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Microwave assisted solution combustion synthesis of strontium phosphate (SrP) whiskers
Materials Letters 116 (2014) 286–288
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave assisted solution combustion synthesis of strontium phosphate (SrP) whiskers Huan Zhou a,n, Michael Nedley b, Sarit B. Bhaduri a,b a b
Department of Mechanical, Industrial and Manufacturing Engineering, The University of Toledo, Toledo, OH, USA Division of Dentistry, The University of Toledo, Toledo, OH, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 2 June 2013 Accepted 3 November 2013 Available online 9 November 2013
A rapid microwave assisted solution combustion process is used to synthesize SrCaP and SrP whiskers. It is observed that SrP whiskers composed of Sr2P2O7 and Sr5(PO4)3Cl can be efficiently synthesized. However, co-existence of Ca2 þ and Sr2 þ ions in the reaction can inhibit one-dimensional crystal growth. The introduction of Cl ions to the reacting system is the key to induce the crystal formation towards to one-dimensional structure. The as-synthesized SrP whiskers were characterized and evaluated for potential biomedical applications. & 2013 Elsevier B.V. All rights reserved.
Keywords: Microwave Whiskers Strontium phosphate
1. Introduction In recent years, there is an increasing interest in adding strontium (Sr) to calcium phosphate (CaP) for performance improvements. First, as a member of alkaline earth elements like Ca, Sr2 þ can be easily combined into the CaP structure [1]. Second, Sr2 þ is known to stimulate osteoblast bone cells proliferation and inhibit the activity of osteoclast bone cells for bone regeneration [2]. Third, Sr2 þ can provide radio-opacity to implanted matrix for in vivo tracking [3]. Finally, Sr2 þ potentially possesses antibacterial property [4]. CaP whiskers are used as fillers for matrices to improve mechanical and biological performances [5,6]. Its one-dimensional structure is expected to provide enhanced mechanical properties and high surface/volume ratio. Recognizing these facts, there is a motivation to synthesize both strontium doped calcium phosphate (SrCaP) and strontium phosphate (SrP) whiskers as implant reinforcements. However, hydrothermal method is the only reported technique for the preparation of both materials [1,7,8]. Shen et al. synthesized strontium doped hydroxyapatite (Sr-HA) nano whiskers via incubating aqueous solutions containing Ca2 þ , Sr2 þ , HPO42 , acetamide at 180 1C autoclave for 10 h [7]. The products had widths of 0.2–8 μm and lengths up to 155 μm. Similarly, Lam et al. prepared strontium phosphate chloride nanowires via incubating strontium tri-polyphosphate and Collin salt in 1,4-dioxane at 150 1C for 24 h [8]. Therefore, there is a need to explore a more efficient process to synthesize one-dimensional SrP and SrCaP materials. Previously, our group reported the rapid synthesis of HA, tricalcium phosphate (TCP), biphasic calcium phosphate (BCP),
n
Corresponding author. Tel.: þ 1 4195306081. E-mail address: [email protected] (H. Zhou).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.013
chlorapatite (CA) whiskers via microwave-assisted solution combustion synthesis (MASCS) process [9–11]. In the process, aqueous solutions containing appropriate reagents were irradiated in a household microwave oven for minutes. The solidified substances were then simply stirred in water at room temperature to obtain the whiskers of the desired CaP phase. Realizing the chemical similarity between Ca2 þ and Sr2 þ , it is hypothesized that SrCaP and SrP whiskers can also be synthesized through MASCS. This paper reports the results of preliminary experiments. 2. Experiments Chemicals used in experiments were from Fisher Scientific. The chemical compositions of aqueous mixture for synthesizing SrCaP and SrP whiskers are listed in Table 1. In the composition SrCaP, the molar ratio of Sr/Ca is 1/9, and (Sr þCa)/P is 1.5/1. In the composition SrP, the molar ratio of Sr/P is 1.5/1. The chemicals were added in 10 mL of de-ionized water (DI water) in a 30 mL Pyrex beaker under constant stirring for 30 min. It was noted that once KCl was added, the final mixture was a suspension instead of transparent solution. The beaker was positioned on a 10 10 1 cm3 alumina fiberboard and covered with a 250 mL inverted Pyrex beaker. The assemblage was placed in a household 1200 W microwave (Panasonic) and heated at full power for 5 min. The assemblage was subsequently cooled in air. The beaker with solidified substance was soaked in DI water to suspend the as-synthesized particles. The suspension was filtered and washed with 1 L of DI water to remove residual ions followed by drying overnight. The as-synthesized particles were examined by scanning electron microscope (SEM, S4800, Hitachi) equipped with energy
H. Zhou et al. / Materials Letters 116 (2014) 286–288
287
Table 1 Compositions of aqueous mixture in 10 ml DI water.
