Aqueous two-phase systems (ATPS) are liquid-liquid extraction methods that utilize two immiscible aqueous solutions, typically formed by mixing polymers or a polymer and a salt, to separate biomolecules based on their differential partitioning. These systems are advantageous in biotechnology and bioseparation processes due to their biocompatibility, low interfacial tension, and ability to maintain the native state of proteins and other sensitive biomolecules.