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Preparation, biocompatibility and wear resistance of microstructured Zr and ZrO2 alloyed layers on 316L stainless steel
Materials Letters 203 (2017) 24–27
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Preparation, biocompatibility and wear resistance of microstructured Zr and ZrO2 alloyed layers on 316L stainless steel Jianfang Li, Xiangyu Zhang ⇑, Xiaojing He, Ruiqiang Hang, Xiaobo Huang, Bin Tang College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 2 December 2016 Received in revised form 13 May 2017 Accepted 25 May 2017 Available online 26 May 2017 Keywords: Biomaterials Microstructure 316L stainless steel Zr Biocompatibility Wear resistance
a b s t r a c t The surface properties of implanted materials play critical roles in clinical application. An ideal biomaterial is required to possess both excellent biocompatibility and appropriate load-bearing capacity. To this end, microstructured zirconium (Zr) and ZrO2 alloyed layers were prepared on 316L stainless steel (316L SS). Firstly, a Zr alloyed layer was prepared on 316L SS by using plasma surface alloying technique. And then, a ZrO2 alloyed layer was obtained on the Zr alloyed layer after annealing at 850 °C for 3 h. The microstructure, biocompatibility and wear resistance of the surface alloyed layers were investigated. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction 316L stainless steel (SS), a common orthopedic and dental implants materials [1], has attracted tremendous attention due to its excellent mechanical strength, high corrosion resistance and low cost [2]. Unfortunately, the low biocompatibility has limited the development of 316L SS as a biomaterial [3,4]. And the loadbearing capacity of SS is another major concern to potential users. Many approaches have been applied to enhance the biocompatibility, among which adding new elements to improve surface microstructure is an extremely promising method. Plasma surface alloying technique is an effective technology to introduce the desired alloying elements to be sputtered and diffused into the matrix [5,6]. It can form an alloyed layer with adjustable thickness according various requirement [7] and change material surface microstructure, such as microtopography and surface roughness. Microtopography has demonstrated the ability to enable cell attachment, differentiation and proliferation [8]. As is well known, zirconium (Zr) shows excellent biocompatibility [9]. An extremely stable oxide ZrO2 can be formed on the surface of Zr, and ZrO2 coating, especially monoclinic ZrO2 [10], can enhance implant osseointegration and cytocompatibility [11]. Zr alloyed layers have been prepared on 440B SS and Ti-6Al-4V for improved hardness [12,13]. In this work, Zr alloyed layer was ⇑ Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.matlet.2017.05.106 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
prepared on 316L SS by plasma surface alloying technology, subsequently was oxidized in the oxygen to obtain ZrO2 alloyed layer. We will focus on the biocompatibility and the load-bearing capacity of the Zr and ZrO2 alloyed layers. The microstructure of the Zr and ZrO2 alloyed layers were investigated by scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD). The osteoblast cell adhesion and wear resistance were also discussed.
2. Experiment 316L SS specimen disks (U14 mm 3 mm) were mechanically polished to the extent of Ra 0.25 lm, and were successively cleaned in acetone and ethanol. Zr alloyed layer (labeled as 316LZr) was fabricated on the surface of 316L SS using plasma surface alloying apparatus [14]. 316L-Zr was oxidized at 850 °C for 3 h and labeled as 316L-ZrO2. Surface and cross-section microstructure were observed by a scanning electron microscopy (FE-SEM, JSM-7001F, JEOL). The average surface roughness was measured by confocal laser scanning microscopy (CLSM, C2 Plus, Nikon). The phase structure was analyzed by X-ray diffraction spectroscopy (XRD, DX-2700X). The MC3T3-E1 pre-osteoblast cells were seeded on the surface of specimens at a density of 4.0 104 cells/well in a 24-well plate. Fluorescent images were taken to observe F-actin cytoskeleton and cell nuclei using laser scanning confocal microscope (CLSM). The scratch test was conducted to evaluate the bonding strength between the layers and the substrate using diamond
J. Li et al. / Materials Letters 203 (2017) 24–27
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Fig. 1. A Surface morphology of 316L SS, 316L-Zr and 316L-ZrO2; B Cross-sectional image of the Zr and ZrO2 alloyed layers and the corresponding EDS elemental map results.
indenters. The wear resistance was measured by MFT-R4000 reciprocating frication and wear tester with a load of 5 N, sliding duration of 10 min and a frequency of 2 Hz. The wear scar length was 5 mm with Al2O3 balls used. Then the depth and width of wear scar were evaluated by white light interferometry. 3. Result and discussion
Fig. 2. XRD patterns of 316L SS, 316L-Zr and 316L-ZrO2.
