Manu Prakash, Stanford University professor who had earlier built the less than a dollar foldscope — a paper microscope that can be used for diagnosing blood-borne diseases such as malaria, African sleeping sickness and Chagas — has now developed another ultra-low cost device that can revolutionise public health.

The human-powered paper centrifuge, which can attain a maximum speed of up to 1,25,000 revolutions per minute (RPM), could enable simple blood tests for diseases such as malaria for just 20 cents. The paper centrifuge can separate pure plasma from whole blood in less than 90 seconds and isolate malaria parasites in 15 minutes. The power-free centrifuges have the potential to become point-of-care diagnostics in resource-poor settings.

The results were published in the journal Nature Biomedical Engineering. Prof. Prakash is the corresponding author of the paper.

Centrifuges are routinely used for analysing the concentration of pathogens and parasites in blood, urine and stool, and is the first step in detecting diseases such as malaria, tuberculosis and other diseases. Commercially available centrifuges need electricity and are expensive. This prompted Prof. Praksh to look for alternatives.

After trying several other items including the yo-yos and tops, the breakthrough came in early 2016 when Saad Bhamla from the Department of Bioengineering at Stanford University and the first author of the paper tried a button whirligig. He realised the potential of the whirligig as a centrifuge when a high speed camera clocked the speed of the button at 10,000-15,000 rpms. “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,” Bhamla says in a press release about the speed at which the button was spinning.

The paper centrifuge or “paperfuge”, as Prof. Prakash calls it, is the improvised whirligig — circular discs spun by pulling the strings passing through two holes in the discs.

The paperfuge spins at very high rpm by going through winding and unwinding phases. The disc unwinds when the strings are pulled; since there is no force applied during the winding phase it allows the inertia of the disc to rewind the strings. Since the strings are very flexible they wind beyond a point that the string gets into a tightly packed supercoiled state and the spinning stops. At this point when the string is pulled outwards they unwind and spin the disc in the opposite direction.

Though commercial centrifuges spin only in one direction, same results can be achieved when the paperfuge spins in one direction, stops and then spins in the reverse direction. Prof. Prakash is confident that performance-wise the paperfuge can match centrifuges that cost $1,000-5,000. “The simplicity of manufacturing our proposed device will enable immediate mass distribution of a solution urgently needed in the field,” they write. It is yet one more example of frugal science “leveraging the complex physics of a simple toy for global health applications”.

“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 in the release.

Health-care implications

The paperfuge can separate red blood cells from plasma in about 90 seconds and cost about 20 cents. The paperfuge with two capillaries loaded with blood samples had a maximum speed of 20,000 rpm. The haematocrit (volume percentage of RBCs in blood) value obtained was in “good agreement” with control experiments carried out for 120 seconds on a commercial electric centrifuge. The commercially available centrifuge has a speed of 16,000 rpm.

Prof. Prakash and his team used the paperfuge to test blood samples containing malaria parasites.  The Plasmodium falciparum parasitemia present in 30 microlitre of blood sample could be isolated from blood samples in 15 minutes. They went a step further by using capillaries precoated with acridine orange dye; the malaria parasites glow under fluorescent microscopy making the identification easy and simple.

Prof. Prakash in collaboration with nonprofit health care Pivot based in Boston is all set to test the paperfuge in a region of rural Madagascar in full-scale trials in March. “I would guess that 90 per cent of labs in Madagascar don’t have a working centrifuge. If it works this could be a game changer,” Pivot co-CEO Matthew Bonds told Science.

“The simplicity and robustness of the paperfuge device makes it possible to design and construct devices from materials beyond paper, including wood, plastic and polymers,” they write. The authors printed lightweight prototypes of 3D-fuges using 3D printer.  These with a maximum rpm of 10,000 “opens up opportunities to mas-manufacture millions of centrifuges using injection-moulding techniques”.