On-demand production of femtoliter drops in microchannels and their use as biological reaction compartments.

We present a method allowing to produce monodisperse droplets with volumes in the femtoliter range in a microchannel on demand. The method utilizes pulsed electric fields deforming the interface between an aqueous and an oil phase and pinch-ing off droplets. Water and xanthan gum solutions are considered as disperse-phase liquids, and it is shown that the method can be applied even to solutions with a zero-shear rate viscosity more than 104 times higher than that of water. The droplet formation regimes are explored by systematically varying the pulse amplitude and duration as well as the salt concentration. The dependence of the process on the pulse amplitude can be utilized to tune the droplet size. To demonstrate the applica-bility of the electric-field-driven droplet generator, it is shown that the droplets can be used as versatile biological reaction compartments. It is proven that droplets containing a cell-free transcription-translation system execute gene transcription and protein biosynthesis in a timely and programmable fashion. Moreover, it is verified that biomolecules inside the aque-ous droplets such as small RNAs can be diffusionally activated from the outside to induce a ligand-driven biochemical switch.