- Email: [email protected]
Preparation of bioactive titania films on titanium metal via anodic oxidation
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 80–86
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Preparation of bioactive titania films on titanium metal via anodic oxidation X. Cui a , H.-M. Kim b , M. Kawashita c,∗ , L. Wang a , T. Xiong a , T. Kokubo d , T. Nakamura e a
Institute of Metal Research, Chinese Academy of Sciences, Wenhua Road 72, Shenyang 110016, China Department of Ceramic Engineering, School of Advanced Materials Engineering, Yonsei University 134, Shinchon-dong Seodaemun-gu, Seoul 120-749, Republic of Korea c Center for Research Strategy and Support, Tohoku University, Aoba-ku, Sendai 980-8579, Japan d Department of Biomedical Sciences, College of Life and Health Sciences, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan e Department of Orthopedic Surgery, Graduate School of Medical, Kyoto University, Shogoin, Kawahara-cho 54, Sakyo-ku, Kyoto 606-8507, Japan b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To research the crystal structure and surface morphology of anodic films on
Received 1 May 2007
titanium metal in different electrolytes under various electrochemical conditions and inves-
Accepted 15 April 2008
tigate the effect of the crystal structure of the oxide films on apatite-forming ability in simulated body fluid (SBF). Methods. Titanium oxide films were prepared using an anodic oxidation method on the sur-
Keywords:
face of titanium metal in four different electrolytes: sulfuric acid, acetic acid, phosphoric
Titanium
acid and sodium sulfate solutions with different voltages for 1 min at room temperature.
Anodic oxidation
Results. Anodic films that consisted of rutile and/or anatase phases with porous structures
Rutile
were formed on titanium metal after anodizing in H2 SO4 and Na2 SO4 electrolytes, while
Anatase
amorphous titania films were produced after anodizing in CH3 COOH and H3 PO4 electrolytes.
Apatite
Titanium metal with the anatase and/or rutile crystal structure films showed excellent apatite-forming ability and produced a compact apatite layer covering all the surface of titanium after soaking in SBF for 7 d, but titanium metal with amorphous titania layers was not able to induce apatite formation. Significance. The resultant apatite layer formed on titanium metal in SBF could enhance the bonding strength between living tissue and the implant. Anodic oxidation is believed to be an effective method for preparing bioactive titanium metal as an artificial bone substitute even under load-bearing conditions. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In the past 20 years, titanium and its alloys have been successfully used as dental and orthopedic biomaterials because
∗
of their good mechanical properties, corrosion resistance and biocompatibility with living tissue after implantation into the bone [1–3]. However, being bioinert metallic materials, they cannot bond to living bone directly after implantation into a
Corresponding author. Tel.: +81 22 795 3937; fax: +81 22 795 3937. E-mail address: [email protected] (M. Kawashita). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2008.04.012
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 80–86
host body [4,5]. Therefore, various surface modifications have been attempted to improve the bioactive bone-bonding ability of titanium metal [6–8]. Recently, Kokubo’s group showed that titanium and its alloys were able to form a bonelike apatite layer on their surfaces in the body environment and bond to living bones after implantation in the body when they were treated in NaOH solution and then heat-treated in a furnace [9–12]. The apatite-forming ability of the metal was due to the formation of amorphous sodium titanate on the titanium metal by the NaOH and heat treatments, and then the formation of the Ti–OH group through the exchange of Na+ and H3 O+ in SBF or real body fluid. Uchida et al. showed that the apatite-forming ability of titanium induced by NaOH and heat treatments was greatly enhanced by combining hot water and subsequent heat treatments [13]. They ascribed the enhancement of the apatite-forming ability to the formation of anatase from the conversion of the sodium titanate gel on the NaOH-treated titanium metal after hot water and heat treatments. Moreover, it was found that titania gels with the specific structure of anatase and rutile prepared by a sol–gel process showed higher apatite-forming ability in SBF, while amorphous titania did not [14,15]. These results implied that both the formation of Ti–OH groups and the crystal structure of titania would greatly affect apatite-forming ability on the surface of titania gels. Electrochemical anodic oxidation has been a traditional surface modification method to modify the structures and chemical properties of anodic films on titanium by controlling the electrochemical parameters, such as electrolyte composition and concentration, applied potential or current, temperatures, etc. [16–19]. Recently, anodic oxidation has become an attractive method for preparing oxide films on titanium, because the oxide films improved apatite formation in SBF [7,8,20–24]. On the basis of the above research, titanium metal after anodic oxidation would produce the anatase and/or rutile crystal structure of titania by conditioning the electrochemical parameters, which would contribute to forming titanium metal with high apatite-forming ability. However, in a previous study, a subsequent heat treatment or hydrothermal treatment was required to provide the titania layers with the desired apatite-forming ability after anodic oxidation. In this study, the authors treated the titanium metal in sulfuric acid, acetic acid, phosphoric acid and sodium sulfate solution by anodic oxidation without any subsequent treatment. The purpose of this study was to research the crystal structure and surface morphology of anodic films on titanium metal in different electrolytes with various electrochemical conditions and investigate the effect of the crystal structure of the oxide films on apatite-forming ability in SBF solutions.
