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Synthesis and characterisation of sol gel derived bioactive glass for biomedical applications
Materials Letters 60 (2006) 3752 – 3757 www.elsevier.com/locate/matlet
Synthesis and characterisation of sol gel derived bioactive glass for biomedical applications A. Balamurugan a,⁎, G. Sockalingum b , J. Michel a , J. Fauré a , V. Banchet c , L. Wortham a , S. Bouthors a , D. Laurent-Maquin a , G. Balossier a a
INSERM ERM 0203, Laboratoire de Microscopie Electronique Analytique, Université de Reims, 21, Rue Clément Ader, 51685 Reims Cedex 2, France b Unité MéDIAN, CNRS UMR 6142, UFR de Pharmacie, Université de Reims, 51, rue Cognacq-Jay, 51096 Reims Cedex, France c Laboratoire de Tribologie et de Dynamique des Surfaces (LTDS), UMR CNRS 5513, Ecole Centrale de Lyon, département STMS, 36 avenue Guy de Collongue, 69134 Ecully, France Received 19 October 2005; accepted 26 March 2006 Available online 2 May 2006
Abstract Bioactive glasses have been used successfully as bone-filling materials in orthopaedic and dental surgery, but their poor mechanical strength limits their applications in load-bearing positions. Approaches to strengthen materials decrease their bioactivity. In order to realize the optimal matching between mechanical and biological properties, the sol-gel-self propagating method is adopted to prepare gel-derived bioglass bulk: 58S in the system SiO2–CaO–P2O5. The obtained glass was analysed for its composition, crystalinity and morphology through FT-IR, Raman, XRD, STEM and X-ray microanalysis. © 2006 Elsevier B.V. All rights reserved. Keywords: Sol gel; Bioglass; Bioactive
1. Introduction Bone regeneration is required in many clinical issues addressed by orthopaedic and dental medicine. The autogeneous bone graft is the gold standard, but host tissue is often scarce and can hardly be modeled to the shape required for successful reconstruction [1]. Then people's attention was transferred to implantation. Although bioactive ceramics have achieved great application successes in bone repairing, their elastic modulus mismatch and stress shielding of human bone cannot fill the longer and longer lifetimes. Tissue engineering provides a new approach for solving these problems. The biomaterials should be the scaffold to assist or enhance the body's own reparative capacity [2]. Bioglasses could elicit a specific biological in vivo response at the interface and attach to the tissues, such as soft tissue and
bone, with a strong chemical bond. So they have been widely used for a number of different applications. Certain compositions of bioactive glasses containing SiO2–CaO–P2O5 bond to both soft and hard tissue without an intervening fibrous layer. Results of in vivo implantation show that these compositions produce no local or systemic toxicity, no inflammation, and no foreign-body response [3]. At present, it is possible to develop some new biomaterials due to extensive overlaps between sol-gel chemistry and biochemistry. The sol-gel networks are excellent model systems for studying and controlling biochemical interactions within constrained matrices with well-defined textures. It is believed that enhanced bioactivity can be achieved in gel-derived materials because of their residual hydroxyl ions and micro pores, and large specific surface [4]. 2. Experimental
⁎ Corresponding author. Tel.: +33 326823586; fax: +33 326051900. E-mail address: [email protected] (A. Balamurugan). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.102
The composition studied was bioactive glass 58 wt.% SiO2-33 wt.% CaO-9 wt.% P2O5 (58S). The sol-gel precursors
A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757 Table 1 Composition of the Hank's balanced salt solution Reagents
Composition
NaCl KCl Na2HPO4 KH2PO4 CaCl2 MgSO4 NaHCO3
0.137 M 5.4 mM 0.25 mM 0.44 mM 1.3 mM 1.0 mM 4.2 mM
