Materials Science and Engineering C 54 (2015) 1–7
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
In vitro biocompatibility of Ti–Mg alloys fabricated by direct current magnetron sputtering Junko Hieda ⁎, Mitsuo Niinomi, Masaaki Nakai, Ken Cho Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
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Available online 22 April 2015 Keywords: Ti–Mg alloy Biomedical application Magnetron sputtering Dissolution amount Hard-tissue compatibility
a b s t r a c t Ti–xMg (x = 17, 33, and 55 mass%) alloy films, which cannot be prepared by conventional melting processes owing to the absence of a solid-solution phase in the phase diagram, were prepared by direct current magnetron sputtering in order to investigate their biocompatibility. Ti and Mg films were also prepared by the same process for comparison. The crystal structures were examined by X-ray diffraction (XRD) analysis and the surfaces were analyzed by X-ray photoelectron spectroscopy. The Ti, Ti–xMg alloy, and Mg films were immersed in a 0.9% NaCl solution at 310 K for 7 d to evaluate the dissolution amounts of Ti and Mg. In addition, to evaluate the formation ability of calcium phosphate in vitro, the Ti, Ti–xMg alloy, and Mg films were immersed in Hanks' solution at 310 K for 30 d. Ti and Mg form solid-solution alloys because the peaks attributed to pure Ti and Mg do not appear in the XRD patterns of any of the Ti–xMg alloy films. The surfaces of the Ti–17Mg alloy and Ti–33Mg alloy films contain Ti oxides and MgO, whereas MgO is the main component of the surface oxide of the Ti–55Mg alloy and Mg films. The dissolution amounts of Ti from all films are below or near the detection limit of inductively coupled plasma-optical emission spectroscopy. On the other hand, the Ti–17Mg alloy, Ti–33Mg alloy, Ti–55Mg alloy, and Mg films exhibit Mg dissolution amounts of approximately 2.5, 1.4, 21, and 41 μg/cm2, respectively. The diffraction peaks attributed to calcium phosphate are present in the XRD patterns of the Ti–33Mg alloy, Ti–55Mg alloy, and Mg films after the immersion in Hanks' solution. Spherical calcium phosphate particles precipitate on the surface of the Ti–33Mg film. However, many cracks are observed in the Ti–55Mg film, and delamination of the film occurs after the immersion in Hanks' solution. The Mg film is dissolved in Hanks' solution and calcium phosphate particles precipitate on the glass substrate. Consequently, it is revealed that the Ti–33Mg alloy film evaluated in this study is suitable for biomedical applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Titanium (Ti) and Ti alloys exhibit good mechanical properties, high corrosion resistance, and excellent biocompatibility [1,2], making these metals ideal for structural and biomedical applications. Various metals are alloyed with Ti in order to improve the mechanical properties of Ti or to add desirable properties to Ti. For example magnesium (Mg) is a lightweight structural metal and its alloys are used for structural applications [3]. In addition, Mg is an essential element for life and plays an important role in our bodies. Mg alloys have been studied for biomedical applications because of their degradability [4]. However, the corrosion resistance of Mg alloys is lower than that of other biometallic materials. When Ti and Mg are alloyed, the resulting Ti–Mg alloys are expected to be quite useful as lightweight metals and metallic biomaterials with good mechanical properties and good corrosion resistance. The high corrosion resistance of the Ti–0.2 at.%Mg alloy against fluoride ions has been reported [5]. It has been also reported that the formation ⁎ Corresponding author. E-mail address: [email protected] (J. Hieda).
http://dx.doi.org/10.1016/j.msec.2015.04.029 0928-4931/© 2015 Elsevier B.V. All rights reserved.
ability of calcium phosphate is improved with Mg ion-implanted Ti [6] and that the attachment and spreading of human bone-derived cells are enhanced on Mg ion-implanted alumina [7], which indicates that the presence of Mg on the surface of materials enhances hard-tissue compatibility. However, Ti–Mg alloys with high Mg content cannot be prepared by conventional melting processes owing to the absence of a solid-solution phase in the phase diagram [8]. Therefore, nonequilibrium processes such as sputtering [9,10], electron beam deposition [11,12], and mechanical alloying [13–15] have been employed to fabricate Ti–Mg alloys. The sputtering and electron beam deposition enable fabrication of Ti–Mg alloy films with a wide range of designed Mg content. Although Ti–Mg alloys can make good metallic biomaterials, few studies have focused on their biomedical applications. The purpose of this study is to investigate the possibility of Ti–Mg alloys as metallic biomaterials used for bone prosthesis. For this purpose, the biocompatibility, especially the hard-tissue compatibility, of Ti–Mg alloys was evaluated in vitro in this study. Ti–xMg alloy films with various Mg contents were prepared by direct current (DC) magnetron sputtering. Their crystal structures were examined and the surfaces were analyzed. In addition, the dissolution amounts of Ti and Mg from
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the Ti–Mg alloy films were examined by immersing the films in a 0.9% NaCl solution, and the hard-tissue compatibility was evaluated in vitro by immersion of the films in simulated body fluid (Hanks' solution) in order to investigate the possibilities for biomedical applications. In general, to evaluate the dissolution amounts and the hard-tissue compatibility of biomedical films, Ti and Ti alloys are used as substrates. However, in this study, in order to exclude the influences of Ti from the substrates, Si and glass substrates were used for the dissolution test in 0.9% NaCl solution and the immersion test in Hanks' solution, respectively. 2. Experimental procedures
order to prevent the influence of dissolved Si ions on the precipitation of calcium phosphate, all films were prepared on glass substrates. Hanks' solution was prepared by mixing the following reagents: NaCl, KCl, Na 2 HPO 4·2H2 O, KH2 PO 4, MgSO 4 ·7H2 O, NaHCO3, and CaCl2 in deionized water in order (pH 7.4). The Ti, Ti– xMg (x = 17, 33, and 55 mass%) alloy, and Mg films were immersed in 90 mL of Hanks' solution in PTFE vessels at 310 K for 30 d in the incubator. After immersion, the films were rinsed in deionized water and dried in atmosphere. Changes in the crystal structures of the films were examined using an XRD before and after immersion. The surfaces of the films were observed by scanning electron microscopy (SEM) and precipitates were analyzed by energy dispersive X-ray spectroscopy (EDS).
