High-throughput injection with microfluidics using picoinjectors

Abstract

Adding reagents to drops is one of the most important functions in droplet-based microfluidic systems; however, a robust technique to accomplish this does not exist. Here, we introduce the picoinjector, a robust device to add controlled volumes of reagent using electro-microfluidics at kilohertz rates. It can also perform multiple injections for serial and combinatorial additions.

Fig. 1.

Microfluidic device consisting of a droplet spacer and picoinjector. The spacer adds oil from a side channel to space the drops. The picoinjector injects fluid by merging the drops with a pressurized channel containing the reagent. Picoinjection is triggered by an electric field, which is applied by the electrodes.

Fig. 2.

The picoinjector can be switched on and off. This is possible because the injector channel begins wide and narrows to a small orifice; to pass through the orifice, the fluid must adopt a bulge with high curvature. This creates a Laplace pressure that balances the pressure differential between the injection and oil channels. (A) When the electrode is off, the drop slides over the bulge without injection, because surfactants prevent merger. By contrast, (B) when the electric field is activated, the drop merges with the reagent, causing fluid to be injected. By switching the field on and off, drops can be injected selectively. This can be used to inject every other drop (Upper) or to inject a more complex pattern (Lower).

Fig. 3.

The picoinjector allows the injection volume to be controlled. (A) By increasing the injection channel pressure, more fluid is injected because the flow velocity into the drop is larger. (B) By increasing the spacer oil flow rate, less reagent is injected because the drop spends less time by the picoinjector. (C) Electrode voltage can switch picoinjection on and off, not injecting below 30 V, and injecting a constant volume above. The error bars correspond to the standard deviation in the volume injected, as measured for 100 injections.

Fig. 4.

Picoinjectors can be used in combination for serial and combinatorial injection. In this example, three picoinjectors are used in series. Each is activated by the electrode opposite, while ground electrodes on either side shield the picoinjectors nearby. This allows each to be actuated independently, to inject different combinations.