Building a centrifuge on a shoestring

A team of Stanford engineers has taken the dime store approach to kitting out their lab. Using twine, paper and plastic tubing, they have created the world’s cheapest centrifuge. It costs just 20 cents to make. With a little creativity and some thrifty spending, the team hope to produce a full field lab for even the most remote areas.

Inspired by whirligig toys, the “paperfuge” spins at speeds of up to 125,000rpm and hits centrifugal forces of around 30,000 Gs. That’s 30,000 times stronger than the pull of gravity! Manu Prakash, assistant professor of bioengineering at Stanford and the study’s senior author, believes this makes it the fastest human-powered spinning object on earth.

Made in exactly the same way as the simple toys, the hand-spun centrifuge separates blood into component parts in 90 seconds flat.

Off-grid diagnosis

A centrifuge is just one of the crucial tools needed for the speedy detection of diseases. Some of the deadliest include malaria, African sleeping sickness, tuberculosis and HIV. The centrifuge typically works by spinning liquid samples inside an electric-powered, rotating drum.

When used to detect diseases in blood, the centrifugal forces cause the sample to split. The heavy red blood cells collect at the bottom of the tube and the light, watery plasma floats to the top. Any parasites, like those that cause malaria, settle in the middle.

In the remote, off-grid areas where these diseases thrive, keeping the kit running can prove impossible. Prakash tells of a trip to a rural clinic in Uganda. With no electricity to power the machine, the pricey centrifuge was relegated to propping the door open.

“There are more than a billion people around the world who have no infrastructure, no roads, no electricity. I realised that if we wanted to solve a critical problem like malaria, we needed to design a human-powered centrifuge that costs less than a cup of coffee,” said Prakash.

Inspired by spinning toys, Prakash and Saad Bhamla, a post-doctoral research fellow in his lab, began experimenting.

“One night I was playing with a button and string, and out of curiosity, I set up a high-speed camera to see how fast a button whirligig would spin. I couldn’t believe my eyes,” said Bhamla. The low-tech toy span at a dizzying 10-15,000 revolutions per minute.

Feeling Frugal

After testing different prototypes, Bhamla mounted a narrow plastic tube onto a paper disc whirligig. Filling each with a sample of blood, he successfully it into layers. Next, he recruited three undergraduate engineering students from MIT and Stanford to help him unravel the maths. Together, the team created a computer model to analyse the paperfuge. This looked at the perfect paper disc size, the elasticity of the twine and the pulling force applied. By picking the best results, they created a prototype with a rotational speed of up to 125,000rpm.

“From a technical spec point of view, we can match centrifuges that cost from $1,000 to $5,000,” said Prakash.In their lab tests, the team separated malaria parasites from red blood cells in just 15 minutes. They also span samples in a tube pre-coated with a special orange dye. The resulting glowing malaria parasites could then be identified using a microscope.

At five-to-the-dollar, the centrifuge could bring precise diagnosis and treatment to those who need it the most.

Accompanying his cheap-as-chips centrifuge, Prakash has designed a range of other frugal field tools. His ultimate aim is to give health workers, field ecologists and children in the world’s most remote areas a whole laboratory in a backpack.

“Frugal science is about democratising scientific tools to get them out to people around the world,” he said.