Point-of-care blood tests with microfluidics

Blood tests are widely used to diagnose and monitor diseases, medication effectiveness, and organ function. Most analytes reside in the plasma, blood’s liquid component, and the separation of plasma from blood’s solid components typically entails sending blood samples to processing labs for centrifugation. But microfluidics offers the prospect of running tests on location, at low cost, with mere droplets of blood fresh from a pricked finger—by spinning (“lab on a CD”), wicking in ordinary paper, or incorporating filters into “labs on a chip.” Jasmina Casals-Terré (Technical University of Catalonia) and colleagues have now addressed a major hurdle for so-called cross-flow microfluidic filters and greatly improved their efficiency. In the team’s device, blood from an injected drop flows through a narrow, 2-cm-long channel. An electric field applied to the channel helps drive the flow electro-osmotically (see the article by George Whitesides and Abraham Stroock, Physics Today, June 2001, page 42 ). On each side of the channel, half a millimeter away, are parallel channels to carry plasma to the testing region. Separating the channels is an array of diamond-shaped micropillars—each 50 µm wide, 200 µm long, and separated by 30 µm—that filters out the red blood cells. As in other filtration devices, the cells will clog the filter entrance. But every 30 seconds, the team reverses the field direction for 5 seconds, just long enough to break up the accumulating cells. With those electro-osmotic flow reversals, from a 10-µL droplet of blood the researchers could collect 1 µL of plasma in 5–8 minutes. That’s enough plasma, they show, to run a blood panel comprising multiple tests. (M. Mohammadi, H. Madadi, J. Casals-Terré, Biomicrofluidics 9, 054106, 2015, doi:10.1063/1.4930865 .)