Mechanism of Apatite Nucleation in Biomimetic Process

本工作的目的是阐明在SBF仿生过程中各种生物活性材料上磷灰石的形成机理。钛和钽金属经过氢氧化钠和随后的热处理在其表面形成无定形的钛酸钠或钽酸钠。采用常规熔融-淬火工艺制备了安娜-SiO_2玻璃。这些金属和玻璃在模拟体液(SBF)中浸泡不同时间，离子浓度与血浆中的离子浓度几乎相同。用X射线光电子能谱和透射电子显微镜结合能谱分析了SBF浸泡后表面结构的变化。用电感耦合等离子体原子发射光谱仪研究了浸泡金属和玻璃后SBF中元素浓度的变化，发现金属和玻璃在SBF中释放出Na~+，在其表面形成了Ti-OH、Ta-OH和Si-OH等官能团。这些官能团在形成后立即与流体中的钙离子结合，形成无定形钛酸钙、钽酸钙和硅酸钙等钙化合物。随后，这些钙化合物与流体中的磷酸根离子结合，形成低钙磷原子比的无定形磷酸钙。这些结果表明，生物活性材料上的官能团，如Ti-OH、Ta-OH和Si-OH，不是直接诱导磷灰石的形成，而是通过无定形钛酸钙、钽酸钙和硅酸钙等钙化合物的形成，进而形成无定形的磷酸钙。

The purpose of this work is to clarify the mechanism of apatite formation on various bioactive materials in biomimetic process using SBF.Titanium and tantalum metals were subjected to NaOH and subsequent heat treatments to form amorphous sodium titanate or sodium tantalate on their surfaces. An Na_2O-SiO_2 glass was prepared by conventional melting-quenching technique. The metals and glass were soaked in simulated body fluid (SBF) with ion concentrations nearly equal to those in blood plasma for various periods. Changes in surface structure due to soaking in SBF were analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy accompanied with energy-dispersive X-ray spectrometry. Changes in element concentrations of SBF due to soaking of the metals and glass were examined by inductively coupled plasma atomic emission spectroscopy.The metals and glass were found to release Na^+ ions in SBF to form functional groups such as Ti-OH, Ta-OH and Si-OH groups on their surfaces. These functional groups, immediately after their formation, combined with Ca^<2+> ions in the fluid to form calcium compounds such as amorphous calcium titanate, calcium tantalate and calcium silicate. Later, these calcium compounds combined with phosphate ions in the fluid to form an amorphous calcium phosphate with low Ca/P atomic ratio. Thereafter, this amorphous calcium phosphate increased its Ca/P ratio and incorporated minor components such as Na^+ and Mg^<2+> to eventually transform into bonelike apatite crystal.These results indicate that functional groups on bioactive materials, such as Ti-OH, Ta-OH and Si-OH groups, induce apatite formation not directly, but through formations of calcium compounds such as amorphous calcium titanate, calcium tantalate and calcium silicate, and subsequently amorphous calcium phosphate.