used in this study were tetraethyl orthosilicate, triethyl phosphate, and calcium nitrate, hydrolyzed in the presence of 2 mol/l of nitric acid. The initial procedure involved mixing distilled water, the appropriate alkoxide precursors and salts, and the catalyst. On completion of the hydrolysis, the sols were aged in a drying oven at 60 °C to reach high viscosity near the gel point. After aging for 1 day, the gels were dried in an environment containing 50:50% mixture of water/ethanol, and the temperature was raised to 120 °C slowly and kept for 48 h. Finally, the dried gels were heat treated in step wise fashion up to 900 °C for stabilization [5]. 3. Methods of testing The functional group analysis was performed by Fourier Transform Infrared Spectroscopy (FT-IR). The measurements were carried out in the transmission mode in the mid-infrared range (400–4000 cm− 1) at the resolution of 4 cm− 1. The studies were performed using the FT-IR, Spot light 300, Imaging system, Perkin-Elmer, France. For FT-IR measurements, KBr pellets containing the exactly weighted amount of the substance examined were prepared. For the collection of Raman spectra a confocal Raman microscope (LabRam, Horiba, Jobin-Yvon, France) consisting a
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point-focus ≈785 nm He–Ne laser source, a microscope (Olympus BX41) and a stigmatic spectrometer was used. The sample was placed on the stage of the microscope using a custom-made fixture such that the sample thickness was oriented perpendicularly to the laser beam incident from a 50× microscope objective. Scanning electron microscopy (SEM) (JEOL 5610 SEM and LEO 1525 field emission gun, FEG-SEM) was used to analyse the bioglass powders. The samples were coated with gold before the examination. SEM was mainly used to characterize the microstructure and grain size of the bioglass. X-ray powder diffraction method (Rigaku D/max-IIB). X-ray radiation of Cu K∝ (1.548 Å) to gain information about the phases. Scanning transmission electron microscope (Philips CM30) operating at a voltage of 100 kV. The microscope is fitted with an energy dispersive X-ray spectrometer (EDXS 30 mm2 Si (Li) RSUTW detector) and a parallel electron energy-loss spectrometer (Gatan model enfina 1000) placed under the STEM column. Analysis is carried out using a beryllium specimen holder with a 30° tilt. The concentration profiles were made across three different locations. The elemental composition was determined [6]. The biocompatibility of the bioglass particles were evaluated in simulated body fluid, Hank's balanced salt solution (HBSS) with ionic concentration nearly equal to the human blood plasma at static condition (Table 1). Immersion technique was adopted for the analysis, the glass particles were immersed into the SBF for various time periods finally the particles were analysed in XRD for the growth of HAP on the surface of the bioglass particles. 4. Results and discussion 4.1. FT-IR analysis The Fig. 1 shows the complex bands of the bioglass 58S, the bands in the region 1200–900 cm− 1 includes absorption due to both PO and
Fig. 1. FT-IR spectrum of the 58S bioglass sintered at 900 °C.
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A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757
Fig. 2. Raman spectrum of the 58S bioglass sintered at 900 °C.
SiO groups. As for the SiO groups, these can be grouped as SiO ‘bridging’ and ‘non-bridging’ to other SiO4 tetrahedra. In fact, some of the oxygen atoms are not directly connected to another Si atom in the glassy network, because the presence of cations induces the formation of SiO-groups. Non-bridging oxygens (NBO) are also related to hydroxyl groups (SiOH groups that break the silica bulk network). It is very difficult to distinguish between the frequency of absorption of each of these groups, and in the literature different hypotheses have been proposed [7]. Usually, in the presence of some NBOs, SiO frequency of absorption falls in the range 1040–940 cm− 1. In the spectrum of 58S, no bands are seen at frequencies higher than 1100 cm− 1. This means that most of SiO4 groups possess at least some NBOs. It is possible to assign the peak at 1040 cm− 1 observed in 58S to Si– O–Si groups, because when the sample is heat treated the relative intensity of this peak with respect to the band at 962 cm− 1 decreases.
