Formation and properties of liposome-stabilized all-aqueous emulsions based on PEG/dextran, PEG/Ficoll, and PEG/sulfate aqueous biphasic systems

Vesicle-stabilized all-aqueous emulsion droplets are appealing as bioreactors because they provide uniform encapsulation via equilibrium partitioning without restricting diffusion in and out of the interior. These properties rely on the composition of the aqueous two-phase system (ATPS) chosen for the emulsion and the structure of the interfacial liposome layer, respectively. Here, we explore how changing the aqueous two-phase system from a standard poly(ethyleneglycol), PEG, 8 kDa/dextran 10 kDa ATPS to PEG 8 kDa/Ficoll 70 kDa or PEG 8 kDa/Na2SO4 systems impacts droplet uniformity and partitioning of a model solute (U15 oligoRNA). We also compare liposomes formed by two different methods, both of which begin with multilamellar, polydisperse vesicles formed by gentle hydration: (1) extrusion, which produced vesicles of 150 nm average diameter, and (2) vortexing, which produced vesicles of 270 nm average diameter. Our data illustrate that while droplet uniformity and stability are somewhat better for samples based on extruded vesicles, extrusion is not necessary to create functional microreactors, as emulsions stabilized with vortexed liposomes are just as effective at solute partitioning and allow diffusion across the droplet's liposome corona. This work expands the compositions possible for liposome-stabilized, all-aqueous emulsion droplet bioreactors, making them amenable to a wider range of potential reactions. Replacing the liposome extrusion step with vortexing can reduce time and cost of bioreactor production with only modest reductions in emulsion quality.