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Journal of Non-Crystalline Solids

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Journal of Non-Crystalline Solids

  1. 1. Evaluation of zinc and magnesium doped 45S5 mesoporous bioactive glass system for the growth of hydroxyl apatite layer Vikas Anand, K.J. Singh ⁎, Kulwinder Kaur Department of Physics, Guru Nanak Dev University, Amritsar 143005, India a b s t r a c ta r t i c l e i n f o Article history: Received 9 July 2014 Received in revised form 21 September 2014 Accepted 28 September 2014 Available online xxxx Keywords: Mesoporous bioactive glass; Tris SBF; Apatite layer Bioactive glass samples of xZnO(22.4 − x)Na2O·46.1SiO2·26.9CaO.2·6P2O5·2MgO system have been prepared by using the sol gel technique. Investigations of structural and bioactive properties of these samples have been undertaken by using X-ray diffraction, field emission scanning electron microscopy, Raman and energy- dispersive X-ray spectroscopy, Brunauer, Emmett and Teller technique, thermogravimetry, differential thermal analysis and derivative thermogravimetry methods and pH studies. X-ray diffraction spectra of prepared samples indicate the formation of hydroxyl apatite layer after 7 and 14 days during in vitro analysis. Observed peaks of Raman spectra confirm the growth of hydroxyl apatite layer during in vitro analysis. The formation of apatite phase is responsible for change in the morphology of samples which has been studied by field emission scanning electron microscopy. Ca/P ratio and ion dissolution rate have been evaluated from energy dispersive X-ray spec- troscopy results. The observed Ca/P ratio further confirms the growth of apatite layer with time during in vitro analysis. The effect of the addition of zinc on surface area and mesoporous nature of all the samples has been investigated by the Brunauer, Emmett and Teller technique. All glass samples have been found to be thermally stable from thermogravimetry, differential thermal analysis and derivative thermogravimetry techniques. pH studies indicate the non-acidic nature of all the prepared glass samples. Drug release properties of the glass sam- ples have been investigated by using gentamycin as an antibiotic. Prepared samples have shown excellent drug release properties which can be related to the porous nature of the samples. Due to slow dissolution rate, good drug delivery properties and formation of apatite phase within 7 to 14 days, our glass samples can be promising candidates for bone regeneration applications. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Fracture in the bone may cause a serious problem when its impact is on a large scale. Sometimes, fracture can lead to the loss of the bone and there can be many other incidents like cancer therapies, accidents, and age factors (old age) responsible for the elimination of the bone from the human body. In a survey, it has been estimated that in women and men (specially over the age of 50 years), the lifetime risk of devel- oping a fracture is 40% and 13% respectively [1]. Replacement of dam- aged bone is a major problem in orthopedics surgery and in order to meet this purpose, bioinert metallic implants are used. These implants act like a foreign particle for host body and sometimes, they may get rejection. Moreover, metallic implants may not stimulate the osteoblas- tic and osteoconductive phenomena in the host body. Due to metallic nature, these implants are temperature sensitive and start to cause the pain on surrounding tissues under hot and cold conditions. To overcome all these problems, a special kind of glass has been developed which has the ability to bond with bone and soft tissues [2]. This glass is popularly known as ‘bioactive glass’. Bioactive glasses are one of the categories of bio-materials and they can be potential materials for applications in orthopedics and surgery. Bioactive glasses are bioresorbable