2.1. Preparation and evaluation of Ti, Ti–xMg, and Mg films 3. Results and discussion Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films were prepared by DC magnetron sputtering. Ti–17Mg, Ti–33Mg, and Ti– 55Mg mass% approximately correspond to Ti–30Mg, Ti–50Mg, and Ti–70Mg at.%. Ti (99.999%) and Mg (99.9%) (Furuuchi Chemical Corporation) targets with diameters of 50.7 mm and thicknesses of 5.1 mm were employed. The distance between the targets and substrates was about 200 mm. The base pressure was approximately 2.0 × 10− 7 Pa. Ar gas was introduced to the chamber at a flow rate of 15 mL/min. The operating pressure was 1.8 × 10 − 1 Pa. The sputtering powers for the preparation of the Ti, Ti–xMg, and Mg films are shown in Table 1. The substrates were rotated at a rotation speed of 5 rpm. The thickness of the obtained films was 300 nm. The Ti, Ti–xMg alloy, and Mg films were prepared on Si(100) wafers with a diameter of 101.6 mm and then cut into 10 × 10 mm samples in order to evaluate crystal structures and analyze the surface chemical states. The compositions of the obtained films were examined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The crystal structures were evaluated using an X-ray diffractometer (XRD) with Cu Kα (λ = 1.54 nm) radiation. The tube voltage and current were 40 kV and 40 mA, respectively. The surface chemical states were evaluated by X-ray photoelectron spectroscopy (XPS). 2.2. Immersion in 0.9% NaCl solution In order to evaluate the dissolution amounts of Ti and Mg from Ti, Ti– xMg (x = 17, 33, and 55 mass%) alloy, and Mg films, the films prepared on Si substrate were immersed in a 0.9% NaCl solution, which was adjusted by the addition of NaCl into deionized water. The size of these films was 25 × 25 mm. The films were cleaned ultrasonically in acetone for 10 min and then immersed in 30 mL of the 0.9% NaCl solution in a polytetrafluoroethylene (PTFE) vessel at 310 K for 7 d in an incubator. After immersion, the films were rinsed with deionized water and dried in atmosphere. The crystal structures of the films after immersion were measured using XRD. The dissolution amounts of Ti and Mg from these films were evaluated by ICP-OES and statistically analyzed by oneway ANOVA and Tukey's test. 2.3. Evaluation of hard-tissue compatibility in vitro The Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films with 10 × 10 mm in size were immersed in Hanks' solution at 310 K for 30 d for the evaluation of hard-tissue compatibility. In Table 1 Sputtering powers for preparation of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films. Metal
Ti Mg
Alloy Ti
Ti–17Mg
Ti–33Mg
Ti–55Mg
Mg
150 W –
172 W 10 W
133 W 18 W
88 W 28 W
– 100 W
The compositions of the obtained films examined by ICP-OES are shown in Table 2. Fig. 1 shows the XRD patterns of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films. The peak at approximately 69.4°, which appears in the XRD patterns of all samples, corresponds to the Si(400) plane of the Si substrate. In the XRD pattern of the Ti film, peaks corresponding to the Ti(002) and (004) planes are present at approximately 38.4 and 82.3°, respectively. The obtained Ti film exhibits an orientation along the (002) plane parallel to the surface of the Ti film. Peaks corresponding to the Ti(100) and (101) planes are seen in the XRD pattern of the Ti–17Mg alloy film, and peaks corresponding to the Ti (100), (002), and (101) planes are seen in the XRD pattern of the Ti–33Mg alloy film. The peak at approximately 39° is attributed to TiO2 in the XRD pattern of the Ti–17Mg alloy film. The XRD pattern of the Ti–55Mg alloy film shows peaks at approximately 36.1 and 76.5°, which correspond to the Mg(002) and (004) planes, respectively. The peaks at approximately 34.7 and 73.0° correspond to the Mg(002) and (004) planes, respectively, in the XRD pattern of the Mg film. The Ti–55Mg alloy and Mg films also exhibit orientation of the (002) plane parallel to the surfaces of the films. From these results, it is supposed that Ti and Mg form solid-solution alloys because the peaks attributed to pure Ti and Mg do not appear in the XRD patterns of any of the Ti–xMg alloy films. In terms of the (002) plane, the peaks corresponding to Ti(002) in the Ti and Ti–33Mg alloy films and to Mg(002) in the Ti–55Mg alloy and Mg films shift to a lower diffraction angle with increasing Mg content because the atomic radius of Mg is larger than that of Ti, which causes expansion of the lattice parameter of the (002) plane. The peak positions and the tendency of orientation are in agreement with the results for films prepared by similar physical vapor deposition processes [9,10]. The Ti 2p XPS spectra of the Ti, Ti–17Mg alloy, Ti–33Mg alloy, Ti– 55Mg alloy, and Mg films prepared on Si substrates are shown in Fig. 2. The Ti 2p XPS spectra of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films show component peaks corresponding to Ti0, Ti2 +, Ti3 +, and Ti4+, which indicates that Ti oxides exist on the surfaces of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films. The ratio of the peak areas of Ti0 increases in comparison to those of Ti2 +, Ti3 +, and Ti4 + with increasing Mg content. Finally, component peaks corresponding to only Ti0 are present in the Ti 2p XPS spectrum of the Ti–55Mg alloy film. There is no contamination of Ti on the surface of the Mg film. Fig. 3 Table 2 Compositions of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films. Alloys
Ti Ti–17Mg Ti–33Mg Ti–55Mg Mg
Element (mass%) Ti
Mg
99.99 82.84 67.18 45.09 0.06
0.01 17.16 32.83 54.91 99.94
J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7 Si (400)
Mg (004)
Mg (004) Ti-55Mg
Mg (002) Ti (002) Ti (101) Ti (100)
Ti-33Mg
Ti (101)
Ti-17Mg
Ti (100)
Ti (004)
Ti (002)
20
Mg
30
40
50
60
70
80
Ti
90
2θ(degree) Fig. 1. XRD patterns of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on Si substrates.
shows the Mg 2p XPS spectra of the Ti, Ti–17Mg alloy, Ti–33Mg alloy, Ti–55Mg alloy, and Mg films prepared on Si substrates. The component peaks corresponding to Mg2+ and Mg0, which appear in the Mg 2p XPS spectrum of the Ti film, are attributed to contamination. The peak areas of Mg2+ are clearly larger than those of Mg0 in the Mg 2p XPS spectra of the Ti–17Mg alloy, Ti–33Mg alloy, and Ti–55Mg alloy films. The ratio of the peak area of Mg0 increases with increasing Mg content. These results indicate that the surface oxide layers of Ti–17Mg alloy and Ti– 33Mg alloy contain Ti oxides and MgO. MgO is a main component of the surface oxide layers on the surfaces of the Ti–55Mg alloy and Mg films. It is supposed that the increase in the Mg content leads to the increase in the peak area of Ti0 because Mg is more easily oxidized than Ti. After immersion in the 0.9% NaCl solution at 310 K for 7 d, no change was observed visually in the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films,
(b)
(c)
Ti3+ 2+ Ti
Ti0
466 464 462 460 458 456 454 452 450 Binding Energy, E/ eV
Intensity (a.u.)