SrCaP SrP
NaNO3 (g)
Ca(NO3)2 4H2O (g)
KH2PO4 (g)
HNO3 (ml)
KCl (g)
SrCl2 6H2O (g)
Sr/(Sr þ Ca)
5 5
0.6 –
0.384 0.384
1.5 1.5
8.752 8.752
0.07523 0.75232
0.1 1
Fig. 1. Visualization of SrCaP and SrP particles (a) SEM of SrCaP; (b) SEM of SrP; and (c) TEM of SrP.
dispersive x-ray spectroscopy (EDS) to determine the morphologies and related chemistry. They were further characterized using x-ray diffraction (XRD, Ultima III, Rigaku) and transmission electron microscope (TEM, HD-2300A, Hitachi). The zeta potential measurement of SrP was performed using zeta-sizer (380 ZLS, Nicomp) via loading SrP-DI water (0.001 g/50 ml) solution. The surface activity of the particles was studied by incubating powders in simulated body fluid (SBF) [9] for 7 days. To study the reinforcement of whiskers to matrices, the synthesized whiskers were added to a monetite–silica (CaHPO4– SiO2) bone cement. The CaHPO4–SiO2 and whiskers mixtures with whiskers content of 0 and 2.5 wt% were prepared. Then DI water was used to set the cements, and the weight ratio of mixtures to H2O was set at 2:1. The initial (ti) and final (tf) setting times of cements at 37 1C with 100% humidity were measured using Gillmore needle (Humboldt Mfg. Co.) and related compressive strength after 24 h setting were tested using a universal testing machine (model 5569, Instron).
3. Results and discussion As shown in Fig. 1a, particles from SrCaP are a mixture of whiskers and irregular bead-like materials. In Fig. 1b, the synthesized SrP products are a mixture of micro- and nano-whiskers. The EDS analysis indicates these whiskers are mainly composed of Sr, P, O and Cl. Fig. 1c is the TEM of SrP whiskers, showing a dense inner structure of the asformed whiskers. The XRD characterization (Fig. 2a) shows SrP whiskers are mainly composed of distrontium diphosphate (Sr2P2O7, JCPDs PDF#97-003-1004) and pentastrontium tris(phosphate) chloride (Sr5(PO4)3Cl, JCPDs PDF#97-000-2089). The relevent zeta potential value is highly negative ( 25.0373.76 mV). After 7 days SBF incubation, no change is found on the XRD patterns of SrP whiskers (Fig. 2b). In simultaneous SEM examination, the morphologies of whiskers were preserved, and no needle-like apatite coating formed on the surface of whiskers. Instead, some amorphous apatite nano-granules were observed to randomly appear on the surface of whiskers. Fig. 3 is the evaluation data of SrP whisker containing DCPA-SiO2 cement. It is shown the addition of SrP whiskers can delay the setting time (ti from 2.570.5 to 3.570.5 min, tf from 8.570.5 to 1971 min) and improve the compressive strength of DCPA-SiO2 cement from 10.3571.36 MPa to 15.6470.67 MPa. Co-existence of Ca2 þ and Sr2 þ inhibits both one-dimensional crystal growth, resulting in micro-sized irregular bead-like particles
Fig. 2. (a) XRD patterns of synthesized SrP whiskers and related zeta potential result, (h k l) and “□” refers to indices of Sr2P2O7 planes, and (h k l) and “●” refers to indices of Sr5(PO4)3Cl planes; (b) XRD patterns of SrP whiskers after 7 days incubation in SBF and related SEM image.
instead of whiskers. Similar inhibiting phenomenon was observed in Ca–Mg system, in which Mg2 þ inhibited the transition of amorphous calcium phosphate (ACP) in highly crystallized hydroxyapatite (HA) [12,13]. Therefore, synthesis of SrCaP using this MASCS is not applicable. The introduction of Cl to the reacting system seems to induce the crystal formation towards to one-dimensional structure as expected, matching previous reports [8,10]. The XRD results confirm the synthesized micro- and nano-whiskers are composed
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H. Zhou et al. / Materials Letters 116 (2014) 286–288
element in the SBF solution does not reach the threshold necessary to induce CaP crystallization [17]. The high negative zeta potential value of SrP whiskers indicates the formed particles are naturally deagglomerated, thus providing some advantages in matrix reinforcement. It is known that agglomeration of fillers in the matrix can result in inhomogeneous phase distribution and stress concentration sites in matrix, causing loss in mechanical properties [18]. The cement study shows SrP whiskers can improve the compressive strength of cement as expected. Simultaneously, the setting time is delayed due to the interruption of Sr2 þ on the CaP crystal growth.
