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A number of materials have shown to be capable of inducing apatite formation in metasatble calciumand phosphate-containing solutions known as simulated body fluids (SBFs). All these apatite inducers have a great number of acidic hydroxyl groups on their surfaces that include SiOH, TiOH and COOH . A hypothesis is thus proposed that a material could induce apatite formation when there are a large number of acidic OH groups present on its surface. These acidic OH groups could be introduced in the material via sol-gel processing routes and/or generated on the surface of a material as a result of its interaction with the solutions. The ktnowledge has been applied to develop a new process of growing an apatite film on titanium implants for enhancement of their fixation in bone . INTRODUCTION Bone mineral is a carbonated apatite formed in an organic matrix . Bone cells, namely osteoblasts, synthesize the organic matrix that turns to induce the apatite formation from the physiological solution. The physiological solution is supersaturated with respect to apatite. But it is a metastable solution and requires an external stimulation to initiate the apatite nucleation. Once apatite nuclei are formed by the organic matrix (nucleator) , they spontaneously grow by uptake of calcium and phosphate present in the physiological solution (the source of apatite). The formation of apatite in metastable calciumand phosphate-containing solutions could happen on materials other than the organic matrix of bone. The objective of this paper is to demonstrate the apatite induction by a group of materials including ceramics, metal and polymers. The fundamental aspects of the apatite induction is then discussed and the knowledge is finally applied to grow an apatite film on titanium implants for enhancement of their fixation in bone. MATERIALS AND METHODS Simulated body fluids (SBFs) were prepared by dissolving reagent-grade NaCl, Phosphorus Research Bulletin Vol. 10 (1999), 232 NaHCO3, KCl, K2HPO4.3H2O, MgCl2.6H20, CaCl2 and Na2SO4 in de-ionized water. The solutions were buffered by tris(hydroxymethyl)aminomethane ((CH2OH)3CNH2) and IN HCl, with the concentration of tris being 50 mM and the concentration of HCl adjustable depending on the target pH value. They were designed to imitate the human blood plasma (HBP) in terms of the inorganic composition. The compositions of the SBFs used in this study are listed in Table 1. According to the calculation by Ohtsuki et al [1], these solutions are supersaturated with respect to hydroxyapatite. However, no spontaneous precipitation was noticed, indicating that the solutions are at steady states. Three materials are investigated in this paper regarding their ability to induce apatite formation in SBFs. They are silica (SiO2), titania (TiO2) and a polymer called Polyactive (R). The silica and titania were prepared by sol-gel processing routes. The details of preparation had been described before [2,3]. Polyactive (R) is a copolymer consisting of 80 wt% of a polyethylene oxide (PEO) soft segment and 20 wt% of a polybutylene terephthalate (PBT) hard segment. More information on Polyactive (R) could be found elsewhere [4]. Each 15-mm by 10-mm specimen of silica was soaked in 30 mL of SBF1for 7 days while every titania sample was immersed in 30 mL of SBF2 for 2 weeks. One gram of Polyactive (R) powders (< 300ƒÊm) were incubated in 100 mL of SBF3 for 2 days. Interaction of each material with the respective SBF was set at 37•Ž. All samples were extensively rinsed with de-ionized water after removed of the solutions. They were then dried at 60•Ž over night and analyzed by several techniques including scanning electron microscopy (SEM) linked with energy dispersive of X-ray analysis (EDX), x-ray diffraction analysis (XRD) and Fourier transform infrared (FT-IR) reflection spectroscopy. RESULTS Figure 1 shows a SEM micrograph of the silica after exposed to SBF1 for 7 days. A new ceramic layer is seen on top of the silica. It was identified to be a carbonated apatite TABLE 1. The compositions of simulated body fluids (SBFs) and human blood plasma (HBP) in mM Phosphorus Research Bulletin Vol. 10 (1999), 233 by thin film-XRD and FT-IR whose results are shown on the right of Figure 1. As a result of the apatite growth, the porous silica is fully covered