Ti4+
Ti4+
Ti3+ 2+ Ti
Ti0 Ti0
466 464 462 460 458 456 454 452 450
Ti4+ Ti3+ 2+ Ti
466 464 462 460 458 456 454 452 450
Binding Energy, E/ eV
(d)
Binding Energy, E/ eV
(e)
Ti0 Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
(a)
whereas part of the Ti–55Mg alloy film was delaminated, exposing the surface of the underlying Si substrate. In the case of the Mg film, the surface of sample turned black after immersion in the 0.9% NaCl solution. The XRD patterns of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on Si substrates after immersion in the 0.9% NaCl solution are shown in Fig. 4. The diffraction peaks attributed to the Ti(002) and (004) planes in the Ti film, the Ti(100) and (101) planes in the Ti–17Mg alloy film, and the Ti(100), (002), and (101) planes in the Ti–33Mg alloy film, all remained after immersion in the 0.9% NaCl solution. At a diffraction angle of approximately 39°, the peaks attributed to TiO2 are present in the XRD patterns of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films. It is supposed that the Ti, Ti–17Mg alloy, and Ti– 33Mg alloy films do not react with the NaCl solution because of the Ti oxides present on the surfaces of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films as passive layers. The intensity of the diffraction peak of Mg(002) decreases and the diffraction peak of Mg(004) disappears in the XRD pattern of the Ti–55Mg alloy film. A broad peak appears in the range of diffraction angles from 20–30°. The Ti–55Mg alloy film reacts with the NaCl solution and then is easily peeled off from the Si substrate. The corrosion product formed on the surface of Ti–55Mg alloy is not unclear. The diffraction peaks of Mg(002) and (004) completely disappear in the XRD pattern of the Mg film. The peak at approximately 19°, which appears after immersion in the 0.9% NaCl solution, is attributed to Mg(OH)2 [16,17]. This result reveals that the Mg film dissolves in the NaCl solution and Mg(OH)2 is formed on the surface of the Si substrate. The corrosion resistances of the Ti–55Mg alloy and Mg films are lower than those of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films. Fig. 5 shows the dissolution amounts of Ti and Mg after immersion in the 0.9% NaCl solution at 310 K for 7 d. The dissolution amounts of Ti from the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films are below the detection limit of ICP-OES (b 0.014 μg/cm2). The dissolution amount of Ti from the Ti–55Mg alloy film is 0.068 μg/cm 2 , which is near the detection limit of ICP-OES. On the other hand, the dissolution amounts of Mg from the Ti–17Mg alloy, Ti–33Mg alloy,
Intensity (a.u.)
Intensity (a.u.)
Mg (002)
3
466 464 462 460 458 456 454 452 450 Binding Energy, E/ eV
466 464 462 460 458 456 454 452 450 Binding Energy, E/ eV
Fig. 2. Ti 2p XPS spectra of surfaces of (a) Ti, (b) Ti–17Mg alloy, (c) Ti–33Mg alloy, (d) Ti–55Mg alloy, and (e) Mg films prepared on Si substrates.
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J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7
(a)
(b)
(c)
Mg2+ Mg0
Mg2+
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
Mg2+
Mg0
Mg0 58 56 54 52 50 48 46 44 42 40
Binding Energy, E/ eV
Binding Energy, E/ eV
(d)
(e) Intensity (a.u.)
58 56 54 52 50 48 46 44 42 40
Binding Energy, E/ eV
Intensity (a.u.)
58 56 54 52 50 48 46 44 42 40
Mg2+
Mg0
Mg2+
Mg0
58 56 54 52 50 48 46 44 42 40
58 56 54 52 50 48 46 44 42 40
Binding Energy, E/ eV
Binding Energy, E/ eV
Fig. 3. Mg 2p XPS spectra of surfaces of (a) Ti, (b) Ti–17Mg alloy, (c) Ti–33Mg alloy, (d) Ti–55Mg alloy, and (e) Mg films prepared on Si substrates.
Ti–55Mg alloy, and Mg films are approximately 2.5, 1.4, 21, and 41 μg/cm2 , respectively. The amount of Mg in 30 mL of the 0.9% NaCl solution without immersion of the films (blank solution) was below 0.01 μg. The dissolution amounts of Mg are similar in the Ti– 17 Mg alloy and Ti–33Mg alloy films (p N 0.05), and they increase rapidly in the Ti–55Mg alloy and Mg films (p b 0.05). The surface oxide layers consisting of Ti oxides and MgO, which are present on the surface of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films, prevent the dissolution of Ti and Mg from these films. Fig. 6 shows XRD patterns of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on glass substrates before and after immersion in Hanks' solution at 310 K for 30 d. The broad peak at 20–40°, which appears in all samples, is attributed to the glass substrate. The diffraction peaks corresponding to the Ti(002) and (101) planes appear in the XRD pattern of the Ti film. The peak attributed to
TiO2 is also seen at approximately 39°. The Ti–17Mg alloy and Ti– 33Mg alloy films show the diffraction peaks corresponding to the Ti(100) and (101) planes. The diffraction peak of the Mg(002) plane is present in the XRD patterns of the Ti–55Mg alloy and Mg films. These results indicate that the crystal orientations of these films are similar as those of the films prepared on the Si substrate. There is no change in the diffraction peaks attributed to Ti in the XRD patterns of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films before or after the immersion in Hanks' solution. The diffraction peak corresponding to Mg(002), which is observed in the XRD patterns of the Ti–55Mg alloy and Mg films before immersion, disappears after immersion in Hanks' solution. In the XRD patterns of the Ti–33Mg alloy, Ti–55Mg alloy, and Mg films, the diffraction peaks attributed to calcium phosphate are present. SEM images of the surfaces of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films after immersion in Hanks' solution are shown in 5
5x10
Intensity (a.u.)
2
Dissolution amount ( μg/m )
Si (400)
Mg(OH)2
Mg Mg (002)
Ti-55Mg
Ti (002) Ti (100)
Ti (101)
Ti-33Mg
Ti (100)
Ti (101)
Ti-17Mg
Ti (002)
20
30
Ti (004)
40
50
60
70
80
Ti
90
2θ (degree) Fig. 4. XRD patterns of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on Si substrates after immersion in 0.9% NaCl solution at 310 K for 7 d.
5
4x10
Ti Mg
5
3x10
5
2x10
5
1x10
0 100 90 80 70 60 50 40 30 20 10 0 Ti content (mass%)
Fig. 5. Dissolution amounts of Ti and Mg in 0.9% NaCl aqueous solution from Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films after immersion in 0.9% NaCl solution at 310 K for 7 d.
J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7
(a) Intensity(a.u.)
