Biomaterials 25 (2004) 1003–1010
ARTICLE IN PRESS
*Correspondin
E-mail addres
0142-9612/$ - see
doi:10.1016/S014
Preparation of bioactive titanium metal via anodicoxidation treatment
Bangcheng Yanga, Masaiki Uchidab, Hyun-Min Kimc,*, Xingdong Zhanga,Tadashi Kokubod
aEngineering Research Center for Biomaterials, Sichuan University, Wangjiang Road, No. 29, Chengdu, Sichuan 610064, ChinabDepartment of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
cDepartment of Ceramic Engineering, School of Advanced Materials Engineering, Yonsei University 134, Shinchon-dong,
Seodaemun-gu, Seoul 120-749, South KoreadResearch Institute for Science and Technology, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan
Received 7 June 2003; accepted 14 July 2003
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.
r 2003 Elsevier Ltd. All rights reserved.
Keywords: Titanium; Bioactivity; Anodic oxidation; Anatase; Apatite
1. Introduction
It is well known that bioactive materials such assintered hydroxyapatite and glass-ceramic A-W formbioactive bonding with the living bone by forming anapatite layer on their surfaces after they are implanted inthe bony site. But their fracture resistance is not enoughto replace bones under load-bearing conditions in clinic[1–4]. At the time being, biomedical metals such asstainless steel, Co–Cr alloys, titanium and titaniumalloys with higher fracture toughness are used in clinicfor this purpose, but they are non-bioactive [5,6].Recently, it was found that titanium metal and its
alloys subjected to NaOH and heat treatments show theapatite-forming ability and integrate with the living
g author.
s: [email protected] (H.-M. Kim).
front matter r 2003 Elsevier Ltd. All rights reserved.
2-9612(03)00626-4
bone after implanted in bone. The apatite-formingability of the metal is attributed to the amorphoussodium titanate that is formed on the metal during theNaOH and heat treatment [7–9]. More recently, it wasfound in vitro that sodium-free titania with specificstructure of anatase and rutile possesses much higherapatite-forming ability than sodium-containing titanate[10,11]. This result implies a possibility of preparationmore bioactive titanium metal and its alloys bytransforming the surface sodium titanate into sodium-free titania via combination of a sodium removaltreatment with the NaOH and heat treatment, or byforming such titania through different methodology.Anodic oxidation is a traditional method to modify
the surface structure and properties of titanium forcatalyst and valve metal. In biomedical field, thismethod was employed to produce calcium phosphatecoatings on metallic implants via multi-step oxidation in
ARTICLE IN PRESSB. Yang et al. / Biomaterials 25 (2004) 1003–10101004
complex calcium and/or phosphate-containing electro-lyte [12,13]. This study surmises upon the basis of theabove results that anodic oxidation of titanium metaland its alloys in simple electrolyte would produceanatase and/or rutile on their surfaces by conditioningthe anodic oxidation process [14–20], which might besufficient for preparation titanium metal and its alloyswith high apatite-forming ability.This paper presents the effect of anodic oxidation in
H2SO4 solution combined subsequently with furtherheat treatment on the structure and the apatite-formingability of the titanium metal.
2. Materials and methods
Substrates of commercially pure titanium metal(Kobe Steel Ltd., Kobe, Japan) 10� 10� 1mm in sizewere grinded with No. 400 diamond plate, and cleanedin pure acetone and distilled water. An anodic oxidationsystem was composed of anode and cathode plates oftitanium, respectively, 3� 300� 1mm and 10� 300�1mm in size, electrolyte of H2SO4 solution contained inglass chamber and an extended range direct current(DC) power supply system (EX1500H, Takasago Co.,Kawasaki, Kanagawa, Japan). Substrates of titaniummetal were fixed on the anode using titanium wire andimmersed in 1m H2SO4 solution for 5min to dissolve theair-formed oxide film on the surface. Then, they wereimmersed in electrolyte-contained glass chamber, andsubjected to anodic oxidation by applying DC for 1minat room temperature. The anodic oxidation wasprocessed at different DC voltages of 90, 155 and
2 µm
2 µm
(A)
(C) (
(
Fig. 1. SEM photographs of (A) titanium metals without treatment and titan
H2SO4 for 1min.
