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Highly ordered mesoporous bioactive glasses with Im3m symmetry
Materials Letters 61 (2007) 4569 – 4572 www.elsevier.com/locate/matlet
Highly ordered mesoporous bioactive glasses with Im3m symmetry Hui-suk Yun ⁎, Seung-eon Kim, Yong-teak Hyeon Department of Future Technology, Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon-si, Gyeongnam, 641-831, Republic of Korea Received 14 February 2007; accepted 20 February 2007 Available online 12 March 2007
Abstract Highly 3D body centered cubic (Im3m) ordered mesoporous bioactive glasses (MBG) were synthesized by evaporation-induced self-assembly (EISA) in the presence of a nonionic triblock copolymer, EO100PO65EO100 (F127), template. The influence of the F127 concentration on the mesostructure was examined. MBG calcined at 600 °C possessed a large specific surface area (∼ 520 m2 g− 1) and pore volume (0.51 cm3 g− 1) and a uniformly distributed pore size (5.4 nm). In vitro bioactivity studies were carried out in simulated body fluid (SBF). © 2007 Elsevier B.V. All rights reserved. Keywords: Bioactive glass; Mesoporous; 3D cubic structure; Nanomaterials; Sol–gel preparation
1. Introduction Since the discovery of KSW-1 [1] and M41S [2], highly ordered mesoporous materials have attracted the attention of many scientists, mainly due to their potential technological applications [3]. Currently, mesoporous materials are coveted for their various applications in biomaterials science, such as drug delivery systems (DDS) and bone tissue regeneration, because their unique structural properties, which consist of ordered open pore structures and large specific surface areas and pore volumes, may enhance their bioactive behavior [4–16]. The DDS applications of mesoporous materials have experienced a remarkable breakthrough during the past several years [4–6]. The study of the bone tissue regeneration applications of mesoporous materials began after Vallet-Regi et al. first described the in vitro bioactivity of various mesoporous silica [7]. Studies related to tissue regeneration accelerated after the development of MBG by Zhao et al. in 2004 [8]. BGs have been widely studied because they have the ability to chemically bond with living bone tissue and have consequently been used in a variety of medical applications [9,10]. Increasing the specific surface area and pore volume of BGs may greatly accelerate the kinetic deposition process of hydroxycarbonate apatite and therefore enhance their bone-forming bioactivity. Zhao et al. successfully synthesized highly hexagonally ordered MBGs by ⁎ Corresponding author. Tel.: +82 55 280 3351; fax: +82 55 280 3399. E-mail address: [email protected] (H.-S. Yun). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.02.075
templating with a triblock copolymer, EO20PO70EO20 (P123) and demonstrated their superior bone-forming bioactivities in vitro compared to normal BG [8,11,12]. Although several other studies related to the synthesis of MBGs have since been carried out, all of the synthetic methods involved employed the same template polymer, P123, which generally leads to 2D hexagonal symmetry [13–16]. MBGs with 3D cubic pore structures have different accessibilities [17] and may facilitate the transportation of bioactive agents such as protein and nutrients from their hexagonal pore structures. MBGs with a tunable pore size and pore structure may greatly influence the protein adsorption and nutrient delivery behavior, thereby allowing them to have better in vivo bioactivity. A 3D cubic structure can be produced using F127 as a template. Zhao et al. reported using F127 to form a 3D cubic structure, but they only obtained a wormlike pore structure [8]. The successful control of the mesostructure of MBGs adds a new dimension to the art of the designed synthesis of MBGs. Herein, we demonstrate the successful synthesis of highly 3D cubic ordered MBGs with a large BET surface area and good bioactivity in vitro by templating with F127. 2. Experimental 2.1. Preparation of MBG In a typical synthesis, 2.88 g of F127 is dissolved in 18.1 ml of ethanol (EtOH). Stock solutions, which were prepared by mixing 1.36 g of calcium nitrate tetrahydrate (CaNT), 0.26 ml of triethyl
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gel films were easily separated from the vessel after aging [18] and were calcined at 600 °C for 6 h in air to remove the template. In addition, a series of MBGs with different amounts of F127 (denoted as MBGF6, MBGF12, MBGF24, MBGF36, and MBGF48 according to the weight percent of F127 to TEOS) were prepared to study the formation of the MBG. 2.2. Characterizations
Fig. 1. XRD patterns of calcined MBGF6 (a), MBGF12 (b), MBGF24 (c), MBGF36 (d) and MBGF48 (e).