2.
Materials and methods
2.1.
Preparation of samples
Samples of commercially pure titanium metal (purity: 99.9%, Nilaco Co., Tokyo, Japan), 10 mm × 10 mm × 1 mm in size were mechanically abraded with No. 400 diamond plate, and then washed in an ultrasonic cleaner with pure acetone, ethanol and distilled water.
2.2.
81
Anodic oxidation process
Anodic oxidation was performed under potentiostatic control in the present experiments, and measurements were made in 2.0 M H2 SO4 , CH3 COOH and H3 PO4 solutions and in 1.0 M Na2 SO4 solutions in a glass chamber with a direct current power supply system (EX1500H, Takasago Co., Japan). The voltages applied were 100, 150 and 180 V for 1 min at room temperature. Titanium plates (200 mm × 15 mm × 1 mm) were used as the cathode, and titanium samples were fixed with titanium wires on a titanium anode (200 mm × 4 mm × 1 mm). In the anodic oxidation process, a magnetic stirrer at a given rotation speed was employed to achieve a homogeneous electrolyte and to accelerate escape of gas produced in the electrochemical reaction from the surface of the titanium substrates by the magnetic mixer (MD500, Yamato, Japan). After the anodic oxidations, the samples were rinsed with distilled water and dried in an oven at 40 ◦ C for 24 h.
2.3.
Evaluation of apatite-forming ability
After anodic oxidation, the specimens were immersed in 30 ml of an acellular simulated body fluid (SBF) with pH 7.40, whose ion concentrations (Na+ 142.0, K+ 5.0, Mg2+ 1.5, Ca2+ 2.5, Cl− 147.8, HCO3 − 4.2, HPO4 2− 1.0, SO4 2− 0.5 mM) are nearly equal to those of human blood plasma at 36.5 ◦ C [25]. The SBF was prepared by dissolving reagent grade chemicals of NaCl, NaHCO3 , KCl, K2 HPO4 ·3H2 O, MgCl2 ·6H2 O, CaCl2 and Na2 SO4 in distilled water and buffering at pH 7.40 with tris(hydroxymethyl)aminomethane and 1 M HCl at 36.5 ◦ C. After soaking for a given period of 1, 3 or 7 d, the substrates were removed from the fluid and washed with distilled water, and then dried on a clean bench.
2.4.
Analyses of samples
Before and after anodic oxidation and soaking in SBF, the surface of the titanium substrates was analyzed and measured by thin film X-ray diffraction (TF-XRD RINT2500, Rigaku, Japan) and field-emission scanning electron microscopy (FE-SEM S4700, Hitachi, Japan).
3.
Results
3.1. Crystal structure and surface morphology of titania formed on the surface of titanium metal after anodic oxidation After anodic oxidation in H2 SO4 solutions, a titania layer with a three-dimensional oxide structure consisting of numerous open pores formed on the surface of titanium metal from 100 to 180 V, which leads to increased surface roughness. In the process of electrochemistry, gas evolution and spark discharge occurred. The porosity and the pore size increased with increasing voltage from 100 to 150 V, while the porous structure of the titania layers did not change with increasing voltage from 150 to 180 V (Fig. 1). TF-XRD patterns showed that the oxide films on the surface of titanium mainly consisted of the anatase phase after anodizing at 100 V, rutile and
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d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 80–86
Fig. 1 – FE-SEM photographs of the surfaces of titanium metals untreated and subjected to anodic oxidation in H2 SO4 solution at 100, 150 and 180 V.
