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5. Picoinjection

After drop formation, perhaps the most important microfluidic module is picoinjection, which is used to add reagents to drops after they have been formed. Picoinjection is needed for almost any assay of practical importance, because most assays require several reagents to be added at different times. Picoinjection works by flowing drops past a channel containing the pressurized reagent to be added; due to a surfactant layer present on the drops, however, the fluid will not immediately enter the drop. To trigger the picoinjection, the surfactant layer must be destabilized, which can be achieved by applying an electric field. The field ruptures the film separating the droplet and reagent, allowing the reagent to be injected, as shown in this movie:  

and in this higher-magnification, faster frame-rate movie:

Picoinjection is a simple, robust, and controlled way to add reagents to drops. It also is insensitive to variations in the periodicity of the drops, allowing uniform injection even if the drops enter irregularly. This is a particular advantage of picoinjection compared to other addition techniques such as droplet merger. Another advantage is that picoinjection allows the volume of reagent added to be adjusted. This is possible by either varying the time of the injection, or the injection pressure, as shown in these plots:

(Click on image for high-res version.)

Because picoinjection is triggered using an electric field, it can be switched on and off; to stop picoinjecting, the electric field need only be switched off, allowing the surfactants already present on the drops to stabilize the interface, and block injection. By combining switching with a droplet detection techniques, it is possible to selectively inject drops. This is especially useful for creating libraries of compounds in the drops, for application like combinatorial screening and chemical synthesis.