Over the past couple decades, microfluidic devices have become a staple of medical and biological diagnostics and analysis. Tests that once required large and specialized equipment can now be completed closer to a patient, using only a few drops of sample fluid. Running multiple tests on a single chip can become difficult, though, since flow through the device tends to dissolve and mix the dried reagents used for tests. But a new method cleverly uses fluidic forces to keep reagents separated without the need for complicated microfluidic structures.
The basic concept is outlined in the illustration above. You’re looking down on a microfluidic channel that’s long and very thin. A shallow groove down the middle serves as a barrier by pinning the contact line of the incoming fluid. So when the sample fluid flows in through the inlet on the left, it will only fill the top half of the cell. When it reaches the far right side, it turns the corner and flows to the left, encountering the first of the dried reagents it must dissolve for the device’s tests. The fluid will fill the lower channel quickly and then come to rest while the reagents dissolve.
With both sides of the channel full of liquid, the shallow barrier can no longer hold, and the fluid will take up the full width of the channel, with two well-dispersed – but separated – regions of reagents. Once that’s happened, a valve – represented by the pale blue line near the right side of the illustration – releases the fluid into the next section of the chip, allowing the analysis to proceed. (Image credit: Nature; research credit: O. Gökçe et al.; submitted by Kam-Yung Soh)