180V under a constant electrolyte concentration of 1mor at different electrolyte concentrations of 0.5, 1 and3m under a constant DC voltage of 155V. After theanodic oxidations, the titanium metals were rinsed withdistilled water and dried in an oven at 40�C. Some of thetitanium metals after the anodic oxidation in 1melectrolyte at different DC voltage were subjected toheat treatment at 600�C for 1 h.After the anodic oxidation and subsequent heat
treatment, the titanium metals were soaked in 30ml ofsimulated 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 SO4
2� 0.5mm) nearly equal to humanblood plasma for 3 and 6d. The SBF was prepared bydissolving reagent grade chemicals of NaCl, NaHCO3,KCl, K2PO4.3H2O,MgCl2.6H2O, CaCl2, and Na2SO4 intodistilled water and buffered at pH 7.40 with tris(hydrox-ymethyl) aminomethane and 1m HCl at 36.5�C.Before and after anodic oxidation, heat treatment and
soaking in SBF, the surfaces of titanium metals wereanalyzed with field-emission scanning electron micro-scopy (SEM: S-4700, Hitachi Co, Tokyo, Japan) andthin film X-ray diffraction (TF-XRD: Rint-2500,Rigaku Co, Tokyo, Japan).
3. Results
3.1. Structures of titanium metals surfaces after anodic
oxidation and heat treatment
When titanium metals were anodically oxidized in 1mH2SO4 solution, titanium metals surfaces become
1 µm
2 µmD)
B)
ium metals anodically oxidized at (B) 90V, (C) 155V, (D) 180V in 1m
ARTICLE IN PRESSB. Yang et al. / Biomaterials 25 (2004) 1003–1010 1005
porous at DC voltage from 90 to 180V and spark-discharge occurred when the DC voltage was higherthan 105V. The porosity and the pore size increasedwith increasing voltage from 90 to 155V and the porousstructure did not change with increasing voltage from155 to 180V (Fig. 1). On the titanium metals surfaces,
10 20 30 40 50 60
AnataseRutileTi
Inte
nsity
(110
)
(101
)
(111
)
(211
)
(101
)
(200
)
(A)
(B)
(C)
(D)
2θ/degree
i
0
1
1
Fig. 2. TF-XRD patterns of (A) titanium metals without treatment
and titanium metals anodically oxidized at (B) 90V, (C) 155V, (D)
180V in 1m H2SO4 for 1min.
2 µm
(A)
(B) (C
Fig. 4. SEM photographs of titanium metals anodically oxidized at 155V
titania with anatase structure appeared at 90V, titaniaof anatase and rutile phase formed at 155V and onlyrutile appeared at 180V. The peaks of Ti on the TF-XRD pattern decreased with increasing voltage, whichindicated the amount of titania with the structure ofanatase and/or rutile on the titanium surfaces increasedwith increasing voltage. The ratio of the relative strengthof the rutile (1 0 1) crystal plane to the rutile (1 1 0)
2 µm
2 µm)
in H2SO4 with concentration of (A) 0.5m, (B) 1m, (C) 3m for 1min.
10 20 30 40 50 60
AnataseRutileTi
(C)
(B)
(A)
(110
)
(101
)
(111
) (211
)
(101
)
(200
)
u
(110
)
(101
)
(111
) (211
)
(101
)
(200
)
Inte
nsity
2θ/degree
Fig. 3. TF-XRD patterns of titanium metals after they were anodically
oxidized in 1m H2SO4 solution at (A) 90V, (B) 155V, (C) 180V for
1min and heat treated.
ARTICLE IN PRESSB. Yang et al. / Biomaterials 25 (2004) 1003–10101006
crystal plane was about 0.7 (Fig. 2), while that of thestandard rutile powder XRD pattern was 0.5 [21]. Thismeant that the rutile formed on the titanium metalsduring the anodic oxidation was oriented to the (1 0 1)crystal plane.After the titanium metals were oxidized in 1m H2SO4
solution at DC voltage from 90 to 180V and heattreated, compared with those titanium metals withoutheat treatment, they had more titania on their surfaces.The heat treatment made rutile, besides anatase, formedon titanium oxidized at 90V and more rutile form on thetitanium anodically oxidized at 155 and 180V. Theorientation of the rutile to the (1 0 1) crystal plane didnot be changed by the heat treatment (Fig. 3). The SEM
10 20 30 40 50 60
anataserutileTi
(A)
(B)
(C)
(101
)(1
10)
(101
)
(111
)
(200
)
(211
)
Inte
nsity
(
0
1
1)
2θ/degree
Fig. 5. TF-XRD patterns of titanium metals anodically oxidized at
155V in H2SO4 with concentration of (A) 0.5m, (B) 1m, (C) 3m for
1min.