4. Conclusion
Fig. 3. Results of compressive strength and setting time of cements CaHPO4–SiO2 and CaHPO4–SiO2–SrP.
of Sr2P2O7 and Sr5(PO4)3Cl, different from the composition of hydrothermal synthesized SrP whiskers as reported before [8]. The growth of SrP probably follows a “dissolution-crystallization-whisker-growth” process [9]. Regular SrP precipitates were formed during boiling and evaporation of water in the early stage of microwave irradiation. The molten NaNO3 wraps all the initially formed SrP precipitates to form uniform liquid phase under continuous microwave irradiation. Embryonic particles of SrP phases are formed through the nucleation and growth processes during the cooling of the molten ionic bath. During the cooling, rapid crystallization occurs along the preferred growth axes of the SrP phase, especially in the presence of Cl ions. It is expected that the morphology of particles growing out of the cooling molten salt bath could be strongly dependent on soaking time at the peak temperature, as well as cooling rate. Therefore, the rapid cooling after microwave heating is a possible reason resulting in formation of both micro and nano sized whiskers. As to various applications, it has been reported both Sr2P2O7 and Sr5(PO4)3Cl can be a potential luminescent agent after doping lanthanide series elements into its crystal structure [14,15]. Indeed, we have successfully prepared luminescent CaP whiskers via doping europium (Eu) via MASCS before [10,11]. Therefore, preparation of luminescent SrP whiskers is also possible via adding lanthanide salts to MASCS. The SBF incubation shows SrP whiskers are stable and no apatite coating were deposited on the surfaces of them. This phenomenon can be attributed to facts: (1) Sr2 þ can significantly inhibit the growth of crystallized Ca-apatite in SBF [16]; (2) there is no Ca2 þ in SrP composition and so the concentration of this
SrP whiskers can be synthesized using MASCS by controlling the composition of reactants. The presence of Cl ions in the reaction system is a key factor to induce the formation of whiskers. It is expected this efficient whisker synthesis process and resultant products have potential in bone cement and luminescent material applications.
Acknowledgment This work is partially supported by the NSF funding CMMI 0753479 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Kim T-G, Park B. Inorg Chem 2005;44:9895–901. Bonnelye E, Chabadel A, Saltel F, Jurdic P. Bone 2008;42:129–38. Wang X, Ye J, Wang Y. Acta Biomater 2007;3:757–63. Ravi ND, Balu R, Kumar TSSampath. J Ame Ceram Soc 2012;95:2700–8. Suchanek W, Yashima M, Kakihana M, Yoshimura M. Biomaterials 1996;17: 1715–23. Roeder RK, Sproul MM, Turner CH. J Biomed Mater Res Part A 2003;67A: 801–12. Shen Y, Liu J, Lin K, Zhang W. Mater Lett 2012;70:76–9. Lam WM, Wong CT, Li ZY, Luk KDK, Chan WK, Yang C, et al. J Cryst Growth 2007;306:129–34. Jalota S, Bhaduri SB, Tas AC. J Biomed Mater Res Part A 2006;78A:481–90. Nabiyouni M, Zhou H, Bhaduri SB. Mater Lett 2013;108:54–7. Wagner DE, Eisenmann KM, Nestor-Kalinoski AL, Bhaduri SB. Acta Biomater 2013;9:8422–32. Boskey AL, Posner AS. Mater Res Bull 1974;9:907–16. Zhou H, Bhaduri S. J Biomed Mater Res Part B: Appl Biomater 2012;100B:1142–50. Hou D, Han B, Chen W, Liang H, Su Q, Dorenbos P, et al. J Appl Phys 2010;108:083527-1–6. Guo C, Luan L, Ding X, Zhang F, Shi FG, Gao F, et al. Appl. Phys. B 2009;95:779–85. Bracci B, Torricelli P, Panzavolta S, Boanini E, Giardino R, Bigi A. J Inorg Biochem 2009;103:1666–74. Castro AGB, Almeida JC, Salvado IMM, Margaça FMA, Fernandes MHV. Mater Lett 2012;88:12–5. Harris P. Int Mater Rev 2004;49:31–43.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave assisted solution combustion synthesis of strontium phosphate (SrP) whiskers Huan Zhou a,n, Michael Nedley b, Sarit B. Bhaduri a,b a b
Department of Mechanical, Industrial and Manufacturing Engineering, The University of Toledo, Toledo, OH, USA Division of Dentistry, The University of Toledo, Toledo, OH, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 2 June 2013 Accepted 3 November 2013 Available online 9 November 2013
A rapid microwave assisted solution combustion process is used to synthesize SrCaP and SrP whiskers. It is observed that SrP whiskers composed of Sr2P2O7 and Sr5(PO4)3Cl can be efficiently synthesized. However, co-existence of Ca2 þ and Sr2 þ ions in the reaction can inhibit one-dimensional crystal growth. The introduction of Cl ions to the reacting system is the key to induce the crystal formation towards to one-dimensional structure. The as-synthesized SrP whiskers were characterized and evaluated for potential biomedical applications. & 2013 Elsevier B.V. All rights reserved.
