Abstract
Aqueous two-phase system (ATPS) droplets have demonstrated superior compatibility over conventional water-in-oil droplets for various biological assays. However, the ultralow interfacial tension hampers efficient and stable droplet generation, limiting further development and more extensive use of such approaches. Here, we present a simple strategy to employ oil as a transient medium for ATPS droplet generation. Two methods based on passive flow focusing and active pico-injection are demonstrated to generate water-water-oil double emulsions, achieving a high generation frequency of ~2.4 kHz. Through evaporation of the oil to break the double emulsions, the aqueous core can be released to form uniform-sized water-in-water droplets. Moreover, this technique can be used to fabricate aqueous microgels, and the introduction of the oil medium enables integration of droplet sorting to produce single-cell-laden hydrogels with a harvest rate of over 90%. We believe that the demonstrated high-throughput generation and sorting of ATPS droplets represent an important tool to advance droplet-based tissue engineering and single-cell analyses.
Highlights
Droplet microfluidics has demonstrated vast promise in processing and analyzing biological samples due to its capacity to confine individual targets in microscale volumes and perform high-throughput manipulations[1,2,3,4]
A spacing stream of pure polyethylene glycol (PEG)-rich solution is introduced at a flow rate of 100 μL/h to separate the two reagent streams, thereby enabling stable droplet generation for over 1 h and forming monodisperse double emulsions (Fig. 1b, c)
The core concept of this technique is to first use oil as a continuous phase to rapidly generate W-W-O doubleemulsion drops and remove the oil to release the inner cores for water-in-water droplet formation
Summary
Droplet microfluidics has demonstrated vast promise in processing and analyzing biological samples due to its capacity to confine individual targets in microscale volumes and perform high-throughput manipulations[1,2,3,4]. ATPS is typically an aqueous solution of two incompatible polymers or one polymer and one salt, which phase-separates to form two immiscible phases when the ATPSs as a promising candidate for various biological assays, including cell culture[9], protein delivery[10], and DNA partitioning[11], but a strategy to generate water-inwater droplets is needed. Interfaces of two aqueous phases can possess ultralow interfacial tension (typically in the range of 10−4 ~ 10−1 mN/m), challenging the efficient and stable generation of droplets in microfluidic systems: The droplets can be formed only under ultralow flow rates (below 3 μL/h) at frequencies below 10 Hz, and the droplet size distribution is undesirably large[13]. When polyelectrolytes or nanoparticles are added to stabilize the droplets, their fast assembly at the interface hampers pinch-off of the interface, further lowering the generation efficiency[14,15]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
