Preparation of bioactive titanium metal via anodic oxidation treatment

Bangcheng Yang^a, Masaiki Uchida^b, Hyun-Min Kim^{, , c}, Xingdong Zhang^a and Tadashi Kokubo<sup>d

a</sup> Engineering Research Center for Biomaterials, Sichuan University, Wangjiang Road, No. 29, Chengdu, Sichuan 610064, China
^b Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
^c Department of Ceramic Engineering, School of Advanced Materials Engineering, Yonsei University 134, Shinchon-dong, Seodaemun-gu, Seoul 120-749, South Korea
^d Research Institute for Science and Technology, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan

Received 7 June 2003;  accepted 14 July 2003. ; Available online 28 September 2003.
Biomaterials
Volume 25, Issue 6 , March 2004, Pages 1003-1010

1. Abstract

Titania with specific structures of anatase and rutile was found to induce apatite formation in vitro. In this study, anodic oxidation in H₂SO₄ solution, which could form anatase and rutile on titanium metal surface by conditioning the process, was employed to modify the structure and bioactivity of biomedical titanium. After the titanium metal was subjected to anodic oxidation treatment, thin film X-ray diffraction and scanning electron microscopy results showed the titanium metals surfaces were covered by porous titania of anatase and/or rutile. In simulated body fluid (SBF), the titanium anodically oxidized under the conditions with spark-discharge could induce apatite formation on its surface. The induction period of apatite formation was decreased with increasing amount of either anatase or rutile by conditioning the anodic oxidation. After the titanium metal, anodically oxidized under the conditions without spark-discharge, was subjected to heat treatment at 600°C for 1 h, it could also induce apatite formation in SBF because the amount of anatase and/or rutile was increased by the heat treatment. Our results showed that induction of apatite-forming ability on titanium metal could be attained by anodic oxidation conjoined with heat treatment. So it was believed that anodic oxidation in H₂SO₄ solution was an effective way to prepare bioactive titanium.

Author Keywords: Titanium; Bioactivity; Anodic oxidation; Anatase; Apatite

2. Article Outline

1. Introduction

2. Materials and methods

3. Results

3.1. Structures of titanium metals surfaces after anodic oxidation and heat treatment

3.2. Apatite-forming abilities of the treated titanium metals

4. Discussion

5. Conclusion

Acknowledgements

References

3. 1. Introduction

It is well known that bioactive materials such as sintered hydroxyapatite and glass-ceramic A-W form bioactive bonding with the living bone by forming an apatite layer on their surfaces after they are implanted in the bony site. But their fracture resistance is not enough to replace bones under load-bearing conditions in clinic [1, 2, 3 and 4]. At the time being, biomedical metals such as stainless steel, Co-Cr alloys, titanium and titanium alloys with higher fracture toughness are used in clinic for this purpose, but they are non-bioactive [5 and 6].

Recently, it was found that titanium metal and its alloys subjected to NaOH and heat treatments show the apatite-forming ability and integrate with the living bone after implanted in bone. The apatite-forming ability of the metal is attributed to the amorphous sodium titanate that is formed on the metal during the NaOH and heat treatment [7, 8 and 9]. More recently, it was found in vitro that sodium-free titania with specific structure of anatase and rutile possesses much higher apatite-forming ability than sodium-containing titanate [10 and 11]. This result implies a possibility of preparation more bioactive titanium metal and its alloys by transforming the surface sodium titanate into sodium-free titania via combination of a sodium removal treatment with the NaOH and heat treatment, or by forming such titania through different methodology.

Anodic oxidation is a traditional method to modify the surface structure and properties of titanium for catalyst and valve metal. In biomedical field, this method was employed to produce calcium phosphate coatings on metallic implants via multi-step oxidation in complex calcium and/or phosphate-containing electrolyte [12 and 13]. This study surmises upon the basis of the above results that anodic oxidation of titanium metal and its alloys in simple electrolyte would produce anatase and/or rutile on their surfaces by conditioning the anodic oxidation process [14, 15, 16, 17, 18, 19 and 20], which might be sufficient for preparation titanium metal and its alloys with high apatite-forming ability.