The surface microstructure of 316L SS, 316L-Zr and 316L-ZrO2 is shown in Fig. 1 A. The surface of 316L-Zr appears the polygonal grain boundaries with the width of about 8–12 lm. There are two basic functions for the plasma in plasma surface alloying process. One makes the Zr element sputtered from the Zr target and one bombards the surface of SS. The bombardment, the interdiffusion between Zr and SS, and together with the formation of new phases results in the distinctive microstructure of 316L-Zr.
Fig. 3. Fluorescent images of MC3T3-E1 cells cultured on surface of 316L SS, 316L-Zr and 316L-ZrO2 for 24 h with F-actin stained with FITC (green) and the cell nuclei stained with DAPI (blue) at a cell density of 1 104 cells cm 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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J. Li et al. / Materials Letters 203 (2017) 24–27
Fig. 4. The acoustic emission vs. normal load curves (A) and wear scars profile curves (B) and corresponding surface morphology (C) of 316L SS, 316L-Zr and 316L-ZrO2 against Al2O3.
J. Li et al. / Materials Letters 203 (2017) 24–27
After oxidation, the surface of 316L-ZrO2 displays distinct contourlike structure. The measured surface roughness are in the order: 316L-ZrO2 (0.74 ± 0.05 lm) > 316L-Zr (0.65 ± 0.03 lm) > polished samples (0.06 ± 0.02 lm). The cross-sectional image and the corresponding EDS elemental mapping results (Fig. 1B) show that the continuous, dense and homogeneous alloyed layers with the thickness of 38 lm are tightly attached to the 316L SS substrate. However, the oxidation part (about 18 lm) and unoxidation part are obviously distinguished on 316L-ZrO2. EDS elemental mapping suggests that Zr in the two layers is evenly distributed over the alloyed layer on 316L SS. The XRD patterns in the Fig. 2 show that 316L-Zr mainly consists of metallic Zr, FeZr2 and ZrC. It can be inferred that the formation of the Zr alloyed layer is mainly due to the outward diffusion of iron, while the inward diffusion of Zr is contributed to the growth of the Zr alloyed layer. In addition, Zr is easier to combine with carbon compared to Cr, thus replacing Cr from the chromium compounds to form ZrC [15]. 316L-ZrO2 mainly contains monoclinic ZrO2 and a minor amount of Fe2O3. To investigate the cell adhesion and spreading activity, osteoblast cells were cultured on the surfaces of 316L SS, 316L-Zr and 316L-ZrO2 to visualize the F-actin and cell nuclei. As shown in Fig. 3, the F-actin stress fibers spanning the entire length of the cells can be clearly observed on all sample surfaces, but the number and the spreading areas of cells on the surfaces of 316L-Zr and 316L-ZrO2 are larger than those on the 316L SS. It can infer that both 316L-Zr and 316L-ZrO2 is beneficial to cell adhesion and spreading, which is mainly ascribed to the introduction of Zr element and the micro-scale structure [16]. Study has shown that the microstructure can promote the growth of certain cells [17]. Our results also indicate that the number of cells is increased as the roughness increased. It is possible that the combined effect of surface topography, surface roughness and the Zr element is associated for the improvement of cell attachment. However, the precise mechanisms remain to be defined. In practical applications, the ideal biomaterial surfaces should possess excellent biocompatibility and appropriate load-bearing capacity. The scratch test is a widely used method for testing the adhesion of the coating. Fig. 4A shows the critical loads of 316LZr and 316L-ZrO2 are 131.2 N, 152.4 N, respectively. In generally, a critical load of above 30 N in scratch testing is believed to be enough for the biomaterials. Thus, the two layers are suitable for long-term stability in the human body under load-bearing conditions. To test the load-bearing capacity of the alloyed surfaces, the wear resistance was also investigated by a ball-on-disk test. Fig. 4B and C shows the wear scars profile curves and corresponding wear track. The width and depth on 316L SS of wear scars were 0.52 mm and 14.2 lm, respectively. However, the depth of wear scars increased to 17.8 lm after plasma surface alloying, and then