Keywords: Microwave Whiskers Strontium phosphate
1. Introduction In recent years, there is an increasing interest in adding strontium (Sr) to calcium phosphate (CaP) for performance improvements. First, as a member of alkaline earth elements like Ca, Sr2 þ can be easily combined into the CaP structure [1]. Second, Sr2 þ is known to stimulate osteoblast bone cells proliferation and inhibit the activity of osteoclast bone cells for bone regeneration [2]. Third, Sr2 þ can provide radio-opacity to implanted matrix for in vivo tracking [3]. Finally, Sr2 þ potentially possesses antibacterial property [4]. CaP whiskers are used as fillers for matrices to improve mechanical and biological performances [5,6]. Its one-dimensional structure is expected to provide enhanced mechanical properties and high surface/volume ratio. Recognizing these facts, there is a motivation to synthesize both strontium doped calcium phosphate (SrCaP) and strontium phosphate (SrP) whiskers as implant reinforcements. However, hydrothermal method is the only reported technique for the preparation of both materials [1,7,8]. Shen et al. synthesized strontium doped hydroxyapatite (Sr-HA) nano whiskers via incubating aqueous solutions containing Ca2 þ , Sr2 þ , HPO42 , acetamide at 180 1C autoclave for 10 h [7]. The products had widths of 0.2–8 μm and lengths up to 155 μm. Similarly, Lam et al. prepared strontium phosphate chloride nanowires via incubating strontium tri-polyphosphate and Collin salt in 1,4-dioxane at 150 1C for 24 h [8]. Therefore, there is a need to explore a more efficient process to synthesize one-dimensional SrP and SrCaP materials. Previously, our group reported the rapid synthesis of HA, tricalcium phosphate (TCP), biphasic calcium phosphate (BCP),
n
Corresponding author. Tel.: þ 1 4195306081. E-mail address: [email protected] (H. Zhou).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.013
chlorapatite (CA) whiskers via microwave-assisted solution combustion synthesis (MASCS) process [9–11]. In the process, aqueous solutions containing appropriate reagents were irradiated in a household microwave oven for minutes. The solidified substances were then simply stirred in water at room temperature to obtain the whiskers of the desired CaP phase. Realizing the chemical similarity between Ca2 þ and Sr2 þ , it is hypothesized that SrCaP and SrP whiskers can also be synthesized through MASCS. This paper reports the results of preliminary experiments. 2. Experiments Chemicals used in experiments were from Fisher Scientific. The chemical compositions of aqueous mixture for synthesizing SrCaP and SrP whiskers are listed in Table 1. In the composition SrCaP, the molar ratio of Sr/Ca is 1/9, and (Sr þCa)/P is 1.5/1. In the composition SrP, the molar ratio of Sr/P is 1.5/1. The chemicals were added in 10 mL of de-ionized water (DI water) in a 30 mL Pyrex beaker under constant stirring for 30 min. It was noted that once KCl was added, the final mixture was a suspension instead of transparent solution. The beaker was positioned on a 10 10 1 cm3 alumina fiberboard and covered with a 250 mL inverted Pyrex beaker. The assemblage was placed in a household 1200 W microwave (Panasonic) and heated at full power for 5 min. The assemblage was subsequently cooled in air. The beaker with solidified substance was soaked in DI water to suspend the as-synthesized particles. The suspension was filtered and washed with 1 L of DI water to remove residual ions followed by drying overnight. The as-synthesized particles were examined by scanning electron microscope (SEM, S4800, Hitachi) equipped with energy
H. Zhou et al. / Materials Letters 116 (2014) 286–288
287
Table 1 Compositions of aqueous mixture in 10 ml DI water.
