Sol-gel bioactive glass-ceramics Part II: Glass-ceramics in the CaO-SiO2-P2O5-MgO system

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1 Cent. Eur. J. Chem. 7(3) DOI: /s Central European Journal of Chemistry Sol-gel bioactive glass-ceramics Part II: Glass-ceramics in the CaO-SiO2-P2O5-MgO system Research Article Lachezar Radev*, Vladimir Hristov, Irena Michailova, Bisserka Samuneva University of Chemical Technology and Metallurgy, 1756, Sofia, Bulgaria Received 23 July 2008; Accepted 11 November 2008 Abstract: C eramics, with basic composition based on the CaO-SiO2-P2O5-MgO system with different Ca+ Mg/P+Si molar ratio (R), were prepared via polystep sol-gel technique. The structure of the obtained ceramic materials has been studied by XRD, FTIR spectroscopy, and SEM. X-ray diffraction showed the presence of akermanite and HA for the sample with R = 1.68 and Mg substituted β-tcp and silicocarnotite for the sample with R = 2.16, after thermal treatment at 1200 C/2 h. The obtained results are in good agreement with FTIR. In vitro test for bioactivity in static condition proved that the carbonate containing hydroxyapatite (CO3HA) can be formed on the surface of the synthesized samples. CO3HA consisted of both A- and B-type CO32- ions. SEM micrographs depicted different forms of HA particles, precipitated on the surface after soaking in 1.5 simulated body fluid (SBF). Keywords: Sol-gel Bioceramics In vitro bioactivity Versita Warsaw and Springer-Verlag Berlin Heidelberg. 1. Introduction The bioglass of SiO2-CaO-P2O5-Na2O glass developed by Hench et al. [1] released ions and formed thin hydroxyapatite (HA) layers on the glass surface. It has been established that the HA layers directly bonded to the bone tissues. The bioglass conducted new bone formation on the glass surfaces without any inclusions such as fibrous tissues, and possessed an osteoconductivity and osteoinductivity. The osteoconductive and osteoinductive ceramics and glasses such as HA and A/W showed similar phenomena in bone tissues. They were also categorized as bioactive materials [2]. Moreover, almost all of the osteoconductive and osteoinductive materials exhibited apatite formabilities in immersion test using SBF, advocated by Oyane et al. [3]. Based on the SBF estimations, the glasses containing CaO and SiO2 were recognized to be osteoconductiveand osteoinductive [4]. There is enough information in literature about the investigation of gel glasses in the systems SiO2-CaO [5], SiO2-CaO-P2O5 [5-8], and SiO2-P2O5-CaO-MgO [9,10] as potential bioactive materials and drug carriers. In the fundamental works of Kokubo et al. [2,11] Apatite/Wollastonite (A/W) including crystalline HA and β-wollastonite with MgO-CaO-SiO2 formed residual glassy-matrix. The dissolution of calcium and silicate ions from the obtained glass-ceramic play an important role in forming the surface HA layer. In the presence of Al2O3, Cho et al. [12] prepared A/W (Al) by heat treatment. The obtained ceramics has a bioactivity after HCl and NaOH treatment with different concentrations. Up to now, there are several reports about a well-defined crystalline phases and in vitro bioactivity in CaO-SiO2-P2O5-MgO system. The surface crystallization of akermanite was observed by [13]. Wollastonite, (Ca, Mg)3(PO4)2 and Mg2SiO4 was detected in the prepared samples after thermal treatment at 1000 and 1100 C. Amorphous calcium phosphate was formed after 21 day soaking in SBF [14]. When the CaO-SiO2-P2O5-MgO was doped with CaF2, akermanite, wollastonite, and HA were found to precipitate simultaneously at different * l_radev@abv.bg 322