Fig. 2 – TF-XRD patterns of the surfaces of titanium metals untreated and subjected to anodic oxidation in H2 SO4 solution at 100, 150 and 180 V.
anatase phases at 150 V, and only the rutile phase at 180 V (Fig. 2). It was also shown that the intensity of the anatase phase decreased with increasing applied voltage, and eventually the anatase phase completely converted into the rutile phase after anodizing at 180 V. Fig. 3 shows FE-SEM photographs of the surface of titanium metal after anodic oxidation in CH3 COOH solutions. Numerous pores can be seen on the sample surface, and the quantity and size of the pores increased with increasing voltage from 100 to 180 V. A mass of small white substrates appeared on the surface of the oxide film after anodizing at 180 V, which was
produced by the evolution of gas from the pores in the process of anodic oxidation. After the titanium metals were anodized in H3 PO4 solutions, the surfaces of the titania were uniform compact oxide films at 100 and 150 V, while a thread-like pore structure was formed at 180 V, as shown in Fig. 4. It can be seen from Fig. 5 that no anatase and/or rutile phases appeared on the surface of titanium metal with an increase in applied voltage after anodizing in CH3 COOH and H3 PO4 solutions. Compared with the TF-XRD patterns of untreated titanium, it can be concluded that amorphous titania layers were formed on the surface of titanium after anodizing in CH3 COOH and H3 PO4 solutions. Fig. 6 shows the FE-SEM photographs of titanium subjected to anodic oxidation in Na2 SO4 solution at various applied voltages from 100 to 180 V. It can be seen from Fig. 6 that a gel-like titania film with a pore structure formed on the sample surface after anodizing in Na2 SO4 solution, accompanying the occurrence of gas evolution and spark discharge. The surface of the oxide films became rougher with increasing applied voltage from 100 to 150 V, while the surface morphology of the substrates showed no apparent change. The TF-XRD patterns in Fig. 7 show that the crystal structure of titania was mainly composed of the rutile phase at 100 V, along with a small amount of the anatase phase, and consisted completely of the rutile phase after anodizing at 150 and 180 V, respectively. This indicated that the anatase phase converted into the rutile phase entirely with the increase of applied voltage from 100 to 150 V.
3.2. Apatite-forming ability of titanium metal after anodic oxidation in SBF After anodizing in H2 SO4 solution and then soaking in SBF solution for 1 d, apatite with small spheres started to form on
Fig. 3 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in CH3 COOH solution at 100, 150 and 180 V.
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 80–86
83
Fig. 4 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in H3 PO4 solution at 100, 150 and 180 V.
Fig. 5 – TF-XRD patterns of the surfaces of titanium metals untreated and subjected to anodic oxidation in CH3 COOH and H3 PO4 solutions at 100, 150 and 180 V.
the surface of the titanium oxidized at 150 and 180 V, while there was no apatite formation on the surface of the titanium oxidized at 100 V (Fig. 8). After soaking in SBF for 7 d, the surfaces of titanium anodized at 150 and 180 V were fully covered with a dense apatite layer that consumed the Ca2+ and PO4 3− from the surrounding solutions, while only sparse apatite formed on the titanium anodized at 100 V.
After anodic oxidation in CH3 COOH and H3 PO4 solutions and subsequent soaking in SBF solutions for 7 d, no apatite phase appeared on the TF-XRD patterns (Fig. 9). From these results, one could draw the conclusion that there was no apatite-forming ability for titanium soaked in SBF even for 7 d after anodizing in CH3 COOH and H3 PO4 solutions.
Fig. 6 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in Na2 SO4 solution at 100, 150 and 180 V.
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Fig. 9 – TF-XRD patterns of the surfaces of titanium metals subjected to anodic oxidation in CH3 COOH and in H3 PO4 solution at 100, 150 and 180 V, and then soaked in SBF for 7 d. Fig. 7 – TF-XRD patterns of the surfaces of titanium metals untreated and subjected to anodic oxidation in Na2 SO4 solution at 100, 150 and 180 V.
4.
After anodic oxidation in 1.0 M Na2 SO4 solution and subsequent soaking in SBF solutions for 7 d, a dense apatite layer was formed on the surface of the titanium metal (Fig. 10). Fig. 11 shows TF-XRD patterns for the surfaces of titanium metal after anodic oxidation in H2 SO4 and Na2 SO4 solutions with different voltages. It can be seen that broad halo patterns appeared at 25–27◦ and 30–34◦ in 2Â ascribed to the apatite (0 0 2) and (2 1 1) crystal planes on the surface of titanium metal. These results indicate that dense apatite layers were formed on anodized titanium metal in H2 SO4 and Na2 SO4 solutions after soaking in SBF for 7 d. Compared with Fig. 9, it can be seen that the crystal structure of the titania layers strongly affects the ability to induce apatite formation in SBF solutions.
