T. Kokubo et al. I Biomaterials 24(2003)2161-2175 Fig. 5. SEM photographs of the surfaces of titania gels with amorphous(A), anatase(B) and rutile( C)structures after soaking in an SBF for 7d. Apatite ca/P=1.65 100nm Energy /keV Energy /keV Fig. 6. TEM-EDX profiles of the nano-sized apatite formed on amorphous sodium titanate(A)and Ha sintered at 800C(B). after soaking in an SBF for 72 and 48 h, respectively. (center of the electron diffraction). 2.4. Mechanism of apatite nucleation by functiona the fluid to form amorphous calcium titanate. This calcium titanate later combines with phosphate ions in the fluid to form amorphous calcium phosphate with a and glass-ceramic A-W are multi-compo- low Ca/P ratio. The calcium phosphate transforms into nent systems that are based upon the Na2O-SiO2 or apatite, which exhibits a Ca/P ratio of 1.65, and contains Cao-SiO2 binary system. As a model of these materials, a small concentration of Mg and Na, similar to bone apatite formation on a simple binary sodium silicate mineral, as shown in Fig 6A (20Na20-80SiO2 in mol%) glass was investigated in To reveal the reasons why this complex process is detail using X-ray photoelectron spectroscopy (XPS) required for apatite formation, the zeta potential of the Ind transmission electron microscopy accompanied by surface of sodium titanate was measured by laser energy-dispersive X-ray analysis (TEM-EDX). This electrophoresis at various SBF soaking times [47]. It glass releases Na ions into SBF via an exchange with was found that the surface of the sodium titanate was the H3O ions in the fluid to form Si-OH groups on its highly negatively charged immediately after it was surface [43, 44]. The Si-OH groups formed immediately soaked in the SBF, as shown in Fig. 7. The surface combine with Ca- ions in the fluid to form an potential increased with increasing soaking time up to a amorphous calcium silicate on the glass surface. After maximum positive value. Thereafter, it decreased with a long soaking period, this calcium silicate combines increasing soaking time, reached a negative value with phosphate ions in the fluid to form an amorphous and finally converged to a constant negative value. On calcium phosphate with a low Ca/P atomic ratio, and basis of this finding, the complex process of apatite his phase later transforms into bone-like apatite crystal, formation described above is well interpreted in terms of increasing its Ca/P ratio and incorporating minor ions the electrostatic interaction of the functional groups such as Na, Mg+, and CI with the ions in the fluid. The Ti-Oh groups formed on In a different model, the mechanism of apatite the surface of sodium titanate after soaking in SBF are formation on an amorphous sodium titanate for negatively charged and, hence, combine selectively with on titanium metal was also examined by XPS and TEM- the positively charged Ca- ions in the fluid to form EDX 145, 46]. In the sBf, the sodium titanate releases calcium titanate, as shown in Fig. 7. As the calcium ions Na ions via exchange with the H3o ions in the fluid accumulate on the surface, the surface gradually gains to form Ti-OH groups on its surface. The Ti-OH an overall positive charge. As a result, the positively groups formed immediately combine with Ca- ions in charged surface combines with negatively charged2.4. Mechanism of apatite nucleation by functional groups Bioglasss and glass-ceramic A-W are multi-compo￾nent systems that are based upon the Na2O–SiO2 or CaO–SiO2 binary system. As a model of these materials, apatite formation on a simple binary sodium silicate (20Na2O
80SiO2 in mol%) glass was investigated in detail using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy accompanied by energy-dispersive X-ray analysis (TEM-EDX). This glass releases Na+ ions into SBF via an exchange with the H3O+ ions in the fluid to form Si–OH groups on its surface [43,44]. The Si–OH groups formed immediately combine with Ca2+ ions in the fluid to form an amorphous calcium silicate on the glass surface. After a long soaking period, this calcium silicate combines with phosphate ions in the fluid to form an amorphous calcium phosphate with a low Ca/P atomic ratio, and this phase later transforms into bone-like apatite crystal, increasing its Ca/P ratio and incorporating minor ions such as Na+, Mg2+, and Cl. In a different model, the mechanism of apatite formation on an amorphous sodium titanate formed on titanium metal was also examined by XPS and TEM￾EDX [45,46]. In the SBF, the sodium titanate releases Na+ ions via exchange with the H3O+ ions in the fluid to form Ti–OH groups on its surface. The Ti–OH groups formed immediately combine with Ca2+ ions in the fluid to form amorphous calcium titanate. This calcium titanate later combines with phosphate ions in the fluid to form amorphous calcium phosphate with a low Ca/P ratio. The calcium phosphate transforms into apatite, which exhibits a Ca/P ratio of 1.65, and contains a small concentration of Mg and Na, similar to bone mineral, as shown in Fig. 6A. To reveal the reasons why this complex process is required for apatite formation, the zeta potential of the surface of sodium titanate was measured by laser electrophoresis at various SBF soaking times [47]. It was found that the surface of the sodium titanate was highly negatively charged immediately after it was soaked in the SBF, as shown in Fig. 7. The surface potential increased with increasing soaking time up to a maximum positive value. Thereafter, it decreased with increasing soaking time, reached a negative value again, and finally converged to a constant negative value. On basis of this finding, the complex process of apatite formation described above is well interpreted in terms of the electrostatic interaction of the functional groups with the ions in the fluid. The Ti–OH groups formed on the surface of sodium titanate after soaking in SBF are negatively charged and, hence, combine selectively with the positively charged Ca2+ ions in the fluid to form calcium titanate, as shown in Fig. 7. As the calcium ions accumulate on the surface, the surface gradually gains an overall positive charge. As a result, the positively charged surface combines with negatively charged Fig. 5. SEM photographs of the surfaces of titania gels with amorphous (A), anatase (B), and rutile (C) structures after soaking in an SBF for 7 d. Fig. 6. TEM-EDX profiles of the nano-sized apatite formed on amorphous sodium titanate (A) and HA sintered at 800C (B), after soaking in an SBF for 72 and 48 h, respectively. (* center of the electron diffraction). T. Kokubo et al. / Biomaterials 24 (2003) 2161–2175 2165