Since thermal treatment induces sample dehydration (through condensation of surface hydroxyls: 2 SiOH → SiOSi + H2O), the lower relative intensity of the 1040 cm− 1 band should be related to the lower amount of hydroxyl groups. The band at 940 cm− 1 can then be assigned to SiO NBO formed because of the presence of cations, in particular Ca, since it was shown in the literature that SiO groups with 2 NBOs can absorb at this frequency. The band observed at 962 cm− 1 is assinged to the typical PO strech. The peak at 940 cm− 1 related to SiO NBO decreases, and a band at 1200 cm− 1 due to νSi–O–Si is formed. In particular, this band is typical of high surface area silica structures, since it was shown in the past that low surface silica presents a low intensity for the 1200 cm− 1 shoulder in the Si–O–Si νas peak [8]. 4.2. Raman analysis The typical Raman spectrum of 58S reveals the presence of three transverse-optical (TO) and longitudinal-optical (LO) pairs, at
Fig. 3. X-ray diffraction patterns of 58S bioglass at various sintering temperatures.
Fig. 4. SEM image of 58S bioglass at 300 °C.
A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757
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Fig. 5. SEM images of the cross-section of 58S bioglass sintered at 900 °C.
approximately the following frequencies: 410–460, 470, 720 and 1060–1190 cm− 1, respectively. In each pair, the lowest frequency is associated with TO excitations and the higher frequencies reveal the existence of Raman-active LO modes [9]. Using the nomenclature, the lowest frequency band is associated with a ‘bond-rocking’ vibration, in which the oxygen atom moves perpendicular to the Si– O–Si plane. The small TO–LO splitting observed at 960 cm− 1 is attributed to a ‘bond-bending’ vibration, in which the oxygen move approximately at right angles to the Si–Si lines and in the Si–O–Si plane; and finally the TO–LO splitting pair at high frequencies is assigned to a ‘bond-stretching’ vibration, in which the bridging oxygen atom moves parallel to the Si–Si lines in the opposite direction to their Si neighbours. Fig. 2 shows the Raman spectra of the bioglass, the well defined vibrational bands characteristic of the Si–O–Si stretching and bending modes which are associated with the crystalline phases of the samples were observed. The spectra normalized on the band centered at 470 cm− 1 (νsymm Si–O–Si), this band and the peak at 960 cm− 1 (δSi–
O–Si) are related to the amount of silica present. The phosphate (600 cm− 1 νPOO) bands are also observed at 505–610 cm− 1. As confirmed by the results, Raman spectroscopy is very sensitive to changes in the Si–O–Si environment of silica based glasses of different compositions and it is a powerful technique for the identification of Si–O–NBO bonds in bioactive glasses. The frequency shifting and intensity variations of the Raman bands observed for bioactive glasses are due to a decrease of the local symmetry originated by the addition of alkali and alkali earth oxides to the silica network. Higher concentrations of network modifiers promote the increase of intensity of the Raman lines and a dramatic emergence of the band at 950 cm− 1 due to the increase of the number of structural units including Si–O–NBO bonds. These effects are explained by the fundamentals of the Raman technique. Changes of the molecular local symmetry lead to variations in the Raman activity of the vibrational modes. As shown in Fig. 2, the Si–O–Si vibrational modes of the amorphous silica present a low Raman activity. These results are in agreement with those reported in the literature [9,10].
Fig. 6. EDXS spectrum of the sintered 58S bioglass at 900 °C.
