Bio-Medical Materials and Engineering 24 (2014) 1793–1802 DOI 10.3233/BME-140990 IOS Press

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Fabrication of hydroxyapatite thin films on zirconia using a sputtering technique K. Ozeki a,∗ , T. Goto b , H. Aoki c and T. Masuzawa a a

Department of Mechanical Engineering, Ibaraki University, Ibaraki, Japan Department of Biosciences, Division of Anatomy, Kyushu Dental College, Kitakyushu, Japan c International Apatite Institute Co., Ltd, Tokyo, Japan b

Received 17 January 2014 Accepted 11 March 2014 Abstract. Hydroxyapatite (HA) thin films were prepared on a zirconia (ZrO2 ) substrate using a sputtering technique, and the film was also coated on a titanium (Ti) substrate for comparison. The coated films were recrystallised using a hydrothermal treatment to reduce film dissolution. The films were then characterised by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The osteocompatiblity of the films was evaluated by investigating the alkaline phosphatase (ALP) activity and the size of the bone formation area of osteoblast cells. In the XRD patterns of the as-sputtered films on the ZrO2 substrate, there are no peaks except for those from the ZrO2 substrate. After the hydrothermal treatment, HA peaks appeared in the patterns. Nanoparticles (less than 20 nm) were observed on the ZrO2 substrates in the SEM images of the as-sputtered films. After the hydrothermal treatment, particles of 20–40 nm were observed on the film, whereas the HA film on the Ti substrate was covered by a larger number of globular particles (20– 60 nm). In the osteoblast cell cultures, the ALP activity and bone formation area on the HA films on both the ZrO2 and Ti substrates increased after the hydrothermal treatment of the films, and the values for the ZrO2 substrate were higher than those for the Ti substrate. Keywords: Sputtering, hydroxyapatite, zirconia, titanium, osteoblast cell

1. Introduction Titanium (Ti) is regarded as the standard for contemporary dental implant materials because of its excellent biocompatibility and mechanical properties. However, Ti has some disadvantages; for example, its use can result in poor aesthetics, especially in anterior sites in the mouth, due to its greyish colour, and it could elicit a potential allergic response [1,2]. Zirconia (ZrO2 ) has been studied for its potential use in dentistry in abutments and implants. ZrO2 has positive properties such as high fracture toughness, high bending strength, high hardness, excellent resistance corrosion, and it does not elicit an allergic response. With regard to aesthetics, the colour of ZrO2 is white, and its optical properties are closer to those of the natural tooth than those of Ti. In the last fifteen years, the use of ZrO2 in new ceramic implants has been reported [3]. Surface modification of the ZrO2 implants by alumina sandblasting substantially enhanced the osseointegration capacity of machined zirconia surfaces [4]. However, Gahlert et al. reported that sandblasted ZrO2 implants *

Address for correspondence: K. Ozeki, Department of Mechanical Engineering, Ibaraki University, 4-12-1, Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan. E-mail: [email protected] 0959-2989/14/$27.50 © 2014 – IOS Press and the authors. All rights reserved

