Abstract
We demonstrate the dynamic control of aqueous two phase system (ATPS) droplets in shrinking, growing, and dissolving conditions. The ATPS droplets are formed passively in a flow focusing microfluidic channel, where the dextran-rich (DEX) and polyethylene glycol-rich (PEG) solutions are introduced as disperse and continuous phases, respectively. To vary the ATPS equilibrium condition, we infuse into a secondary inlet the PEG phase from a different polymer concentration ATPS. We find that the resulting alteration of the continuous PEG phase can cause droplets to shrink or grow by approximately 45 and 30 %, respectively. This volume change is due to water exchange between the disperse DEX and continuous PEG phases, as the system tends towards new equilibria. We also develop a simple model, based on the ATPS binodal curve and tie lines, that predicts the amount of droplet shrinkage or growth, based on the change in the continuous phase PEG concentration. We observe a good agreement between our experimental results and the model. Additionally, we find that, when the continuous phase PEG concentration is reduced such that PEG and DEX phases no longer phase separate, the ATPS droplets are dissolved into the continuous phase. We apply this method to controllably release encapsulated microparticles and cells, and we find that their release occurs within 10 seconds. Our approach uses the dynamic equilibrium of ATPS to control droplet size along the microfluidic channel. By modulating the ATPS equilibrium, we are able to shrink, grow, and dissolve ATPS droplets in situ. We anticipate that this approach may find utility in many biomedical settings, for example, in drug and cell delivery and release applications.
Highlights
Over the last decade, droplet-microfluidics using pico- to nanoliter sample volumes[1] has been widely used in applications for bioassays,[2] single cell encapsulation,[3] and high-throughput analysis.[4]
DEX droplets are formed by the Rayleigh-Plateau instability-induced breakup of the DEX phase thread near the DEX-polyethylene glycol (PEG) flow focusing junction. Downstream of this junction, we introduce a different concentration of the PEG phase, via a secondary inlet, to perturb the equilibrium condition of the Aqueous two phase systems (ATPS), and modulate the ATPS droplet behavior
We propose a simple model, based on the ATPS binodal curve and tie lines, that predicts the change in the ATPS droplet size based on the PEG phase supplied at the secondary inlet
Summary
Droplet-microfluidics using pico- to nanoliter sample volumes[1] has been widely used in applications for bioassays,[2] single cell encapsulation,[3] and high-throughput analysis.[4]. Monodisperse water-in-oil or oil-in-water droplets are generated in T-junctions or flow focusing channel geometries. The size of the generated droplets is tuned by changing the orifice size, sample flow rates, and fluid viscosities.[5]. Once the droplets are formed in the microchannel, the droplets can be manipulated, for example by coalescing or splitting at a T-junction, to create larger or smaller droplets.[6–8]. ATPS are composed of water plus at least two incompatible polymers, for example, polyethylene glycol (PEG) and dextran (DEX). The biocompatible nature of both ATPS phases makes ATPS suitable for many biomedical applications, including cell patterning,[9] cell encapsulation,[10] protein extraction,[11] and DNA separation.[12]
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