As shown in Figs. 8–11, apatite was formed on the surfaces of titanium metal subjected to anodic oxidation in H2 SO4 and Na2 SO4 solutions, while no apatite was formed on titanium metal after anodizing in CH3 COOH and H3 PO4 solutions. TFXRD results in Figs 2, 5 and 7, show that the titania formed on titanium metal consisted of anatase and/or rutile structures after anodizing in H2 SO4 and Na2 SO4 solutions, while amorphous titania layers appeared on titanium metal after anodizing in CH3 COOH and H3 PO4 solutions. These results demonstrate that the crystal structure of titania affects the apatite-forming ability in SBF. It is interesting to find that the ratio of relative intensity for the rutile crystal plane (1 0 1) to (1 1 0) is about 0.9 after anodizing in H2 SO4 at 150 and 180 V (Fig. 2) and about 0.7 after anodizing in Na2 SO4 solutions at 150 and 180 V (Fig. 7), while that for the standard rutile powder XRD pattern is 0.5 [26].
Discussion
Fig. 8 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in H2 SO4 solution at 100, 150 and 180 V, and then soaked in SBF for 1 or 7 d.
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 80–86
85
Fig. 10 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in Na2 SO4 solution at 100, 150 and 180 V, and then soaked in SBF for 7 d.
This means that the rutile phase of titania formed on titanium metal was oriented to the (1 0 1) crystal plane in the process of anodic oxidation [27]. On the other hand, it can be seen from Fig. 11 that the ratio of the relative intensity of the apatite (0 0 2) crystal plane to the apatite (2 1 1) crystal plane is about 0.8, while that of the standard apatite powder XRD pattern is 0.38 [28]. This implies that the apatite formed on titanium metal was oriented to the (0 0 2) crystal plane. Yang et al. [27] analyzed the distance of O atoms on the rutile (1 0 1) crystal plane and the OH group on the hydroxyapatite (0 0 2) crystal plane, and indicated there was a matching relationship of crystal orientation between the rutile (1 0 1) and apatite (0 0 2) crystal planes. Therefore, it can be interpreted as the rutile phase having good apatite-forming ability when it has a crystal orientation in the (1 0 1) plane, because the matching structure could easily promote the nuclei and epitaxy for crystal growth [29]. Recently, Wu et al. [30] reported that lattice matching between the O–O distance in the (1 1 0) plane of rutile and the Ca–Ca distance in the (1 0 0) plane of apatite was important for apatite deposition, because the calcium ions in the fluid were supposed to come first to the oxygen sites of the titania layers on which they attracted the phosphate ions in the fluid to form apatite [31–34]. When using lattice parameters for carbonateincorporated hydroxyapatite prepared from aqueous solution [35], which were very similar in both composition and structure to the apatite induced in SBF, the difference between the O–O distance in rutile and the Ca–Ca distance in apatite
was even smaller. This meant the carbonate-incorporated apatite gives better lattice matching with the rutile phase than does hydroxyapatite. In short, from the viewpoint of lattice matching, the rutile structure of titania formed on anodized titanium metal would exhibit good apatite-forming ability, namely bioactivity. The amorphous titania layers that appeared on the surface of titanium metal after anodizing in CH3 COOH and H3 PO4 solutions could not induce apatite formation in SBF. These results coincided with previous research findings [14,15,30,36]. Therefore, it can be concluded that the amorphous titania layers cannot show apatite-forming ability in SBF, regardless of the method used, such as sol–gel or anodic oxidation.
5.
Conclusion
After anodic oxidation in H2 SO4 and Na2 SO4 solutions, porous titania layers mainly consisted of rutile or rutile/anatase phases produced on the surface of titanium, which could effectively induce apatite formation in a short time in SBF. However, amorphous titania layers on titanium metal subjected to anodizing in CH3 COOH and H3 PO4 solutions could not induce apatite formation in SBF. It was found that bioactive titanium metals could be prepared via anodic oxidation in H2 SO4 and Na2 SO4 solutions. The rutile structure of titania played an important role in inducing the apatite deposition because of the relationship of lattice matching between rutile and apatite.
Acknowledgement This work was partially supported by a Grant-in-Aid for Scientific Research by the Ministry of Education, Science, Sports and Culture, Japan.
references
Fig. 11 – TF-XRD patterns of the surface of titanium metals subjected to anodic oxidation in H2 SO4 and Na2 SO4 solutions at 100, 150 and 180 V, and then soaked in SBF for 7 d.