This paper presents the effect of anodic oxidation in H₂SO₄ solution combined subsequently with further heat treatment on the structure and the apatite-forming ability of the titanium metal.

4. 2. Materials and methods

Substrates of commercially pure titanium metal (Kobe Steel Ltd., Kobe, Japan) 10×10×1 mm in size were grinded with No. 400 diamond plate, and cleaned in pure acetone and distilled water. An anodic oxidation system was composed of anode and cathode plates of titanium, respectively, 3×300×1 mm and 10×300×1 mm in size, electrolyte of H₂SO₄ solution contained in glass chamber and an extended range direct current (DC) power supply system (EX1500H, Takasago Co., Kawasaki, Kanagawa, Japan). Substrates of titanium metal were fixed on the anode using titanium wire and immersed in 1 
H₂SO₄ solution for 5 min to dissolve the air-formed oxide film on the surface. Then, they were immersed in electrolyte-contained glass chamber, and subjected to anodic oxidation by applying DC for 1 min at room temperature. The anodic oxidation was processed at different DC voltages of 90, 155 and 180 V under a constant electrolyte concentration of 1 
or at different electrolyte concentrations of 0.5, 1 and 3 
under a constant DC voltage of 155 V. After the anodic oxidations, the titanium metals were rinsed with distilled water and dried in an oven at 40°C. Some of the titanium metals after the anodic oxidation in 1 
electrolyte at different DC voltage were subjected to heat treatment at 600°C for 1 h.

After the anodic oxidation and subsequent heat treatment, the titanium metals were soaked in 30 ml of simulated body fluid (SBF) with ion concentrations (Na⁺ 142, K⁺ 5.0, Mg²⁺ 1.5, Ca²⁺ 2.5, Cl⁻ 147.8, HCO₃⁻ 4.2, HPO₄²⁻ 1.0, and SO₄²⁻ 0.5 m
) nearly equal to human blood plasma for 3 and 6 d. The SBF was prepared by dissolving reagent grade chemicals of NaCl, NaHCO₃, KCl, K₂PO₄.3H₂O, MgCl₂.6H₂O, CaCl₂, and Na₂SO₄ into distilled water and buffered at pH 7.40 with tris(hydroxymethyl) aminomethane and 1 
HCl at 36.5°C.

Before and after anodic oxidation, heat treatment and soaking in SBF, the surfaces of titanium metals were analyzed with field-emission scanning electron microscopy (SEM: S-4700, Hitachi Co, Tokyo, Japan) and thin film X-ray diffraction (TF-XRD: Rint-2500, Rigaku Co, Tokyo, Japan).

5. 3. Results

3.1. Structures of titanium metals surfaces after anodic oxidation and heat treatment

When titanium metals were anodically oxidized in 1 
H₂SO₄ solution, titanium metals surfaces become porous at DC voltage from 90 to 180 V and spark-discharge occurred when the DC voltage was higher than 105 V. The porosity and the pore size increased with increasing voltage from 90 to 155 V and the porous structure did not change with increasing voltage from 155 to 180 V (Fig. 1). On the titanium metals surfaces, titania with anatase structure appeared at 90 V, titania of anatase and rutile phase formed at 155 V and only rutile appeared at 180 V. The peaks of Ti on the TF-XRD pattern decreased with increasing voltage, which indicated the amount of titania with the structure of anatase and/or rutile on the titanium surfaces increased with increasing voltage. The ratio of the relative strength of the rutile (1 0 1) crystal plane to the rutile (1 1 0) crystal plane was about 0.7 (Fig. 2), while that of the standard rutile powder XRD pattern was 0.5 [21]. This meant that the rutile formed on the titanium metals during the anodic oxidation was oriented to the (1 0 1) crystal plane.

(58K)

Fig. 1. SEM photographs of (A) titanium metals without treatment and titanium metals anodically oxidized at (B) 90 V, (C) 155 V, (D) 180 V in 1 
H₂SO₄ for 1 min.

(10K)

Fig. 2. TF-XRD patterns of (A) titanium metals without treatment and titanium metals anodically oxidized at (B) 90 V, (C) 155 V, (D) 180 V in 1 
H₂SO₄ for 1 min.

