Pb. Messersmith et S. Starke, THERMALLY TRIGGERED CALCIUM-PHOSPHATE FORMATION FROM CALCIUM-LOADED LIPOSOMES, Chemistry of materials, 10(1), 1998, pp. 117-124
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.