THERMALLY TRIGGERED CALCIUM-PHOSPHATE FORMATION FROM CALCIUM-LOADED LIPOSOMES

A thermally triggered liposome-based mineralization system is describe d 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 phosph ate resulted in a stable Liposome suspension whose bulk ionic concentr ation was highly supersaturated with respect to hydroxyapatite and oth er calcium phosphate minerals. The mineralization activity of metastab le liposome suspensions was found to be strongly dependent both on tem perature and on the composition of the phospholipids that comprise the vesicle membrane. No detectable mineral formation occurred in the met astable liposome suspensions duping storage for several weeks at room temperature. However, when the liposome suspensions were heated to nea r the lipid chain melting transition (T-m), Ca2+ was released from the lipid vesicles and reacted with extravesicular phosphate to form calc ium phosphate mineral. Infrared spectroscopy and powder X-ray diffract ion analysis of the reaction products indicated the formation of both apatite and brushite minerals, a finding that is consistent with the c hanging pH conditions of thermally triggered mineralization. Mixtures of miscible phospholipids (dipalmitoylphosphatidylcholine and dimyrist oylphosphatidylcholine) were used to tailor T-m to physiologic tempera ture. This strategy was employed for the preparation of metastable lip osome suspensions that were stable for long periods of time at room te mperature but that mineralized rapidly when heated to 37 degrees C. Th e potential medical and dental significance of thermally triggered lip osomal mineralization is discussed.