and they provide friendly environment for the growth of bone (apatite layer) and soft tissues. Therefore, these can be good alternatives of metallic implants. First, bio-active glass was prepared by using the conventional melt-quenching technique by Larry Hench. This glass was termed as 45S5 Bioglass®. Lower Ca/P ratios do not bond to bone [3]. Bioactive glasses can also be prepared by the sol–gel technique. Recently, borate glasses have also been investigated for biomedical ap- plications by removing silica from the bioactive glass compositions [4,5]. Silica free glass samples with the composition (45B2O3, 24.5Na2O, 24.5CaO, 6P2O5) were prepared with additional 2% of fluoride cations which were added separately. It has been observed that dissolution rate is better for LiF as compared to other fluorides (ZnF2, CaF2 and NaF) [4]. Oxide glass samples of the B2O3, Na2O, CaO, K2O and LiO constituents have also been prepared in the laboratory [5]. It has been noticed that dissolution rate proceeds rapidly than as observed in silicate Hench's bioglass (45S5). It has been inferred that the corrosion mechanisms of borate glasses in aqueous environments, generally undergo hydration, hydrolysis, and ion exchange reactions. Journal of Non-Crystalline Solids 406 (2014) 88–94 ⁎ Corresponding author. E-mail address: kanwarjitsingh@yahoo.com (K.J. Singh). http://dx.doi.org/10.1016/j.jnoncrysol.2014.09.050 0022-3093/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
  2. 2. In order to check the bioactivity of the samples, the following is the conventional procedure. Samples are soaked in the TRIS Simulated Body Fluid (SBF). SBF has concentration of ions equal to human blood plasma. The creation of apatite layer on the samples (from few hours up to few days) can confirm the bioactive nature of prepared samples. It has been found that formation rate of apatite layer improves at higher concentra- tion of calcium ions when the phosphate ions remain in the range of 4– 6 mol% in the composition. When the composition of phosphate ions is increased from 6 mol% onwards, it shows a negative effect in the rate of formation of apatite layer. Applications of bioactive glass in human body depend upon its bio- activity, structural and dissolution properties. Bioactivity of glass pro- vides the information regarding the growth of apatite layer, whereas, structural and dissolution rate properties inform about the strength of bioactive glass. 45S5 has been the most successful bioactive glass. More- over, 45S5 bio-glass cannot be used as a good scaffold owing to high dissolution rate. In this work, authors have made an attempt to improve the dissolution rate of 45S5 bio-glass along with constant Ca and P ratio. All glass samples have been prepared in the laboratory by the sol gel technique. 2. Materials and method Glass system xZnO(22.4 − x)Na2O·46.1SiO2·26.9CaO.2·6P2O5· 2MgO was prepared in the laboratory by using the sol gel method. Details of the chemical composition of the glass samples are provided in Table 1. The solutions to obtain glasses were prepared from the stoichio- metric amounts of tetraethyl orthosilicate (TEOS), triethyl phosphate (TEP), Ca(NO3)2·4H2O, NaNO3, Mg(NO3)·6H2O and Zn(NO3)·4H2O (AR grade). 1 M HNO3 was used as the catalyst for hydrolysis process. TEOS was added into 1 M HNO3 solution (TEOS and H2O molar ratio equal to eight) and the mixture was stirred up to 1 h for complete hydrolysis. TEP, calcium nitrate tetra hydrate and magnesium nitrate hexa hydrate were dissolved in 1 M HNO3 solution and stirred up to more than 40 min. Both solutions were mixed under vigorous stirring and sodium nitrate was added into the solution. After 1 h of vigorous stirring, transparent solution was obtained. Solution was kept in an air tight beaker for 5 days for aging. Gel was heated up to 60 °C for 12 h, 120 °C for 12 h and the product had been calcinated up to 700 °C for 4 h. Prepared samples had been crushed in