Preparation and characterization of semi-solid phospholipid dispersions and dilutions thereof

Highly concentrated, viscous to semi-solid phospholipid dispersions with phosphatidylcholine (PC) contents up to 600 mg/g or 780 mM were obtained by high-pressure homogenization. Dilution of these pastes with excess buffer led to ‘classical' liposome dispersions. The dilution technique determined the homogeneity of the liposome dispersions. Handshaking yielded heterogeneous dispersions, which according to cryo-electron microscopy contained large multivesicular vesicles (MVVs) as well as small unilamellar vesicles (SUVs). By using a ball mill for dilution, however, the phospholipid pastes could be completely transferred into uniform SUVs with mean diameters of about 20–40 nm. The absence of bigger particles could be demonstrated both by a membrane filtration test through 0.2 μm pore filters and photon correlation spectroscopy. Lipid paste formation and subsequent dilution into liposomes led to high encapsulation efficiencies of the hydrophilic model compound 5,6-carboxyfluorescein. For true SUV dispersions, encapsulation efficiencies rose with increasing lipid contents up to a maximum of over 45% at original lipid contents of 600 mg/g. According to geometrical considerations, the packing of SUVs reaches densest sphere packing at this lipid content. In conclusion, semi-solid, vesicular PC pastes can be diluted by ball milling into homogeneous SUV dispersions with high encapsulation efficiency for hydrophilic compounds.