Stabilization of all-aqueous droplets by interfacial self-assembly of fatty acids bilayers.

All-aqueous microdroplets produced by liquid-liquid phase separation have emerged as promising models of artificial cells, and offer new approaches for the solvent-free encapsulation of fragile solutes. Yet, the lack of a membrane on such droplets makes them intrinsically unstable against coarsening, and precludes a fine control over chemical localization, as solutes can freely diffuse through the interface. Herein, we report the construction of stable and impermeable water-in-water emulsions via the interfacial self-assembly of mixed sodium oleate/1-decanol bilayers on dextran-rich droplets produced by segregative liquid-liquid phase separation with poly(ethylene glycol). Lipids spontaneously self-assemble as multilamellar structures at the surface of the droplets as revealed by freeze-fracture transmission electron microscopy and small-angle X-ray scattering. We further demonstrate that the lipid-based membrane is impermeable to oligonucleotides and proteins, but also to a low molecular weight dye, so that a strict chemical encapsulation can be achieved by spontaneous partitioning within the droplets before membrane self-assembly. Taken together, our results highlight the ease of production of fatty acid-stabilized all-aqueous emulsions droplets able to encapsulate a range of solutes without the need of oil or organic solvents, paving the way to the construction of robust membrane-bounded, polymer-rich artificial cells.

Building micro-capsules using water-in-water emulsion droplets as templates.

The use of templates in materials chemistry is a well-established approach for producing membrane-bounded hollow spheres used for microencapsulation applications, but also in synthetic biology to assemble artificial cell-like compartments. Sacrificial solid or gel micro-particles, but also liquid-like oil-in-water or water-in-oil emulsion droplets are routinely used as templates to produce capsules. Yet, disruption of the core sacrificial material often requires harsh experimental conditions, such as organic solvents, which limits the use of such approach to encapsulate fragile solutes, including biomolecules. Recently, water-in-water emulsion droplets have emerged as promising alternative templates to produce capsules in solvent-free conditions. These water-in-water droplets result from liquid-liquid phase separation in dilute aqueous polymer or surfactants solutions. Their ease of preparation, the large palette of components they can be assembled from and the lack of harsh solvent or oil used for their production make water-in-water emulsions of practical importance in materials chemistry. Water-in-water droplets can also spontaneously sequester solutes by equilibrium partitioning, which provides a simple strategy to locally accumulate molecules of interest and encapsulate them in capsules after interfacial shell formation. Here, we review recent works that employ water-in-water emulsion droplets to prepare capsules and suggest possible additional applications in materials chemistry.

Self-coacervation of ampholyte polymer chains as an efficient encapsulation strategy.

Coacervation is a phase separation process involving two aqueous phases, one solute-phase and one solute-poor phase. It is frequently observed among oppositely-charged polyelectrolyte systems. In this study, we focus on self-coacervation involving a single polymer chain and investigate its potential for encapsulation applications. Negatively charged polyacrylic acid polymer chains were partially cationized using diamine and carbodiimide chemistry affording ampholytes, named PAA-DA, with tunable charge ratio. When dispersed in water, at pH 7, PAA-DA was soluble but a phase separation occurs when decreasing pH close to the isoelectric point. Coacervation is found only for a given amine-to-acid ratio otherwise precipitation is observed. Increasing the pH above 4 yielded progressive destruction of the coacervates droplets via the formation of vacuoles within droplets and subsequent full homogeneous redispersion of PAA-DA in water. However, addition of calcium allowed increasing the coacervate droplet stability upon increasing the pH to 7 as the divalent ion induced gelation within droplets. Moreover, the coacervate droplets present the ability to spontaneously sequestrate a broad panel of entities, from small molecules to macromolecules or colloids, with different charges, size and hydrophobicity. Thanks to the reversible character of the coacervates, triggered-release could be easily achieved, either by varying the pH or by removing calcium ions in the case of calcium-stabilized coacervates. Self-coacervation presents the advantage of pathway-independent preparation, offering a real output interest in pharmacy, water treatment, food science or diagnostics.