agar and mortar for 1 h. X-ray diffraction (XRD) study has been undertaken by using BRUKER D8 FOCUS XRD machine. Raman studies of prepared samples have been performed by the RENISHAW IN VIA REFLEX MICRO RAMAN SPEC- TROMETER with 785 nm laser beam of exposure time of 40 s with the range of 300 to 1500 cm−1 . Field emission scanning electron microsco- py (FESEM) study has been carried out by ZEISS SUPERA 55. In order to get FESEM images, samples have been filtered from SBF and washed with acetone and DI water four times. Moisture has been removed from samples by drying them up to 60 °C. Platinum coating has been used to make the samples conductive. Gold coating of samples has been avoided due to overlapping of gold and phosphorus peaks during energy dispersive X-ray (EDX) analysis. Brunauer–Emmett–Teller (BET) analysis has been undertaken by micrometrics ASAP 2020. Thermogravimetry, differential thermal analysis and derivative thermogravimetry (TG–DTA–DTG) techniques have been used to inves- tigate the thermal behavior of prepared bioactive glasses by EXSTAR TG/ DTA 6300 instrument up to 1400 °C with the increase in temperature of 10 °C per minute. 3. Assessment of in vitro bioactivity Bioactivity nature of samples had been evaluated with the help of 27-Tris SBF solution. SBF solution was prepared as per the recipe report- ed elsewhere [6]. 3 mg of powder sample was soaked in 30 ml of 27-Tris SBF solution for 7 and 14 days under 37 °C temperature. After every 12 h, sample was replaced by a fresh TRIS SBF solution. pH of Tris SBF was measured after regular interval of time to check the ion exchange process in between the surface of sample and Tris SBF solution. Ion con- centration of Tris SBF and human blood plasma is provided in Table 2. The results of in vitro bioactivity were analyzed with the help of XRD, Raman, SEM and EDX studies. 4. Results and discussion 4.1. XRD studies XRD patterns of the samples are provided in Fig. 1. All samples show amorphous nature of material before soaking into SBF. As shown in the Fig., sharp peaks of Calcium Hydroxide Phosphate [Hydroxylapatite (HAp)] (JCPDS No. 00-074-0566) are observed after 7 days. Intensity of peaks has increased when the number of days is increased to 14 days. Along with HAp layer, calcium carbonate and calcium phos- phate peaks are also observed in all three XRD patterns. The formation of HAp layer on the surface of prepared glass samples and shifting of amorphous to crystalline behavior after soaking of glass samples in SBF solution with the passage of time indicate the bioactive behavior of prepared samples. Sometimes, there may be slight shifting of peaks of HAp due to the small substitution of calcium magnesium in biological apatite [7]. 4.2. Raman spectroscopy It is shown in the Raman spectra that before the in vitro analysis, one sharp peak appears around 1070 cm−1 indicating the presence of car- bonate ions (Fig. 2). Same peak splits into two small peaks after 7 and 14 days during in vitro test which indicate the presence of phosphate ions. Peaks around 1089–1091 and 1052–1055 cm−1 in the Raman spectra corresponds to the asymmetric stretching (ν3) of the PO bond in phosphate. The peak around 963–965 cm−1 corresponds to the symmetric stretching mode (ν1) of the PO bond of the phosphate group. This peak is generally observed around 963 cm−1 in hydroxylap- atite [8]. The minor shift from 963 cm−1 may be due to minor incorpo- ration of magnesium or zinc ions into the HAp lattice. The resultant compressive stresses in the lattice may lead to this difference in peak position [9]. Raman data support the inferences of XRD studies in terms of observation of bioactive nature of the glass samples. 4.3. FESEM and EDX studies Morphology of samples has been confirmed by FESEM before and after the immersion of samples in SBF solution. FESEM micrographs have observed that after 7 days, the content of calcium and phosphate Table 1 Composition (in mol%) of samples. Sample code ZnO Na2O SiO2 CaO P2O5 MgO BG-1 6 16.4 46.1 26.9 2.6 2 BG-2 8 14.4 46.1 26.9 2.6 2 BG-3 10 12.4 46.1 26.9 2.6 2 Table 2 Ion concentration of Tris SBF and Human plasma. S. no. Ions Ion conc. (in 27-Tris SBF) Ion conc. (in human plasma) 1 Na+ 142 142 2 K+ 5 5 3 Mg2+ 1.5 1.5 4 Ca2+ 2.5 2.5 5 HPO4 2− 1 1 6 HCO3 − 27 27 7 Cl− 125 103 8 SO4 2− 0.5 0.5 89V. Anand et al. / Journal of Non-Crystalline Solids 406 (2014) 88–94
