Polyion complex micelle formation from double-hydrophilic block copolymers composed of charged and non-charged segments in aqueous media

Polymeric micelles are representative self-assembly structures of block copolymers and have widely been investigated from both fundamental and applied aspects. In 1995, we discovered polyion complex (PIC) micelles formed from a pair of oppositely charged block copolymers with poly(ethylene glycol) segments through electrostatic interactions in aqueous media, which expanded the concept of polymeric micelle formation in selective solvents. Hereafter, extensive studies have been carried out on PIC micelles, for example, fundamental characterizations as a novel class of self-assembly systems and applications as nanocarrier systems for the delivery of charged molecules with therapeutic efficacies, including nucleic acids and proteins. This review mainly focuses on physicochemical studies on the formation of PIC micelles, particularly the critical molecular factors that have a role to determine the self-assembly scheme of charged block copolymers for micelle structures.