Injecting picoliter-volume reagents into droplets

Shuttled around a microfluidic circuit, water droplets can serve as test tubes in which controlled chemical reactions occur. That microenvironment has proven useful in applications such as genetic sequencing and the screening of huge libraries of molecular compounds in the search for new drugs. The central challenge in reactor applications is controllably adding reagents to the drops, which is particularly problematic in those cases where drops are coated with an oily surfactant layer to ensure that the small chemical factories remain isolated from each other. A Harvard University team led by David Weitz has now designed a pressurized microfluidic system that injects, with subpicoliter precision, an aqueous reagent into individual drops in the presence of an electric field. The figure illustrates the technique. As tightly packed drops pass through a narrow channel in single file, carrier oil is added to space them apart. Downstream, a reagent-filled injector channel tapers to a small opening and connects to the main channel. The watery reagent and oily carrier fluid are immiscible, and interfacial tension keeps the reagent, except for a small bulge, out of the main channel. As a drop flows under the bulge and voltage is applied to the electrodes, the electric field destabilizes the drop’s oil-water interface and reagent flows into it. By adjusting the drop’s velocity, the injection pressure, and the rate at which the field is switched on and off, the researchers can control how much reagent is added—between 0.1 and 3 pL in their device—and selectively target particular drops. (A. R. Abate et al., Proc. Natl. Acad. Sci. USA 107 , 19163, 2010 http://dx.doi.org/10.1073/pnas.1006888107 .)