  3. 3. starts to increase which indicates that the apatite layer (calcium phos- phate) starts to generate on the surface of samples (shown in Fig. 3) After 14 days, growth of layer has been observed to increase and almost the whole surface of the sample has been found to be covered by the ap- atite layer. This growth phenomenon is common in all three prepared samples but the growth rate of the layer is high at the surface of BG-1 and low at the surface of BG-3. It may be due to two reasons: (i) Increase in ZnO content (BG-3 sample). The presence of ZnO or MgO in the samples may have slowed down the deposition rate of HAp. This observation is consistent with the results of Li et al. [10] and Du and Chang [11]; and (ii) lesser surface area for growth of apatite layer (BG-3 sample, Table 4). Due to small surface area in BG-3 than BG-1, the rate of growth of ap- atite layer may have slowed down. It is reported by Li et al. that leaching of ions is one of the reasons to slow down the rate of apatite layer but when zinc is added in the presence of magnesium with the replacement sodium content, there is a decrease in the leaching of Mg, Zn and silicon ions as shown in EDX (Fig. 4). This can be explained as follows. Both Mg and Zn ions have a dual character to behave like a network former or network modifier. If they act like a former then SiOSi bond is replaced by SiOMg or SiOZn bond and as a result, structure becomes weak because the newly formed bond has less strength as compared to the old one. But in case, Mg and Zn behave like modifiers they give strength to the structure which reduces the solubility of glass in SBF [12]. Enhancement in growth of apatite layer may be due to the presence of magnesium ions along with zinc ions. Magnesium being a major ele- ment in the bone, may increase the growth of apatite layer. Moreover, magnesium plays a vital role in providing the apoptosis control to cell. EDX study of the prepared samples has been undertaken before and after 7 and 14 days of immersion in SBF. In EDX study, it has been Fig. 1. XRD pattern of (a) BG-1, (b) BG-2 and (c) BG-3 glass samples before and after the in vitro analysis. Fig. 2. Raman peaks of (a) BG-1, (b) BG-2 and (c) BG-3 glass samples before and after the in vitro analysis. 90 V. Anand et al. / Journal of Non-Crystalline Solids 406 (2014) 88–94
  4. 4. observed that there is an increase in the concentration of calcium and phosphorus with time due to the formation of apatite layer on the surface of the sample but the dissolution of silicon ions is very slow in all three samples. Due to this low solubility rate, these samples may show good scaffold properties. Exchange of ions is also confirmed by EDX graphs (Fig. 4). The graphs show the change of calcium and phos- phorus ions from Tris SBF and the growth of apatite layer on the surface of glass has been established within seven days. At the end of 14th day, the concentration of calcium and phosphorus ions increases due to the formation of calcium phosphate phases on the surface of glass. Phase formation has also been confirmed with the weight (%) data of EDX (Table 3). The results indicate that Ca/P ratio of samples changes before and after in vitro analysis. Ca/P ratio has been reported as 1.67 for the stoichiometric composition of HAp [13]. Synthetic calcium phosphate materials have also been prepared and studied extensively in vitro and in vivo. Some materials have been called “hydroxylapatite” with Ca/P ratios ranging from 1.3 up