2 L. Radev et al. temperatures. The obtained material demonstrates a higher flexural strength, fracture toughness, and a high in vitro bioactivity [15]. When B2O3, Na2O, and CaF2 were used as additives, Agathopoulos et al. [16] studied that the presence of diopside and wollastonite were dominated, but weak peaks, assigned to akermanite and fruorapatite were also registered. Apatite and the mica crystalline phases are observed by others [17]. Alizadeh et al. [18] denote the presence of diopside and HA in CaO-SiO2-P2O5-MgO system. When Fe2O3 was used as an additive, HA, whitlokite, and diopside can also be observed [19]. The addition of ZnO can lead to crystallization of wollastonite [20]. In our previous work we have synthesized via solgel technique some bioactive ceramics, containing HA and other different phases such as: mullite [21-23], anorthite-gehlenite [24]; HA and other crystalline phases from the systems ZnO-SiO2, ZnO-Al2O3-SiO2, ZnO-CaO2SiO2 [25]; HA and crystalline structures from the MgOSiO2 and MgO-Al2O3-SiO2 systems [26]. We have also shown that bioactive hybrid materials were synthesized in SiO2-P2O5-TiO2-CaO-PVA system. When Ca/P+Si was 1.96, two calcium phosphate silicate phases could be observed after thermal treatment at 1200 C/2 h [27]. The aims of this study were to obtain two types of ceramic materials in the system CaO-SiO2-P2O5-MgO with different Ca+Mg/P+Si molar ratio, as well as to investigate the phase formation in the materials and to evaluate its bioactivity in SBF. Table 1. Chemical compositions of the obtained materials. Composition of synthesized samples, mol% CaO SiO2 P2O5 MgO Ca+Mg/ P+Si, molar ratio (R) Ca Ca Samples In vitro bioactivity of the obtained materials was evaluated by examining the apatite formation on their surfaces in SBF with ph = 7.4. The used SBF solution in our investigation was 1.5 times higher from standard SBF solution [28]. 1.5 SBF solution was prepared from reagents as follows: (CH2OH)3CNH2 = g, NaCl = g, NaHCO3 = g, KCl = 0,3360 g, K2HPO4 3H2O = g, MgCl2 6H2O = g, CaCl2 2H2O = 0,5520 g, Na2SO4 = g, HCl (2 mol L-1) = 30 ml in distilled water. The structural evolution and phase formation of the obtained materials have been studied by using XRD (Bruker D8 Advance) with Cu Kα radiation, FTIR (MATSON 800 FTIR) and SEM (Philips-515). 2. Experimental Procedures The obtained materials have been synthesized by polystep sol-gel method. The first step was to prepare SiO2 sol from tetraethoxysilane (TEOS). TEOS was stirred under the mixed solvent of C2H5OH and H2O with a small amount of HCl as a catalyst in a volume ratio TEOS : C2H5OH : H2O: HCl = 1:1:1:0.01. After recognizing transparent solution of above mixture in approximately 1 h, the magnesium salt dissolved in water was mixed with prehydrolysed TEOS under stirring for 14 h. The second step was to prepare calcium phosphate (CP) solution. CP was prepared by mixing of Ca(OH)2 and H3PO4 at ph = The calcium and phosphate mixture was added drop by drop into silica and magnesium sol under intensive stirring. The prepared mixed sol was stirred at 24 h. The obtained sol was gelated at 120 C/12 h and thermally treated at 1200 C for 2 h in a tubular furnace. Chemical compositions of the obtained materials are given in Table 1. Figure 1. XRD of Ca-1 sample after thermal treatment at 1200oC/2 h Figure 2. XRD of Ca-2 sample after thermal treatment at 1200oC/2 h 323

3 Sol-gel bioactive glass-ceramics Part II: Glass-ceramics in the CaO-SiO2-P2O5-MgO system 3. Results and Discussion X-ray diffraction analyses of the synthesized Ca-1 and Ca-2 samples are given in Figs. 1 and 2. XRD diffractograms of synthesized and thermally treated samples are quite different, due to the different ratio values. In Ca-1 sample with R = 1.68 (Fig. 1), XRD detects the presence of two phases: akermanite (PDF ) and hydroxyapatite (PDF ). In the case of Ca-2 sample (Fig. 2), in which R = 2.16, XRD observed the presence of Mg-substituted β-tcp (PDF ) and silicocarnotite (PDF ). On one hand, the depicted results show that the introducing of magnesium salts into CaO-P2O5-SiO2 system can lead to the formation of akermanite and (Ca,Mg)3(PO4)2 as a main phases. On the other hand, the excess of Ca2+ or Si4+ ions tend to form hydroxyapatite and silicocarnotite phases. Our results are in good agreement with Kannan et al. [29]. FTIR spectra of thermally treated Ca-1 and Ca-2 samples are given in Fig. 3. In the FTIR spectra, the bands corresponding between 1022 (1033) cm (1063) cm-1, together