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decreased to 7.8 lm after oxidation. Compared with 316L SS, the wear volume of 316L-Zr has increased, this is due to the relatively low hardness of Zr and serious adhesive wear occurred on the Zr surface (Fig. 4C). However, the wear resistance of 316L-ZrO2 is improved and only slight plastic deformation occurred on the ZrO2 surface. This is probably because of the relatively high hardness and the good stability of ZrO2. 4. Conclusions In summary, Zr and ZrO2 alloyed layers have been successfully prepared on 316L SS. Micro-scale structures have also formed on the surface of the alloyed layers. The alloyed layers are uniform, continuous and compact and the total thickness is about 38 lm. The 316L-Zr and 316L-ZrO2 can evidently improve the adhesion and spreading of osteoblast cells. Meanwhile, ZrO2 layer can tremendously enhance the wear resistance of 316L SS. Acknowledgment This work was supported by the National Natural Science Foundation of China (51671140 and 31400815), the Natural Science Foundation of Shanxi Province (2015021063), the Qualified Personnel Foundation of Taiyuan University of Technology (QPFT) (tyutrc-2011257a) and the Research Project Supported by Shanxi Scholarship Council of China (2015-034). References [1] A. Srinivasan, N. Rajendran, RSC Adv. 5 (2015) 26007–26016. [2] H.D. Jung, T.S. Jang, L. Wang, H.E. Kim, Y.H. Koh, J. Song, Biomaterials 37 (2015) 49–61. [3] S.V. Muley, A.N. Vidvans, G.P. Chaudhari, S. Udainiya, Acta Biomater. 30 (2016) 408–419. [4] K. Rokosz, T. Hryniewicz, R. Rokicki, Teh. Vjesn. 21 (2014) 799–805. [5] X. Zhang, M. Li, X. He, X. Huang, R. Hang, B. Tang, Mater. Des. 65 (2015) 600– 605. [6] B.Z. Duan, P.Z. Zhang, X.F. Wei, L. Wang, D.B. Wei, D.D. Zhen, Surf. Eng. 31 (2014) 942–948. [7] X. Zhang, X. Huang, L. Jiang, Y. Ma, A. Fan, B. Tang, Appl. Surf. Sci. 258 (2011) 1399–1404. [8] B. Zhu, Q. Zhang, Q. Lu, Y. Xu, J. Yin, J. Hu, et al., Biomaterials 25 (2004) 4215– 4223. [9] G.C. Yeo, M. Santos, A. Kondyurin, J. Liskova, A.S. Weiss, M.M.M. Bilek, ACS Biomater. Sci. Eng. 2 (2016) 662–676. [10] G. Wang, F. Meng, C. Ding, P.K. Chu, X. Liu, Acta Biomater. 6 (2010) 990–1000. [11] L. Zhang, S. Wang, Y. Han, Surf. Coat. Technol. 212 (2012) 192–198. [12] H.H. Shen, L. Liu, X.Z. Liu, Q. Guo, T.X. Meng, Z.X. Wang, H.J. Yang, X.P. Liu, Appl. Surf. Sci. 388 (2016) 126–132. [13] X. Li, B. Tang, J. Ye, Appl. Surf. Sci. 258 (2012) 1981–1984. [14] K. Chen, X. Liu, X. Liu, T. Meng, Q. Guo, Z. Wang, et al., J. Wuhan Univ. Technol. Mater. Sci. Ed. 31 (2016) 1086–1092. [15] H. Cai, Y. Gao, Z. Ma, C. Wang, Chin. J. Rare Met. 6 (2013) 915–921. [16] H.F. Li, Y.B. Wang, Y.F. Zheng, J.P. Lin, J. Biomed. Mater. Res. B Appl. Biomater. 100 (2012) 1721–1728. [17] L. Zhao, S. Mei, P.K. Chu, Y. Zhang, Z. Wu, Biomaterials 31 (2010) 5072–5082.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Preparation, biocompatibility and wear resistance of microstructured Zr and ZrO2 alloyed layers on 316L stainless steel Jianfang Li, Xiangyu Zhang ⇑, Xiaojing He, Ruiqiang Hang, Xiaobo Huang, Bin Tang College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 2 December 2016 Received in revised form 13 May 2017 Accepted 25 May 2017 Available online 26 May 2017 Keywords: Biomaterials Microstructure 316L stainless steel Zr Biocompatibility Wear resistance