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A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757
4.3. X-ray diffraction analysis The diffraction patterns of the 58S bioglass is shown in Fig. 3. It can be seen that the starting 58S bioglass powder was amorphous, proving the sol-gel method could prepare pure glasses. As the temperature increased up to 600 °C, only a broad peak existed in the pattern, showing that the bioglass powder still kept the amorphous state. With further increase of the temperature, sharp peaks appeared at 800 °C, indicating crystallization occurred in the 58S bioglass. The peaks matched the pattern of wollastonite CaSiO3 (JCPDS #27-0088). The CaSiO3 peaks became much sharper at 900 °C, corresponding to further crystallization. 4.4. Scanning electron microscopy analysis SEM. Fig. 4 shows the CaSiO3 bioglass surface after thermal treatment at 300 °C. With the increase of temperature to 900 °C, the precipitation of well-defined spherical and sheet crystals 2–5 μm in size appear in a glassy matrix. Microstructures of the cross-section have also been analyzed by SEM (Fig. 5). From the micrographs of the sample, the cross-sections are coarse. The sample has more but smaller pits and less sidestep surfaces on its surface, which suggests the crystalinity and less residual glass phase of the bioglass. With the increasing of thermal treatment temperature, there is a process of nucleation and crystal growth in 58S bioglass. The composition of materials will affect the process importantly at the same temperature. The velocity of the crystal growth is slower after it forms a nuclei. Sample treated at 1100 °C for 4 h, the 58S will form quantity of crystals with sheet and layer shapes on its surface. The average size of the crystal granule is 2–5 μm. The surface crystallization mechanism was confirmed through the SEM of crosssections of 58S.
Fig. 8. XRD patterns of the 58S bioglass after immersion into the Hank's solution.
analysis in STEM, absorption correction requires the knowledge of the samples' local mass thickness for EELS was used as a tool for the Xrays absorption correction. Combining EELS measured relative thickness and X-ray characteristic peak intensities the absorption corrected elemental weight concentrations were obtained with an iterative process the local mass thickness. The analysis was performed on bioglass sample in STEM. The Fig. 7 shows the micrograph of STEM, observed the homogenious distribution of the elements. The concentration profiles were also made on various grains and obtained homogenity.
4.5. EDXS and STEM analysis 4.6. Biocompatibility analysis in SBF The X-ray microanalysis was performed on 58S bioglass. The Fig. 6 represents the elemental composition of the bioglass. It can be noticed that no peaks correspond to any impurity element, other than those supposed to be present in the 58S. From the EDXS analysis the purity of the material was confirmed. The elemental composition of the bioglass is quantified by EDXS. The quantification of low Z elements requires X-ray absorption correction in the case of bulk substance
The X-ray diffraction analysis results of the glass are shown in Fig. 8. The bioglass particles were soaked into the HBSS up to 7 days, the substance formed on glasses were detected after 1 day immersion in SBF, new peaks (at 2θ 32°, 26°) were assigned to be (2 1 1), (0 0 2) apatite according to the standard JCPDS (09-0432). After 3rd day of immersion, the two peaks were intensified and the other peaks of apatite at 40°, 46°, 49° also appeared. After 7 days immersion all the peaks became more apparent. The widen diffraction peak at 2θ ranging from 30° to 34°, which should be distinguished the (2 1 1), (1 1 2) and (3 0 0) planes for well-crystallized hydroxyapatite. The X-ray diffraction patterns are indicative of the biocompatibility of the bioglass in SBF medium.
5. Conclusion The bioglass 58S was successfully synthesised by the sol-gel low temperature process and the composition was ensured from the analytical results. Further the biocompatibility analysis confirms the growth of HAP on the surface of the bioglass particles. References Fig. 7. STEM image of the bioglass particles.
[1] P. Sepulveda, J.R. Jones, L.L. Hench, J. Biomed. Mater. Res. 61 (2002) 301.
A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757 [2] P. Li, C. Ohtsuki, T. Kokubo, K. Nakanishi, N. Soga, J. Am. Ceram. Soc. 75 (1992) 2094. [3] D.C. Greenspan, J.P. Zhong, G.P. Latorre, Bioceramics 7 (1994) 28. [4] T. Peltola, M. Jokinen, H. Rahiala, E. Levanen, J.B. Rosenholm, I. Kangasniemi, A. Yli-urpo, J. Biomed. Mater. Res. 44 (1999) 12. [5] J.P. Zhong, D.C. Greenspan, J. Biomed. Mater. Res. 53 (2000) 694.