Table 1
Properties of samples after anodic oxidation and soaking in SBF
Parameters for study Sample number Treatment condition A
A
Voltage 1 90V +
2 155V +
3 180V �
Heat treatment 4 90V+HT +
5 155V+HT +
6 180V+HT �
Concentration of H2SO4 7 0.5m +
8 1m +
9 3m +
‘‘+, ++, +++’’: The ability to form anatase, rutile or apatite on the me
‘‘���’’: Without ability to form anatase, rutile or apatite on the metal surf
investigation showed the morphology of the materialsurfaces had not change after the heat treatment.When titanium metals were anodically oxidized at
155V in different H2SO4 solutions with concentrationsof 0.5–3m, they also became porous and spark-dischargeoccurred in all of these cases. The pore size and porosityof the structure increased with increasing concentrationfrom 0.5 to 1m, and it did not change with increasingconcentration from 1 to 3m (Fig. 4). Anatase and rutileformed on all the material surfaces. The amount ofanatase decreased and the amount of rutile increasedgradually on the surfaces with increasing the concentra-tion of H2SO4. The rutile was also oriented to the (1 0 1)crystal plane (Fig. 5).In Table 1, the effects of DC voltages, concentration
of electrolyte and heat treatment on the structures of thetitania formed on titanium metals surfaces are shown.
3.2. Apatite-forming abilities of the treated titanium
metals
After the titanium metals oxidized in 1m H2SO4solution at different voltages were soaked in SBF for 3 d,apatite formed on the titanium metals oxidized at 155and 180V, while there was no apatite on the titaniummetal oxidized at 90V. After they were soaked in SBFfor 6 d, no apatite could be found on the titanium metaloxidized at 90V at this time, but the surfaces of thetitanium metals oxidized at 155 and 180V were almostfully covered by apatite (Fig. 6). It is interesting to notethat the ratio of the relative strength of the apatite(0 0 0 2) crystal plane to the apatite (0 2 1 1) crystal planeis about 1 (Fig. 7), while that of the standard apatitepowder XRD pattern is 0.4 [22]. This meant that theapatite formed on the titanium metals was oriented tothe (0 0 0 2) crystal plane.After heat-treated titanium metals were soaked in
SBF for 3 d, all of them induced apatite on their
fter anodic oxidation Apatite formation ability in SBF
natase Rutile 3 days 6 days
��� ��� ���+ +++ + +++
�� ++++ ++ +++
+ + ++ +++
+ +++ ++ +++
�� +++ ++ +++
++ ++ + +++
+ +++ + +++
+++ ++ +++
tal surface, respectively.
ace.
ARTICLE IN PRESS
10 µm
10 µm10 µm
(A)
(B) (C)
Fig. 6. SEM photographs of titanium metals soaked in SBF for 6 d after they were anodically oxidized in 1m H2SO4 at (A) 90V, (B) 155V, (C) 180V
for 1min.
10 20 30 40 50 60
ApatiteAnataseTi
Inte
nsity
(000
2)
(021
1)
it
(A)
(B)
(C)
2θ/degree
Fig. 7. TF-XRD patterns of titanium metals soaked in SBF for 6 d
after they were anodically oxidized in 1m H2SO4 at (A) 90V, (B) 155V,
(C) 180V for 1min.
Inte
nsity
RutileApatiteTi
(A)
(B)
(C)
(021
1)
(000
2)
10 20 30 40 50 60
2θ/degree
Fig. 8. TF-XRD patterns of titanium metals soaked in SBF for 6 d
after they were anodically oxidized in 1m H2SO4 solution at (A) 90V,
(B) 155V, (C) 180V for 1min and then heat treated.
B. Yang et al. / Biomaterials 25 (2004) 1003–1010 1007
surfaces. After they were soaked in SBF for 6 d, a lot ofapatite formed on their surfaces. Compared with thesamples without heat treatment, the titanium oxidized at90V also induced apatite formation after it was heattreated. The apatite formed on the titanium was alsooriented to the (0 0 0 2) crystal plane (Fig. 8).After the titanium metals oxidized at 155V in
different solutions were soaked in SBF for 3 d, apatiteformed on all the surfaces. The apatite on the titanium
metal oxidized in 3m H2SO4 solutions was more thanthat on the titanium metals oxidized in 0.5 and 1mH2SO4 solutions, which means the period of inductionof apatite formation for the titanium metals oxidized inH2SO4 solutions with higher concentration was shorter.After they were soaked in SBF for 6 d, apatite coveredalmost all the surfaces of the titanium metals (Fig. 9).