Structural characterization was carried out using X-ray diffraction (XRD; θ–2θ scanning, Philips-X'pert MPD 3040), using Cu Kα (λ = 1.5406 Å) radiation (40 kV-40 mA), transmission electron microscopy (TEM; JEOL-JEM2100F) with field emission gun at 200 kV, and field emission scanning electron microscopy (FE-SEM; Hitachi-S5500 and JEOL-5800) at an accelerating voltage of 1–5 kV. The specific surface area was measured by the N2-gas adsorption method using a BET apparatus (BelJapan-Belsorp mini II). 2.3. In vitro bioactivity test
phosphate (TEP), 6 ml of tetraethyl orthosilicate (TEOS), 0.95 ml of HCl (1 M), 7.62 ml of EtOH and 2.86 ml of H2O, were added to this solution after stirring them separately for 1 h, and were vigorously stirred together for another 4 h at 40 °C. The molar composition was TEOS:CaNT:TEP:F127 = 1:0.2:0.05:0.008 in this case. The reactant solution was transferred to a polystyrene vessel without a cap, and aged at 40 °C, 40 RH% for 48 h without stirring. Gel films were obtained on evaporating the solvent. These
The MBG films were ground and sieved (d b 25 μm). The assessment of the in vitro bioactivity of the MBGs was carried out in SBF at 37 °C. Before immersing MBGs in SBF, they were treated using an alternate soaking process [19]: The MBGs were soaked in 300 ml of calcium chloride (CaCl2, 200 mM) for 10 s and then rinsed with excess water. The MBGs were subsequently soaked in a solution of potassium hydrogen
Fig. 2. TEM images of calcined MBGF6 (a), MBGF12 (b), MBGF24 (c), and MBGF48 (d). The inset shows the Fourier transform patterns of (d).
H.-S. Yun et al. / Materials Letters 61 (2007) 4569–4572
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Fig. 3. FE-SEM images (a)–(d) and optical images (e)–(h) of as-synthesized MBGF6 (a), (e), MBGF12 (b), (f), MBGF24 (c), (g) and MBGF48 (d), (h).
phosphate trihydrate (K2HPO4·3H2O, 200 mM) and then rinsed with excess water. These steps were repeated 3 times and MBGs rinsed well. The MBGs were immersed in SBF after drying. The SBF contained 142.0 mM Na+ , 5 mM K+ ,1.5 mM Mg2+ , 2.5 mM Ca2+ , 147.8 mM HCl− , 4.2 mM HCO3− , 1.0 mM HPO42− , and 0.5 mM SO42− [20]. Its chemical composition was similar to that of human plasma. The solution had a pH of 7.4 and was kept at 37 °C before use. 3. Results and discussion The XRD patterns of the MBGs produced using five different amounts of polymer template (MBGF6–MBGF48) calcined at 600 °C are shown in Fig. 1. The weight ratio of the triblock copolymer, F127, to TEOS significantly affected the formation of the mesostructure. MBGF6 and MBGF12 present no diffraction peaks (Fig. 1a and b). XRD peaks, indicating the formation of a mesostructure, can be detected from the samples with an amount of F127 greater than 24% (MBGF24; Fig. 1c). The intensity of the peaks increased with increasing amount of F127, corresponding to a better and longer range mesostructure arrangement. MBGF48 exhibits three diffraction peaks in the small-angle regime (2θ = 0.95, 1.3, 1.61°), which can be indexed to the (110), (200), and (211) diffractions of a 3D Im3m lattice, respectively (Fig. 1e) (d100 = 13.1 nm). The TEM observations of these samples also confirm this conclusion as shown in Fig. 2. No mesostructure was observed in the case of the MBGF6 sample. MBGF12 has a wormhole-like mesostructure, while MBGF24 has an ordered mesostructure. The TEM image along the [001] direction of MBGF48 reveals a highly ordered cubic arrangement (Fig. 2d). Fig. 3 shows the FE-SEM and optical images of the as-synthesized films. Micrometer sized pores with heterogeneous size distribution were observed in the case of MBGF6 (Fig. 3a). These pores apparently resulted from the aggregation of F127, owing to the heterogeneous reactions between F127 and the inorganic species, which led to the sample having inferior optical properties (Fig 3e). The large pores became increasingly homogeneous in size and gradually disappeared with increasing amount of F127 (Fig. 3b–d). As a result, the films evolved to become transparent, continuous, and flexible (Fig. 3f–h).
