Mechanism of apatite formation on pure titanium treated with alkaline solution

Transcription

1 Bio-Medical Materials and Engineering 14 (2004) IOS Press Mechanism of apatite formation on pure titanium treated with alkaline solution C.X. Wang a,,x.zhou b and M. Wang a a School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore , Singapore b Department of Inorganic Materials, Sichuan University, Chengdu , Sichuan, China Received 10 March 2003 Abstract. The mechanism of bone-like apatite formation on the surface of pure titanium pretreated with NaOH solution is still being investigated. The apatite formation may depend on the solution that is used. In the present study, several types of solutions such as simulated body fluid (SBF), calcium aqueous solution (CAS), and phosphate aqueous solution (PAS) were used to investigate bone-like apatite formation on alkali-treated titanium. In order to observe the effect of hydrolysis on the apatite formation, experiments of pretreated titanium immersed in distilled water before the immersion in SBF were also conducted. The results showed that the mechanism of apatite formation was the hydrolysis reaction of sodium titanate which induced the apatite formation. The pre-precipitation of either calcium or phosphate could prevent the apatite formation on the surface of alkaline treated titanium. Keywords: Alkaline treatment, pure titanium, bone-like apatite 1. Introduction Due to enhanced biocompatibility and mechanical properties, calcium phosphate ceramics such as hydroxyapatite (HA) coated titanium and its alloys are among the most promising implant materials for orthopedic and dental applications [1]. However, problems such as low bond strength between the coating and the substrate and non-uniformity across the thickness of the coating are often encountered with these coatings [2]. Recently, it has been reported that chemically treated titanium can induce bone-like apatite formation in vitro and in vivo [3 10], which means that titanium and its alloys have potential bioactivity. The reagents most frequently employed in the treatments are NaOH [3 8] and hydrogen peroxide (H 2 O 2 ) solution [9], and HCl and H 2 SO 4 are also used to etch titanium before alkaline treatment [10]. Treatment with a NaOH solution produces a sodium titanate gel layer on titanium surface while H 2 O 2 treatment produces a titania gel layer. Both gel layers have the ability to induce formation of bone-like apatite during immersing in simulated body fluid (SBF) and thus are considered bioactive. The gel layers can initiate apatite nucleation on itself. Once apatite nucleation occurs, it spontaneously grows by taking calcium and phosphate ions from the surrounding environment. However, some aspects of the mechanism of bone-like apatite formation on the surface of pure titanium pretreated with NaOH solution are not clear, such as how the hydrolysis reaction affects apatite * Corresponding author: Dr C. Wang, current address: School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom. Tel.: +44 (0) ; Fax: +44 (0) ; c.wang@ bham.ac.uk /04/$ IOS Press and the authors. All rights reserved

2 6 C.X. Wang et al. / Mechanism of apatite formation on pure titanium formation, how the phosphate and calcium ions are adsorbed onto the titanium surface. In order to gain some insights into these, several types of solutions such as simulated body fluid (SBF), calcium aqueous solution (CAS), and phosphate aqueous solution (PAS) were used to investigate bone-like apatite formation on alkali-treated titanium. In order to observe the effect of hydrolysis on the apatite formation, experiments of pretreated titanium immersed in distilled water before the immersion in SBF were also conducted. 2. Materials and methods 2.1. Surface treatment of titanium substrates A commercially available pure titanium substrate (discs of dimensions φ 15 mm 3mm)wasmechanically polished and ultrasonically cleaned with acetone and alcohol. These discs were soaked in 5.0 M NaOH solution at 60 C for 24 h, then gently washed with distilled water, and finally dried at 37 C for 24 h Immersing of pretreated substrates in the solutions Table 1 lists ion concentrations of the SBF, CAS and PAS. SBF, CAS and PAS were prepared by dissolving reagent grade chemicals of NaCl, NaHCO 3,KCl,K 2 HPO 4 3H 2 O, MgCl 2 6H 2 O, CaCl 2 and Na 2 SO 4 into distilled water and buffering at ph 7.40 with tris-hydrochlomethyl-amminomethane ((CH 2 OH) 3 -CNH 3 ) and hydrochloric acid at 36.5 C. Some of the pretreated titanium substrates were immersed in the SBF at 36.5 C for 4 weeks. Several pretreated titanium substrates were immersed in distilled water for 1 to 4 days and then in SBF for 4 weeks. Some of the pretreated titanium substrates were firstly immersed in CAS or PAS for 7 days, respectively, and then immersed in SBF for 4 weeks. Several pretreated titanium substrates were firstly immersed in CAS for 7 days, and then immersed in PAS for 4 weeks. Also several pretreated titanium substrates were firstly immersed in PAS for 7 days, and then immersed in CAS for 4 weeks. The specimens were removed from the solutions used, washed with distilled water and acetone, and dried at room temperature Microstructural characterizations X-ray diffraction (XRD) was employed to analyze the structure of titanium substrate, gel layer and bone-like apatite. A thin-film X-ray diffractometer (XRD, Rigaku X-ray diffractometer) was used. The morphologies of the specimens were examined under scanning electron microscopy (SEM, JEOL JSM 5600LV). Table 1 Ionic concentrations of SBF, CAS and PAS (mm) Ions Na K Ca P Mg HCO 3 SO 4 SBF CAS PAS