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[6] V. Banchet, J. Michel, J. Jallot, D. Laurent-Maquin, G. Balossier, J. Phys., D. Appl. Phys. 36 (2003) 1599. [7] F. Devreux, P. Baroboux, J. Nucl. Mater. 298 (2001) 145. [8] M. Cerruti, D. Greenspan, K. Powers, Biomaterials 26 (2005) 1665. [9] G. Penel, G. Leroy, C. Rey, E. Bres, Calcif. Tissue Int. 63 (1998) 475. [10] D. Qin, R.T. Kean, Appl. Spectrosc. 52 (1998) 488.
Synthesis and characterisation of sol gel derived bioactive glass for biomedical applications A. Balamurugan a,⁎, G. Sockalingum b , J. Michel a , J. Fauré a , V. Banchet c , L. Wortham a , S. Bouthors a , D. Laurent-Maquin a , G. Balossier a a
INSERM ERM 0203, Laboratoire de Microscopie Electronique Analytique, Université de Reims, 21, Rue Clément Ader, 51685 Reims Cedex 2, France b Unité MéDIAN, CNRS UMR 6142, UFR de Pharmacie, Université de Reims, 51, rue Cognacq-Jay, 51096 Reims Cedex, France c Laboratoire de Tribologie et de Dynamique des Surfaces (LTDS), UMR CNRS 5513, Ecole Centrale de Lyon, département STMS, 36 avenue Guy de Collongue, 69134 Ecully, France Received 19 October 2005; accepted 26 March 2006 Available online 2 May 2006
Abstract Bioactive glasses have been used successfully as bone-filling materials in orthopaedic and dental surgery, but their poor mechanical strength limits their applications in load-bearing positions. Approaches to strengthen materials decrease their bioactivity. In order to realize the optimal matching between mechanical and biological properties, the sol-gel-self propagating method is adopted to prepare gel-derived bioglass bulk: 58S in the system SiO2–CaO–P2O5. The obtained glass was analysed for its composition, crystalinity and morphology through FT-IR, Raman, XRD, STEM and X-ray microanalysis. © 2006 Elsevier B.V. All rights reserved. Keywords: Sol gel; Bioglass; Bioactive
1. Introduction Bone regeneration is required in many clinical issues addressed by orthopaedic and dental medicine. The autogeneous bone graft is the gold standard, but host tissue is often scarce and can hardly be modeled to the shape required for successful reconstruction [1]. Then people's attention was transferred to implantation. Although bioactive ceramics have achieved great application successes in bone repairing, their elastic modulus mismatch and stress shielding of human bone cannot fill the longer and longer lifetimes. Tissue engineering provides a new approach for solving these problems. The biomaterials should be the scaffold to assist or enhance the body's own reparative capacity [2]. Bioglasses could elicit a specific biological in vivo response at the interface and attach to the tissues, such as soft tissue and
bone, with a strong chemical bond. So they have been widely used for a number of different applications. Certain compositions of bioactive glasses containing SiO2–CaO–P2O5 bond to both soft and hard tissue without an intervening fibrous layer. Results of in vivo implantation show that these compositions produce no local or systemic toxicity, no inflammation, and no foreign-body response [3]. At present, it is possible to develop some new biomaterials due to extensive overlaps between sol-gel chemistry and biochemistry. The sol-gel networks are excellent model systems for studying and controlling biochemical interactions within constrained matrices with well-defined textures. It is believed that enhanced bioactivity can be achieved in gel-derived materials because of their residual hydroxyl ions and micro pores, and large specific surface [4]. 2. Experimental
⁎ Corresponding author. Tel.: +33 326823586; fax: +33 326051900. E-mail address: [email protected] (A. Balamurugan). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.102
The composition studied was bioactive glass 58 wt.% SiO2-33 wt.% CaO-9 wt.% P2O5 (58S). The sol-gel precursors
A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757 Table 1 Composition of the Hank's balanced salt solution Reagents
Composition
NaCl KCl Na2HPO4 KH2PO4 CaCl2 MgSO4 NaHCO3
0.137 M 5.4 mM 0.25 mM 0.44 mM 1.3 mM 1.0 mM 4.2 mM