After the titanium metals were oxidized in 1 
H₂SO₄ solution at DC voltage from 90 to 180 V and heat treated, compared with those titanium metals without heat treatment, they had more titania on their surfaces. The heat treatment made rutile, besides anatase, formed on titanium oxidized at 90 V and more rutile form on the titanium anodically oxidized at 155 and 180 V. The orientation of the rutile to the (1 0 1) crystal plane did not be changed by the heat treatment (Fig. 3). The SEM investigation showed the morphology of the material surfaces had not change after the heat treatment.

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Fig. 3. TF-XRD patterns of titanium metals after they were anodically oxidized in 1 
H₂SO₄ solution at (A) 90 V, (B) 155 V, (C) 180 V for 1 min and heat treated.

When titanium metals were anodically oxidized at 155 V in different H₂SO₄ solutions with concentrations of 0.5-3 
, they also became porous and spark-discharge occurred in all of these cases. The pore size and porosity of the structure increased with increasing concentration from 0.5 to 1 
, and it did not change with increasing concentration from 1 to 3 
(Fig. 4). Anatase and rutile formed on all the material surfaces. The amount of anatase decreased and the amount of rutile increased gradually on the surfaces with increasing the concentration of H₂SO₄. The rutile was also oriented to the (1 0 1) crystal plane (Fig. 5).

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Fig. 4. SEM photographs of titanium metals anodically oxidized at 155 V in H₂SO₄ with concentration of (A) 0.5 
, (B) 1 
, (C) 3 
for 1 min.

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Fig. 5. TF-XRD patterns of titanium metals anodically oxidized at 155 V in H₂SO₄ with concentration of (A) 0.5 
, (B) 1 
, (C) 3 
for 1 min.

In Table 1, the effects of DC voltages, concentration of electrolyte and heat treatment on the structures of the titania formed on titanium metals surfaces are shown.

Table 1. Properties of samples after anodic oxidation and soaking in SBF

"+, ++, +++": The ability to form anatase, rutile or apatite on the metal surface, respectively.

"−−−": Without ability to form anatase, rutile or apatite on the metal surface.

3.2. Apatite-forming abilities of the treated titanium metals

After the titanium metals oxidized in 1 
H₂SO₄ solution at different voltages were soaked in SBF for 3 d, apatite formed on the titanium metals oxidized at 155 and 180 V, while there was no apatite on the titanium metal oxidized at 90 V. After they were soaked in SBF for 6 d, no apatite could be found on the titanium metal oxidized at 90 V at this time, but the surfaces of the titanium metals oxidized at 155 and 180 V were almost fully covered by apatite (Fig. 6). It is interesting to note that the ratio of the relative strength of the apatite (0 0 0 2) crystal plane to the apatite (0 2 1 1) crystal plane is about 1 (Fig. 7), while that of the standard apatite powder XRD pattern is 0.4 [22]. This meant that the apatite formed on the titanium metals was oriented to the (0 0 0 2) crystal plane.

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Fig. 6. SEM photographs of titanium metals soaked in SBF for 6 d after they were anodically oxidized in 1 
H₂SO₄ at (A) 90 V, (B) 155 V, (C) 180 V for 1 min.

(8K)

Fig. 7. TF-XRD patterns of titanium metals soaked in SBF for 6 d after they were anodically oxidized in 1 
H₂SO₄ at (A) 90 V, (B) 155 V, (C) 180 V for 1 min.

After heat-treated titanium metals were soaked in SBF for 3 d, all of them induced apatite on their surfaces. After they were soaked in SBF for 6 d, a lot of apatite formed on their surfaces. Compared with the samples without heat treatment, the titanium oxidized at 90 V also induced apatite formation after it was heat treated. The apatite formed on the titanium was also oriented to the (0 0 0 2) crystal plane (Fig. 8).

(9K)

Fig. 8. TF-XRD patterns of titanium metals soaked in SBF for 6 d after they were anodically oxidized in 1 
H₂SO₄ solution at (A) 90 V, (B) 155 V, (C) 180 V for 1 min and then heat treated.

