Hybrid polymer/lipid vesicles: state of the art and future perspectives

Vesicles are microscopic sacs that enclose a volume with a molecularly thin membrane. The membranes are generally self-directed assemblies of amphiphilic molecules with a dual hydrophilic-hydrophobic character. Biological amphiphiles form vesicles central to cell function and are principally lipids of molecular weight less than 1 kilodalton. Block copolymers that mimic lipid amphiphilicity can also self-assemble into vesicles in dilute solution, but polymer molecular weights can be orders of magnitude greater than those of lipids. Structural features of vesicles, as well as properties including stability, fluidity, and intermembrane dynamics, are greatly influenced by characteristics of the polymers. Future applications of polymer vesicles will rely on exploiting unique property-performance relations, but results to date already underscore the fact that biologically derived vesicles are but a small subset of what is physically and chemically possible.

Vesicles were made from amphiphilic diblock copolymers and characterized by micromanipulation. The average molecular weight of the specific polymer studied, polyethyleneoxide-polyethylethylene (EO40-EE37), is several times greater than that of typical phospholipids in natural membranes. Both the membrane bending and area expansion moduli of electroformed polymersomes (polymer-based liposomes) fell within the range of lipid membrane measurements, but the giant polymersomes proved to be almost an order of magnitude tougher and sustained far greater areal strain before rupture. The polymersome membrane was also at least 10 times less permeable to water than common phospholipid bilayers. The results suggest a new class of synthetic thin-shelled capsules based on block copolymer chemistry.

In the last past dozen years, polymersomes (Ps) have attracted tremendous attention as versatile carriers because of their colloidal stability, tunable membrane properties and ability in encapsulating or integrating a broad range of drugs and molecules. Relatively long blood circulation times of Ps can be accomplished when block copolymers with a poly(ethylene glycol) (PEG) are used for the formation of Ps. A number of Ps has been developed for new possibilities and applications in drug delivery, medical imaging, electronics and nanoreactors. In particular, Ps prepared by using biodegradable and/or stimuli-sensitive block copolymers that are responsive to various internal or external stimuli are of great interest for such applications. In this review, recent advances of Ps as drug delivery systems are discussed. Critical factors that influence the formation of Ps are also addressed. The review describes preparative methods and characterization techniques for Ps. Moreover, protein and cell interactions with Ps, in vivo circulation kinetics and biodistribution of Ps are addressed.