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available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Preparation of bioactive titania films on titanium metal via anodic oxidation X. Cui a , H.-M. Kim b , M. Kawashita c,∗ , L. Wang a , T. Xiong a , T. Kokubo d , T. Nakamura e a
Institute of Metal Research, Chinese Academy of Sciences, Wenhua Road 72, Shenyang 110016, China Department of Ceramic Engineering, School of Advanced Materials Engineering, Yonsei University 134, Shinchon-dong Seodaemun-gu, Seoul 120-749, Republic of Korea c Center for Research Strategy and Support, Tohoku University, Aoba-ku, Sendai 980-8579, Japan d Department of Biomedical Sciences, College of Life and Health Sciences, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan e Department of Orthopedic Surgery, Graduate School of Medical, Kyoto University, Shogoin, Kawahara-cho 54, Sakyo-ku, Kyoto 606-8507, Japan b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To research the crystal structure and surface morphology of anodic films on
Received 1 May 2007
titanium metal in different electrolytes under various electrochemical conditions and inves-
Accepted 15 April 2008
tigate the effect of the crystal structure of the oxide films on apatite-forming ability in simulated body fluid (SBF). Methods. Titanium oxide films were prepared using an anodic oxidation method on the sur-
Keywords:
face of titanium metal in four different electrolytes: sulfuric acid, acetic acid, phosphoric
Titanium
acid and sodium sulfate solutions with different voltages for 1 min at room temperature.
Anodic oxidation
Results. Anodic films that consisted of rutile and/or anatase phases with porous structures
Rutile
were formed on titanium metal after anodizing in H2 SO4 and Na2 SO4 electrolytes, while
Anatase
amorphous titania films were produced after anodizing in CH3 COOH and H3 PO4 electrolytes.
Apatite
Titanium metal with the anatase and/or rutile crystal structure films showed excellent apatite-forming ability and produced a compact apatite layer covering all the surface of titanium after soaking in SBF for 7 d, but titanium metal with amorphous titania layers was not able to induce apatite formation. Significance. The resultant apatite layer formed on titanium metal in SBF could enhance the bonding strength between living tissue and the implant. Anodic oxidation is believed to be an effective method for preparing bioactive titanium metal as an artificial bone substitute even under load-bearing conditions. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In the past 20 years, titanium and its alloys have been successfully used as dental and orthopedic biomaterials because
∗
of their good mechanical properties, corrosion resistance and biocompatibility with living tissue after implantation into the bone [1–3]. However, being bioinert metallic materials, they cannot bond to living bone directly after implantation into a
Corresponding author. Tel.: +81 22 795 3937; fax: +81 22 795 3937. E-mail address: [email protected] (M. Kawashita). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2008.04.012
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 80–86
host body [4,5]. Therefore, various surface modifications have been attempted to improve the bioactive bone-bonding ability of titanium metal [6–8]. Recently, Kokubo’s group showed that titanium and its alloys were able to form a bonelike apatite layer on their surfaces in the body environment and bond to living bones after implantation in the body when they were treated in NaOH solution and then heat-treated in a furnace [9–12]. The apatite-forming ability of the metal was due to the formation of amorphous sodium titanate on the titanium metal by the NaOH and heat treatments, and then the formation of the Ti–OH group through the exchange of Na+ and H3 O+ in SBF or real body fluid. Uchida et al. showed that the apatite-forming ability of titanium induced by NaOH and heat treatments was greatly enhanced by combining hot water and subsequent heat treatments [13]. They ascribed the enhancement of the apatite-forming ability to the formation of anatase from the conversion of the sodium titanate gel on the NaOH-treated titanium metal after hot water and heat treatments. Moreover, it was found that titania gels with the specific structure of anatase and rutile prepared by a sol–gel process showed higher apatite-forming ability in SBF, while amorphous titania did not [14,15]. These results implied that both the formation of Ti–OH groups and the crystal structure of titania would greatly affect apatite-forming ability on the surface of titania gels. Electrochemical anodic oxidation has been a traditional surface modification method to modify the structures and chemical properties of anodic films on titanium by controlling the electrochemical parameters, such as electrolyte composition and concentration, applied potential or current, temperatures, etc. [16–19]. Recently, anodic oxidation has become an attractive method for preparing oxide films on titanium, because the oxide films improved apatite formation in SBF [7,8,20–24]. On the basis of the above research, titanium metal after anodic oxidation would produce the anatase and/or rutile crystal structure of titania by conditioning the electrochemical parameters, which would contribute to forming titanium metal with high apatite-forming ability. However, in a previous study, a subsequent heat treatment or hydrothermal treatment was required to provide the titania layers with the desired apatite-forming ability after anodic oxidation. In this study, the authors treated the titanium metal in sulfuric acid, acetic acid, phosphoric acid and sodium sulfate solution by anodic oxidation without any subsequent treatment. The purpose of this study was to research the crystal structure and surface morphology of anodic films on titanium metal in different electrolytes with various electrochemical conditions and investigate the effect of the crystal structure of the oxide films on apatite-forming ability in SBF solutions.