SrCaP SrP
NaNO3 (g)
Ca(NO3)2 4H2O (g)
KH2PO4 (g)
HNO3 (ml)
KCl (g)
SrCl2 6H2O (g)
Sr/(Sr þ Ca)
5 5
0.6 –
0.384 0.384
1.5 1.5
8.752 8.752
0.07523 0.75232
0.1 1
Fig. 1. Visualization of SrCaP and SrP particles (a) SEM of SrCaP; (b) SEM of SrP; and (c) TEM of SrP.
dispersive x-ray spectroscopy (EDS) to determine the morphologies and related chemistry. They were further characterized using x-ray diffraction (XRD, Ultima III, Rigaku) and transmission electron microscope (TEM, HD-2300A, Hitachi). The zeta potential measurement of SrP was performed using zeta-sizer (380 ZLS, Nicomp) via loading SrP-DI water (0.001 g/50 ml) solution. The surface activity of the particles was studied by incubating powders in simulated body fluid (SBF) [9] for 7 days. To study the reinforcement of whiskers to matrices, the synthesized whiskers were added to a monetite–silica (CaHPO4– SiO2) bone cement. The CaHPO4–SiO2 and whiskers mixtures with whiskers content of 0 and 2.5 wt% were prepared. Then DI water was used to set the cements, and the weight ratio of mixtures to H2O was set at 2:1. The initial (ti) and final (tf) setting times of cements at 37 1C with 100% humidity were measured using Gillmore needle (Humboldt Mfg. Co.) and related compressive strength after 24 h setting were tested using a universal testing machine (model 5569, Instron).
3. Results and discussion As shown in Fig. 1a, particles from SrCaP are a mixture of whiskers and irregular bead-like materials. In Fig. 1b, the synthesized SrP products are a mixture of micro- and nano-whiskers. The EDS analysis indicates these whiskers are mainly composed of Sr, P, O and Cl. Fig. 1c is the TEM of SrP whiskers, showing a dense inner structure of the asformed whiskers. The XRD characterization (Fig. 2a) shows SrP whiskers are mainly composed of distrontium diphosphate (Sr2P2O7, JCPDs PDF#97-003-1004) and pentastrontium tris(phosphate) chloride (Sr5(PO4)3Cl, JCPDs PDF#97-000-2089). The relevent zeta potential value is highly negative ( 25.0373.76 mV). After 7 days SBF incubation, no change is found on the XRD patterns of SrP whiskers (Fig. 2b). In simultaneous SEM examination, the morphologies of whiskers were preserved, and no needle-like apatite coating formed on the surface of whiskers. Instead, some amorphous apatite nano-granules were observed to randomly appear on the surface of whiskers. Fig. 3 is the evaluation data of SrP whisker containing DCPA-SiO2 cement. It is shown the addition of SrP whiskers can delay the setting time (ti from 2.570.5 to 3.570.5 min, tf from 8.570.5 to 1971 min) and improve the compressive strength of DCPA-SiO2 cement from 10.3571.36 MPa to 15.6470.67 MPa. Co-existence of Ca2 þ and Sr2 þ inhibits both one-dimensional crystal growth, resulting in micro-sized irregular bead-like particles
Fig. 2. (a) XRD patterns of synthesized SrP whiskers and related zeta potential result, (h k l) and “□” refers to indices of Sr2P2O7 planes, and (h k l) and “●” refers to indices of Sr5(PO4)3Cl planes; (b) XRD patterns of SrP whiskers after 7 days incubation in SBF and related SEM image.
instead of whiskers. Similar inhibiting phenomenon was observed in Ca–Mg system, in which Mg2 þ inhibited the transition of amorphous calcium phosphate (ACP) in highly crystallized hydroxyapatite (HA) [12,13]. Therefore, synthesis of SrCaP using this MASCS is not applicable. The introduction of Cl to the reacting system seems to induce the crystal formation towards to one-dimensional structure as expected, matching previous reports [8,10]. The XRD results confirm the synthesized micro- and nano-whiskers are composed
288
H. Zhou et al. / Materials Letters 116 (2014) 286–288
element in the SBF solution does not reach the threshold necessary to induce CaP crystallization [17]. The high negative zeta potential value of SrP whiskers indicates the formed particles are naturally deagglomerated, thus providing some advantages in matrix reinforcement. It is known that agglomeration of fillers in the matrix can result in inhomogeneous phase distribution and stress concentration sites in matrix, causing loss in mechanical properties [18]. The cement study shows SrP whiskers can improve the compressive strength of cement as expected. Simultaneously, the setting time is delayed due to the interruption of Sr2 þ on the CaP crystal growth.
