Tracking the formation of vaterite particles containing aminopropyl-functionalized silsesquioxane and their structure for bone regenerative medicine.

Abstract

Vaterite particles containing aminopropyl-functionalized silsesquioxane (SiV) were prepared as osteogenic devices for bone regeneration. The SixV particles (x = 0, 2.6 and 4.9 wt%) were synthesized by reacting a slurry of calcium hydroxide with carbon dioxide gas in the presence of γ-aminopropyltriethoxysilane (APTES), a source of soluble silica which would genetically enhance osteogenesis. The obtained Si2.6V and Si4.9V particles were monodispersed with a diameter of 1.4 and 1.5 μm, respectively. The Si2.6V particles showed spherical morphologies. On the surface of the Si4.9V particle small particles were aggregated, resulting in the formation of irregular textures. Transmission electron microscopy of a sectioned Si2.6V particle revealed that the vaterite particles were present as lamellae with a length of 5-20 nm and surrounded by silsesquioxane from APTES. Moreover, the vaterite lamellae were relatively orientated to the c face of the unit lattice, where it is known to be highly polarized, compared to pure vaterite, due to the exposure of the uni-ionic plane with positive (Ca2+) or negative (CO3 2-) charge. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed the co-existence of amorphous calcium carbonate (ACC) in the SiV particles. On contact with physiological pH buffer solution, the vaterite was transiently stabilized and subsequently dissolved and released after the dissolution of silsesquioxane from the particles. This stabilization time was significantly increased with the increase in silicon content. The vaterite was observed in Si2.6V particles up to 3 h of soaking, which extended up to 12 h in Si4.9V particles. The formation of the particles from the precursor gel was monitored by laser Raman spectroscopy and ATR-FTIR. During the initial 1 to 2 h of the aging step, maturation of ACC into vaterite and condensation of monomeric APTES molecules were found to begin simultaneously. These reactions proceeded up to 7 h of the analysis period. The condensation of hydrolyzed APTES is suggested to occur in the vicinity of growing vaterite, which might play a role in the enclosure of vaterite in silsesquioxanes.