Mg (002)
Mg Mg (002)
Ti-55Mg Ti (100)
Ti (101)
Ti-33Mg
Ti (100)
Ti-17Mg
Ti (101) Ti (002)
Ti
Ti (101)
20
30
40
50
60
70
2θ (degrees)
(b)
(002)
Calcium phosphate
(112)
(004)
Intensity (a.u.)
(213)
Mg Ti-55Mg Ti (100) Ti (101)
Ti-33Mg
Ti (101)
Ti-17Mg
Ti (100) Ti (002) Ti (101)
20
30
40
Ti
50
60
70
2θ (degrees) Fig. 6. XRD patterns of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on glass substrates (a) before and (b) after immersion in Hanks' solution at 310 K for 30 d.
Fig. 7. No precipitates were observed on the surfaces of the Ti and Ti– 17Mg alloy films, which is consistent with the XRD patterns shown in Fig. 6. However, many precipitates are observed on the surface of the
(a)
5
Ti–33Mg alloy film, and the Ti–55Mg alloy film has many cracks and large areas are peeled off from the glass substrate. Small precipitates are observed on the surface of the Ti–55Mg alloy film and the underlying glass substrate. In the case of the Mg film, the precipitates cover the surface of the sample. However, the color of the sample is transparent, which suggests that there is no Mg film on the glass substrate. The Mg film reacts with the Hanks' solution in an early stage and calcium phosphate subsequently precipitates and covers the glass substrate. Fig. 8 shows SEM image and EDS elemental maps of the precipitates formed on the surface of the Ti–33Mg alloy film at high magnification. Many spherical precipitates exist on the Ti–33Mg alloy film. The EDS elemental maps of Ca and P reveal that many spherical precipitates contain Ca and P. Thus these precipitates are calcium phosphate. These results indicate that Mg in the Ti–xMg alloy films promotes the formation of calcium phosphate and that improvement of the formation ability of calcium phosphate depends on the Mg content in Ti–xMg alloy films. However, the corrosion resistance of the Ti–xMg alloy films decreases with increasing Mg content above 55 mass%. The optimum Mg content for biomedical applications among the Ti–xMg alloy films fabricated in this study, which is based on the balance between corrosion resistance and improved formation ability of calcium phosphate, is that of the Ti–33Mg alloy film. Mg contained in the surface of the Ti–33Mg alloy films accelerates the formation of calcium phosphate. It is supposed that the surface of the Ti–33Mg alloy film, which contains Ti, Mg, and Ti oxides, reacts with H2O and Ti(OH)4, Mg(OH)2, TiO2, and MgO are formed on the surface of the film after immersion in Hanks' solution. It is noted that pH value increases when Mg dissolves in aqueous solution [6,16]. The pH value near the surface of the Ti–33Mg alloy film increases simultaneously, which leads to the formation of calcium phosphate. 4. Conclusions The biocompatibility of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films fabricated by DC magnetron sputtering was investigated in this study. XRD analysis reveals that Ti and Mg form solid-solution alloys because the peaks attributed to pure Ti and Mg do not appear in the XRD patterns of any of the Ti–xMg alloy films. XPS analysis reveals that the surfaces of the Ti–17Mg alloy and Ti–33Mg alloy films contain
(b)
(c)
10 μm
10 μm (d)
10 μm
(e)
10 μm
10 μm
Fig. 7. SEM images of surfaces of (a) Ti, (b) Ti–17Mg alloy, (c) Ti–33Mg alloy, (d) Ti–55Mg alloy, and (e) Mg films prepared on glass substrates after immersion in Hanks' solution at 310 K for 30 d.
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J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7
(a)
10 μm (b)
(c)
(d)
(e)
Fig. 8. (a) SEM image of precipitates formed on surface of Ti–33Mg alloy film prepared on glass substrate after immersion in Hanks' solution at 310 K for 30 d and EDS elemental maps of (b) Ti, (c) Mg, (d) Ca, and (e) P.
Ti oxides and MgO, whereas MgO is the main component of the surface oxide of the Ti–55Mg alloy and Mg films. The dissolution amounts of Ti from all films are below or near the detection limit of ICP-OES. The dissolution amounts of Mg from the Ti–17Mg alloy, Ti–33Mg alloy, Ti–55Mg alloy, and Mg films are approximately 2.5, 1.4, 21, and 41 μg/cm2, respectively. Calcium phosphate is precipitated on the surfaces of the Ti–33Mg alloy and Ti–55Mg alloy films. However, the Ti–55Mg alloy film peels off from a large area of the substrate. Consequently, the Ti–33Mg alloy film exhibits high corrosion resistance and good biocompatibility and is thus the most suitable for biomedical applications. Acknowledgment This study was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Grant Numbers 24246111, 24656401, and 25820348). This study was also funded by the Tohoku Leading Women's JUMP UP Project, Tohoku University; the inter-university cooperative research program “Innovative Research for Biosis-Abiosis Intelligent Interface,” by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan; a grant from Tokuyama Science Foundation; and the Cooperative Research and
Development Center for Advanced Materials, especially the support from Mr. K. Saito and Ms. T. Sugiyama for preparation of the Ti–Mg alloy films, Institute for Materials Research, Tohoku University. References [1] H.J. Rack, J.I. Qazi, Titanium alloys for biomedical applications, Mater. Sci. Eng. C 26 (2006) 1269–1277. [2] M. Niinomi, Mechanical biocompatibilities of titanium alloys for biomedical applications, J. Mech. Behav. Biomed. Mater. 1 (2008) 30–42. [3] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys—a critical review, J. Alloys Compd. 336 (2002) 88–113. [4] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: a review, Biomaterials 27 (2006) 1728–1734. [5] T. Haruna, D. Motoya, Y. Nakagawa, N. Yamashita, T. Oishi, Corrosion resistance of titanium–magnesium alloy in weak acid solution containing fluoride ions, Mater. Trans. 54 (2013) 143–148. [6] Y.Z. Wan, Y. Huang, F. He, Y.L. Wang, Z.G. Zhao, H.F. Ding, Effect of Mg ion implantation on calcium phosphate formation on titanium, Surf. Coat. Technol. 201 (2006) 2904–2909. [7] C.R. Howlett, H. Zreiqat, R. O'Dell, J. Noorman, P. Evans, The effect of magnesium ion implantation into alumina upon the adhesion of human bone derived cells, J. Mater. Sci.: Mater. Med. 5 (1994) 715–722. [8] J.L. Murray, Phase Diagrams of Binary Titanium Alloys, ASM International, Metals Park, Ohio, 1986. 156. [9] G.L. Song, D. Haddad, The topography of magnetron sputter-deposited Mg–Ti alloy thin films, Mater. Chem. Phys. 125 (2011) 548–552.