with 479 (485) cm-1 bands can be assigned to the vibration of the Si-O-Si bond [16,30]. The absorption bands near 950 and 900 cm-1 (for Ca-1 and Ca-2 samples) have been assigned to νas SiO- bonds in silicate tetrahedral units with 2 and 3 non-bridging oxygens [16]. On the other hand, the absorption band envelope in cm-1 frequency is well defined at lower frequencies upon the polymerization of silicate network [31]. As can be seen from the depicted FTIR spectra, the frequency of the cm-1 band increased with increasing of the silica content (Table 1), or decreasing polymerization of the silicate unit. The presence of the absorption bands at 910 cm-1 (Ca-1 sample) with small intensity can be ascribed to the presence of Si2O7 structure [32], which is in accordance with XRD data. The peak with low intensity at 485 cm-1 (for Ca-2 sample) suggests a high degree of modification to the silicate network [16]. At first sight, Si-O-Si assigned to symmetric stretch 1052 cm-1 peak tends to shift slightly to lesser wavenumber 1063 cm-1. In addition, the high quality of phosphorus in Ca-2 sample (Table 1) caused a shift in the Si-O-Si band. The second important feature of the synthesized samples is that the intensity of the peak of 932 (939) cm-1, due to its SiOH/Si-O-Ca vibration mode, tends to diminish in comparison with unary and binary sol-gel ceramics. The obtained results suggest that the change in this band is attributed to P-containing glasses. These observations are in a good agreement with Federman et al. [33]. On the other hand, the crystalline silicate assignment was more difficult due to several overlaps with PO43vibrations, however the main vibration modes were identified [34]. In Ca-1 sample there were three sites for ν4 PO43- vibrational mode centered at 570, 605 and 636 cm-1. The absorption bands centered at 472 cm-1 and 965 cm-1 were assigned to ν2 PO43- and ν1 PO43- vibrational modes. These absorption bands were observed in the case of commercial HA powders, referred by Rehman et al. [35]. In Ca-2 sample, the phosphate ν4, ν3, ν2 and ν1 bands are present at and 616 cm-1 (for ν4 PO43-); 1022 and 1052 cm-1 (for ν3 PO43-); 485 cm-1 (for ν2 PO43-) and 986 cm-1 (for ν1 PO43-) can be ascribed to the presence of β-tcp [36-38]. In Ca-1 and Ca-2 samples the absorption bands at 400, 402, 417, and 518 cm-1 could be ascribed to the presence of Mg-O bond [39]. From the presented data, it can be seen that the FTIR data are in good agreement with X-ray diffractograms. Fig. 4 presents the FTIR spectra of Ca-1 and Ca-2 samples after immersion in 1.5 SBF. Figure 3. FTIR spectra of synthesized and thermally treated Ca-1 Figure 4. FTIR spectra of synthesized Ca-1 and Ca-2 spectra after and Ca-2 samples immersion in 1.5 SBF 324

4 L. Radev et al. It can be obviously seen that the FTIR spectra were very similar to those of the carbonate substituted hydroxyapatite (CO3HA). The absorption peaks located at 1090 and 1038 cm-1 for two samples originated from asymmetric stretching (ν3) of PO43- [40,41]. The absorption bands at 564 (530) cm-1 [30,40,41] and 600 cm-1 [30] were attributed to bending modes (ν4) of PO43-, respectively. The symmetric stretching modes (ν1 and ν3) of PO43- were also observed around 972 cm-1 [41,42]. This peak is a characteristic for HA structures [43]. The bands located at 666 (671) cm-1 and 1632 (1630) cm-1 were associated with hydroxyl groups [40]. The broad bands at 1296 cm-1 can be attributed to the presence of CO32- groups [44]. The ν3 CO32- was observed at 1400 cm-1 [40,46]. On one hand, the ν2 CO32- bands at 875 (870) cm-1 can also be detected. This is because of the substitution of PO43- by CO32species (B-type substitution) presented in [46,47]. On the other hand, the small absorption bands at 875 (870) cm-1 and 1060 (1058) cm-1 could be assigned to the presence of HPO42- in biological and synthetic apatites [48,49]. Other authors denote that these features are not clearly evident in the spectrum, as the absorption band due to PO43- of HA is intense and broad [50]. The small bands