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A Portable, Cheap Blood-Clotting Test

A new microsensor could help millions of patients who rely on blood-thinning drugs safely treat themselves.
November 10, 2008

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.

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.

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.

MicroVisk’s current prototype requires blood to be applied using a pipette. But the company is about to engineer a device that captures and analyzes a sample in one go, similar to a glucose-testing kit. The company was recently granted a European patent and secured $1.7 million in funding, and its aim, says Curtis, is to come to market within three years.

Given that similar approaches have been used to measure viscosity at a macroscopic level, the approach should work well at a microscopic level too, says Todd Przybycien, head of biochemical and chemical engineering at Carnegie Mellon University, in Pittsburgh. He adds that measuring viscosity, rather than trying to detect specific particulates in blood, is a more manageable task.

In March, the Centers for Medicare and Medicaid Services (CMS) extended its health-insurance policy to allow Warfarin patients in the United States to be covered for more-frequent testing at home. “The market for testing patients on Warfarin is huge, with a global value at over $2.25 billion,” Curtis says. “This is going to end up much like the glucose-testing market.”

“By limiting doctor visits, MicroVisk will make life much more manageable for people taking Warfarin,” says Phil Cooper, director of the Sensors and Instrumentation Knowledge Transfer Network, an academic and industry group based at the National Physical Laboratory, in Oxfordshire, U.K., which supported MicroVisk in its early stages.

Other devices that perform a similar job are becoming available but are less accurate, Curtis says, because they measure coagulation through indirect biochemical changes–for example, by applying a current to the blood and measuring impedance. The other advantage of using a MEMS sensor is that it requires less blood. “We require about one microliter of blood, as opposed to between 10 and 15 microliters,” Curtis adds. This means that each blood test requires a smaller pinprick. “You’re going to cut through a lot less nerve endings, so it hurts less,” he says.

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