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Fabrication of porous titanium scaffolds with high compressive strength using camphene-based freeze casting
Materials Letters 63 (2009) 1502–1504
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of porous titanium scaffolds with high compressive strength using camphene-based freeze casting Se-Won Yook a, Hyoun-Ee Kim a, Young-Hag Koh b,⁎ a b
Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Republic of Korea Department of Dental Laboratory Science and Engineering, Korea University, Seoul, 136-703, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2009 Accepted 29 March 2009 Available online 5 April 2009 Keywords: Metals and alloys Porosity Mechanical properties Titanium (Ti) Freeze casting
a b s t r a c t We herein report the fabrication of highly porous titanium (Ti) scaffolds with unusually high compressive strength by freezing a titanium hydride (TiH2)/camphene slurries at 42 °C. As the freezing time was increased from 1 to 7 days, the pore size obtained was increased significantly from 143 to 271 μm due to the continual overgrowth of camphene dendrites. However, interestingly, the formation of the micro-pores inside the Ti walls was suppressed at longer freezing time. This resulted in a significant increase in compressive strength up to 110 ± 17 MPa with a porosity of 64%. It is believed that this unusually high compressive strength with large interconnected pores makes this material suitable for applications as loadbearing parts. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the past few decades, titanium (Ti) and its alloys have been used successfully in dental and orthopedic implants, on account of their excellent mechanical properties, chemical stability, and biocompatibility [1]. However, these materials often suffer from relatively high stiffness compared to the surrounding bones, which can cause bone resorption and eventually loosening of the implant [2,3]. One of the most promising approaches for overcoming this limitation is to create pores in the bulks that can allow matching of the mechanical properties of materials to those of the surrounding bone [4,5]. Furthermore, these interconnected pores can provide a favorable environment for bone ingrowth, which would lead to a tight fixation between the implant and surrounding bone Thus far, considerable effort has been made to develop new manufacturing processes for the production of porous Ti scaffolds [6– 9]. More recently, freeze casting has been also developed to tailor the pore structure of porous materials, such as porosity, pore size, pore shape, and pore orientation [10,11]. This method makes full use of the dendritic growth of a freezing vehicle including water [10] and camphene [11], which can be removed by freeze-drying to create interconnected pores in the bulk. In this study, we examined how the freezing time affects the development of the pore structure and compressive strength of porous Ti scaffolds using the camphenebased freeze casting. To accomplish this, a titanium hydride (TiH2) / camphene slurry with an initial TiH2 content of 10 vol.% was frozen at 42 °C for various times (1, 4, and 7 days), followed by freeze-drying
⁎ Corresponding author. Tel.: +82 2 940 2844. E-mail address: [email protected] (Y.-H. Koh). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.056
and heat-treatment in a vacuum at 1300 °C for 2 h. The porous structures (e.g., porosity, pore size, pore shape, and densification of Ti walls) were examined. In addition, compressive strength tests were performed to evaluate the mechanical property of the samples. 2. Experimental procedure Commercially available titanium hydride powder (TiH2, Alfa Aesar, Ward Hill, MA, USA) was used as the source for Ti metal [9,11]. Camphene (C10H16, Sigma Aldrich, St. Louis, MO, USA) with a purity of 95% was used as the freezing vehicle without further purification. A titanium hydride/ camphene slurry with a TiH2 content of 10 vol.% was prepared by ballmilling at 60 °C for 24 h using 1 wt.% of oligomeric polyester (Hypermer KD-4; UniQema, Everburg, Belgium) as a dispersant. The optimum freezing temperature was determined by examining the solidification of the slurry using differential scanning calorimetry (DSC; DSC Q 1000, TA Instruments, West Sussex, UK). Based on this observation, the warm slurry prepared was poured into aluminum molds with dimensions of 15 × 15 × 15 mm and kept in an oven at 42 °C for various times (1, 4, and 7 days). After freeze-drying to remove the frozen camphene, the samples were heated to 400 °C at a heating rate of 3 °C/min in a vacuum and kept at this temperature for 2 h to induce the complete hydride decomposition of the TiH2 powder, followed by subsequent heat-treatment at 1300 °C for 2 h to consolidate the Ti powder. The porous structures of the porous Ti scaffolds and densifications of the Ti walls were characterized by scanning electron microscopy (FE-SEM, JSM-6330F, JEOL Techniques, Tokyo, Japan). The pore size was also analyzed from the SEM images of the samples prepared by infiltrating the porous Ti scaffolds with an epoxy resin (Spurrs epoxy,
S.-W. Yook et al. / Materials Letters 63 (2009) 1502–1504
1503
Fig. 1. SEM micrographs of the porous Ti scaffolds produced with various freezing times of (A) 1 day, (B) 4 days, and (C) 7 days, at 42 °C.
