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...