A Portable, Cheap Blood-Clotting Test

A new microsensor could help millions of patients who rely on blood-thinning drugs safely treat themselves.

Millions of patients who take the blood-thinning drug Warfarin could soon use a home testing kit to measure the thickness of their own blood. This portable device, featuring a new micromechanical sensor, would make it far easier for these patients to safely treat themselves.

Good vibrations: The MEMS blood sensor has two cantilevers. One directly measures blood thickness, while the other detects background vibrations that are subtracted to provide a clearer signal. Credit: MicroVisk

Warfarin is used to treat patients suffering from a range of conditions, from pulmonary embolism and heart conditions to thrombosis and excessive blood clotting. But the drug's use needs to be constantly monitored because of a tendency to react with other drugs and metabolic molecules, says John Curtis, chief executive of MicroVisk, the U.K.-based company behind the new device. Diet, alcohol consumption, exercise, and infection can all influence Warfarin's effectiveness, and hence the body's ability to form clots. To avoid the risk of severe internal or external bleeding, drug doses must be managed carefully through regular monitoring of blood coagulation.

Usually, this means taking a blood sample at a doctor's office, sending it off for laboratory testing, and waiting weeks for the results. MicroVisk's aim is to provide a point-of-care or home testing kit that works almost instantly with the same accuracy, Curtis says.

Coagulation is normally measured by recording the thickness of blood at 100-millisecond intervals after adding a reagent called thromboplastin that initiates coagulation. In the lab, this is done by measuring the way that light scatters through a blood sample. But MicroVisk achieves the same result using a pair of vibrating cantilevers, which are immersed in a blood sample and vibrated quickly.

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The device consists of a micro-electromechanical system (MEMS) made up of two 600-micrometer-long cantilevers. These silicon-based devices contain multiple layers of a conducting polymer and a separate piezoelectric material that is coiled along the length of each cantilever. "When we run a current through the conductor, it heats the coils, which causes them to flex," says Curtis. Pulsing current through the coils makes the cantilevers vibrate, while the piezoelectric coils generate a small current that reveals how much they flex.

Two cantilevers are needed, Curtis explains, because one samples the blood directly, while the other detects background vibrations that are subtracted to cancel out vibrations in the surrounding environment.