(229aj) Inkjet Printed Emulsions and Microparticles for Single-Cell Analysis

The field of droplet or emulsion microfluidics has seen rapid expansion in recent years, enabling novel techniques to address challenging problems in biology such as single-cell analysis. Droplet microfluidcs typically relies upon microfluidic channels with flow-focusing or T-junctions to break up an aqueous stream into a stream of monodisperse droplets surrounded by an immiscible oil phase, with each droplet (typically ranging from picoliters to a few nanoliters in volume) serving as a discrete reaction chamber. While numerous variations upon droplet generation devices have been described, almost all rely upon fluid mechanical effects to acheive periodic breakup of the aqueous stream into uniform droplets. This can be quite effective but is ultimately dependent upon precise generation of highly stable flows. In practice this can be difficult to acheive reliably, and almost invariably results in some stabilization time for each experiment, and furthermore presents challenges with coating devices for compatibility with immiscible liquid phases.

We describe an elegant alternative in which electrical signals rather than fluid mechanics are used to acheive extremely regular, periodic generation of droplets. This is acheived using a commercial, off-the-shelf piezoelectric inkjet dispenser suspended above a stirred bath of surfactant-laden oil. Stable and precise droplet generation, at frequencies up to 2 kHz, and with sizes somewhat tunable in the 30-50 micron range, is acheived at the flip of a switch in the "drop-on-demand" mode. Larger droplets (80-100 microns) are produced at 10-20 kHz using the jet breakup mode by applying a constant head pressure to the fluid. Since the aqueous phase (in the inkjet dispenser) and the oil phase are not directly in contact, the two phases can contain incompatible components such as reactive monomers. We demonstrate the use of this system to produce standard water-in-oil emulsions with a variety of hydrocarbon oil and fluorinated oil phases, as well as core-shell microcapsules, and photopolymerized hydrogel microbeads. We demonstrate applications to genetic analysis of single microbial cells. We will discuss practical considerations of the inkjetting setup, and we will discuss unique advantages as well as disadvantages to the inkjetting approach.