The nitrogen adsorption and desorption isotherms were also changed when the amount of F127 was increased. MBGF6 shows H3-type hysteresis loop (corresponding to slit-like pores), as shown in Fig. 4a. That is, it has no mesostructure. It is observed that the samples, MBGF12–MBGF48, exhibit similar type IV isotherms typical of mesoporous materials and that increasing the amount of template results in a progressive augmentation in their nitrogen sorption capacity and mesopore volume (Fig. 4b–d). In particular, the mesopore volume of MBGF48 was considerably increased. MBGF48 has a type IV isotherm with a sharp capillary condensation step at high relative pressures and a broad H2 hysteresis loop that is indicative of large uniform cage-type pores [21]. It has a BET surface area of 520 m2 g− 1, a total pore volume of 0.51 cm3 g− 1, and a pore size of 5.4 nm. The bone-forming activity of MBGF48 in vitro was tested in SBF to monitor the formation of hydroxyl apatite (HA) on the surface of the MBG over time. The FE-SEM image of MBGF48 before soaking shows a smooth and homogeneous surface (Fig. 5a). The FE-SEM image reveals that the surface of the MBG undergoes important changes
Fig. 4. N2 adsorption–desorption isotherms of MBGF6 (a), MBGF12 (b), MBGF24 (c), and MBGF48 (d).
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Fig. 5. SEM images of MBGF48 after immersing in SBF for 0 h (a) and 24 h (b).
during the reaction with SBF. The growth of HA-like nanoparticles is observed after soaking the sample for 1 h, and HA with a rod-like morphology composed of rods with a length of 100–200 nm, which is similar to the morphology of HA in human bones, is subsequently observed after 24 h (Fig. 5b).
4. Conclusions Highly 3D cubic ordered MBG with large specific surface area and pore volume, and superior bone-forming bioactivity in vitro were obtained by means of the EISA method, using a nonionic triblock copolymer, F127, as the structure-directing agent. The formation of a long-ranged cubic mesostructure largely depends on the concentration of F127. These cubic ordered MBGs as a new family of biomaterials, may find potential applications in drug delivery systems, implant coating materials, and functional bone tissue engineering. References [1] T. Yanagisama, Z. Shimizu, K. Kuroda, Bull. Chem. Soc. Jpn. 63 (1990) 988. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [3] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403.
