Preparation of calcium-loaded liposomes and their use in calcium phosphate formation

Liposome encapsulation technology has been used to entrap aqueous calcium salts within dipalmitoylphosphatidylcholine lipid vesicles, which were then used to form calcium phosphate minerals. The calcium encapsulation efficiency was found to depend upon a number of factors that included calcium salt concentration, vesicle size, lipid concentration, and method of vesicle preparation. Removal of unencapsulated calcium by ion exchange resulted in calcium-loaded liposome suspensions with calcium concentration as high as 85 mM. Addition of inorganic phosphate to the calcium-loaded liposomes resulted in liposome suspensions that, although highly supersaturated with respect to calcium phosphates, exhibited metastability as a result of the physical separation of calcium (intravesicular) and phosphate (extravesicular) ions. Calcium ion release and mineral formation was activated by a bilayer-to-micelle mesophase transformation induced by the addition of lipid surfactant. Characterization of the mineral product by infrared spectroscopy, X-ray diffraction, and electron microscopy revealed the formation of apatite and brushite mineral under basic and acidic conditions, respectively. The potential use of calcium-loaded liposomes for in situ formation of mineral for medical and dental applications is discussed.