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exhibited much lower removal torque from porcine maxilla bone than sandblasted acid etched (SLA) Ti implants in animal tests; the authors suggested that this occurred because the surface roughness of the ZrO2 implant was lower than that of the SLA Ti implant [5]. The treatment of the ZrO2 to create the high surface roughness by sandblasting may have led to the degradation of the ZrO2 . Bhargava et al. reported that the sandblasted ZrO2 with greater surface roughness has a higher amount of transformation of the crystal phase of ZrO2 (from tetragonal to monoclinic) than does ZrO2 with lower surface roughness [6]. This finding indicates that excessive surface roughness created by the sandblasting treatment should be limited in order to minimise the surface degradation of ZrO2 . Taking the preceding point into consideration, hydroxyapatite (HA) coating is a good potential alternative to excessive sandblasting as a surface modification of ZrO2 for the improvement of osseointegration because of an excellent osteocompatibility of HA. Many HA coating techniques have been reported, such as plasma spraying, sol-gel, sputtering, laser ablation, biomimetic and electro deposition [7]. The radio frequency (RF) sputtering technique is a particularly attractive method because it results in a homogeneous thin film less than 1 µm thick that exhibits a high bond strength to the substrate [8]. Sputter-coated HA thin films showed superior bone bonding strength and have been used clinically for dental implants [9,10]. In the present study, we coated HA films on a ZrO2 substrate to investigate the use of HA-sputter coating on ZrO2 implants. The coated films were characterised using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The adhesion strength of the HA films to the ZrO2 substrate was evaluated using a pull-out test and compared with HA films on a Ti substrate. The bone formation areas and alkaline phosphatase (ALP) activity of osteoblast cells on the films were measured to evaluate the osteocompatiblity of the HA-coated ZrO2 . 2. Materials and methods 2.1. Materials The substrates used were Ti plates (10×10×1 mm3 ) and Y-TZP ZrO2 plates (3 mol% Y2 O3 ; 10×10× 1 mm3 ). Powdered HA was used as a sputtering target. The HA powder was purchased from Taihei Corp. (Tokyo, Japan). The powder was heated to 800◦ C. HA powder (0.01 g) was dissolved in 0.1 M HNO3 solution (100 ml), and elemental analysis (Ca and P) of the solution was performed using ICP-AES (ICPS-7510, Shimadzu Corp, Japan). The average Ca/P molar ratio of the HA solution was 1.67 ± 0.05. 2.2. Methods 2.2.1. Deposition RF magnetron sputter coating was performed using an Anelva Model L-210HS-D magnetron sputtering system (Anelva Corp., Japan). The distance between the target and the substrate was approximately 60 mm, and the diameter of the target was 50 mm. Both the target and the substrate were water-cooled during the sputtering process. An RF generator operating at 13.56 MHz was coupled to the target electrode (cathode) through an impedance matching network. The sputtering chamber was evacuated to a pressure below 1 × 10−5 Pa using an oil-diffusion pump equipped with a liquid nitrogen trap. Argon gas (99.999%) was then introduced into the chamber using a mass flow controller at a constant flow rate of 10 sccm. The substrate was at floating potential without any extra heating or biasing during the deposition process. Before deposition took place, the target was covered with a shield and pre-sputtered using