used in this study were tetraethyl orthosilicate, triethyl phosphate, and calcium nitrate, hydrolyzed in the presence of 2 mol/l of nitric acid. The initial procedure involved mixing distilled water, the appropriate alkoxide precursors and salts, and the catalyst. On completion of the hydrolysis, the sols were aged in a drying oven at 60 °C to reach high viscosity near the gel point. After aging for 1 day, the gels were dried in an environment containing 50:50% mixture of water/ethanol, and the temperature was raised to 120 °C slowly and kept for 48 h. Finally, the dried gels were heat treated in step wise fashion up to 900 °C for stabilization [5]. 3. Methods of testing The functional group analysis was performed by Fourier Transform Infrared Spectroscopy (FT-IR). The measurements were carried out in the transmission mode in the mid-infrared range (400–4000 cm− 1) at the resolution of 4 cm− 1. The studies were performed using the FT-IR, Spot light 300, Imaging system, Perkin-Elmer, France. For FT-IR measurements, KBr pellets containing the exactly weighted amount of the substance examined were prepared. For the collection of Raman spectra a confocal Raman microscope (LabRam, Horiba, Jobin-Yvon, France) consisting a
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point-focus ≈785 nm He–Ne laser source, a microscope (Olympus BX41) and a stigmatic spectrometer was used. The sample was placed on the stage of the microscope using a custom-made fixture such that the sample thickness was oriented perpendicularly to the laser beam incident from a 50× microscope objective. Scanning electron microscopy (SEM) (JEOL 5610 SEM and LEO 1525 field emission gun, FEG-SEM) was used to analyse the bioglass powders. The samples were coated with gold before the examination. SEM was mainly used to characterize the microstructure and grain size of the bioglass. X-ray powder diffraction method (Rigaku D/max-IIB). X-ray radiation of Cu K∝ (1.548 Å) to gain information about the phases. Scanning transmission electron microscope (Philips CM30) operating at a voltage of 100 kV. The microscope is fitted with an energy dispersive X-ray spectrometer (EDXS 30 mm2 Si (Li) RSUTW detector) and a parallel electron energy-loss spectrometer (Gatan model enfina 1000) placed under the STEM column. Analysis is carried out using a beryllium specimen holder with a 30° tilt. The concentration profiles were made across three different locations. The elemental composition was determined [6]. The biocompatibility of the bioglass particles were evaluated in simulated body fluid, Hank's balanced salt solution (HBSS) with ionic concentration nearly equal to the human blood plasma at static condition (Table 1). Immersion technique was adopted for the analysis, the glass particles were immersed into the SBF for various time periods finally the particles were analysed in XRD for the growth of HAP on the surface of the bioglass particles. 4. Results and discussion 4.1. FT-IR analysis The Fig. 1 shows the complex bands of the bioglass 58S, the bands in the region 1200–900 cm− 1 includes absorption due to both PO and
Fig. 1. FT-IR spectrum of the 58S bioglass sintered at 900 °C.
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A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757
Fig. 2. Raman spectrum of the 58S bioglass sintered at 900 °C.
SiO groups. As for the SiO groups, these can be grouped as SiO ‘bridging’ and ‘non-bridging’ to other SiO4 tetrahedra. In fact, some of the oxygen atoms are not directly connected to another Si atom in the glassy network, because the presence of cations induces the formation of SiO-groups. Non-bridging oxygens (NBO) are also related to hydroxyl groups (SiOH groups that break the silica bulk network). It is very difficult to distinguish between the frequency of absorption of each of these groups, and in the literature different hypotheses have been proposed [7]. Usually, in the presence of some NBOs, SiO frequency of absorption falls in the range 1040–940 cm− 1. In the spectrum of 58S, no bands are seen at frequencies higher than 1100 cm− 1. This means that most of SiO4 groups possess at least some NBOs. It is possible to assign the peak at 1040 cm− 1 observed in 58S to Si– O–Si groups, because when the sample is heat treated the relative intensity of this peak with respect to the band at 962 cm− 1 decreases.