4. Conclusion
Fig. 3. Results of compressive strength and setting time of cements CaHPO4–SiO2 and CaHPO4–SiO2–SrP.
of Sr2P2O7 and Sr5(PO4)3Cl, different from the composition of hydrothermal synthesized SrP whiskers as reported before [8]. The growth of SrP probably follows a “dissolution-crystallization-whisker-growth” process [9]. Regular SrP precipitates were formed during boiling and evaporation of water in the early stage of microwave irradiation. The molten NaNO3 wraps all the initially formed SrP precipitates to form uniform liquid phase under continuous microwave irradiation. Embryonic particles of SrP phases are formed through the nucleation and growth processes during the cooling of the molten ionic bath. During the cooling, rapid crystallization occurs along the preferred growth axes of the SrP phase, especially in the presence of Cl ions. It is expected that the morphology of particles growing out of the cooling molten salt bath could be strongly dependent on soaking time at the peak temperature, as well as cooling rate. Therefore, the rapid cooling after microwave heating is a possible reason resulting in formation of both micro and nano sized whiskers. As to various applications, it has been reported both Sr2P2O7 and Sr5(PO4)3Cl can be a potential luminescent agent after doping lanthanide series elements into its crystal structure [14,15]. Indeed, we have successfully prepared luminescent CaP whiskers via doping europium (Eu) via MASCS before [10,11]. Therefore, preparation of luminescent SrP whiskers is also possible via adding lanthanide salts to MASCS. The SBF incubation shows SrP whiskers are stable and no apatite coating were deposited on the surfaces of them. This phenomenon can be attributed to facts: (1) Sr2 þ can significantly inhibit the growth of crystallized Ca-apatite in SBF [16]; (2) there is no Ca2 þ in SrP composition and so the concentration of this
SrP whiskers can be synthesized using MASCS by controlling the composition of reactants. The presence of Cl ions in the reaction system is a key factor to induce the formation of whiskers. It is expected this efficient whisker synthesis process and resultant products have potential in bone cement and luminescent material applications.
Acknowledgment This work is partially supported by the NSF funding CMMI 0753479 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Kim T-G, Park B. Inorg Chem 2005;44:9895–901. Bonnelye E, Chabadel A, Saltel F, Jurdic P. Bone 2008;42:129–38. Wang X, Ye J, Wang Y. Acta Biomater 2007;3:757–63. Ravi ND, Balu R, Kumar TSSampath. J Ame Ceram Soc 2012;95:2700–8. Suchanek W, Yashima M, Kakihana M, Yoshimura M. Biomaterials 1996;17: 1715–23. Roeder RK, Sproul MM, Turner CH. J Biomed Mater Res Part A 2003;67A: 801–12. Shen Y, Liu J, Lin K, Zhang W. Mater Lett 2012;70:76–9. Lam WM, Wong CT, Li ZY, Luk KDK, Chan WK, Yang C, et al. J Cryst Growth 2007;306:129–34. Jalota S, Bhaduri SB, Tas AC. J Biomed Mater Res Part A 2006;78A:481–90. Nabiyouni M, Zhou H, Bhaduri SB. Mater Lett 2013;108:54–7. Wagner DE, Eisenmann KM, Nestor-Kalinoski AL, Bhaduri SB. Acta Biomater 2013;9:8422–32. Boskey AL, Posner AS. Mater Res Bull 1974;9:907–16. Zhou H, Bhaduri S. J Biomed Mater Res Part B: Appl Biomater 2012;100B:1142–50. Hou D, Han B, Chen W, Liang H, Su Q, Dorenbos P, et al. J Appl Phys 2010;108:083527-1–6. Guo C, Luan L, Ding X, Zhang F, Shi FG, Gao F, et al. Appl. Phys. B 2009;95:779–85. Bracci B, Torricelli P, Panzavolta S, Boanini E, Giardino R, Bigi A. J Inorg Biochem 2009;103:1666–74. Castro AGB, Almeida JC, Salvado IMM, Margaça FMA, Fernandes MHV. Mater Lett 2012;88:12–5. Harris P. Int Mater Rev 2004;49:31–43.