Thermally Triggered Calcium Phosphate Formation from Calcium-Loaded Liposomes

Abstract

A thermally triggered liposome-based mineralization system is described that is metastable at ambient temperature but rapidly forms calcium phosphate mineral upon warming to physiologic temperature. Mixing of a calcium-loaded lipid vesicle suspension with aqueous inorganic phosphate resulted in a stable liposome suspension whose bulk ionic concentration was highly supersaturated with respect to hydroxyapatite and other calcium phosphate minerals. The mineralization activity of metastable liposome suspensions was found to be strongly dependent both on temperature and on the composition of the phospholipids that comprise the vesicle membrane. No detectable mineral formation occurred in the metastable liposome suspensions during storage for several weeks at room temperature. However, when the liposome suspensions were heated to near the lipid chain melting transition (Tm), Ca 2+ was released from the lipid vesicles and reacted with extravesicular phosphate to form calcium phosphate mineral. Infrared spectroscopy and powder X-ray diffraction analysis of the reaction products indicated the formation of both apatite and brushite minerals, a finding that is consistent with the changing pH conditions of thermally triggered mineralization. Mixtures of miscible phospholipids (dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine) were used to tailor Tm to physiologic temperature. This strategy was employed for the preparation of metastable liposome suspensions that were stable for long periods of time at room temperature but that mineralized rapidly when heated to 37 °C. The potential medical and dental significance of thermally triggered liposomal mineralization is discussed.