to 2.0 [14]. Dur- ing the EDX analysis for our glass samples, authors have found that after 14 days, Ca/P ratio lies in the range of 1.68–1.77 for all the glass samples prepared in the laboratory. These values are very close to Ca/P ratio (1.67) for stoichiometric composition of HAp and also, lie in the range of already reported values of Ca/P ratio (1.3–2.0) for hydroxyl apatite. These results confirm the growth of HAp on the surface of the glass sam- ples. Moreover, these results also support the inferences of XRD and Raman studies. No major change in the concentration of Si, Mg and Zn ions has been observed even after 14 days of samples in Tris SBF solution. 4.4. Brunauer–Emmett–Teller studies BET study has been undertaken to find out the surface area and porous nature of prepared samples (Fig. 5). N2 adsorption–desorption phenomena have been studied to obtain the desired results. BET surface area is calculated by using all adsorption data points from .01 to 1.0 (total points 46) in the relative pressure (P/Po). In order to calculate the pore size, the Barrett–Joyner–Halenda (BJH) desorption method (relative pressure from 1.0 to .12, total 16 points) has been preferred over the adsorption method because desorption is carried out at a low relative pressure which is useful for thermodynamic equilibrium [15]. Passage of time indicates the bioactive behavior of prepared samples. Sometimes, there may be slight shifting of peaks of HAp due to small substitution of calcium magnesium in biological apatite [7]. Surface area, pore volume and BJH desorption average pore volume of prepared samples are provided in Table 4. As shown in the table, pore volume and pore size of samples decrease with the increase of the amount of Zn. This is due to the role of Zn as the network modifier to increase the strength of glass samples. Fig. 5 shows the N2 physisorption i.e. adsorption and desorption isotherms and pore size distribution of prepared bioactive samples. The pore size of samples (Table 4) comes in the range of type II (2–50 nm). According to International Union of Pure and Applied Chemistry (IUPAC) classification of adsorption isotherm, type II de- scribes the presence of mesoporous and microspores which exhibits a hysteresis loop and a variation point at lower pressure in a material [16]. Fig. 3. SEM images of (a) BG-1, (b) BG-2 and (c) BG-3 glass samples before and after in vitro study. 91V. Anand et al. / Journal of Non-Crystalline Solids 406 (2014) 88–94
  5. 5. 4.5. TG/DTA/DTG studies Graphical representation of thermal behavior of samples is given in Fig. 6. There is no sharp change in the mass of samples. For BG-1, BG-2 and BG-3, there is 5.60%, 5.24% and 2.6% loss of weight up to 300 °C respectively which is due to the evaporation of adsorbed water and ethanol molecules. BG-1 has high porosity which may be a reason of highest weight loss among all three samples. After that, from 300 to 700 °C, the weight loss is 2.81%, 1.86% and 2.09% for BG-1, BG-2 and BG-3 respectively. This may be due to the decompo- sition of precursor and removal of nitrate from samples. Further, samples are heated up to 1400 °C with 0.57%, 0.72%, and 0.79% weight loss of BG-1, BG-2 and BG-3 respectively. Weight loss from (700 to 1400 °C) may be due to the decomposition of carbonate and formation of crystalline phases in the sample. All three samples show a stable thermal response of up to 1400 °C. 4.6. pH studies pH study of samples gives an idea about ion exchange process in be- tween the surface of sample and Tris SBF solution. Variation of pH of samples in Tris SBF is shown in Fig. 7. A sudden rise in the pH of solution indicates the exchange of ions between sample and Tris SBF which started in the first hour. Total variation of pH of solution is from 7.4 up to 8.74 within 180 h. The pH of solution changes in a narrow range of 1.34 which shows that samples are chemically stable in Tris SBF solution and do not show any acidic nature in Tris SBF. Fig. 4. EDX results of (a) BG-1, (b) BG-2 and (c) BG-3 glass samples before and after in vitro study. Table 3 Weight percentage of the elements of samples from EDX data. EDX results are average (Avg.) of 5 measurements and the observed error (Er) percentage is also presented. Element Standard used BG-1 (wt.