Recent advances on water-in-water emulsions in segregative systems of two water-soluble polymers

The present paper reviews the most recent knowledge on water-in-water (W/W) emulsions formed in aqueous two-phase systems based on incompatibility between two polymers. The interfaces of these systems are ill-defined, relatively thick, and interfacial tensions are extremely low. Consequently, small molecules do not adsorb in W/W interfaces and emulsions are inherently unstable and the main challenge is achieving a proper colloidal stability at long times. The most widely used strategy is the addition of particles and/or macromolecules able to adsorb at the W/W interfaces, but often the stability of these emulsions is still not satisfactory in the long term. More recently, stabilization of W/W emulsions has been improved by ionic complexation and/or autoaggregation, forming membranes at the interfaces. The proper colloidal stabilization of W/W is paving the way for novel applications, such as carriers of living cells or the development of new 3D cell cultures and cell organoids.