A passive microfluidic system based on step emulsification allows the generation of libraries of nanoliter-sized droplets from microliter droplets of varying and known concentrations of a sample

Abstract

We present a novel geometry of microfluidic channels that allows us to passively generate monodisperse emulsions of hundreds of droplets smaller than 1 nL from collections of larger (ca. 0.4 μL) mother droplets. We introduce a new microfluidic module for the generation of droplets via passive break-up at a step. The module alleviates a common problem in step emulsification with efficient removal of the droplets from the vicinity of the step. In our solution, the droplets are pushed away from the step by a continuous liquid that bypasses the mother droplets via specially engineered bypasses that lead to the step around the main channel. We show that the bypasses tighten the distribution of volume of daughter droplets and eliminate subpopulations of daughter droplets. Clearing away the just produced droplets from the vicinity of the step provides for similar conditions of break-up for every subsequent droplet and, consequently, leads to superior monodispersity of the generated emulsions. Importantly, this function is realized autonomously (passively) in a protocol in which only a sequence of large mother droplets is forced through the module. Our system features the advantage of step emulsification systems in that the volumes of the generated droplets depend very weakly on the rate of flow through the module - an increase in the flow rate by 300% causes only a slight increase of the average diameter of generated droplets by less than 5%. We combined our geometry with a simple T-junction and a simple trap-based microdroplet dilutor to produce a collection of libraries of droplets of gradually changing and known concentrations of a sample. The microfluidic system can be operated with only two syringe pumps set at constant rates of flow during the experiment.

Controlled droplet microfluidic systems for multistep chemical and biological assays

“…An interesting and recent example is the generation of libraries of picoliter droplets by step emulsification of large plugs with arbitrarily defined compositions. 32 Another possible synergistic improvement might be a hybrid device composed of EWOD based modules for universal and versatile operations on individually addressed droplets combined with parts of controlled droplet channel microfluidics for the increase of the throughput and capacity of the whole system. First examples of such systems were already presented, 52 but the potential of the synergistic combination of DMF and channel droplet microfluidics has not yet been fully exploited.…”

“…The appropriate and careful design of various geometries of barriers and bypasses allowed for the development of several versions of traps to perform operations such as (i) dilution, 34 (ii) metering and diluting, 33 (iii) merging and releasing, 33 (iv) locking and shifting, 33 (v) derailing and storing, 18 (vi) metering and storing 17 and (vii) splitting large droplets into a library of subnanoliter droplets. 32 These functions are realized by a set of hard-wired geometric modules that can be assembled into networks -systems that perform multistep complex liquid handling protocols. Importantly, in contrast to SlipChips and self-digitization systems, these systems allow complex and sequential operations for, e.g., serial dilution that spans a wide range of concentrations of the samples or reagents.…”

Droplet microfluidics for high‐sensitivity and high‐throughput detection and screening of disease biomarkers

“…Step-emulsification devices typically do not require co-flow of two phases, as only the aqueous phase is driven to produce droplets (Dutka, Opalski, & Garstecki, 2016). These devices are less resistant to variations in the aqueous flow rate, and the single-input nature of these devices make them amenable to parallelization for increased throughputs, without the need for channel distribution layers or multiple inlets (Ofner et al, 2016; Postek, Kaminski, & Garstecki, 2017). However, the lack of control on the continuous phase can result in lack of control on droplet movement following droplet generation.…”

Microfluidic Chamber Design for Controlled Droplet Expansion and Coalescence

“…Only the pressure-driven flow drives the droplet operations in these cases [38,39]. Different examples such as droplet (a) formation [40,41], (b) merging [42,43], (c) splitting [44], (d) sorting [45,46], (e) synchronization [47] and (f) trapping [48][49][50][51][52] have already been described.…”