PREPARATION OF CALCIUM-LOADED LIPOSOMES AND THEIR USE IN CALCIUM-PHOSPHATE FORMATION

Liposome encapsulation technology has been used to entrap aqueous calc ium salts within dipalmitoylphosphatidylcholine lipid vesicles, which were then used to form calcium phosphate minerals. The calcium encapsu lation efficiency was found to depend upon a number of factors that in cluded calcium salt concentration, vesicle size, lipid concentration, and method of vesicle preparation. Removal of unencapsulated calcium b y ion exchange resulted in calcium-loaded liposome suspensions with ca lcium 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, ex hibited metastability as a result of the physical separation of calciu m (intravesicular) and phosphate (extravesicular) ions. Calcium ion re lease and mineral formation was activated by a bilayer-to-micelle meso phase transformation induced by the addition of lipid surfactant. Char acterization of the mineral product by infrared spectroscopy, X-ray di ffraction, and electron microscopy revealed the formation of apatite a nd brushite mineral under basic and acidic conditions, respectively. T he potential use of calcium-loaded liposomes for in situ formation of mineral for medical and dental applications is discussed.