is spun by pulling on strings that pass through its center—rotates at extremely high rates.

The team designed a whirligig, which they call a “paperfuge,” made with a paper disk and braided fishing line for the string. They analyzed the mechanics of the device, which consists of successive winding and unwinding phases. Based on their modeling, they optimized the components, including the disk radius and width, and the string radius and length. Using a high-speed camera, they showed that the paperfuge could reach speeds of 125,000 revolutions per minute (rpm) using only human power.

“There are more than a billion people around the world who have no infrastructure, no roads, no electricity. 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,” Prakash says.

The researchers used drinking straws to hold tubes containing blood samples and found that the paperfuge could separate pure plasma from whole blood in less than 1.5 minutes of spinning. This separation provides a reading of hematocrit, which is used to diagnose anemia. With 15 minutes of spinning, they could separate out a layer known as the buffy coat. This layer is used for diagnosing conditions where a parasite is in the blood—such as malaria and African trypanosomiasis (sleeping sickness). The device weighed 2 grams and could be made for 20 cents.

The scientists also made paperfuge-like devices from other materials, including plastic and 3-D printed polymers. Such devices could open opportunities for other types of diagnostic tests in resource-poor settings. They could be used for science education and field ecology as well.

The team has tested the paperfuge in a community health setting in Madagascar and is now clinically validating the device.

—by Carol Torgan, Ph.D.

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