Since thermal treatment induces sample dehydration (through condensation of surface hydroxyls: 2 SiOH → SiOSi + H2O), the lower relative intensity of the 1040 cm− 1 band should be related to the lower amount of hydroxyl groups. The band at 940 cm− 1 can then be assigned to SiO NBO formed because of the presence of cations, in particular Ca, since it was shown in the literature that SiO groups with 2 NBOs can absorb at this frequency. The band observed at 962 cm− 1 is assinged to the typical PO strech. The peak at 940 cm− 1 related to SiO NBO decreases, and a band at 1200 cm− 1 due to νSi–O–Si is formed. In particular, this band is typical of high surface area silica structures, since it was shown in the past that low surface silica presents a low intensity for the 1200 cm− 1 shoulder in the Si–O–Si νas peak [8]. 4.2. Raman analysis The typical Raman spectrum of 58S reveals the presence of three transverse-optical (TO) and longitudinal-optical (LO) pairs, at
Fig. 3. X-ray diffraction patterns of 58S bioglass at various sintering temperatures.
Fig. 4. SEM image of 58S bioglass at 300 °C.
A. Balamurugan et al. / Materials Letters 60 (2006) 3752–3757
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Fig. 5. SEM images of the cross-section of 58S bioglass sintered at 900 °C.
approximately the following frequencies: 410–460, 470, 720 and 1060–1190 cm− 1, respectively. In each pair, the lowest frequency is associated with TO excitations and the higher frequencies reveal the existence of Raman-active LO modes [9]. Using the nomenclature, the lowest frequency band is associated with a ‘bond-rocking’ vibration, in which the oxygen atom moves perpendicular to the Si– O–Si plane. The small TO–LO splitting observed at 960 cm− 1 is attributed to a ‘bond-bending’ vibration, in which the oxygen move approximately at right angles to the Si–Si lines and in the Si–O–Si plane; and finally the TO–LO splitting pair at high frequencies is assigned to a ‘bond-stretching’ vibration, in which the bridging oxygen atom moves parallel to the Si–Si lines in the opposite direction to their Si neighbours. Fig. 2 shows the Raman spectra of the bioglass, the well defined vibrational bands characteristic of the Si–O–Si stretching and bending modes which are associated with the crystalline phases of the samples were observed. The spectra normalized on the band centered at 470 cm− 1 (νsymm Si–O–Si), this band and the peak at 960 cm− 1 (δSi–
O–Si) are related to the amount of silica present. The phosphate (600 cm− 1 νPOO) bands are also observed at 505–610 cm− 1. As confirmed by the results, Raman spectroscopy is very sensitive to changes in the Si–O–Si environment of silica based glasses of different compositions and it is a powerful technique for the identification of Si–O–NBO bonds in bioactive glasses. The frequency shifting and intensity variations of the Raman bands observed for bioactive glasses are due to a decrease of the local symmetry originated by the addition of alkali and alkali earth oxides to the silica network. Higher concentrations of network modifiers promote the increase of intensity of the Raman lines and a dramatic emergence of the band at 950 cm− 1 due to the increase of the number of structural units including Si–O–NBO bonds. These effects are explained by the fundamentals of the Raman technique. Changes of the molecular local symmetry lead to variations in the Raman activity of the vibrational modes. As shown in Fig. 2, the Si–O–Si vibrational modes of the amorphous silica present a low Raman activity. These results are in agreement with those reported in the literature [9,10].
Fig. 6. EDXS spectrum of the sintered 58S bioglass at 900 °C.