2.
Materials and methods
2.1.
Preparation of samples
Samples of commercially pure titanium metal (purity: 99.9%, Nilaco Co., Tokyo, Japan), 10 mm × 10 mm × 1 mm in size were mechanically abraded with No. 400 diamond plate, and then washed in an ultrasonic cleaner with pure acetone, ethanol and distilled water.
2.2.
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Anodic oxidation process
Anodic oxidation was performed under potentiostatic control in the present experiments, and measurements were made in 2.0 M H2 SO4 , CH3 COOH and H3 PO4 solutions and in 1.0 M Na2 SO4 solutions in a glass chamber with a direct current power supply system (EX1500H, Takasago Co., Japan). The voltages applied were 100, 150 and 180 V for 1 min at room temperature. Titanium plates (200 mm × 15 mm × 1 mm) were used as the cathode, and titanium samples were fixed with titanium wires on a titanium anode (200 mm × 4 mm × 1 mm). In the anodic oxidation process, a magnetic stirrer at a given rotation speed was employed to achieve a homogeneous electrolyte and to accelerate escape of gas produced in the electrochemical reaction from the surface of the titanium substrates by the magnetic mixer (MD500, Yamato, Japan). After the anodic oxidations, the samples were rinsed with distilled water and dried in an oven at 40 ◦ C for 24 h.
2.3.
Evaluation of apatite-forming ability
After anodic oxidation, the specimens were immersed in 30 ml of an acellular simulated body fluid (SBF) with pH 7.40, whose ion concentrations (Na+ 142.0, K+ 5.0, Mg2+ 1.5, Ca2+ 2.5, Cl− 147.8, HCO3 − 4.2, HPO4 2− 1.0, SO4 2− 0.5 mM) are nearly equal to those of human blood plasma at 36.5 ◦ C [25]. The SBF was prepared by dissolving reagent grade chemicals of NaCl, NaHCO3 , KCl, K2 HPO4 ·3H2 O, MgCl2 ·6H2 O, CaCl2 and Na2 SO4 in distilled water and buffering at pH 7.40 with tris(hydroxymethyl)aminomethane and 1 M HCl at 36.5 ◦ C. After soaking for a given period of 1, 3 or 7 d, the substrates were removed from the fluid and washed with distilled water, and then dried on a clean bench.
2.4.
Analyses of samples
Before and after anodic oxidation and soaking in SBF, the surface of the titanium substrates was analyzed and measured by thin film X-ray diffraction (TF-XRD RINT2500, Rigaku, Japan) and field-emission scanning electron microscopy (FE-SEM S4700, Hitachi, Japan).
3.
Results
3.1. Crystal structure and surface morphology of titania formed on the surface of titanium metal after anodic oxidation After anodic oxidation in H2 SO4 solutions, a titania layer with a three-dimensional oxide structure consisting of numerous open pores formed on the surface of titanium metal from 100 to 180 V, which leads to increased surface roughness. In the process of electrochemistry, gas evolution and spark discharge occurred. The porosity and the pore size increased with increasing voltage from 100 to 150 V, while the porous structure of the titania layers did not change with increasing voltage from 150 to 180 V (Fig. 1). TF-XRD patterns showed that the oxide films on the surface of titanium mainly consisted of the anatase phase after anodizing at 100 V, rutile and
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Fig. 1 – FE-SEM photographs of the surfaces of titanium metals untreated and subjected to anodic oxidation in H2 SO4 solution at 100, 150 and 180 V.