Polysciences Inc., Warrington, PA). The mechanical properties were evaluated by compressing 5 × 5 × 7 mm sized specimens using a screw-driven load frame (Instron 5565, Instron Corp., Canton, MA, USA) at a crosshead speed of 5 mm/min. The compressive strengths were calculated from the compressive stress-strain curves. Five samples were tested to obtain the average values along and its standard deviation.
3. Results and discussion The solidification behavior of a TiH2/camphene slurry with an initial TiH2 content of 10 vol.% was first examined using differential scanning calorimetry (DSC), since a new type of camphene with a higher melting point of 48–52 °C (from the manufacturer's specifications) was used in this study. A strong exothermic peak was observed at 42.7 °C (data not shown here), suggesting that the optimum freezing temperature is 42 °C, which would be expected to allow the
Fig. 2. Pore size of the porous Ti scaffolds as a function of the freezing time.
continual overgrowth of the camphene dendrites, while enabling the complete solidification of the slurry. In order to examine how the freezing time can affect the growth of camphene dendrites, the slurry was kept at 42 °C for various times, ranging from 1 to 7 days. All of the fabricated samples showed a highly porous structure with large interconnected pores, as shown in Fig. 1 (A)–(C). This suggests that the new type of camphene with a higher melting point can induce the successful continual overgrowth of camphene dendrites and the redistribution of TiH2 particles during freezing at 42 °C, which is similar to previous reports [11–13]. It should be noted that the pore size was increased remarkably with increasing the freezing time. However, regardless of the freezing time, all of the fabricated samples showed a similar porosity of 64%, indicating negligible sedimentation of the TiH2 particles during freezing. The pore sizes of the porous Ti scaffolds prepared using various freezing times were calculated from SEM images of the epoxy-filled samples that were further digitally colored for more accurate measurement. As expected, the pore size was increased remarkably from 144 ± 21 to 271 ± 15 µm with increasing the freezing time from 1 to 7 days, as shown in Fig. 2. A longer freezing time might lead to a larger pore size, but would, inevitably, cause the considerable evaporation of the frozen camphene, because the freezing temperature would be very close to its melting point. The freezing time significantly affected the microstructures of the Ti walls, as shown in Fig. 3 (A)–(C). Interestingly, the micro-pores formed as a replica of relatively small camphene dendrites disappeared with in creasing the freezing time. In particular, the sample prepared using a freezing time of 1 day showed many micro-pores inside the Ti walls (Fig. 3 (A)). However, the formation of these micropores was suppressed by increasing the freezing time to 4 days (Fig. 3 (B)). Eventually, the micro-pores disappeared after freezing for 7 days, allowing the achievement of fully dense Ti walls (Fig. 3 (C)). To the best of our best knowledge, this singular phenomenon has been never observed for the freeze casting method using either camphene or water as the freezing medium. We herein speculate that the particle size of TiH2 (1–3 μm) was much larger than conventional ceramic particles with a size in the submicron range. The packing density of the ceramic walls should be determined by the balance
Fig. 3. SEM micrographs of the microstructures of the Ti walls produced with various freezing times of (A) 1 day, (B) 4 days, and (C) 7 days, at 42 °C.
