In a Spin

How can a length of string, two pieces of paper, and a couple of handles become a low-cost alternative to a traditional centrifuge? Researchers at Stanford University have the answer (1), and their efforts could have positive implications for diagnostics in resource-poor areas.

The “paperfuge” was inspired by whirligigs – toys invented thousands of years ago – and, rather suitably, it appears to be child’s play to operate. How does it work? A plastic capillary tube is used to collect a blood sample before being sealed and securely fixed to a circular piece of paper with two holes in the center. A similar piece of paper is secured on top to trap the capillary between the two sheets. String is then threaded through the holes in the paper, and held via a grip at each end. The disc is “wound” up and then unwound by pulling the grips apart; the inertia allows the disc to rewind once more before the cycle begins again. Despite the featherweight device being simple to put together and operate – and costing a mere 20 cents to produce – it is highly effective, spinning up to 125,000 rpm (the authors cite 1,000,000 rpm as the theoretical maximum!) and reaching g-forces of approximately 30,000. Such force allows the separation of blood cells from plasma in less than two minutes, and malaria parasites can be isolated in 15 minutes. The investigators noted that in regions without access to traditional centrifuges, researchers and medical professionals often use eggbeaters or salad spinners as alternatives – but those solutions don’t spin rapidly enough to be effective substitutes – and they are bulkier than the paperfuge. As the Stanford investigators have shown, sometimes to move forward, you need to look back – even thousands of years. Simple, cost-effective tools like the paperfuge (and the paper-based biosensor here) have the potential to open up modern diagnostic capabilities to the people and places that often need them most.

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