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Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
In vitro biocompatibility of Ti–Mg alloys fabricated by direct current magnetron sputtering Junko Hieda ⁎, Mitsuo Niinomi, Masaaki Nakai, Ken Cho Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e
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Available online 22 April 2015 Keywords: Ti–Mg alloy Biomedical application Magnetron sputtering Dissolution amount Hard-tissue compatibility
a b s t r a c t Ti–xMg (x = 17, 33, and 55 mass%) alloy films, which cannot be prepared by conventional melting processes owing to the absence of a solid-solution phase in the phase diagram, were prepared by direct current magnetron sputtering in order to investigate their biocompatibility. Ti and Mg films were also prepared by the same process for comparison. The crystal structures were examined by X-ray diffraction (XRD) analysis and the surfaces were analyzed by X-ray photoelectron spectroscopy. The Ti, Ti–xMg alloy, and Mg films were immersed in a 0.9% NaCl solution at 310 K for 7 d to evaluate the dissolution amounts of Ti and Mg. In addition, to evaluate the formation ability of calcium phosphate in vitro, the Ti, Ti–xMg alloy, and Mg films were immersed in Hanks' solution at 310 K for 30 d. Ti and Mg form solid-solution alloys because the peaks attributed to pure Ti and Mg do not appear in the XRD patterns of any of the Ti–xMg alloy films. The surfaces of the Ti–17Mg alloy and Ti–33Mg alloy films contain Ti oxides and MgO, whereas MgO is the main component of the surface oxide of the Ti–55Mg alloy and Mg films. The dissolution amounts of Ti from all films are below or near the detection limit of inductively coupled plasma-optical emission spectroscopy. On the other hand, the Ti–17Mg alloy, Ti–33Mg alloy, Ti–55Mg alloy, and Mg films exhibit Mg dissolution amounts of approximately 2.5, 1.4, 21, and 41 μg/cm2, respectively. The diffraction peaks attributed to calcium phosphate are present in the XRD patterns of the Ti–33Mg alloy, Ti–55Mg alloy, and Mg films after the immersion in Hanks' solution. Spherical calcium phosphate particles precipitate on the surface of the Ti–33Mg film. However, many cracks are observed in the Ti–55Mg film, and delamination of the film occurs after the immersion in Hanks' solution. The Mg film is dissolved in Hanks' solution and calcium phosphate particles precipitate on the glass substrate. Consequently, it is revealed that the Ti–33Mg alloy film evaluated in this study is suitable for biomedical applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Titanium (Ti) and Ti alloys exhibit good mechanical properties, high corrosion resistance, and excellent biocompatibility [1,2], making these metals ideal for structural and biomedical applications. Various metals are alloyed with Ti in order to improve the mechanical properties of Ti or to add desirable properties to Ti. For example magnesium (Mg) is a lightweight structural metal and its alloys are used for structural applications [3]. In addition, Mg is an essential element for life and plays an important role in our bodies. Mg alloys have been studied for biomedical applications because of their degradability [4]. However, the corrosion resistance of Mg alloys is lower than that of other biometallic materials. When Ti and Mg are alloyed, the resulting Ti–Mg alloys are expected to be quite useful as lightweight metals and metallic biomaterials with good mechanical properties and good corrosion resistance. The high corrosion resistance of the Ti–0.2 at.%Mg alloy against fluoride ions has been reported [5]. It has been also reported that the formation ⁎ Corresponding author. E-mail address: [email protected] (J. Hieda).
http://dx.doi.org/10.1016/j.msec.2015.04.029 0928-4931/© 2015 Elsevier B.V. All rights reserved.
ability of calcium phosphate is improved with Mg ion-implanted Ti [6] and that the attachment and spreading of human bone-derived cells are enhanced on Mg ion-implanted alumina [7], which indicates that the presence of Mg on the surface of materials enhances hard-tissue compatibility. However, Ti–Mg alloys with high Mg content cannot be prepared by conventional melting processes owing to the absence of a solid-solution phase in the phase diagram [8]. Therefore, nonequilibrium processes such as sputtering [9,10], electron beam deposition [11,12], and mechanical alloying [13–15] have been employed to fabricate Ti–Mg alloys. The sputtering and electron beam deposition enable fabrication of Ti–Mg alloy films with a wide range of designed Mg content. Although Ti–Mg alloys can make good metallic biomaterials, few studies have focused on their biomedical applications. The purpose of this study is to investigate the possibility of Ti–Mg alloys as metallic biomaterials used for bone prosthesis. For this purpose, the biocompatibility, especially the hard-tissue compatibility, of Ti–Mg alloys was evaluated in vitro in this study. Ti–xMg alloy films with various Mg contents were prepared by direct current (DC) magnetron sputtering. Their crystal structures were examined and the surfaces were analyzed. In addition, the dissolution amounts of Ti and Mg from
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the Ti–Mg alloy films were examined by immersing the films in a 0.9% NaCl solution, and the hard-tissue compatibility was evaluated in vitro by immersion of the films in simulated body fluid (Hanks' solution) in order to investigate the possibilities for biomedical applications. In general, to evaluate the dissolution amounts and the hard-tissue compatibility of biomedical films, Ti and Ti alloys are used as substrates. However, in this study, in order to exclude the influences of Ti from the substrates, Si and glass substrates were used for the dissolution test in 0.9% NaCl solution and the immersion test in Hanks' solution, respectively. 2. Experimental procedures
order to prevent the influence of dissolved Si ions on the precipitation of calcium phosphate, all films were prepared on glass substrates. Hanks' solution was prepared by mixing the following reagents: NaCl, KCl, Na 2 HPO 4·2H2 O, KH2 PO 4, MgSO 4 ·7H2 O, NaHCO3, and CaCl2 in deionized water in order (pH 7.4). The Ti, Ti– xMg (x = 17, 33, and 55 mass%) alloy, and Mg films were immersed in 90 mL of Hanks' solution in PTFE vessels at 310 K for 30 d in the incubator. After immersion, the films were rinsed in deionized water and dried in atmosphere. Changes in the crystal structures of the films were examined using an XRD before and after immersion. The surfaces of the films were observed by scanning electron microscopy (SEM) and precipitates were analyzed by energy dispersive X-ray spectroscopy (EDS).