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Ar ions for 10 min. The deposition was performed using an Ar gas pressure of 0.5 Pa and a discharge power of 100 W. The film was coated onto the Ti and ZrO2 substrates to fabricate 1 µm thick HA film samples. The as-sputtered films were evaluated using XRD (Ultima IV, Rigaku Corp., Japan) with a CuKα radiation source operating at 40 kV and 30 mA excitation current. The films’ surfaces were examined by SEM (S-4300, Hitachi, Japan) with an accelerating voltage of 5 kV. 2.2.2. Hydrothermal treatment The hydrothermal treatment was performed at 120◦ C and 0.202 MPa in an NaOH solutions in a stainless steel vessel for 24 h. The NaOH solution (pH = 9.5) was used to reduce film dissolution during the treatment [11]. Four as-sputtered films were placed in 800 ml of the NaOH solution during the hydrothermal treatment. After the treatment, the films were washed in distilled water and characterised using XRD and SEM as was performed for the as-sputtered films. Elemental analysis (Zr and Ti) of the solution after the treatment was performed using ICP-AES to evaluate a dissolution of Ti and ZrO2 substrates. The final results was obtained from an average of five treatments. 2.2.3. Pull-out tests to determine the adhesion strength of the films to the substrate Six-millimetre diameter Al rods were glued onto the as-sputtered and hydrothermally treated films using an epoxy resin adhesive (SW2214, Sumitomo 3M Corp., Japan). The as-sputtered films on the Ti and ZrO2 substrates were labelled as HA/Ti (sp) and HA/ZrO2 (sp), and the hydrothermally treated films on the Ti and ZrO2 substrates were labelled as HA/Ti (hyd) and HA/ZrO2 (hyd). The rods were pulled from each film at a crosshead speed of 0.5 mm/min using a precise tensile tester (LCS-1, Tokyo Testing Machine Corp., Japan). After the tests, the substrates were examined with an optical microscope to determine the location of the failure. The adhesion strength was calculated by dividing the pull-out force by the observed contact area. The strength was determined from an average of 16 tests. 2.2.4. Cell culture Rat osteoblast cells were isolated as described by Bellows et al. [12]. Briefly, calvaria from 3-day-old Wistar rats were dissected aseptically, minced, and digested in a collagenase-containing enzyme mixture at 37◦ C for 10, 20, 30 or 50 min, yielding populations I through IV, respectively. Cells retrieved from each step of the digestion sequence were plated in 60-mm culture dishes in αminimal essential medium (α-MEM) containing 10% heat-inactivated foetal bovine serum (FBS; Wako Pure Chemical Industries, Osaka, Japan), 0.3 µg/ml Fungizone (Gibco, Grand Island, NY), 100 µg/ml penicillin G (Wako), and 50 µg/ml gentamicin (Gibco). After 2 days, the cultures were washed with phosphate-buffered saline (PBS) to remove nonviable cells, incubated with 0.25% trypsin (Gibco), and then counted using a haemocytometer. The cells from populations II through IV were pooled, resuspended in α-MEM containing 10% FBS and antibiotics, and seeded on the samples in 35-mm culture dishes at 1 × 104 cells/dish. The experimental samples were HA/Ti (sp), HA/ZrO2 (sp), HA/Ti (hyd) and HA/ZrO2 (hyd), and Ti and ZrO2 substrates were used as controls. After 24 h, the culture medium was replaced with the above medium, supplemented with 50 µg/ml ascorbic acid (Wako), 10 nM dexamethasone (Wako), and 10 mM α-glycerophosphate (Nacalai Tesque, Kyoto, Japan). These conditions are optimal for the formation of mineralised osteoid nodules. The culture medium was changed twice each week. 2.2.4.1. Alkaline phosphatase (ALP) activity. Seven days after cell-seeding, the cells were then homogenised for 2 min and centrifuged at 2000 rpm for 20 min. The alkaline phosphatase (ALP) activity of the supernatant was measured using an ALP Activity Assay Kit (Wako) according to the manufacturer’s instructions. Each experimental values was obtained from the average of five samples.

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2.2.4.2. Bone formation. After 20 days of cell culture, 2.0 µg/ml alizarin complexone (Dojindo, Kumamoto, Japan) was added to the culture medium, and the osteoid nodules formed by the osteoblasts were stained with alizarin red for 24 hours. Next, all samples were washed in distilled water three times and dried. The bone formation areas stained with alizarin red were observed using a CCD camera, and the images were transferred to a computer-assisted image analysis system (MCID, Image Research, Brock University, Ontario, Canada). The bone area was determined from an average of five samples. 2.2.5. Statistical analysis For all test groups, the results from each experiment are expressed as the mean ± the standard deviation (SD). Statistically significant differences (p < 0.05 and p < 0.01) were determined using Student’s t-test. 3. Results and discussion 3.1. XRD patterns of the films before and after hydrothermal treatment Figure 1 shows XRD patterns of the sputtered films on the Ti and the ZrO2 substrates before and after the hydrothermal treatment. The HA/Ti (sp) shows a broad peak at 2θ ≈ 30◦ and four strong peaks at 2θ = 35.1, 38.4, 40.1◦ and 53.0◦ , corresponding to titanium (PDF #44-1294) (Fig. 1(a)). The film has low crystallinity, as other studies have reported [13]. In the HA/ZrO2 (sp), there are no peaks except for those from the ZrO2 substrate, which correspond to tetragonal zirconia (PDF #42-1164) (Fig. 1(c)). After the hydrothermal treatment, HA peaks appeared in the film on both the Ti and ZrO2 substrates (Fig. 1(b), (d)). However, a number of peaks from HA on the HA/ZrO2 (hyd) were smaller than those on the HA/Ti (hyd). This finding suggests that the HA/ZrO2 (hyd) has lower crystallinity than the HA/ ZrO2 (hyd). 3.2. SEM photos of the films before and after hydrothermal treatment Figure 2 shows SEM micrographs of the sputtered films on the Ti and the ZrO2 substrates before and after the hydrothermal treatment. In the as-sputtered films on both substrates, nanoparticles of less than 20 nm were observed (Fig. 2(a), (b)). After the hydrothermal treatment, the surface morphology was changed (Fig. 2(c), (d)). The surface of the HA/Ti (hyd) was covered with globular particles, which were composed of thin columnar grains; the grain size was 20–60 nm. By contrast, fewer particles with 20–40 nm grain size were observed on the HA/ZrO2 (hyd) than on the HA/ Ti (hyd). This observation suggests that there was more crystal growth of the HA film on the Ti substrate than that on the ZrO2 substrate and is consistent with the result from Fig. 1, which indicates that the HA/ZrO2 (hyd) has lower crystallinity than the HA/ZrO2 (hyd). This finding suggests that HA crystal growth is more favourable on the Ti surface than on the ZrO2 surface. There are two possible explanations for this difference. The first explanation is that epitaxial growth of HA is favoured on Ti surfaces. Crystal planes of HA are known to grow epitaxially on planes of titanium oxide. Lindberg et al. reported that an HA crystal grew preferentially in the (001) direction on the (001) rutile surface of titanium oxide [14]. Uchida et al. also reported that the fit of the HA (001) plane to the anatase (110) or to the rutile (101) planes showed a close superposition of hydrogen-bonding groups in these crystals [15]. These crystal fittings may lead the crystal growth of HA on Ti. The second explanation may be the inhibition of HA crystal growth by the Zr ion. Both Zr and Ti ions are strong inhibitors to the crystal growth of HA [16]. Table 1 showed

