Cardboard centrifuge separates blood from plasma in 90 seconds
A cheap cardboard centrifuge can separate plasma from blood in 90 seconds. The hand-powered Paperfuge was developed by Stanford bioengineer Man Prakash and can spin samples at up to 125,000 revolutions per minute (rpm). In contrast, a commercial StatSpin MP centrifuge takes up to two minutes to perform a plasma separation, at 15,800 rpm. Prakash was inspired by the design of a toy called a whirligig in making the device, which is a critical tool in the diagnosis of infections such as HIV and malaria.

Both options were bigger, heavier, more expensive, and far less effective than the Paperfuge. “We wanted a much bigger jump,” Prakash says. “For us, it wasn’t about finding the first thing that worked. It was about pushing the limit. We wanted an absolute solution.”

Prakash’s team found that solution in toys. The best ones, he says---the ones that have stood the test of time---distill physical properties into approachable forms. “There’s a sense of simplicity to their mechanisms that causes me to not only enjoy using them but thinking about them,” Prakash says.

The Paperfuge is a great example of how extremely creative people can originate important new ideas by relaxing and observing the world around them. Ray Baughman

His team started by experimenting with tops, but they didn't spin fast enough or long enough to separate biological samples. The researchers tried yo-yos with some success, but the learning curve was way too steep.

“The problem is, you gotta know how to throw a yo-yo,” Prakash says. “And not just any throw. The best throw. Consistently. And ideally, you have to be able to set the yo-yo-spinning again without it returning to your hand.” (This is called looping, and it looks very, very hard.) Even after more than a year of practice, Prakash says he’s never been able to beat the lab record of 4,000 rpm---set by a visiting researcher, a former circus performer and yo-yo master.

The breakthrough came at the beginning of 2016, when Saad Bhamla (a postdoc in Prakash’s lab, and by his own admission a lousy yo-yoer) decided to analyze the spin of a toy from his childhood in India, called a whirligig. The design dates back thousands of years: Suspend a button or disc on a loop of string, and pull the looped ends to set the button spinning. Successive rounds of pulling cause the string to wind and unwind and the button to spin quickly in one direction and then another. (Turns out that even though lab-grade centrifuges spin up, whirl in the same direction, and then spin down, you can get the same results with something that spins one way, stops, and then spins back.)

“It’s a simple design, so one night, I made one myself. My girlfriend lent me a thread from her sewing kit and I strung it through a button,” Bhamla says. He positioned the toy in front of a high speed camera and started spinning. “Later, when I analyzed the footage, I realized that the button was spinning between 10,000 and 15,000 rpms. I remember feeling that this was it, this was what we’d been looking for.”

Bhamla knew that if he understood the whirligig’s mechanics he could optimize it for speed. But when he searched for whether anybody had ever modeled whirligig dynamics, he found only basic simulations. “That was a beautiful paper,” Prakash says, “but the authors weren’t thinking about the toy’s fundamental limits.” The paper’s mathematical models, in other words, were too simple for their purposes. “We wanted to understand the entire design space of the toy.”

Still images of a rotating paperfuge captured with a high-speed camera. The images show the string in a wound, unwound, and rewound state. The insets highlight a phenomenon called supercoiling, whereby the string becomes so tightly wound that it coils over on itself. Creating a coil or coils. A supercoiled string stores more energy, which helps accelerate the disc to higher and higher rpms. Prakash et al.

The team spent the next few months studying the complexities of the whirligig system and converting them into a theoretical model, the details of which they recount in the latest issue of the journal Nature Biomedical Engineering. They discovered that much of the toy’s power hinges on a phenomenon called supercoiling. When the string coils beyond a certain threshold, it starts to form another coil on top of itself. (You can reproduce this phenomenon yourself by twisting your shoelace, or a length of telephone cord.) A supercoiled string stores more energy, which helps accelerate the disc to higher and higher rpms.

The researchers used this and other observations to simulate a wide range of whirligigs. Physical prototypes came next. They tweaked the length of the string and the radius of the disc, and tried a variety of materials, from balsa wood to acrylics. In the end, though, the group settled on the same stuff Prakash used to build his Foldscopes. “It’s synthetic paper, the same thing many countries use in their currency,” Prakash says. “It has polymer films on both front and back that make it waterproof, and it’s incredibly strong, as well.” Plus, they still had plenty of the stuff lying around.