%) BG-2 (wt.%) BG-3 (wt.%) Before After 14 days Before After 14 days Before After 14 days Avg. ±Er%x10 Avg. ±Er%x10 Avg. ±Er%x10 Avg. ±Er%x10 Avg. ±Er%x10 Avg. ±Er%x10 C CaCO3 14.6 0.02 16.44 0.024 14.45 0.031 16.44 0.01 9.62 0.031 13.01 0.039 O SiO2 48.59 0.03 47.02 0.032 49.72 0.034 49.11 0.04 48.35 0.041 49.01 0.021 Na Albite 12.97 0.025 8.48 0.031 8.48 0.023 6.48 0.028 2.63 0.031 1.13 0.041 Mg MgO 2.78 0.031 3.28 0.034 3.28 0.025 3.28 0.032 0.54 0.029 0.41 0.031 Si SiO2 14.65 0.032 14.12 0.031 14.12 0.027 13.12 0.031 23.51 0.028 18.47 0.021 P GaP 1.96 0.031 2.51 0.032 2.68 0.041 3.12 0.049 4.54 0.05 4.98 0.043 Ca Wollastonite 2.11 0.021 4.21 0.041 3.13 0.048 5.31 0.042 4.92 0.031 8.8 0.029 Zn Zn 2.34 0.021 4.14 0.01 4.14 0.014 3.14 0.021 5.89 0.018 4.23 0.022 Ca/P ratio 1.07 1.68 1.17 1.7 1.08 1.77 92 V. Anand et al. / Journal of Non-Crystalline Solids 406 (2014) 88–94
  6. 6. 4.7. In vitro drug release studies In vitro drug release of sample has been studied with the help of gentamicin. 2 g of prepared sample has been immersed in 40 ml of gentamicin solution (10 mg ml−1 ). Sample has been kept in the solution up to 24 h. After filtering the powder and drying at 40 °C up to 48 h, in vitro release of gentamicin from the drug-loaded bioactive glass is carried out in an incubator at 37 °C. 2 g of powder is dipped in the 20 ml of Tris SBF under 37 °C. Gentamicin release was determined by UV analysis. The release medium was withdrawn at the predetermined time intervals and replaced with fresh SBF solution each time. During the drug release mechanism, all three samples show a quick release in the first hour and then there is a decrease in the rate of release of drug in SBF (shown in Fig. 8). All the three samples show similar drug release behavior as reported by mesoporous channel [17] This study shows that prepared samples have a good response in drug delivery phenomena and it is due to their mesoporous behavior. 4.8. Statistical calculations Data for pH and drug delivery studies is the average of 3 times rep- lication of experiment. Error bars have been used in the corresponding figures. EDX measurements have been undertaken 5 times. The ob- served maximum deviation is 0.5%. 5. Conclusions XRD peaks at 25.8°, 28.9°, 31.7°, 32.7°, 32.19°, 32.8°, and 39.78° indi- cate the presence of HAp phase on the surface of samples during in vitro analysis. Growth of HAp phase has been confirmed by HAp(PO bond) peak at 963 cm−1 in Raman spectra. FESEM images further confirm the findings of XRD and Raman investigations. Observed Ca/P ratio (1.68, 1.70 and 1.77) by the EDX technique during in vitro analysis also con- firms the formation of HAp phase. Low dissolution rate of silicon, mag- nesium and zinc ions has been observed by the EDX analysis. From BET study, it can be inferred that the increase in the content of zinc leads to the decrease in the surface area and pore size which can be related to the network modifier behavior of zinc. These results are consistent with the findings of the TGA–DTA–DTG technique which shows a stable thermal behavior of samples having high content of zinc. Drug release properties concluded that glass sample 6ZnO·16.4Na2O·46.1SiO2· 26.9CaO·2.6P2O5·2MgO is better due its higher surface area and pore size. Porosity, surface area and chemical composition of the bioactive glasses have been found to play a significant role in controlling the bio- active behavior of the glass samples. Fig. 5. (a) Adsorption desorption curves and (b) pore volume of bioactive samples. Lines have been drawn as a guide to the eye. Table 4 BET surface area, pore volume and pore size of bioactive samples. Sample code BET surface area (m2 /g) Pore volume (cm3 /g A°) Pore size (nm) BG-1 14.9 7.5 × 10−4 30 BG-2 13.8 3.8 × 10−4 21 BG-3 9.4 1.4 × 10−4 18 Fig. 6. TG–DTA–DTG response of (a) BG-1, (b) BG-2 and (c) BG-3 glass samples. 93V. Anand et al. / Journal of Non-Crystalline Solids 406 (2014) 88–94