2.1. Preparation and evaluation of Ti, Ti–xMg, and Mg films 3. Results and discussion Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films were prepared by DC magnetron sputtering. Ti–17Mg, Ti–33Mg, and Ti– 55Mg mass% approximately correspond to Ti–30Mg, Ti–50Mg, and Ti–70Mg at.%. Ti (99.999%) and Mg (99.9%) (Furuuchi Chemical Corporation) targets with diameters of 50.7 mm and thicknesses of 5.1 mm were employed. The distance between the targets and substrates was about 200 mm. The base pressure was approximately 2.0 × 10− 7 Pa. Ar gas was introduced to the chamber at a flow rate of 15 mL/min. The operating pressure was 1.8 × 10 − 1 Pa. The sputtering powers for the preparation of the Ti, Ti–xMg, and Mg films are shown in Table 1. The substrates were rotated at a rotation speed of 5 rpm. The thickness of the obtained films was 300 nm. The Ti, Ti–xMg alloy, and Mg films were prepared on Si(100) wafers with a diameter of 101.6 mm and then cut into 10 × 10 mm samples in order to evaluate crystal structures and analyze the surface chemical states. The compositions of the obtained films were examined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The crystal structures were evaluated using an X-ray diffractometer (XRD) with Cu Kα (λ = 1.54 nm) radiation. The tube voltage and current were 40 kV and 40 mA, respectively. The surface chemical states were evaluated by X-ray photoelectron spectroscopy (XPS). 2.2. Immersion in 0.9% NaCl solution In order to evaluate the dissolution amounts of Ti and Mg from Ti, Ti– xMg (x = 17, 33, and 55 mass%) alloy, and Mg films, the films prepared on Si substrate were immersed in a 0.9% NaCl solution, which was adjusted by the addition of NaCl into deionized water. The size of these films was 25 × 25 mm. The films were cleaned ultrasonically in acetone for 10 min and then immersed in 30 mL of the 0.9% NaCl solution in a polytetrafluoroethylene (PTFE) vessel at 310 K for 7 d in an incubator. After immersion, the films were rinsed with deionized water and dried in atmosphere. The crystal structures of the films after immersion were measured using XRD. The dissolution amounts of Ti and Mg from these films were evaluated by ICP-OES and statistically analyzed by oneway ANOVA and Tukey's test. 2.3. Evaluation of hard-tissue compatibility in vitro The Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films with 10 × 10 mm in size were immersed in Hanks' solution at 310 K for 30 d for the evaluation of hard-tissue compatibility. In Table 1 Sputtering powers for preparation of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films. Metal
Ti Mg
Alloy Ti
Ti–17Mg
Ti–33Mg
Ti–55Mg
Mg
150 W –
172 W 10 W
133 W 18 W
88 W 28 W
– 100 W
The compositions of the obtained films examined by ICP-OES are shown in Table 2. Fig. 1 shows the XRD patterns of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films. The peak at approximately 69.4°, which appears in the XRD patterns of all samples, corresponds to the Si(400) plane of the Si substrate. In the XRD pattern of the Ti film, peaks corresponding to the Ti(002) and (004) planes are present at approximately 38.4 and 82.3°, respectively. The obtained Ti film exhibits an orientation along the (002) plane parallel to the surface of the Ti film. Peaks corresponding to the Ti(100) and (101) planes are seen in the XRD pattern of the Ti–17Mg alloy film, and peaks corresponding to the Ti (100), (002), and (101) planes are seen in the XRD pattern of the Ti–33Mg alloy film. The peak at approximately 39° is attributed to TiO2 in the XRD pattern of the Ti–17Mg alloy film. The XRD pattern of the Ti–55Mg alloy film shows peaks at approximately 36.1 and 76.5°, which correspond to the Mg(002) and (004) planes, respectively. The peaks at approximately 34.7 and 73.0° correspond to the Mg(002) and (004) planes, respectively, in the XRD pattern of the Mg film. The Ti–55Mg alloy and Mg films also exhibit orientation of the (002) plane parallel to the surfaces of the films. From these results, it is supposed that Ti and Mg form solid-solution alloys because the peaks attributed to pure Ti and Mg do not appear in the XRD patterns of any of the Ti–xMg alloy films. In terms of the (002) plane, the peaks corresponding to Ti(002) in the Ti and Ti–33Mg alloy films and to Mg(002) in the Ti–55Mg alloy and Mg films shift to a lower diffraction angle with increasing Mg content because the atomic radius of Mg is larger than that of Ti, which causes expansion of the lattice parameter of the (002) plane. The peak positions and the tendency of orientation are in agreement with the results for films prepared by similar physical vapor deposition processes [9,10]. The Ti 2p XPS spectra of the Ti, Ti–17Mg alloy, Ti–33Mg alloy, Ti– 55Mg alloy, and Mg films prepared on Si substrates are shown in Fig. 2. The Ti 2p XPS spectra of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films show component peaks corresponding to Ti0, Ti2 +, Ti3 +, and Ti4+, which indicates that Ti oxides exist on the surfaces of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films. The ratio of the peak areas of Ti0 increases in comparison to those of Ti2 +, Ti3 +, and Ti4 + with increasing Mg content. Finally, component peaks corresponding to only Ti0 are present in the Ti 2p XPS spectrum of the Ti–55Mg alloy film. There is no contamination of Ti on the surface of the Mg film. Fig. 3 Table 2 Compositions of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films. Alloys
Ti Ti–17Mg Ti–33Mg Ti–55Mg Mg
Element (mass%) Ti
Mg
99.99 82.84 67.18 45.09 0.06
0.01 17.16 32.83 54.91 99.94
J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7 Si (400)
Mg (004)
Mg (004) Ti-55Mg
Mg (002) Ti (002) Ti (101) Ti (100)
Ti-33Mg
Ti (101)
Ti-17Mg
Ti (100)
Ti (004)
Ti (002)
20
Mg
30
40
50
60
70
80
Ti
90
2θ(degree) Fig. 1. XRD patterns of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on Si substrates.
shows the Mg 2p XPS spectra of the Ti, Ti–17Mg alloy, Ti–33Mg alloy, Ti–55Mg alloy, and Mg films prepared on Si substrates. The component peaks corresponding to Mg2+ and Mg0, which appear in the Mg 2p XPS spectrum of the Ti film, are attributed to contamination. The peak areas of Mg2+ are clearly larger than those of Mg0 in the Mg 2p XPS spectra of the Ti–17Mg alloy, Ti–33Mg alloy, and Ti–55Mg alloy films. The ratio of the peak area of Mg0 increases with increasing Mg content. These results indicate that the surface oxide layers of Ti–17Mg alloy and Ti– 33Mg alloy contain Ti oxides and MgO. MgO is a main component of the surface oxide layers on the surfaces of the Ti–55Mg alloy and Mg films. It is supposed that the increase in the Mg content leads to the increase in the peak area of Ti0 because Mg is more easily oxidized than Ti. After immersion in the 0.9% NaCl solution at 310 K for 7 d, no change was observed visually in the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films,
(b)
(c)
Ti3+ 2+ Ti
Ti0
466 464 462 460 458 456 454 452 450 Binding Energy, E/ eV
Intensity (a.u.)