with an apatite film about 10ƒÊm thick and the concentration of calcium and phosphate ions in the solution decreased. It is clear that the sol-gel prepared silica is capable of inducing apatite formation. No other calcium phosphates were detected in the course of the apatite formation. It is thus believed that the silica induced the apatite nucleation onto its surface in metastable SBF1. The growth of apatite on the titania prepared by a sol-gel process is described in Figure 2. The apatite formation was noticed on the titania surface at day 7 in SBF2. Substantial growth of apatite was observed on the titania in the second week of immersion. This led to the formation of an apatite film approximately 10 p.m on the titania surface. Figure 3 shows a Polyactive (R) granule after immersed in SBF3 for 2 days. The presence of apatite in the polymer was detected in XRD (Fig. 3b). SEM-EDX analyses of a cross-section of the polymer granule (Fig. 3c & d) show the formation of apatite within the polymer matrix. It is clear that the apatite formation was induced on the polymer surface and inside the material as well. DISCUSSION Further studies with over materials have shown that the formation of apatite in Figure 1. Scanning electron micrograph (left) of the sol-gel prepared silica soaked in SBF I for 7 days. Note the formation of an apatite layer on the silica. The growth of the apatite was confirmed by thin-film XRD (upperright) and FT-IR (lowerright) analyses . Phosphorus Research Bulletin Vol. 10 (1999), 234 Figure 2. Scanning electron micrographs of the sol-gel prepared titania after exposed to SBF2 for I week (upperleft) and 2 weeks (lowerleft and upperright). Note the apatite formation (arrow) on the titania in the first week. The considerable growth of the apatite resulted in the full coverage of the titania with an apatite layer that was characterized by FT-IR analysis (lowerright). SBFs is related to the materials [5]. Not every material could initiate the apatite formation in SBFs. For instance, the alumina prepared by a sol-gel process could not induce the apatite formation as the so-gel prepared silica and titania do. The materials and their ability to induce the apatite formation in SBFs is summarized Table 2. Apparently, materials show very different capabilities Figure 3. Scanning electron micrograph (a) and XRD patterns (b) [pattern Abefore and pattern Bafter immersion] of a Polyactive® granule after immersed in SBF3 for 2 days. Note the presence of apatite in the polymer. SEM-EDX (c & d) analysis of a cross section of the particle indicates the formation of apatite inside the polymer. Phosphorus Research Bulletin Vol. 10 (1999), 235 for forming, apatite in SBFs even when they have the same composition. What the materials investigated in this study (sol-gel prepared silica, sol-gel prepared titania and Polyactive (R)) have in common is a great number of acidic OH groups. The sol-gel processes rendered the silica and the titania the abundant SiOH and TiOH groups respectively. In addition , more SiOH groups could be generated as a result of dissolution of silica in SBFs . After Polyactive (R) was immersed in SBF3, the hydrophilic PEO segment absorbed water and the polymer expanded. Accordingly. Polyactive (R) became a hydrogel-like material . More and more carboxylic acid groups (COOH) were generated in the polymer , with the hydrolysis of Poiyactive (R) proceeding in SBF3. In fact, this COOH group is abundant in the organic matrix of bone. Owing, to the partial dissociation of their respective acidic OH groups, the surfaces of the sol-gel prepared silica, the sol-gel prepared titania and Polyactive (R) were negatively charged in SBFs. It appears that the remaining non-dissociated acidic OH groups . and the negatively charged surfaces work together to induce the apatite nucleation in SBFs . It is thus proposed that a material could induce the apatite formation in SBFs when there are a large number of acidic OH groups present on its surface . These acidic OH groups could be introduced in the material via the sol-gel processing routes and/or generated on the surface of a material as a result of its interaction with the solution . Table 2. The capability of materials for inducing apatite formation in SBFs

INDUCTION OF APATITE FILM IN SIMULATED BODY FLUIDS AND ITS APPLICATION TO BONE RECONSTRUCTION