a b s t r a c t The surface properties of implanted materials play critical roles in clinical application. An ideal biomaterial is required to possess both excellent biocompatibility and appropriate load-bearing capacity. To this end, microstructured zirconium (Zr) and ZrO2 alloyed layers were prepared on 316L stainless steel (316L SS). Firstly, a Zr alloyed layer was prepared on 316L SS by using plasma surface alloying technique. And then, a ZrO2 alloyed layer was obtained on the Zr alloyed layer after annealing at 850 °C for 3 h. The microstructure, biocompatibility and wear resistance of the surface alloyed layers were investigated. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction 316L stainless steel (SS), a common orthopedic and dental implants materials [1], has attracted tremendous attention due to its excellent mechanical strength, high corrosion resistance and low cost [2]. Unfortunately, the low biocompatibility has limited the development of 316L SS as a biomaterial [3,4]. And the loadbearing capacity of SS is another major concern to potential users. Many approaches have been applied to enhance the biocompatibility, among which adding new elements to improve surface microstructure is an extremely promising method. Plasma surface alloying technique is an effective technology to introduce the desired alloying elements to be sputtered and diffused into the matrix [5,6]. It can form an alloyed layer with adjustable thickness according various requirement [7] and change material surface microstructure, such as microtopography and surface roughness. Microtopography has demonstrated the ability to enable cell attachment, differentiation and proliferation [8]. As is well known, zirconium (Zr) shows excellent biocompatibility [9]. An extremely stable oxide ZrO2 can be formed on the surface of Zr, and ZrO2 coating, especially monoclinic ZrO2 [10], can enhance implant osseointegration and cytocompatibility [11]. Zr alloyed layers have been prepared on 440B SS and Ti-6Al-4V for improved hardness [12,13]. In this work, Zr alloyed layer was ⇑ Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.matlet.2017.05.106 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
prepared on 316L SS by plasma surface alloying technology, subsequently was oxidized in the oxygen to obtain ZrO2 alloyed layer. We will focus on the biocompatibility and the load-bearing capacity of the Zr and ZrO2 alloyed layers. The microstructure of the Zr and ZrO2 alloyed layers were investigated by scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD). The osteoblast cell adhesion and wear resistance were also discussed.
2. Experiment 316L SS specimen disks (U14 mm 3 mm) were mechanically polished to the extent of Ra 0.25 lm, and were successively cleaned in acetone and ethanol. Zr alloyed layer (labeled as 316LZr) was fabricated on the surface of 316L SS using plasma surface alloying apparatus [14]. 316L-Zr was oxidized at 850 °C for 3 h and labeled as 316L-ZrO2. Surface and cross-section microstructure were observed by a scanning electron microscopy (FE-SEM, JSM-7001F, JEOL). The average surface roughness was measured by confocal laser scanning microscopy (CLSM, C2 Plus, Nikon). The phase structure was analyzed by X-ray diffraction spectroscopy (XRD, DX-2700X). The MC3T3-E1 pre-osteoblast cells were seeded on the surface of specimens at a density of 4.0 104 cells/well in a 24-well plate. Fluorescent images were taken to observe F-actin cytoskeleton and cell nuclei using laser scanning confocal microscope (CLSM). The scratch test was conducted to evaluate the bonding strength between the layers and the substrate using diamond
J. Li et al. / Materials Letters 203 (2017) 24–27
25
Fig. 1. A Surface morphology of 316L SS, 316L-Zr and 316L-ZrO2; B Cross-sectional image of the Zr and ZrO2 alloyed layers and the corresponding EDS elemental map results.
indenters. The wear resistance was measured by MFT-R4000 reciprocating frication and wear tester with a load of 5 N, sliding duration of 10 min and a frequency of 2 Hz. The wear scar length was 5 mm with Al2O3 balls used. Then the depth and width of wear scar were evaluated by white light interferometry. 3. Result and discussion
Fig. 2. XRD patterns of 316L SS, 316L-Zr and 316L-ZrO2.