Fig. 2 – TF-XRD patterns of the surfaces of titanium metals untreated and subjected to anodic oxidation in H2 SO4 solution at 100, 150 and 180 V.
anatase phases at 150 V, and only the rutile phase at 180 V (Fig. 2). It was also shown that the intensity of the anatase phase decreased with increasing applied voltage, and eventually the anatase phase completely converted into the rutile phase after anodizing at 180 V. Fig. 3 shows FE-SEM photographs of the surface of titanium metal after anodic oxidation in CH3 COOH solutions. Numerous pores can be seen on the sample surface, and the quantity and size of the pores increased with increasing voltage from 100 to 180 V. A mass of small white substrates appeared on the surface of the oxide film after anodizing at 180 V, which was
produced by the evolution of gas from the pores in the process of anodic oxidation. After the titanium metals were anodized in H3 PO4 solutions, the surfaces of the titania were uniform compact oxide films at 100 and 150 V, while a thread-like pore structure was formed at 180 V, as shown in Fig. 4. It can be seen from Fig. 5 that no anatase and/or rutile phases appeared on the surface of titanium metal with an increase in applied voltage after anodizing in CH3 COOH and H3 PO4 solutions. Compared with the TF-XRD patterns of untreated titanium, it can be concluded that amorphous titania layers were formed on the surface of titanium after anodizing in CH3 COOH and H3 PO4 solutions. Fig. 6 shows the FE-SEM photographs of titanium subjected to anodic oxidation in Na2 SO4 solution at various applied voltages from 100 to 180 V. It can be seen from Fig. 6 that a gel-like titania film with a pore structure formed on the sample surface after anodizing in Na2 SO4 solution, accompanying the occurrence of gas evolution and spark discharge. The surface of the oxide films became rougher with increasing applied voltage from 100 to 150 V, while the surface morphology of the substrates showed no apparent change. The TF-XRD patterns in Fig. 7 show that the crystal structure of titania was mainly composed of the rutile phase at 100 V, along with a small amount of the anatase phase, and consisted completely of the rutile phase after anodizing at 150 and 180 V, respectively. This indicated that the anatase phase converted into the rutile phase entirely with the increase of applied voltage from 100 to 150 V.
3.2. Apatite-forming ability of titanium metal after anodic oxidation in SBF After anodizing in H2 SO4 solution and then soaking in SBF solution for 1 d, apatite with small spheres started to form on
Fig. 3 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in CH3 COOH solution at 100, 150 and 180 V.
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Fig. 4 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in H3 PO4 solution at 100, 150 and 180 V.
Fig. 5 – TF-XRD patterns of the surfaces of titanium metals untreated and subjected to anodic oxidation in CH3 COOH and H3 PO4 solutions at 100, 150 and 180 V.
the surface of the titanium oxidized at 150 and 180 V, while there was no apatite formation on the surface of the titanium oxidized at 100 V (Fig. 8). After soaking in SBF for 7 d, the surfaces of titanium anodized at 150 and 180 V were fully covered with a dense apatite layer that consumed the Ca2+ and PO4 3− from the surrounding solutions, while only sparse apatite formed on the titanium anodized at 100 V.
After anodic oxidation in CH3 COOH and H3 PO4 solutions and subsequent soaking in SBF solutions for 7 d, no apatite phase appeared on the TF-XRD patterns (Fig. 9). From these results, one could draw the conclusion that there was no apatite-forming ability for titanium soaked in SBF even for 7 d after anodizing in CH3 COOH and H3 PO4 solutions.
Fig. 6 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in Na2 SO4 solution at 100, 150 and 180 V.
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Fig. 9 – TF-XRD patterns of the surfaces of titanium metals subjected to anodic oxidation in CH3 COOH and in H3 PO4 solution at 100, 150 and 180 V, and then soaked in SBF for 7 d. Fig. 7 – TF-XRD patterns of the surfaces of titanium metals untreated and subjected to anodic oxidation in Na2 SO4 solution at 100, 150 and 180 V.
4.
After anodic oxidation in 1.0 M Na2 SO4 solution and subsequent soaking in SBF solutions for 7 d, a dense apatite layer was formed on the surface of the titanium metal (Fig. 10). Fig. 11 shows TF-XRD patterns for the surfaces of titanium metal after anodic oxidation in H2 SO4 and Na2 SO4 solutions with different voltages. It can be seen that broad halo patterns appeared at 25–27◦ and 30–34◦ in 2Â ascribed to the apatite (0 0 2) and (2 1 1) crystal planes on the surface of titanium metal. These results indicate that dense apatite layers were formed on anodized titanium metal in H2 SO4 and Na2 SO4 solutions after soaking in SBF for 7 d. Compared with Fig. 9, it can be seen that the crystal structure of the titania layers strongly affects the ability to induce apatite formation in SBF solutions.
