liposomes

Liposomes are spherical vesicles composed of one or more phospholipid bilayers that enclose an aqueous core. They simulate aspects of biological membranes and are widely used as carriers for drugs, nutrients, and diagnostic agents. Liposomes form spontaneously when amphiphilic lipids are dispersed in water.

They can be classified by lamellarity and size: multilamellar vesicles (MLVs) contain multiple bilayers; unilamellar vesicles have a single bilayer and include small unilamellar vesicles (SUVs, typically 20–100 nm) and large unilamellar vesicles (LUVs, 100–1000 nm). Composition commonly includes phosphatidylcholine and cholesterol.

Preparation methods include thin-film hydration with subsequent size reduction by extrusion or sonication; reverse-phase evaporation; ethanol injection; and newer approaches using microfluidics. Choice of method influences size, lamellarity, encapsulation efficiency, and stability.

Liposome surfaces can be modified to alter pharmacokinetics and targeting. Polyethylene glycol (PEG) chains, or PEGylation, reduce recognition by the immune system and extend circulation time. Targeting ligands (antibodies, aptamers) and cationic lipids enable specific delivery or gene transport.

Applications include drug delivery for cancer and infectious diseases, vaccines and adjuvants, and cosmetic formulations. Several liposomal medicines are approved, such as liposomal doxorubicin (Doxil in the United States; Caelyx in Europe) and liposomal amphotericin B (AmBisome); others include liposomal irinotecan (Onivyde) and verteporfin liposomes (Visudyne).

Limitations include stability challenges and leakage of cargo, potential for rapid clearance by the mononuclear phagocyte system, and manufacturing complexity. Characterization typically uses dynamic light scattering for size and zeta potential, electron microscopy for morphology, and assays for encapsulation efficiency.