The surface microstructure of 316L SS, 316L-Zr and 316L-ZrO2 is shown in Fig. 1 A. The surface of 316L-Zr appears the polygonal grain boundaries with the width of about 8–12 lm. There are two basic functions for the plasma in plasma surface alloying process. One makes the Zr element sputtered from the Zr target and one bombards the surface of SS. The bombardment, the interdiffusion between Zr and SS, and together with the formation of new phases results in the distinctive microstructure of 316L-Zr.
Fig. 3. Fluorescent images of MC3T3-E1 cells cultured on surface of 316L SS, 316L-Zr and 316L-ZrO2 for 24 h with F-actin stained with FITC (green) and the cell nuclei stained with DAPI (blue) at a cell density of 1 104 cells cm 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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J. Li et al. / Materials Letters 203 (2017) 24–27
Fig. 4. The acoustic emission vs. normal load curves (A) and wear scars profile curves (B) and corresponding surface morphology (C) of 316L SS, 316L-Zr and 316L-ZrO2 against Al2O3.
J. Li et al. / Materials Letters 203 (2017) 24–27
After oxidation, the surface of 316L-ZrO2 displays distinct contourlike structure. The measured surface roughness are in the order: 316L-ZrO2 (0.74 ± 0.05 lm) > 316L-Zr (0.65 ± 0.03 lm) > polished samples (0.06 ± 0.02 lm). The cross-sectional image and the corresponding EDS elemental mapping results (Fig. 1B) show that the continuous, dense and homogeneous alloyed layers with the thickness of 38 lm are tightly attached to the 316L SS substrate. However, the oxidation part (about 18 lm) and unoxidation part are obviously distinguished on 316L-ZrO2. EDS elemental mapping suggests that Zr in the two layers is evenly distributed over the alloyed layer on 316L SS. The XRD patterns in the Fig. 2 show that 316L-Zr mainly consists of metallic Zr, FeZr2 and ZrC. It can be inferred that the formation of the Zr alloyed layer is mainly due to the outward diffusion of iron, while the inward diffusion of Zr is contributed to the growth of the Zr alloyed layer. In addition, Zr is easier to combine with carbon compared to Cr, thus replacing Cr from the chromium compounds to form ZrC [15]. 316L-ZrO2 mainly contains monoclinic ZrO2 and a minor amount of Fe2O3. To investigate the cell adhesion and spreading activity, osteoblast cells were cultured on the surfaces of 316L SS, 316L-Zr and 316L-ZrO2 to visualize the F-actin and cell nuclei. As shown in Fig. 3, the F-actin stress fibers spanning the entire length of the cells can be clearly observed on all sample surfaces, but the number and the spreading areas of cells on the surfaces of 316L-Zr and 316L-ZrO2 are larger than those on the 316L SS. It can infer that both 316L-Zr and 316L-ZrO2 is beneficial to cell adhesion and spreading, which is mainly ascribed to the introduction of Zr element and the micro-scale structure [16]. Study has shown that the microstructure can promote the growth of certain cells [17]. Our results also indicate that the number of cells is increased as the roughness increased. It is possible that the combined effect of surface topography, surface roughness and the Zr element is associated for the improvement of cell attachment. However, the precise mechanisms remain to be defined. In practical applications, the ideal biomaterial surfaces should possess excellent biocompatibility and appropriate load-bearing capacity. The scratch test is a widely used method for testing the adhesion of the