Ti4+
Ti4+
Ti3+ 2+ Ti
Ti0 Ti0
466 464 462 460 458 456 454 452 450
Ti4+ Ti3+ 2+ Ti
466 464 462 460 458 456 454 452 450
Binding Energy, E/ eV
(d)
Binding Energy, E/ eV
(e)
Ti0 Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
(a)
whereas part of the Ti–55Mg alloy film was delaminated, exposing the surface of the underlying Si substrate. In the case of the Mg film, the surface of sample turned black after immersion in the 0.9% NaCl solution. The XRD patterns of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on Si substrates after immersion in the 0.9% NaCl solution are shown in Fig. 4. The diffraction peaks attributed to the Ti(002) and (004) planes in the Ti film, the Ti(100) and (101) planes in the Ti–17Mg alloy film, and the Ti(100), (002), and (101) planes in the Ti–33Mg alloy film, all remained after immersion in the 0.9% NaCl solution. At a diffraction angle of approximately 39°, the peaks attributed to TiO2 are present in the XRD patterns of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films. It is supposed that the Ti, Ti–17Mg alloy, and Ti– 33Mg alloy films do not react with the NaCl solution because of the Ti oxides present on the surfaces of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films as passive layers. The intensity of the diffraction peak of Mg(002) decreases and the diffraction peak of Mg(004) disappears in the XRD pattern of the Ti–55Mg alloy film. A broad peak appears in the range of diffraction angles from 20–30°. The Ti–55Mg alloy film reacts with the NaCl solution and then is easily peeled off from the Si substrate. The corrosion product formed on the surface of Ti–55Mg alloy is not unclear. The diffraction peaks of Mg(002) and (004) completely disappear in the XRD pattern of the Mg film. The peak at approximately 19°, which appears after immersion in the 0.9% NaCl solution, is attributed to Mg(OH)2 [16,17]. This result reveals that the Mg film dissolves in the NaCl solution and Mg(OH)2 is formed on the surface of the Si substrate. The corrosion resistances of the Ti–55Mg alloy and Mg films are lower than those of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films. Fig. 5 shows the dissolution amounts of Ti and Mg after immersion in the 0.9% NaCl solution at 310 K for 7 d. The dissolution amounts of Ti from the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films are below the detection limit of ICP-OES (b 0.014 μg/cm2). The dissolution amount of Ti from the Ti–55Mg alloy film is 0.068 μg/cm 2 , which is near the detection limit of ICP-OES. On the other hand, the dissolution amounts of Mg from the Ti–17Mg alloy, Ti–33Mg alloy,
Intensity (a.u.)
Intensity (a.u.)
Mg (002)
3
466 464 462 460 458 456 454 452 450 Binding Energy, E/ eV
466 464 462 460 458 456 454 452 450 Binding Energy, E/ eV
Fig. 2. Ti 2p XPS spectra of surfaces of (a) Ti, (b) Ti–17Mg alloy, (c) Ti–33Mg alloy, (d) Ti–55Mg alloy, and (e) Mg films prepared on Si substrates.
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J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7
(a)
(b)
(c)
Mg2+ Mg0
Mg2+
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
Mg2+
Mg0
Mg0 58 56 54 52 50 48 46 44 42 40
Binding Energy, E/ eV
Binding Energy, E/ eV
(d)
(e) Intensity (a.u.)
58 56 54 52 50 48 46 44 42 40
Binding Energy, E/ eV
Intensity (a.u.)
58 56 54 52 50 48 46 44 42 40
Mg2+
Mg0
Mg2+
Mg0
58 56 54 52 50 48 46 44 42 40
58 56 54 52 50 48 46 44 42 40
Binding Energy, E/ eV
Binding Energy, E/ eV
Fig. 3. Mg 2p XPS spectra of surfaces of (a) Ti, (b) Ti–17Mg alloy, (c) Ti–33Mg alloy, (d) Ti–55Mg alloy, and (e) Mg films prepared on Si substrates.
Ti–55Mg alloy, and Mg films are approximately 2.5, 1.4, 21, and 41 μg/cm2 , respectively. The amount of Mg in 30 mL of the 0.9% NaCl solution without immersion of the films (blank solution) was below 0.01 μg. The dissolution amounts of Mg are similar in the Ti– 17 Mg alloy and Ti–33Mg alloy films (p N 0.05), and they increase rapidly in the Ti–55Mg alloy and Mg films (p b 0.05). The surface oxide layers consisting of Ti oxides and MgO, which are present on the surface of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films, prevent the dissolution of Ti and Mg from these films. Fig. 6 shows XRD patterns of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on glass substrates before and after immersion in Hanks' solution at 310 K for 30 d. The broad peak at 20–40°, which appears in all samples, is attributed to the glass substrate. The diffraction peaks corresponding to the Ti(002) and (101) planes appear in the XRD pattern of the Ti film. The peak attributed to
TiO2 is also seen at approximately 39°. The Ti–17Mg alloy and Ti– 33Mg alloy films show the diffraction peaks corresponding to the Ti(100) and (101) planes. The diffraction peak of the Mg(002) plane is present in the XRD patterns of the Ti–55Mg alloy and Mg films. These results indicate that the crystal orientations of these films are similar as those of the films prepared on the Si substrate. There is no change in the diffraction peaks attributed to Ti in the XRD patterns of the Ti, Ti–17Mg alloy, and Ti–33Mg alloy films before or after the immersion in Hanks' solution. The diffraction peak corresponding to Mg(002), which is observed in the XRD patterns of the Ti–55Mg alloy and Mg films before immersion, disappears after immersion in Hanks' solution. In the XRD patterns of the Ti–33Mg alloy, Ti–55Mg alloy, and Mg films, the diffraction peaks attributed to calcium phosphate are present. SEM images of the surfaces of the Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films after immersion in Hanks' solution are shown in 5
5x10
Intensity (a.u.)
2
Dissolution amount ( μg/m )
Si (400)
Mg(OH)2
Mg Mg (002)
Ti-55Mg
Ti (002) Ti (100)
Ti (101)
Ti-33Mg
Ti (100)
Ti (101)
Ti-17Mg
Ti (002)
20
30
Ti (004)
40
50
60
70
80
Ti
90
2θ (degree) Fig. 4. XRD patterns of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on Si substrates after immersion in 0.9% NaCl solution at 310 K for 7 d.
5
4x10
Ti Mg
5
3x10
5
2x10
5
1x10
0 100 90 80 70 60 50 40 30 20 10 0 Ti content (mass%)
Fig. 5. Dissolution amounts of Ti and Mg in 0.9% NaCl aqueous solution from Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films after immersion in 0.9% NaCl solution at 310 K for 7 d.
J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7
(a) Intensity(a.u.)