3 C.X. Wang et al. / Mechanism of apatite formation on pure titanium 7 3. Results 3.1. Scanning electron microscopy Figure 1 shows scanning electron microscopy (SEM) micrographs of the surfaces of titanium substrates that were soaked in 5.0 M NaOH solution at 60 C for 24 h in comparison to the untreated titanium substrates, and the pretreated titanium substrates immersed in SBF for 4 weeks. It can be seen that a porous network structure was formed on the surface of titanium with the NaOH treatment, and the pretreated titanium substrate was fully covered by bone-like apatite. Figure 2 shows SEM micrographs of the surfaces of NaOH pretreated titanium substrates which were firstly immersed in distilled water for 1 to 4 days and then in SBF at 36.5 C for 4 weeks. It can be seen that the immersion in distilled water has obvious effects on the bone-like apatite formation on the treated titanium surfaces. Only a few of apatite islands formed on the pretreated titanium surface firstly immersed in distilled water for 1 day (Fig. 2). Then much more apatite formed on the treated titanium surfaces with the increase in the immersion time in distilled water, and at 3 days it reached its peak (Fig. 2(c)). Figure 3 shows SEM micrographs of the surfaces of pretreated titanium substrates which were firstly immersed in CAS for 7 days and then in SBF or PAS at 36.5 C for 4 weeks, respectively. Figure 4 shows SEM micrographs of the surfaces of pretreated titanium substrates which were firstly immersed in PAS for 7 days and then in SBF or CAS at 36.5 C for 4 weeks, respectively. There was no bone-like apatite formation on the surfaces of the discs that were firstly immersed in CAS and then in SBF or PAS. And (c) Fig. 1. SEM micrographs of untreated titanium, titanium treated with 5.0 M NaOH solution at 60 C for 24 h and pretreated titanium immersed in SBF for 4 weeks. Untreated titanium substrate; NaOH treated titanium substrates; (c) pretreated titanium immersed in SBF for 4 weeks.

4 8 C.X. Wang et al. / Mechanism of apatite formation on pure titanium (c) (d) Fig. 2. SEM micrographs of the surfaces of pretreated titanium substrates firstly immersed in distilled water, then in SBF solution at 36.5 C for 4 weeks: 1 day, 2 days, (c) 3 days, (d) 4 days. Fig. 3. SEM micrographs of pretreated titanium substrates firstly immersed in CAS, then in SBF and in PAS at 36.5 C for 4 weeks. also no apatite formed on the surfaces of the discs that were firstly immersed in PAS and then in SBF or CAS XRD results Figure 5 shows the TF-XRD pattern of the surface of untreated titanium discs, Fig. 5 shows the TF-XRD pattern of the surface of titanium treated with 5.0 M NaOH solution at 60 C for 24 h, and

5 C.X. Wang et al. / Mechanism of apatite formation on pure titanium 9 Fig. 4. SEM micrographs of pretreated titanium substrates firstly immersed in PAS, then in SBF and in CAS at 36.5 C for 4 weeks. (c) Fig. 5. TF-XRD patterns of untreated titanium substrate, titanium substrate treated with 5.0 M NaOH at 60 C for 24 h, and (c) alkaline treated titanium immersed in the SBF solution at 36.5 C for 4 weeks.

