T. Kokubo et al. Biomaterials 24(2003)2161-2175 aoNa2O如04680cao mol% mo% Fig 3. Compositional dependence of apatite formation on glasses in the systems: CaO-P2O5-SiO2. Na20-CaO-SiOz, and K20-SiOr-TiOz after f for 28d Fig 4. SEM photographs of the surfaces of silica(A), titania(B), zirconia(C), niobium oxide(D), and tantalum oxide(e) gels after soaking in an SBF for 14d concentration and structural arrangements. For exam- however, the amorphous gel forms no apatite, even ple, a silica gel that has been derived by hydrolysis and though it has much more abundant Ti-OH groups than polycondensation of tetraethoxysilane(TEOS) in water the anatase and rutile gels [39), as shown Fig. 5. Sodium- in the presence of polyethylene glycol(PEG-silica) loses or calcium-containing titania gels can release sodium or its apatite-forming ability on heat treatment at tem- calcium ions on immersion in SBf to increase the IAP peratures above 900 C, owing to a decrease in the and thereby provide much more favorable conditions number of Si-OH groups [36]. Silica gels, which are for apatite nucleation than for a pure titania gel derived in water in the absence of polyethylene glycol or However, even these gels do not form apatite if they the different arrangea b ay s theel, have an equivalent do not assume the anatase structure [40, 41]. Similarly in the presence of polyacrylic ad number of Si-OH groups PEG-Silica. but these pure zirconia gel and zirconia gels containing sodium or how no apatite- form ity, presumably because of calcium form apatite in SBF only when they assume a ent of the Si-OH groups [37]. tetragonal and/or a monoclinical structure [42]. Con However, silicate ions dissolved from silica gel can cerning this structural dependence, it has been suggested induce apatite formation, independent of the source that the Ti-Oh or Zr-oh groups in anatase or silica gels [38] the tetragonal /monoclinic structures may provide effec- A titania gel prepared from tetraisopropyl titanate tive epitaxial nucleation sites for apatite crystals (TiPT)assumes an amorphous, anatase, or rutile For example, the arrangement of oxygen ions in the structure, when it is heat-treated at 500"C, 600.C and anatase structure along the (100) plane fits well to that 800C, respectively. Among these, the anatase gel forms of the hydroxide ions in Ha along the(000 1)plane apatite most effectively, followed by the rutile gel; [39, 42concentration and structural arrangements. For exam￾ple, a silica gel that has been derived by hydrolysis and polycondensation of tetraethoxysilane (TEOS) in water in the presence of polyethylene glycol (PEG–silica) loses its apatite-forming ability on heat treatment at tem￾peratures above 900C, owing to a decrease in the number of Si–OH groups [36]. Silica gels, which are derived in water in the absence of polyethylene glycol or in the presence of polyacrylic acid, have an equivalent number of Si–OH groups as the PEG–silica, but these show no apatite-forming ability, presumably because of the different arrangement of the Si–OH groups [37]. However, silicate ions dissolved from silica gel can induce apatite formation, independent of the source silica gels [38]. A titania gel prepared from tetraisopropyl titanate (TiPT) assumes an amorphous, anatase, or rutile structure, when it is heat-treated at 500C, 600C and 800C, respectively. Among these, the anatase gel forms apatite most effectively, followed by the rutile gel; however, the amorphous gel forms no apatite, even though it has much more abundant Ti–OH groups than the anatase and rutile gels [39], as shown Fig. 5. Sodium￾or calcium-containing titania gels can release sodium or calcium ions on immersion in SBF to increase the IAP, and thereby provide much more favorable conditions for apatite nucleation than for a pure titania gel. However, even these gels do not form apatite if they do not assume the anatase structure [40,41]. Similarly, pure zirconia gel and zirconia gels containing sodium or calcium form apatite in SBF only when they assume a tetragonal and/or a monoclinical structure [42]. Con￾cerning this structural dependence, it has been suggested that the Ti–OH or Zr–OH groups in anatase or the tetragonal/monoclinic structures may provide effec￾tive epitaxial nucleation sites for apatite crystals. For example, the arrangement of oxygen ions in the anatase structure along the (1 0 0) plane fits well to that of the hydroxide ions in HA along the (0 0 0 1) plane [39,42]. Fig. 3. Compositional dependence of apatite formation on glasses in the systems: CaO–P2O5–SiO2, Na2O–CaO–SiO2, and K2O–SiO2–TiO2 after soaking in an SBF for 28 d. Fig. 4. SEM photographs of the surfaces of silica (A), titania (B), zirconia (C), niobium oxide (D), and tantalum oxide (E) gels after soaking in an SBF for 14 d. 2164 T. Kokubo et al. / Biomaterials 24 (2003) 2161–2175