Mg (002)
Mg Mg (002)
Ti-55Mg Ti (100)
Ti (101)
Ti-33Mg
Ti (100)
Ti-17Mg
Ti (101) Ti (002)
Ti
Ti (101)
20
30
40
50
60
70
2θ (degrees)
(b)
(002)
Calcium phosphate
(112)
(004)
Intensity (a.u.)
(213)
Mg Ti-55Mg Ti (100) Ti (101)
Ti-33Mg
Ti (101)
Ti-17Mg
Ti (100) Ti (002) Ti (101)
20
30
40
Ti
50
60
70
2θ (degrees) Fig. 6. XRD patterns of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films prepared on glass substrates (a) before and (b) after immersion in Hanks' solution at 310 K for 30 d.
Fig. 7. No precipitates were observed on the surfaces of the Ti and Ti– 17Mg alloy films, which is consistent with the XRD patterns shown in Fig. 6. However, many precipitates are observed on the surface of the
(a)
5
Ti–33Mg alloy film, and the Ti–55Mg alloy film has many cracks and large areas are peeled off from the glass substrate. Small precipitates are observed on the surface of the Ti–55Mg alloy film and the underlying glass substrate. In the case of the Mg film, the precipitates cover the surface of the sample. However, the color of the sample is transparent, which suggests that there is no Mg film on the glass substrate. The Mg film reacts with the Hanks' solution in an early stage and calcium phosphate subsequently precipitates and covers the glass substrate. Fig. 8 shows SEM image and EDS elemental maps of the precipitates formed on the surface of the Ti–33Mg alloy film at high magnification. Many spherical precipitates exist on the Ti–33Mg alloy film. The EDS elemental maps of Ca and P reveal that many spherical precipitates contain Ca and P. Thus these precipitates are calcium phosphate. These results indicate that Mg in the Ti–xMg alloy films promotes the formation of calcium phosphate and that improvement of the formation ability of calcium phosphate depends on the Mg content in Ti–xMg alloy films. However, the corrosion resistance of the Ti–xMg alloy films decreases with increasing Mg content above 55 mass%. The optimum Mg content for biomedical applications among the Ti–xMg alloy films fabricated in this study, which is based on the balance between corrosion resistance and improved formation ability of calcium phosphate, is that of the Ti–33Mg alloy film. Mg contained in the surface of the Ti–33Mg alloy films accelerates the formation of calcium phosphate. It is supposed that the surface of the Ti–33Mg alloy film, which contains Ti, Mg, and Ti oxides, reacts with H2O and Ti(OH)4, Mg(OH)2, TiO2, and MgO are formed on the surface of the film after immersion in Hanks' solution. It is noted that pH value increases when Mg dissolves in aqueous solution [6,16]. The pH value near the surface of the Ti–33Mg alloy film increases simultaneously, which leads to the formation of calcium phosphate. 4. Conclusions The biocompatibility of Ti, Ti–xMg (x = 17, 33, and 55 mass%) alloy, and Mg films fabricated by DC magnetron sputtering was investigated in this study. XRD analysis reveals that Ti and Mg form solid-solution alloys because the peaks attributed to pure Ti and Mg do not appear in the XRD patterns of any of the Ti–xMg alloy films. XPS analysis reveals that the surfaces of the Ti–17Mg alloy and Ti–33Mg alloy films contain
(b)
(c)
10 μm
10 μm (d)
10 μm
(e)
10 μm
10 μm
Fig. 7. SEM images of surfaces of (a) Ti, (b) Ti–17Mg alloy, (c) Ti–33Mg alloy, (d) Ti–55Mg alloy, and (e) Mg films prepared on glass substrates after immersion in Hanks' solution at 310 K for 30 d.
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J. Hieda et al. / Materials Science and Engineering C 54 (2015) 1–7
(a)
10 μm (b)
(c)
(d)
(e)
Fig. 8. (a) SEM image of precipitates formed on surface of Ti–33Mg alloy film prepared on glass substrate after immersion in Hanks' solution at 310 K for 30 d and EDS elemental maps of (b) Ti, (c) Mg, (d) Ca, and (e) P.
Ti oxides and MgO, whereas MgO is the main component of the surface oxide of the Ti–55Mg alloy and Mg films. The dissolution amounts of Ti from all films are below or near the detection limit of ICP-OES. The dissolution amounts of Mg from the Ti–17Mg alloy, Ti–33Mg alloy, Ti–55Mg alloy, and Mg films are approximately 2.5, 1.4, 21, and 41 μg/cm2, respectively. Calcium phosphate is precipitated on the surfaces of the Ti–33Mg alloy and Ti–55Mg alloy films. However, the Ti–55Mg alloy film peels off from a large area of the substrate. Consequently, the Ti–33Mg alloy film exhibits high corrosion resistance and good biocompatibility and is thus the most suitable for biomedical applications. Acknowledgment This study was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Grant Numbers 24246111, 24656401, and 25820348). This study was also funded by the Tohoku Leading Women's JUMP UP Project, Tohoku University; the inter-university cooperative research program “Innovative Research for Biosis-Abiosis Intelligent Interface,” by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan; a grant from Tokuyama Science Foundation; and the Cooperative Research and
Development Center for Advanced Materials, especially the support from Mr. K. Saito and Ms. T. Sugiyama for preparation of the Ti–Mg alloy films, Institute for Materials Research, Tohoku University. References [1] H.J. Rack, J.I. Qazi, Titanium alloys for biomedical applications, Mater. Sci. Eng. C 26 (2006) 1269–1277. [2] M. Niinomi, Mechanical biocompatibilities of titanium alloys for biomedical applications, J. Mech. Behav. Biomed. Mater. 1 (2008) 30–42. [3] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys—a critical review, J. Alloys Compd. 336 (2002) 88–113. [4] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: a review, Biomaterials 27 (2006) 1728–1734. [5] T. Haruna, D. Motoya, Y. Nakagawa, N. Yamashita, T. Oishi, Corrosion resistance of titanium–magnesium alloy in weak acid solution containing fluoride ions, Mater. Trans. 54 (2013) 143–148. [6] Y.Z. Wan, Y. Huang, F. He, Y.L. Wang, Z.G. Zhao, H.F. Ding, Effect of Mg ion implantation on calcium phosphate formation on titanium, Surf. Coat. Technol. 201 (2006) 2904–2909. [7] C.R. Howlett, H. Zreiqat, R. O'Dell, J. Noorman, P. Evans, The effect of magnesium ion implantation into alumina upon the adhesion of human bone derived cells, J. Mater. Sci.: Mater. Med. 5 (1994) 715–722. [8] J.L. Murray, Phase Diagrams of Binary Titanium Alloys, ASM International, Metals Park, Ohio, 1986. 156. [9] G.L. Song, D. Haddad, The topography of magnetron sputter-deposited Mg–Ti alloy thin films, Mater. Chem. Phys. 125 (2011) 548–552.
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