ARTICLE IN PRESS
10 µm
(A)
(B) (C) 10 µm
10 µm
Fig. 9. SEM photographs of titanium metals soaked in SBF for 6 d after they were anodically oxidized in H2SO4 with concentrations of (A) 0.5m, (B)
1m, (C) 3m at 155V for 1min.
10 20 30 40 50 60
apatiterutile
(A)
(B)
(C)
Inte
nsity
apatiterutile
2θ/degree
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) 0.5m, (B) 1m, (C) 3m at 155V for 1min.
B. Yang et al. / Biomaterials 25 (2004) 1003–10101008
The TF-XRD patterns showed apatite was oriented tothe (0 0 0 2) crystal plane (Fig. 10).In Table 1, the effects the structures of the titania on
the apatite-forming ability were also shown.
4. Discussion
Our results showed the apatite formed on the titaniummetal which was anodically oxidized under the condi-
tion with spark-discharge, it means that anodic oxida-tion is an effective method to prepare bioactive titanium.However, no apatite formed on the titanium anodi-
cally oxidized under the condition without spark-discharge, even though the anatase was also producedon its surface. This might indicate that a certain amountof titanium oxide, in other words, a certain thickness fortitanium oxide, was necessary for the bioactivity of thismaterial, because the titanium metal oxidized under thecondition without spark-discharge could also induceapatite formation after the amount of titanium oxide onits surface was increased by the heat treatment. Itsuggested that a three-dimensional structure of themicro-porous titanium oxide structure might be neces-sary for the apatite formation on the surfaces.The titanium oxide that could induce apatite forma-
tion had the structure of anatase and/or rutile withdifferent ratio. It indicated that anatase and rutile wereall effective for apatite formation.The previous studies of our lab have shown that the
bioactivity of anatase might come from the negativecharge on it surface formed in the SBF solution. Thenegative charge absorbed the Ca ions from the SBF first,then the Ca ions absorbed PO4 ions from the solution toform apatite on the surface [23–26].It is interesting to note the orientation of the rutile to
the (1 0 1) crystal plane on the surface of the titaniumand the orientation of apatite to the (0 0 0 2) crystalplane. Actually, the (0 0 0 2) crystal plane of apatite wasparallel to the (0 0 0 4) crystal plane, so the apatiteorientation to the (0 0 0 2) crystal plane meant it alsooriented 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 Oatoms was arranged on [0 1 0] orientation with 4.6 (A
ARTICLE IN PRESS
O OH
16.38¡ Á10-10 m
9.2¡
Á10-1
0m
16.32¡ Á10-10 m
9.42
¡Á10
-10
m
Hydroxyapatite(0004)Rutile(101)
[010]
[101]
a a
O OH
16.38¡ Á10-10 m
9.2¡
Á10-1
0m
16.32¡ Á10-10 m
9.42
¡Á10
-10
m
Hydroxyapatite(0004)Rutile(101)
[010]
[101]
a a
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).
B. Yang et al. / Biomaterials 25 (2004) 1003–1010 1009
distance, which meant the length for two cells at thisorientation was 9.2 (A; and the O atoms were arrangedon the orientation of ½1 0 %1� with a distance of 5.46 (A, sothe length for three cells was 16.38 (A. On the apatite(0 0 0 4) plane, the OH group was arranged with adistance of 16.32 (A between the long diagonal OH groupand a distance of 9.42 (A between the short diagonal OHgroup (Fig. 11). So the structure of rutile (1 0 1) ismatching to the structure of apatite (0 0 0 4). It isreported that the matching structure could be the nucleifor crystal growth [27]. The apatite-forming ability ofrutile on the anodically oxidized titanium might comefrom the orientation of rutile to the (1 0 1) crystal plane.
5. Conclusion
Anodic oxidation with H2SO4 solution was aneffective way to prepare bioactive titanium metal whichis suitable for the applications under the loading-bearingconditions. A certain amount of titania of anatase and/or rutile structures on the oxidized titanium surfaces wasrequired for the apatite formation.
Acknowledgements
This work was supported by a grant-in-aid forscientific research by the Ministry of Education, Science,Sports and Culture, Japan.
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