6 10 C.X. Wang et al. / Mechanism of apatite formation on pure titanium Fig. 5(c) shows the TF-XRD patterns of alkaline treated titanium immersed in the SBF solution at 36.5 C for 4 weeks, respectively. In comparison to the pattern of untreated titanium substrate, a broad bump and small peaks at around 24, 28 and 48 were observed in Fig. 5, indicating that the surface porous network layer, which was formed by the NaOH treatment, is an amorphous sodium titanate phase [3,5]. In comparison to the pattern in Fig. 5, all the new peaks appeared in the patterns in Fig. 5(c) are ascribed to crystalline bone-like apatite, indicating that the network structure formed on the surface of titanium could induce the nucleation and growth of bone-like apatite on titanium. 4. Discussion A broad bump in the TF-XRD pattern of titanium treated with NaOH solution at 60 C for 24 h suggests that the network structure on the surface is an amorphous sodium titanate hydrogel. When the treated titanium discs immersed in simulated body fluid solution at 36.5 C, this hydrogel layer could induce the nucleation and growth of apatite on the surface. The possible mechanism of nucleation and growth of apatite on alkaline treated titanium immersed in SBF solution has been proposed as following [4]. The sodium titanate layer releases its Na + ions into the surrounding fluid via an ion exchange with H 3 O + in the fluid to form Ti OH groups as early as 0.5 h after immersion. The Ti OH groups then immediately interact with the calcium ions in the fluid to form a calcium titanate. The calcium titanate incorporates the phosphate ions, as well as the calcium ions, in the fluid to form apatite nuclei in the SBF solution. At this stage, the crystalline bone-like apatite is first detected TF-XRD analysis. Once formed, the apatite nuclei grow by consuming the calcium and phosphate ions in the SBF solution. In the present study, bone-like apatite formed on the surfaces of pretreated titanium firstly immersed in distilled water then in SBF confirmed that the hydrolysis reaction of the thin sodium titanate layer was the first step for apatite formation. Then calcium ions replaced the position of sodium ions which contributed to the positive charged surface. This made it possible for the calcium titanate incorporated phosphate ions as well as the calcium ions in SBF to form apatite. However, there was no bone-like apatite formation on the surfaces of the discs which were firstly immersed in CAS and then in SBF or PAS. The possible reason maybe that the pre-precipitation of calcium has changed the surface characteristics of the pretreated titanium which could prevent the formation of apatite. For the same reason, the first immersion in PAS also changed the surface characteristics of the pretreated titanium, and apatite formation on the surface has been prevented. Therefore, we think, the dehydration of sodium titanate is the pre-requisite for the apatite formation, and the coexistence of calcium and phosphate ions in the solution is also critical for the apatite formation. The pre-immersion in either calcium or phosphate could prevent the apatite formation on the surface of alkaline treated titanium. 5. Conclusions Bone-like apatite formed on the surface of titanium treated with NaOH solution. The mechanism of apatite formation was the hydrolysis reaction of sodium titanate which induced the apatite formation. The coexistence of calcium and phosphate ions in the solution is also critical for the apatite formation. The pre-precipitation of either calcium or phosphate could prevent the apatite formation on the surface of alkaline treated titanium.

7 C.X. Wang et al. / Mechanism of apatite formation on pure titanium 11 Acknowledgements The authors would like to thank Nanyang Technological University (NTU) for funding the research. Wang Changxiang thanks Nanyang Technological University for providing a research fellowship. Assistance provided by technical staff in the School of MPE, NTU, is gratefully acknowledged. References [1] R.G. Geesink, K. de Groot and C.P. Klein, Bonding of bone to apatite-coated implants, J. Bone Joint Surg., Br. 70 (1988), [2] W.R. Lacefield, Hydroxyapatite coatings, in: Bioceramics: Material Characteristics Versus In Vivo Behavior, P. Ducheyne and J.E. Lemons, eds, Annals of The New York Academy of Science, New York, 1988, pp [3] H.M. Kim, F. Miyaji, T. Kokubo, S. Nishiguchi and T. Nakamura, Graded surface structure of bioactive titanium prepared by chemical treatment, J. Biomed. Mater. Res. 45 (1999), [4] H. Takadama, H.M. Kim, T. Kokubo and T. Nakamura, An X-ray photoelectron spectroscopy study of the process of apatite formation on bioactive titanium metal, J. Biomed. Mater. Res. 55 (2001), [5] H.M. Kim, F. Miyaji, T. Kokubo and T. Nakamura, Preparation of bioactive Ti and its alloys via simple chemical surface treatment, J. Biomed. Mater. Res. 32 (1996), [6] K. Nishio, M. Neo, H. Akiyama, S. Nishiguchi, H.M. Kim, T. Kokubo and T. Nakamura, The effect of alkali- and heattreated titanium and apatite-formed titanium on osteoblastic differentiation of bone marrow cells, J. Biomed. Mater. Res. 52 (2000), [7] C. Ohtsuki, H. Iida, S. Hayakawa and A. Osaka, Bioactivity of titanium treated with hydrogen peroxide solution containing metal chloride, J. Biomed. Mater. Res. 35 (1997), [8] Y. Han, T. Fu, J. Lu and K.W. Xu, Characterization and stability of hydroxyapatite coatings prepared by an electrodeposition and alkaline-treatment process, J. Biomed. Mater. Res. 54 (2001), [9] X.X. Wang, S. Hayakawa, K. Tsuru and A. Osaka, A comparative study of in vitro apatite deposition on heat-, H 2 O 2 -, and NaOH-treated titanium surfaces, J. Biomed. Mater. Res. 54 (2001), [10] H.B. Wen, J.R. de Wijin, F.Z. Cui and K. de Groot, Preparation of calcium phosphate coatings on titanium implant materials by simple chemistry, J. Biomed.l Mater. Res. 41 (1998),

8