  7. 7. Acknowledgments The authors Vikas Anand and Kulwinder Kaur are grateful to the financial assistance provided by the UGC, New Delhi (India) through JRF (NET)[F.17-74/2008 (SA-I)] and DST, New Delhi (India) through IN- SPIRE program [IF-120620] respectively. References [1] A.O. Hoff, R.F. Gagel, Osteoporosis in breast and prostate cancer survivors, Oncology (Williston Park) 19 (2005) 651–658. [2] L.L. Hench, J.M. Polak, Third-generation biomedical materials, Science 295 (2002) 1014–1017. [3] M. Epple, Book review: the chemistry of medical and dental materials By John W. Nicholson, Angew. Chem. Int. Ed. 42 (2003) 2818–2819. [4] A.M. Abdelghany, H. Kamal, Spectroscopic investigation of synergetic bioactivity behavior of some ternary borate glasses containing fluoride anions, Ceram. Int. 40 (6) (2014) 8003–8011. [5] A.M. Abdelghany, H.A. ElBatal, F.M. EzzElDin, Bone bonding ability behavior of some ternary borate glasses by immersion in sodium phosphate solution, Ceram. Int. 38 (2) (2012) 1105–1113. [6] S. Jalota, S.B. Bhaduri, A.C. Tas, Using a synthetic body fluid (SBF) solution of 27 mM HCO3 − to make bone substitutes more osteointegrative, Mater. Sci. Eng. C 28 (2008) 129–140. [7] I. Gutowska, Z. Machoy, B. Machalinski, The role of bivalent metals in hydroxyapa- tite structures as revealed by molecular modeling with the HyperChem software, J. Biomed. Mater. Res. A 75 (2005) 788–793. [8] G. Penel, G. Leroy, C. Rey, E. Bres, MicroRaman spectral study of the PO4 and CO3 vibrational modes in synthetic and biological apatites, Calcif. Tissue Int. 63 (1998) 475–481. [9] G. Gouadec, P. Colomban, Raman spectroscopy of nanomaterials: how spectra relate to disorder, particle size and mechanical properties, Prog. Cryst. Growth Charact. Mater. 53 (2007) 1–56. [10] X. Li, X. Wang, D. He, J. Shi, Synthesis and characterization of mesoporous CaO–MO– SiO2–P2O5 (M = Mg, Zn, Cu) bioactive glasses/composites, J. Mater. Chem. 18 (2008) 4103. [11] R.L. Du, J. Chang, The influence of Zn on the deposition of HA on sol-gel derived bio- active glass, Biomed. Mater. Eng. 16 (2006) 229–236. [12] M.M. Azevedo, G. Jell, M.D. O'Donnell, R.V. Law, R.G. Hill, M.M. Stevens, Synthesis and characterization of hypoxia-mimicking bioactive glasses for skeletal regenera- tion, J. Mater. Chem. 20 (2010) 8854. [13] M. Erol, A. Özyuguran, Ö. Çelebican, Synthesis, characterization, and in vitro bioac- tivity of sol–gel-derived Zn, Mg, and Zn–Mg co-doped bioactive glasses, Chem. Eng. Technol. 33 (7) (2010) 1066–1074. [14] G.A. Stanciu, I. Sandulescu, B. Savu, S.G. Stanciu, K.M. Paraskevopoulos, X. Chatzistavrou, E. Kontonasaki, P. Koidis, Investigation of the hydroxyapatite growth on bioactive glass surface, J. Biomed. Pharm. Eng. 1 (1) (2007) 34–39. [15] E.P. Barrett, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–380. [16] J. Rouquerol, D. Avnir, D.H. Everett, C. Fairbridge, M. Haynes, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, Guidelines for the characterization of porous solids, in: F.R.-R.K.S.W.S.J. Rouquerol, K.K. Unger (Eds.), Studies in Surface Science and Catalysis, Elsevier, 1994, pp. 1–9. [17] M. Vallet-Regi, A. Rámila, R.P. del Real, J. Pérez-Pariente, A new property of MCM-41 drug delivery system, Chem. Mater. 13 (2000) 308–311. Fig. 7. pH variation of samples during in vitro analysis. Lines have been drawn as a guide to the eye. Fig. 8. Gentamicin release by glass samples after regular time intervals. Lines have been drawn as a guide to the eye. 94 V. Anand et al. / Journal of Non-Crystalline Solids 406 (2014) 88–94

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