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Fig. 1. XRD patterns of the HA films on Ti and ZrO2 substrates before and after the hydrothermal treatment. (a) HA.

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Fig. 2. Scanning electron micrographs of: (a) HA/Ti (sp), (b) HA/ZrO2 (sp), (c) HA/Ti (hyd) and (d) HA/ZrO2 (hyd).

Table 1 The concentration of Zr and Ti ions in the solution after the hydrothermal treatment Samples HA/ZrO2 HA/Ti

Concentration of Zr or Ti ions in the solution (ppm) 0.43 ± 0.09 (Zr ion) 0.21 ± 0.05 (Ti ion)

the concentration of Zr and Ti ions in the solution after the hydrothermal treatment. Both Zr and Ti ions were dissolved from ZrO2 and Ti in the solution as some studies have reported [17,18]. However, the concentration of the released Ti ion was less than half of the released Zr ion. The release of Ti ions should be strongly reduced under hydrothermal conditions because of an increase in the titanium oxide layer during the hydrothermal treatment [18,19]. Thus, HA crystal growth on the ZrO2 substrates might be significantly inhibited. 3.3. Adhesion strength of the films to the substrate Figure 3 shows the adhesion strength of the films on the Ti and ZrO2 substrates before and after the hydrothermal treatment. The adhesion strengths of the HA/Ti (sp) and HA/ZrO2 (sp) were 5.8 and 3.8 MPa, respectively, and increased to 10.1 and 13.9 MPa after the hydrothermal treatment. The adhesion strength of the HA film on the ZrO2 substrate increased by more than 10 MPa, which was much greater than the increase for the HA film on the Ti substrate. Yen et al. reported that the OH bonds of Zr(OH)4 contributed to improving bond strength by interaction with the OH bonds of HA [20,21]. In the HA/ZrO2 (hyd), Zr(OH)4 can be formed between the HA film and the ZrO2 substrate from ZrO2 by hydrolysis in alkaline solution under hydrothermal conditions [22].

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Fig. 3. Adhesion strength of the films to the substrate before and after the hydrothermal treatment. ∗ p < 0.05, ∗∗ p < 0.01.