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S.-W. Yook et al. / Materials Letters 63 (2009) 1502–1504
high porosity. These striking features should allow the porous Ti scaffolds produced to find very useful applications as the scaffold for bone regeneration. It is also worth mentioning that their pore structure (e.g., porosity, pore size, pore shape, pore alignment) would be further controlled using the present camphene-based freeze casting method. 4. Conclusions
Fig. 4. Compressive strengths of the porous Ti scaffolds produced with various freezing times at 42 °C.
between the capillary drag force pushing the particles by the molten camphene and the resisting osmotic force by the concentrated particles [14], in which the capillary pressure is inversely proportional to the surface-area equivalent spherical radius for the particles [15]. Therefore, larger particles would be expected to reduce the capillary force for concentrating particles. This would leave a certain amount of molten camphene inside the TiH2 walls, which would allow the additional dendritic growth of camphene, particularly in the early stages of freezing. However, these small camphene dendrites can merge into larger dendrites during freezing at 42 °C, due to the potential for partial re-melting of the camphene dendrites and their continual overgrowth at this temperature. The change in microstructure of the Ti walls affected the compressive strengths of the samples significantly. As the freezing time was increased from 1 to 7 days, the compressive strength was improved remarkably from 48 ± 10 to 110 ± 17 MPa, as shown in Fig. 4. This improvement was attributed mainly to the achievement of the dense Ti walls without any noticeable micro-pores or cracks (see Fig. 3). It should be noted that the highest value of 110 MPa obtained in this study is comparable to that of natural cortical bone [16], which suggests these porous materials can be applied to as load-bearing parts. It should be noted that the porous Ti scaffolds produced in this study are mainly comprised of large interconnected pores with a mean pore size greater than 100 µm, which should provide a favorable environment for bone ingrowth into the pores [17,18]. In addition, they can have unusually high compressive strengths at a reasonably
This study examined the effect of the freezing time on the development of the porous structure, such as the pore size and microstructure of Ti walls, and the compressive strength of the porous Ti scaffolds. As the freezing time was increased from 1 to 7 days, the pore size was increased remarkably from 143 to 271 μm and, at the same time, surprisingly, the compressive strength was also remarkably improved from 48 ± 10 to 110 ± 17 MPa due to the elimination of micro-pores formed in the Ti walls. It was possible to significantly improve the compressive strength of the porous Ti scaffolds with a larger pore size, which is not normally achieved using the conventional freeze casting method. Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] Long M, Rack HJ. Biomaterials 1998;19:1621–39. [2] Wen CE, Yamada Y, Shimojima K, Chino Y, Asahina T, Mabuchi M. J Mater Sci Mater Med 2002;13:397–401. [3] Niinomi M. Mater Sci Eng A 1998;243:231–6. [4] Ryan G, Pandit A, Apatsidis DP. Biomaterials 2006;27:2651–70. [5] St-Pierre JP, Gauthier M, Lefebvre LP, Tabrizian M. Biomaterials 2005;26:7319–28. [6] Oh IH, Nomura N, Masahashi N, Hanada S. Scripta Mater 2003;49:1197–202. [7] Bram M, Stiller C, Buchkremer HP, Stöver D, Baur H. Adv Eng Mater 2000;2:196–9. [8] Li JP, Habibovic P, van den Doel M, Wilson CR, de Wijn JR, van Blitterswijk CA, et al. Biomaterials 2007;28:2810–20. [9] Cachinho SCP, Correia RN. J Mater Sci Mater Med 2008;19:451–7. [10] Chino Y, Dunand DC. Acta Mater 2008;56:105–13. [11] Yook SW, Yoon BH, Kim HE, Koh YH, Kim YS. Mater Lett 2008;62:4506–8. [12] Yoon BH, Choi WY, Kim HE, Kim JH, Koh YH. Scripta Mater 2008;58:537–40. [13] Yoon BH, Koh YH, Park CS, Kim HE. J Am Ceram Soc 2007;90:1744–52. [14] Shanti N, Araki K, Halloran JW. J Am Ceram Soc 2006;89:2444–7. [15] Smith DM, Scherer GW, Anderson JM. J Non-Cryst Solids 1995;188:191–206. [16] Rezwan K, Chen QZ, Blaker JJ, Boccaccini AR. Biomaterials 2006;27:3413–31. [17] Hulbert SF, Young FA, Mathews RS, Klawitter JJ, Talbert D, Stelling FH. J Biomed Mater Res 1970;4:433–56. [18] Lu JX, Flautre B, Anselme K, Hardouin P, Gallur A, Descamps M, et al. J Mater Sci Mater Med 1999;10:111–20.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Fabrication of porous titanium scaffolds with high compressive strength using camphene-based freeze casting Se-Won Yook a, Hyoun-Ee Kim a, Young-Hag Koh b,⁎ a b
Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Republic of Korea Department of Dental Laboratory Science and Engineering, Korea University, Seoul, 136-703, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2009 Accepted 29 March 2009 Available online 5 April 2009 Keywords: Metals and alloys Porosity Mechanical properties Titanium (Ti) Freeze casting
a b s t r a c t We herein report the fabrication of highly porous titanium (Ti) scaffolds with unusually high compressive strength by freezing a titanium hydride (TiH2)/camphene slurries at 42 °C. As the freezing time was increased from 1 to 7 days, the pore size obtained was increased significantly from 143 to 271 μm due to the continual overgrowth of camphene dendrites. However, interestingly, the formation of the micro-pores inside the Ti walls was suppressed at longer freezing time. This resulted in a significant increase in compressive strength up to 110 ± 17 MPa with a porosity of 64%. It is believed that this unusually high compressive strength with large interconnected pores makes this material suitable for applications as loadbearing parts. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the past few decades, titanium (Ti) and its alloys have been used successfully in dental and orthopedic implants, on account of their excellent mechanical properties, chemical stability, and biocompatibility [1]. However, these materials often suffer from relatively high stiffness compared to the surrounding bones, which can cause bone resorption and eventually loosening of the implant [2,3]. One of the most promising approaches for overcoming this limitation is to create pores in the bulks that can allow matching of the mechanical properties of materials to those of the surrounding bone [4,5]. Furthermore, these interconnected pores can provide a favorable environment for bone ingrowth, which would lead to a tight fixation between the implant and surrounding bone Thus far, considerable effort has been made to develop new manufacturing processes for the production of porous Ti scaffolds [6– 9]. More recently, freeze casting has been also developed to tailor the pore structure of porous materials, such as porosity, pore size, pore shape, and pore orientation [10,11]. This method makes full use of the dendritic growth of a freezing vehicle including water [10] and camphene [11], which can be removed by freeze-drying to create interconnected pores in the bulk. In this study, we examined how the freezing time affects the development of the pore structure and compressive strength of porous Ti scaffolds using the camphenebased freeze casting. To accomplish this, a titanium hydride (TiH2) / camphene slurry with an initial TiH2 content of 10 vol.% was frozen at 42 °C for various times (1, 4, and 7 days), followed by freeze-drying
⁎ Corresponding author. Tel.: +82 2 940 2844. E-mail address: [email protected] (Y.-H. Koh). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.056
and heat-treatment in a vacuum at 1300 °C for 2 h. The porous structures (e.g., porosity, pore size, pore shape, and densification of Ti walls) were examined. In addition, compressive strength tests were performed to evaluate the mechanical property of the samples. 2. Experimental procedure Commercially available titanium hydride powder (TiH2, Alfa Aesar, Ward Hill, MA, USA) was used as the source for Ti metal [9,11]. Camphene (C10H16, Sigma Aldrich, St. Louis, MO, USA) with a purity of 95% was used as the freezing vehicle without further purification. A titanium hydride/ camphene slurry with a TiH2 content of 10 vol.% was prepared by ballmilling at 60 °C for 24 h using 1 wt.% of oligomeric polyester (Hypermer KD-4; UniQema, Everburg, Belgium) as a dispersant. The optimum freezing temperature was determined by examining the solidification of the slurry using differential scanning calorimetry (DSC; DSC Q 1000, TA Instruments, West Sussex, UK). Based on this observation, the warm slurry prepared was poured into aluminum molds with dimensions of 15 × 15 × 15 mm and kept in an oven at 42 °C for various times (1, 4, and 7 days). After freeze-drying to remove the frozen camphene, the samples were heated to 400 °C at a heating rate of 3 °C/min in a vacuum and kept at this temperature for 2 h to induce the complete hydride decomposition of the TiH2 powder, followed by subsequent heat-treatment at 1300 °C for 2 h to consolidate the Ti powder. The porous structures of the porous Ti scaffolds and densifications of the Ti walls were characterized by scanning electron microscopy (FE-SEM, JSM-6330F, JEOL Techniques, Tokyo, Japan). The pore size was also analyzed from the SEM images of the samples prepared by infiltrating the porous Ti scaffolds with an epoxy resin (Spurrs epoxy,
S.-W. Yook et al. / Materials Letters 63 (2009) 1502–1504
1503
Fig. 1. SEM micrographs of the porous Ti scaffolds produced with various freezing times of (A) 1 day, (B) 4 days, and (C) 7 days, at 42 °C.