at 1553 (1554) cm-1 suggest there may be another crystallographically distinct CO32site, perhaps associated with the CO32- replacement of OH- (A-type substitution) as suggested by [48]. This observation showed that the CO3HA formed on the synthesized samples contained both A-type and B-type CO32- ions [51]. The presented FTIR spectra are in good agreement with other bioactive ceramics [52]. XRD of the synthesized samples after one day immersion in 1.5 SBF are given in Figs. 5 and 6. From depicted XRD patterns it can be seen that the obtained and thermally treated samples undergo drastic changes after only one day immersion in 1.5 SBF. As can be seen the mean diffraction peaks of hydroxylapatite (PDF ) are presented. The obtained XRD results are in accordance with presented FTIR data. Figure 5. XRD of Ca-1 sample after immersion in 1.5 SBF for 1 day Figure 6. XRD of Ca-2 sample after immersion in 1.5 SBF for 1 day Figure 7. Figure 8. SEM of the Ca-1 sample after thermal treatment at 1200 C/2 h SEM of the Ca-2 sample after thermal treatment at 1200 C/2 h 325

5 Sol-gel bioactive glass-ceramics Part II: Glass-ceramics in the CaO-SiO2-P2O5-MgO system Figure 9. SEM of the Ca-1 sample after 1 day immersion in 1.5 SBF Figure 10. SEM of the Ca-2 sample after 1 day immersion in 1.5 SBF SEM images of synthesized and thermally treated samples are given in Figs. 7 and 8. As pictured, the two prepared and thermally treated samples have different morphologies, depending on the chemical compositions of the gels (Table 1). The Ca-1 surface is more compact, whereas the surface of Ca-2 sample is looser and rougher with irregular pores. These observations are in good agreement with other ceramic materials in the system CaO-SIO2-MgO(P2O5) [14,52,53]. SEM micrographs of synthesized Ca-1 and Ca-2 samples, immersed in 1.5 SBF for 1 day are given in Figs. 9 and 10. SEM image for Ca-1 sample shows that after 1 day immersion in 1.5 SBF of static in vitro test (Fig. 7) a precipitate of apatite phase covered the entire surface. In the case of Ca-2 sample (Fig. 8) SEM depicts that the HA may be crystallized in two forms: longitudinal dendrite arms and irregular assemblies of particles. These results are in good agreement with [13]. to the prehydrolyzed TEOS and magnesium salt. The composition of the synthesized samples was prepared at 1.68 and 2.16 Ca+Mg/P+Si molar ratio. After thermal treatment of the sample with R = 1.68 at 1200 C/2 h, XRD denotes the presence of akermanite and hydroxyapatite. In the other sample with R = 2.16 thermally treated at the same temperature, X-ray diffraction proved the presence of Mg substituted β-tcp and silicocarnotite. Fourier transform infrared analysis indicates the conformation of the obtained XRD data. In vitro essay of bioactivity was carried out in static conditions in 1.5 SBF. FTIR spectra show that the increase in the intensity of CO32and PO43- are associated with apatite formation on the prepared samples. SEM images demonstrate that the apatite phase fully covered the entire surface after 1 day immersion in 1.5 SBF for the sample with R = For the sample with R = 1.47, SEM depicts that the HA may be crystallized in two forms: longitudinal dendrite arms and irregular assemblies of particles. 4. Conclusions Acknowledgements The financial support of the Bulgarian NSF under contract VU-TN-102/2005 is gratefully acknowledged. Two types of bioactive ceramics have been prepared in the CaO-SiO2-P2O5-MgO system by polystep sol-gel method. The calcium phosphate solution was added References [1] L.L. Hench, et al., J. Biomed. Mater. Res. 2, 117 (1972) [2] T. Kokubo, et al., J. Biomed. Mater. Res. 24, 331 (1990) [3] A. Oyane, et al., J. Biomed. Mater. Res. 65A, 188 (2003) [4] C. Ohtsuki, T. Kokubo, T. Yamamuro, J. Non-Cryst. Solids 143, 84 (1992) [5] M. Vallet-Regi, et al., Chem. Mater. 17, 1874 (2005) [6] A. Ramila, F. Balas, M. Vallet-Regi, Chem. Mater. 14, 542 (2002) [7] D. Arcos, J. Pena, M. Vallet-Regi, Chem. Mater. 15, 4132 (2003) 326

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