After the titanium metals oxidized at 155 V in different solutions were soaked in SBF for 3 d, apatite formed on all the surfaces. The apatite on the titanium metal oxidized in 3 
H₂SO₄ solutions was more than that on the titanium metals oxidized in 0.5 and 1 
H₂SO₄ solutions, which means the period of induction of apatite formation for the titanium metals oxidized in H₂SO₄ solutions with higher concentration was shorter. After they were soaked in SBF for 6 d, apatite covered almost all the surfaces of the titanium metals (Fig. 9). The TF-XRD patterns showed apatite was oriented to the (0 0 0 2) crystal plane (Fig. 10).

(43K)

Fig. 9. SEM photographs of titanium metals soaked in SBF for 6 d after they were anodically oxidized in H₂SO₄ with concentrations of (A) 0.5 
, (B) 1 
, (C) 3 
at 155 V for 1 min.

(7K)

Fig. 10. TF-XRD patterns of titanium metals soaked in SBF for 6 d after they were anodically oxidized in H₂SO₄ with concentrations of (A) 0.5 
, (B) 1 
, (C) 3 
at 155 V for 1 min.

In Table 1, the effects the structures of the titania on the apatite-forming ability were also shown.

6. 4. Discussion

Our results showed the apatite formed on the titanium metal which was anodically oxidized under the condition with spark-discharge, it means that anodic oxidation is an effective method to prepare bioactive titanium.

However, no apatite formed on the titanium anodically oxidized under the condition without spark-discharge, even though the anatase was also produced on its surface. This might indicate that a certain amount of titanium oxide, in other words, a certain thickness for titanium oxide, was necessary for the bioactivity of this material, because the titanium metal oxidized under the condition without spark-discharge could also induce apatite formation after the amount of titanium oxide on its surface was increased by the heat treatment. It suggested that a three-dimensional structure of the micro-porous titanium oxide structure might be necessary for the apatite formation on the surfaces.

The titanium oxide that could induce apatite formation had the structure of anatase and/or rutile with different ratio. It indicated that anatase and rutile were all effective for apatite formation.

The previous studies of our lab have shown that the bioactivity of anatase might come from the negative charge on it surface formed in the SBF solution. The negative charge absorbed the Ca ions from the SBF first, then the Ca ions absorbed PO₄ ions from the solution to form apatite on the surface [23, 24, 25 and 26].

It is interesting to note the orientation of the rutile to the (1 0 1) crystal plane on the surface of the titanium and the orientation of apatite to the (0 0 0 2) crystal plane. Actually, the (0 0 0 2) crystal plane of apatite was parallel to the (0 0 0 4) crystal plane, so the apatite orientation to the (0 0 0 2) crystal plane meant it also oriented to the (0 0 0 4) crystal plane.

Based on the crystal structure of the rutile (1 0 1), it could be calculated that on the rutile (1 0 1) plane the O atoms was arranged on [0 1 0] orientation with 4.6 Ĺ distance, which meant the length for two cells at this orientation was 9.2 Ĺ; and the O atoms were arranged on the orientation of
with a distance of 5.46 Ĺ, so the length for three cells was 16.38 Ĺ. On the apatite (0 0 0 4) plane, the OH group was arranged with a distance of 16.32 Ĺ between the long diagonal OH group and a distance of 9.42 Ĺ between the short diagonal OH group (Fig. 11). So the structure of rutile (1 0 1) is matching to the structure of apatite (0 0 0 4). It is reported that the matching structure could be the nuclei for crystal growth [27]. The apatite-forming ability of rutile on the anodically oxidized titanium might come from the orientation of rutile to the (1 0 1) crystal plane.

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Fig. 11. Schematic diagram of the arrangement of oxygen atoms on the rutile (1 0 1) crystal plane and the arrangement of OH group on the apatite (0 0 0 4) crystal plane. It showed the micro-matching structure between rutile (1 0 1) and apatite (0 0 0 4).

7. 5. Conclusion

Anodic oxidation with H₂SO₄ solution was an effective way to prepare bioactive titanium metal which is suitable for the applications under the loading-bearing conditions. A certain amount of titania of anatase and/or rutile structures on the oxidized titanium surfaces was required for the apatite formation.

8. Acknowledgements

This work was supported by a grant-in-aid for scientific research by the Ministry of Education, Science, Sports and Culture, Japan.

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