Polysciences Inc., Warrington, PA). The mechanical properties were evaluated by compressing 5 × 5 × 7 mm sized specimens using a screw-driven load frame (Instron 5565, Instron Corp., Canton, MA, USA) at a crosshead speed of 5 mm/min. The compressive strengths were calculated from the compressive stress-strain curves. Five samples were tested to obtain the average values along and its standard deviation.
3. Results and discussion The solidification behavior of a TiH2/camphene slurry with an initial TiH2 content of 10 vol.% was first examined using differential scanning calorimetry (DSC), since a new type of camphene with a higher melting point of 48–52 °C (from the manufacturer's specifications) was used in this study. A strong exothermic peak was observed at 42.7 °C (data not shown here), suggesting that the optimum freezing temperature is 42 °C, which would be expected to allow the
Fig. 2. Pore size of the porous Ti scaffolds as a function of the freezing time.
continual overgrowth of the camphene dendrites, while enabling the complete solidification of the slurry. In order to examine how the freezing time can affect the growth of camphene dendrites, the slurry was kept at 42 °C for various times, ranging from 1 to 7 days. All of the fabricated samples showed a highly porous structure with large interconnected pores, as shown in Fig. 1 (A)–(C). This suggests that the new type of camphene with a higher melting point can induce the successful continual overgrowth of camphene dendrites and the redistribution of TiH2 particles during freezing at 42 °C, which is similar to previous reports [11–13]. It should be noted that the pore size was increased remarkably with increasing the freezing time. However, regardless of the freezing time, all of the fabricated samples showed a similar porosity of 64%, indicating negligible sedimentation of the TiH2 particles during freezing. The pore sizes of the porous Ti scaffolds prepared using various freezing times were calculated from SEM images of the epoxy-filled samples that were further digitally colored for more accurate measurement. As expected, the pore size was increased remarkably from 144 ± 21 to 271 ± 15 µm with increasing the freezing time from 1 to 7 days, as shown in Fig. 2. A longer freezing time might lead to a larger pore size, but would, inevitably, cause the considerable evaporation of the frozen camphene, because the freezing temperature would be very close to its melting point. The freezing time significantly affected the microstructures of the Ti walls, as shown in Fig. 3 (A)–(C). Interestingly, the micro-pores formed as a replica of relatively small camphene dendrites disappeared with in creasing the freezing time. In particular, the sample prepared using a freezing time of 1 day showed many micro-pores inside the Ti walls (Fig. 3 (A)). However, the formation of these micropores was suppressed by increasing the freezing time to 4 days (Fig. 3 (B)). Eventually, the micro-pores disappeared after freezing for 7 days, allowing the achievement of fully dense Ti walls (Fig. 3 (C)). To the best of our best knowledge, this singular phenomenon has been never observed for the freeze casting method using either camphene or water as the freezing medium. We herein speculate that the particle size of TiH2 (1–3 μm) was much larger than conventional ceramic particles with a size in the submicron range. The packing density of the ceramic walls should be determined by the balance
Fig. 3. SEM micrographs of the microstructures of the Ti walls produced with various freezing times of (A) 1 day, (B) 4 days, and (C) 7 days, at 42 °C.