As shown in Figs. 8–11, apatite was formed on the surfaces of titanium metal subjected to anodic oxidation in H2 SO4 and Na2 SO4 solutions, while no apatite was formed on titanium metal after anodizing in CH3 COOH and H3 PO4 solutions. TFXRD results in Figs 2, 5 and 7, show that the titania formed on titanium metal consisted of anatase and/or rutile structures after anodizing in H2 SO4 and Na2 SO4 solutions, while amorphous titania layers appeared on titanium metal after anodizing in CH3 COOH and H3 PO4 solutions. These results demonstrate that the crystal structure of titania affects the apatite-forming ability in SBF. It is interesting to find that the ratio of relative intensity for the rutile crystal plane (1 0 1) to (1 1 0) is about 0.9 after anodizing in H2 SO4 at 150 and 180 V (Fig. 2) and about 0.7 after anodizing in Na2 SO4 solutions at 150 and 180 V (Fig. 7), while that for the standard rutile powder XRD pattern is 0.5 [26].
Discussion
Fig. 8 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in H2 SO4 solution at 100, 150 and 180 V, and then soaked in SBF for 1 or 7 d.
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Fig. 10 – FE-SEM photographs of the surfaces of titanium metals subjected to anodic oxidation in Na2 SO4 solution at 100, 150 and 180 V, and then soaked in SBF for 7 d.
This means that the rutile phase of titania formed on titanium metal was oriented to the (1 0 1) crystal plane in the process of anodic oxidation [27]. On the other hand, it can be seen from Fig. 11 that the ratio of the relative intensity of the apatite (0 0 2) crystal plane to the apatite (2 1 1) crystal plane is about 0.8, while that of the standard apatite powder XRD pattern is 0.38 [28]. This implies that the apatite formed on titanium metal was oriented to the (0 0 2) crystal plane. Yang et al. [27] analyzed the distance of O atoms on the rutile (1 0 1) crystal plane and the OH group on the hydroxyapatite (0 0 2) crystal plane, and indicated there was a matching relationship of crystal orientation between the rutile (1 0 1) and apatite (0 0 2) crystal planes. Therefore, it can be interpreted as the rutile phase having good apatite-forming ability when it has a crystal orientation in the (1 0 1) plane, because the matching structure could easily promote the nuclei and epitaxy for crystal growth [29]. Recently, Wu et al. [30] reported that lattice matching between the O–O distance in the (1 1 0) plane of rutile and the Ca–Ca distance in the (1 0 0) plane of apatite was important for apatite deposition, because the calcium ions in the fluid were supposed to come first to the oxygen sites of the titania layers on which they attracted the phosphate ions in the fluid to form apatite [31–34]. When using lattice parameters for carbonateincorporated hydroxyapatite prepared from aqueous solution [35], which were very similar in both composition and structure to the apatite induced in SBF, the difference between the O–O distance in rutile and the Ca–Ca distance in apatite
was even smaller. This meant the carbonate-incorporated apatite gives better lattice matching with the rutile phase than does hydroxyapatite. In short, from the viewpoint of lattice matching, the rutile structure of titania formed on anodized titanium metal would exhibit good apatite-forming ability, namely bioactivity. The amorphous titania layers that appeared on the surface of titanium metal after anodizing in CH3 COOH and H3 PO4 solutions could not induce apatite formation in SBF. These results coincided with previous research findings [14,15,30,36]. Therefore, it can be concluded that the amorphous titania layers cannot show apatite-forming ability in SBF, regardless of the method used, such as sol–gel or anodic oxidation.
5.
Conclusion
After anodic oxidation in H2 SO4 and Na2 SO4 solutions, porous titania layers mainly consisted of rutile or rutile/anatase phases produced on the surface of titanium, which could effectively induce apatite formation in a short time in SBF. However, amorphous titania layers on titanium metal subjected to anodizing in CH3 COOH and H3 PO4 solutions could not induce apatite formation in SBF. It was found that bioactive titanium metals could be prepared via anodic oxidation in H2 SO4 and Na2 SO4 solutions. The rutile structure of titania played an important role in inducing the apatite deposition because of the relationship of lattice matching between rutile and apatite.
Acknowledgement This work was partially supported by a Grant-in-Aid for Scientific Research by the Ministry of Education, Science, Sports and Culture, Japan.
references
Fig. 11 – TF-XRD patterns of the surface of titanium metals subjected to anodic oxidation in H2 SO4 and Na2 SO4 solutions at 100, 150 and 180 V, and then soaked in SBF for 7 d.
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