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4.3. X-ray diffraction analysis The diffraction patterns of the 58S bioglass is shown in Fig. 3. It can be seen that the starting 58S bioglass powder was amorphous, proving the sol-gel method could prepare pure glasses. As the temperature increased up to 600 °C, only a broad peak existed in the pattern, showing that the bioglass powder still kept the amorphous state. With further increase of the temperature, sharp peaks appeared at 800 °C, indicating crystallization occurred in the 58S bioglass. The peaks matched the pattern of wollastonite CaSiO3 (JCPDS #27-0088). The CaSiO3 peaks became much sharper at 900 °C, corresponding to further crystallization. 4.4. Scanning electron microscopy analysis SEM. Fig. 4 shows the CaSiO3 bioglass surface after thermal treatment at 300 °C. With the increase of temperature to 900 °C, the precipitation of well-defined spherical and sheet crystals 2–5 μm in size appear in a glassy matrix. Microstructures of the cross-section have also been analyzed by SEM (Fig. 5). From the micrographs of the sample, the cross-sections are coarse. The sample has more but smaller pits and less sidestep surfaces on its surface, which suggests the crystalinity and less residual glass phase of the bioglass. With the increasing of thermal treatment temperature, there is a process of nucleation and crystal growth in 58S bioglass. The composition of materials will affect the process importantly at the same temperature. The velocity of the crystal growth is slower after it forms a nuclei. Sample treated at 1100 °C for 4 h, the 58S will form quantity of crystals with sheet and layer shapes on its surface. The average size of the crystal granule is 2–5 μm. The surface crystallization mechanism was confirmed through the SEM of crosssections of 58S.
Fig. 8. XRD patterns of the 58S bioglass after immersion into the Hank's solution.
analysis in STEM, absorption correction requires the knowledge of the samples' local mass thickness for EELS was used as a tool for the Xrays absorption correction. Combining EELS measured relative thickness and X-ray characteristic peak intensities the absorption corrected elemental weight concentrations were obtained with an iterative process the local mass thickness. The analysis was performed on bioglass sample in STEM. The Fig. 7 shows the micrograph of STEM, observed the homogenious distribution of the elements. The concentration profiles were also made on various grains and obtained homogenity.
4.5. EDXS and STEM analysis 4.6. Biocompatibility analysis in SBF The X-ray microanalysis was performed on 58S bioglass. The Fig. 6 represents the elemental composition of the bioglass. It can be noticed that no peaks correspond to any impurity element, other than those supposed to be present in the 58S. From the EDXS analysis the purity of the material was confirmed. The elemental composition of the bioglass is quantified by EDXS. The quantification of low Z elements requires X-ray absorption correction in the case of bulk substance
The X-ray diffraction analysis results of the glass are shown in Fig. 8. The bioglass particles were soaked into the HBSS up to 7 days, the substance formed on glasses were detected after 1 day immersion in SBF, new peaks (at 2θ 32°, 26°) were assigned to be (2 1 1), (0 0 2) apatite according to the standard JCPDS (09-0432). After 3rd day of immersion, the two peaks were intensified and the other peaks of apatite at 40°, 46°, 49° also appeared. After 7 days immersion all the peaks became more apparent. The widen diffraction peak at 2θ ranging from 30° to 34°, which should be distinguished the (2 1 1), (1 1 2) and (3 0 0) planes for well-crystallized hydroxyapatite. The X-ray diffraction patterns are indicative of the biocompatibility of the bioglass in SBF medium.
5. Conclusion The bioglass 58S was successfully synthesised by the sol-gel low temperature process and the composition was ensured from the analytical results. Further the biocompatibility analysis confirms the growth of HAP on the surface of the bioglass particles. References Fig. 7. STEM image of the bioglass particles.
[1] P. Sepulveda, J.R. Jones, L.L. Hench, J. Biomed. Mater. Res. 61 (2002) 301.
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