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S.-W. Yook et al. / Materials Letters 63 (2009) 1502–1504
high porosity. These striking features should allow the porous Ti scaffolds produced to find very useful applications as the scaffold for bone regeneration. It is also worth mentioning that their pore structure (e.g., porosity, pore size, pore shape, pore alignment) would be further controlled using the present camphene-based freeze casting method. 4. Conclusions
Fig. 4. Compressive strengths of the porous Ti scaffolds produced with various freezing times at 42 °C.
between the capillary drag force pushing the particles by the molten camphene and the resisting osmotic force by the concentrated particles [14], in which the capillary pressure is inversely proportional to the surface-area equivalent spherical radius for the particles [15]. Therefore, larger particles would be expected to reduce the capillary force for concentrating particles. This would leave a certain amount of molten camphene inside the TiH2 walls, which would allow the additional dendritic growth of camphene, particularly in the early stages of freezing. However, these small camphene dendrites can merge into larger dendrites during freezing at 42 °C, due to the potential for partial re-melting of the camphene dendrites and their continual overgrowth at this temperature. The change in microstructure of the Ti walls affected the compressive strengths of the samples significantly. As the freezing time was increased from 1 to 7 days, the compressive strength was improved remarkably from 48 ± 10 to 110 ± 17 MPa, as shown in Fig. 4. This improvement was attributed mainly to the achievement of the dense Ti walls without any noticeable micro-pores or cracks (see Fig. 3). It should be noted that the highest value of 110 MPa obtained in this study is comparable to that of natural cortical bone [16], which suggests these porous materials can be applied to as load-bearing parts. It should be noted that the porous Ti scaffolds produced in this study are mainly comprised of large interconnected pores with a mean pore size greater than 100 µm, which should provide a favorable environment for bone ingrowth into the pores [17,18]. In addition, they can have unusually high compressive strengths at a reasonably
This study examined the effect of the freezing time on the development of the porous structure, such as the pore size and microstructure of Ti walls, and the compressive strength of the porous Ti scaffolds. As the freezing time was increased from 1 to 7 days, the pore size was increased remarkably from 143 to 271 μm and, at the same time, surprisingly, the compressive strength was also remarkably improved from 48 ± 10 to 110 ± 17 MPa due to the elimination of micro-pores formed in the Ti walls. It was possible to significantly improve the compressive strength of the porous Ti scaffolds with a larger pore size, which is not normally achieved using the conventional freeze casting method. Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] Long M, Rack HJ. Biomaterials 1998;19:1621–39. [2] Wen CE, Yamada Y, Shimojima K, Chino Y, Asahina T, Mabuchi M. J Mater Sci Mater Med 2002;13:397–401. [3] Niinomi M. Mater Sci Eng A 1998;243:231–6. [4] Ryan G, Pandit A, Apatsidis DP. Biomaterials 2006;27:2651–70. [5] St-Pierre JP, Gauthier M, Lefebvre LP, Tabrizian M. Biomaterials 2005;26:7319–28. [6] Oh IH, Nomura N, Masahashi N, Hanada S. Scripta Mater 2003;49:1197–202. [7] Bram M, Stiller C, Buchkremer HP, Stöver D, Baur H. Adv Eng Mater 2000;2:196–9. [8] Li JP, Habibovic P, van den Doel M, Wilson CR, de Wijn JR, van Blitterswijk CA, et al. Biomaterials 2007;28:2810–20. [9] Cachinho SCP, Correia RN. J Mater Sci Mater Med 2008;19:451–7. [10] Chino Y, Dunand DC. Acta Mater 2008;56:105–13. [11] Yook SW, Yoon BH, Kim HE, Koh YH, Kim YS. Mater Lett 2008;62:4506–8. [12] Yoon BH, Choi WY, Kim HE, Kim JH, Koh YH. Scripta Mater 2008;58:537–40. [13] Yoon BH, Koh YH, Park CS, Kim HE. J Am Ceram Soc 2007;90:1744–52. [14] Shanti N, Araki K, Halloran JW. J Am Ceram Soc 2006;89:2444–7. [15] Smith DM, Scherer GW, Anderson JM. J Non-Cryst Solids 1995;188:191–206. [16] Rezwan K, Chen QZ, Blaker JJ, Boccaccini AR. Biomaterials 2006;27:3413–31. [17] Hulbert SF, Young FA, Mathews RS, Klawitter JJ, Talbert D, Stelling FH. J Biomed Mater Res 1970;4:433–56. [18] Lu JX, Flautre B, Anselme K, Hardouin P, Gallur A, Descamps M, et al. J Mater Sci Mater Med 1999;10:111–20.