Microneedle-assisted microfluidic flow focusing for versatile and high throughput water-in-water droplet generation

Microdroplets have been utilized for a wide range of applications in biomedicine and biological studies. Despite the importance of such droplets, their fabrication is associated with difficulties in practice that emerge from the incompatible nature of chemicals, such as surfactants and organic solvents, with biological environments. Therefore, microfluidic methods have recently emerged that create biocompatible water-in-water droplets based on aqueous two-phase systems (ATPS), most commonly composed of water and incompatible polymers, dextran (DEX) and polyethylene glycol (PEG). However, so far, DEX- and PEG-based water-in-water droplet generation schemes have been plagued with low throughput, and most systems can only generate DEX-in-PEG droplets; PEG-in-DEX droplets have been elusive due to chemical interactions between the polymers and channel walls. Here, we describe a simple approach to generate water-in-water microdroplets passively at a high throughput of up to 850 Hz, and obtain both DEX-in-PEG and PEG-in-DEX droplets. Specifically, our method involves a simple modification to the conventional microfluidic flow focusing geometry, by the insertion of a microneedle to the flow focusing junction, which causes three-dimensional (3D) flow focusing of the dispersed phase fluid. We observe that the 3D flow focusing of the dispersed phase enables excellent control of droplet diameters, ranging from 5 to 65 µm, and achieves a high throughput. Moreover, we report the passive microfluidic generation of PEG-in-DEX droplets for the first time, because in our system the 3D flow focusing of the disperse phase separates the disperse PEG phase from the channel walls, negating the commonly observed wall wetting issues of the PEG phase. We expect this microfluidic approach to be useful in increasing the versatility and throughput of water-in-water droplet microfluidics, and help enable future biotechnological applications, such as microparticle-based drug delivery, cell encapsulation for single cell analysis, and immunoisolation for cell transplantation.