T. Kokubo et al./ Biomaterials 24(2003)2161-2175 Ti-OH groups ee eleele Formation of TI-OH groups Formation of amorphous Formation of amorphou Before soaking in SBF Formation of Apatite 个不个不 Time/h Fig. 7. Schematic showing the relationship between the changes in surface structure and the potential of amorphous sodium titanate in the apatite formatio on its surface in phosphate ions to form amorphous calcium phosphate. ately after soaking in the SBF, owing to its surface OH This calcium phosphate spontaneously transforms into and PO4 ions, and selectively combines with the the apatite, because the apatite is the stable phase in positively charged Ca- ions in the fluid to form Ca- body environment [48. A similar electrostatic mechan- rich calcium phosphate As the calcium ions accumulate, ism for apatite formation might hold for the other the surface imparts a positive charge and thus combine apatite nucleation, because all these functional groups to form amorphous calcium phosphate with a lonGo functional groups described above that are effective for with the negatively charged phosphate ions in the flui have isoelectric zero points at pH values much lower ratio. This phase is metastable, and eventually trans than 7 and, thus, should be negatively charged in the forms into stable bone-like apatite living body [49 It is has been reported that the bioactivity of HA As already described, even sintered HA forms a bone- decreases with increasing sintering temperature [51] like apatite layer on its surface in the living body, and This can be interpreted in terms of the degree of negative bonds to bone through this apatite layer. HA has none charge on its surface and the rate of bone-like apatite of such functional groups for the apatite nucleation as formation. In the case of HA sintered at 1200C,it described above. Despite this, how does HA form bone- implies a low negative charge after soaking in an SBF, like apatite on its surface in the living body? When HA owing to a smaller number of OH ions on its surface sintered at 800 C is soaked in an SBF, observations of compared with HA sintered at 800C. Thus, this HA its surface using TEM-EDX show that the Ca/P ratio has a low reaction rate in the process described above increases from 1.67 to 1.87 within 3 h [50], as shown in Therefore, the rate of formation of bone-like apatite Fig 8. The Ha then decreases its Ca/P ratio to 1. 41 layer on the Ha decreases with increasing sintering within the next 6h, forming an amorphous calcium temperature phosphate, and then gradually increases its Ca/P ratio to 1.65, forming nano-sized bone-like apatite, as shown in Fig 6B. During this process, the zeta potential of the 3. Preparation and properties of novel bioactive materials urface of the ha varies with soaking time as shown in Fig 8. Initially, it has a negative zeta potential that 3.1. Tough bioactive materials increases to a maximum positive value within 3 h, and then this decreases rapidly to a negative value within 6 h The fundamental findings on bone-like apatite for which gradually converges to a constant negative value. mation on bioactive ceramics provide a basis for the This means that the ha is negatively charged immedi- design of various types of bioactive materials. Forphosphate ions to form amorphous calcium phosphate. This calcium phosphate spontaneously transforms into the apatite, because the apatite is the stable phase in body environment [48]. A similar electrostatic mechan￾ism for apatite formation might hold for the other functional groups described above that are effective for apatite nucleation, because all these functional groups have isoelectric zero points at pH values much lower than 7 and, thus, should be negatively charged in the living body [49]. As already described, even sintered HA forms a bone￾like apatite layer on its surface in the living body, and bonds to bone through this apatite layer. HA has none of such functional groups for the apatite nucleation as described above. Despite this, how does HA form bone￾like apatite on its surface in the living body? When HA sintered at 800C is soaked in an SBF, observations of its surface using TEM-EDX show that the Ca/P ratio increases from 1.67 to 1.87 within 3 h [50], as shown in Fig. 8. The HA then decreases its Ca/P ratio to 1.41 within the next 6 h, forming an amorphous calcium phosphate, and then gradually increases its Ca/P ratio to 1.65, forming nano-sized bone-like apatite, as shown in Fig. 6B. During this process, the zeta potential of the surface of the HA varies with soaking time, as shown in Fig. 8. Initially, it has a negative zeta potential that increases to a maximum positive value within 3 h, and then this decreases rapidly to a negative value within 6 h, which gradually converges to a constant negative value. This means that the HA is negatively charged immedi￾ately after soaking in the SBF, owing to its surface OH and PO4 3 ions, and selectively combines with the positively charged Ca2+ ions in the fluid to form Ca￾rich calcium phosphate. As the calcium ions accumulate, the surface imparts a positive charge, and thus combine with the negatively charged phosphate ions in the fluid to form amorphous calcium phosphate with a low Ca/P ratio. This phase is metastable, and eventually trans￾forms into stable bone-like apatite. It is has been reported that the bioactivity of HA decreases with increasing sintering temperature [51]. This can be interpreted in terms of the degree of negative charge on its surface and the rate of bone-like apatite formation. In the case of HA sintered at 1200C, it implies a low negative charge after soaking in an SBF, owing to a smaller number of OH ions on its surface compared with HA sintered at 800C. Thus, this HA has a low reaction rate in the process described above. Therefore, the rate of formation of bone-like apatite layer on the HA decreases with increasing sintering temperature. 3. Preparation and properties of novel bioactive materials 3.1. Tough bioactive materials The fundamental findings on bone-like apatite for￾mation on bioactive ceramics provide a basis for the design of various types of bioactive materials. For Fig. 7. Schematic showing the relationship between the changes in surface structure and the potential of amorphous sodium titanate in the apatite formation process on its surface in an SBF. 2166 T. Kokubo et al. / Biomaterials 24 (2003) 2161–2175