3.4. Alkaline phosphatase (ALP) activity of osteoblast cells Figure 4 shows the ALP activity of osteoblast cells on films coated onto Ti and ZrO2 substrates before and after the hydrothermal treatment. The ALP activity of osteoblasts on the films on both types of substrates increased after the hydrothermal treatment. The low ALP activity on the HA/Ti (sp) and HA/ZrO2 (sp) can be attributed to their cytotoxicity. The as-sputtered films include CaO because of the decomposition of HA during sputtering, and the presence of CaO leads to an increase in the pH of the culture medium. Then, the highly alkaline pH exerts a cytotoxic effect on the cells [23]. After the hydrothermal treatment, the CaO included in the as-sputtered film was removed into the hydrothermal solution [11]. The activity of HA/ZrO2 (hyd) is higher than that of HA/Ti (hyd), although there is no significant difference in the activity between the two. The ZrO2 substrate only also showed higher activity than the Ti substrate. This finding might be explained by the better cell adhesion properties of ZrO2 . Ko et al. reported the ALP activity of human osteosarcoma cells on TZP discs and Ti discs. The TZP discs showed higher levels of ALP activity than those of the Ti discs after eight days of culture. The authors explained that in general, the cells cultured on ZrO2 showed higher initial adhesion properties and better proliferation and differentiation. The cells on ZrO2 proliferated longer and differentiated more actively than those on Ti [24]. However, there were HA films between the cells and the ZrO2 substrates in our experiment. Even if the cells did not have a direct bond with the ZrO2 substrate, the HA/ZrO2 (hyd)

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Fig. 4. Alkaline phosphatase (ALP) activity of osteoblast cells on the films before and after the hydrothermal treatment. ∗∗ p < 0.01.

showed higher ALP activity than the HA/Ti (hyd). One possible explanation for this result may be the difference in surface topography between the Ti and ZrO2 substrates. According to Ko et al., the surface topography of ZrO2 led to good adhesion of the cells [24]. Even if the Ti and ZrO2 had the same surface roughness, their surface topographies were different due to the different processes used in their manufacture. Surface topography plays an important role in osteoblast adhesion [25], and the differences in surface topography may explain the differences in cell proliferation and ALP activity. The surface topographies of the HA films were almost the same as those of the substrates because the HA films were very thin (approximately 1 µm thickness). 3.5. Bone area measurement of osteoblasts cultured on the films Figure 5 shows the areas of bone formation on the films. The bone areas on HA/Ti (hyd) and HA/ZrO2 (hyd) are larger than those on any of the other samples. The area of bone on HA/ZrO2 (hyd) is larger than that on HA/Ti (hyd), although there is no significant difference in bone area between HA/Ti (hyd) and HA/ZrO2 (hyd). This result is consistent with the results of the ALP activity assay. Generally, ALP activity represents osteoblast differentiation and in vitro mineralisation capacity. Higher ALP activity leads to greater mineralisation by the cells. From these results, the properties of hydrothermally treated HA on the ZrO2 substrate are comparable with those of hydrothermally treated HA on the Ti substrate with regard to film adhesion to the substrate

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Fig. 5. Bone formation area of osteoblast cells on the films before and after the hydrothermal treatment. ∗∗ p < 0.01.

and osteocompatibility. Therefore, sputtered HA films on ZrO2 have high potential for use as dental implants. 4. Conclusions HA thin films were successfully prepared on zirconia substrates using a sputtering technique. The following conclusions were determined: 1. In the XRD patterns of the as-sputtered film on the ZrO2 substrate, there were no peaks except for those from the ZrO2 substrate. After the hydrothermal treatment, HA peaks appeared in the patterns. 2. In the SEM observations, nanoparticles (less than 20 nm) were observed on the as-sputtered films on the ZrO2 substrates. After the hydrothermal treatment, a small number of particles (20–40 nm) was observed on the films on the ZrO2 substrates, whereas a larger number of globular particles (20–60 nm) covered the surface of the films on the Ti substrates. 3. In the pull-out test, the strengths of the as-sputtered films on the Ti and ZrO2 substrates were 5.8 and 3.8 MPa, respectively, which increased to 10.1 and 13.9 MPa after the hydrothermal treatment. 4. In the osteoblast cell culture experiments, the ALP activity and bone formation area on the HA films on the ZrO2 and Ti substrates increased after the hydrothermal treatment, and the values for the ZrO2 substrate were higher than those for the Ti substrate.

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Fabrication of hydroxyapatite thin films on zirconia using a sputtering technique.

Hydroxyapatite (HA) thin films were prepared on a zirconia (ZrO2) substrate using a sputtering technique, and the film was also coated on a titanium (...
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