coating. Fig. 4A shows the critical loads of 316LZr and 316L-ZrO2 are 131.2 N, 152.4 N, respectively. In generally, a critical load of above 30 N in scratch testing is believed to be enough for the biomaterials. Thus, the two layers are suitable for long-term stability in the human body under load-bearing conditions. To test the load-bearing capacity of the alloyed surfaces, the wear resistance was also investigated by a ball-on-disk test. Fig. 4B and C shows the wear scars profile curves and corresponding wear track. The width and depth on 316L SS of wear scars were 0.52 mm and 14.2 lm, respectively. However, the depth of wear scars increased to 17.8 lm after plasma surface alloying, and then
27
decreased to 7.8 lm after oxidation. Compared with 316L SS, the wear volume of 316L-Zr has increased, this is due to the relatively low hardness of Zr and serious adhesive wear occurred on the Zr surface (Fig. 4C). However, the wear resistance of 316L-ZrO2 is improved and only slight plastic deformation occurred on the ZrO2 surface. This is probably because of the relatively high hardness and the good stability of ZrO2. 4. Conclusions In summary, Zr and ZrO2 alloyed layers have been successfully prepared on 316L SS. Micro-scale structures have also formed on the surface of the alloyed layers. The alloyed layers are uniform, continuous and compact and the total thickness is about 38 lm. The 316L-Zr and 316L-ZrO2 can evidently improve the adhesion and spreading of osteoblast cells. Meanwhile, ZrO2 layer can tremendously enhance the wear resistance of 316L SS. Acknowledgment This work was supported by the National Natural Science Foundation of China (51671140 and 31400815), the Natural Science Foundation of Shanxi Province (2015021063), the Qualified Personnel Foundation of Taiyuan University of Technology (QPFT) (tyutrc-2011257a) and the Research Project Supported by Shanxi Scholarship Council of China (2015-034). References [1] A. Srinivasan, N. Rajendran, RSC Adv. 5 (2015) 26007–26016. [2] H.D. Jung, T.S. Jang, L. Wang, H.E. Kim, Y.H. Koh, J. Song, Biomaterials 37 (2015) 49–61. [3] S.V. Muley, A.N. Vidvans, G.P. Chaudhari, S. Udainiya, Acta Biomater. 30 (2016) 408–419. [4] K. Rokosz, T. Hryniewicz, R. Rokicki, Teh. Vjesn. 21 (2014) 799–805. [5] X. Zhang, M. Li, X. He, X. Huang, R. Hang, B. Tang, Mater. Des. 65 (2015) 600– 605. [6] B.Z. Duan, P.Z. Zhang, X.F. Wei, L. Wang, D.B. Wei, D.D. Zhen, Surf. Eng. 31 (2014) 942–948. [7] X. Zhang, X. Huang, L. Jiang, Y. Ma, A. Fan, B. Tang, Appl. Surf. Sci. 258 (2011) 1399–1404. [8] B. Zhu, Q. Zhang, Q. Lu, Y. Xu, J. Yin, J. Hu, et al., Biomaterials 25 (2004) 4215– 4223. [9] G.C. Yeo, M. Santos, A. Kondyurin, J. Liskova, A.S. Weiss, M.M.M. Bilek, ACS Biomater. Sci. Eng. 2 (2016) 662–676. [10] G. Wang, F. Meng, C. Ding, P.K. Chu, X. Liu, Acta Biomater. 6 (2010) 990–1000. [11] L. Zhang, S. Wang, Y. Han, Surf. Coat. Technol. 212 (2012) 192–198. [12] H.H. Shen, L. Liu, X.Z. Liu, Q. Guo, T.X. Meng, Z.X. Wang, H.J. Yang, X.P. Liu, Appl. Surf. Sci. 388 (2016) 126–132. [13] X. Li, B. Tang, J. Ye, Appl. Surf. Sci. 258 (2012) 1981–1984. [14] K. Chen, X. Liu, X. Liu, T. Meng, Q. Guo, Z. Wang, et al., J. Wuhan Univ. Technol. Mater. Sci. Ed. 31 (2016) 1086–1092. [15] H. Cai, Y. Gao, Z. Ma, C. Wang, Chin. J. Rare Met. 6 (2013) 915–921. [16] H.F. Li, Y.B. Wang, Y.F. Zheng, J.P. Lin, J. Biomed. Mater. Res. B Appl. Biomater. 100 (2012) 1721–1728. [17] L. Zhao, S. Mei, P.K. Chu, Y. Zhang, Z. Wu, Biomaterials 31 (2010) 5072–5082.