[4] M. Hartmann, Chem. Mater. 17 (2005) 4577. [5] M. Vallet-Regí, Chem. Eur. J. 12 (2006) 5934. [6] M. Vallet-Regí, L. Ruiz-González, I. Izquierdo-barba, J.M. GonzálezCalbet, J. Mater. Chem. 16 (2006) 26. [7] I. Izquierdo-Barba, L. Ruiz-González, J.C. Doadrio, J.M. GonzálezCalbet, M. Vallet-Regí, Solid State Sci. 7 (2005) 983. [8] X. Yan, C. Yu, X. Zhou, J. Tang, D. Zhao, Angew. Chem., Int. Ed. 43 (2004) 5980. [9] L.L. Hench, R.J. Splinter, W.C. Allen, T.K. Greenlee, J. Biomed. Mater. Res. 2 (1971) 117. [10] L.L. Hench, J. Am. Ceram. Soc. 81 (1998) 1705. [11] X.X. Yan, H.X. Deng, X.H. Huang, G.Q. Lu, S.Z. Quio, D.Y. Zhao, C.Z. Yu, J. Non-Cryst. Solid 351 (2005) 3209. [12] X. Yan, X. Huang, C. Yu, H. Deng, Y. Wang, Z. Zhang, S. Qiao, G. Lu, D. Zhao, Biomaterials (2006) 3396. [13] A. López-Noriega, D. Arcos, I. Izquierdo-Barba, Y. Sakamoto, O. Terasaki, M. Vallet-Regi, Chem. Mater. 18 (2006) 3137. [14] Q. Shi, J. Wang, J. Zhang, J. Fan, G. Stucky, Adv. Mater. 18 (2006) 1038. [15] W. Xia, J. Chang, J. Control. Release 110 (2006) 522. [16] T.A. Ostomel, Q. Shi, C.K. Tsung, H. Liang, G.D. Stucky, Small 2 (2006) 1261. [17] T. Yamada, H. Zhou, H. Uchida, M. Tomita, Y. Ueno, T. Ichino, I. Honma, K. Asai, T. Katsube, Adv. Mater. 14 (2002) 812. [18] H.S. Yun, H. Zhou, I. Honma, Chem. Commun. (2004) 2836. [19] T. Taguchi, Y. Muraoka, H. Matsuyada, A. Kishida, M. Akashi, Biomaterials 22 (2001) 53. [20] T. Kokubo, H. Takadama, Biomaterials 27 (2006) 2907. [21] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169.
Highly ordered mesoporous bioactive glasses with Im3m symmetry Hui-suk Yun ⁎, Seung-eon Kim, Yong-teak Hyeon Department of Future Technology, Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon-si, Gyeongnam, 641-831, Republic of Korea Received 14 February 2007; accepted 20 February 2007 Available online 12 March 2007
Abstract Highly 3D body centered cubic (Im3m) ordered mesoporous bioactive glasses (MBG) were synthesized by evaporation-induced self-assembly (EISA) in the presence of a nonionic triblock copolymer, EO100PO65EO100 (F127), template. The influence of the F127 concentration on the mesostructure was examined. MBG calcined at 600 °C possessed a large specific surface area (∼ 520 m2 g− 1) and pore volume (0.51 cm3 g− 1) and a uniformly distributed pore size (5.4 nm). In vitro bioactivity studies were carried out in simulated body fluid (SBF). © 2007 Elsevier B.V. All rights reserved. Keywords: Bioactive glass; Mesoporous; 3D cubic structure; Nanomaterials; Sol–gel preparation
1. Introduction Since the discovery of KSW-1 [1] and M41S [2], highly ordered mesoporous materials have attracted the attention of many scientists, mainly due to their potential technological applications [3]. Currently, mesoporous materials are coveted for their various applications in biomaterials science, such as drug delivery systems (DDS) and bone tissue regeneration, because their unique structural properties, which consist of ordered open pore structures and large specific surface areas and pore volumes, may enhance their bioactive behavior [4–16]. The DDS applications of mesoporous materials have experienced a remarkable breakthrough during the past several years [4–6]. The study of the bone tissue regeneration applications of mesoporous materials began after Vallet-Regi et al. first described the in vitro bioactivity of various mesoporous silica [7]. Studies related to tissue regeneration accelerated after the development of MBG by Zhao et al. in 2004 [8]. BGs have been widely studied because they have the ability to chemically bond with living bone tissue and have consequently been used in a variety of medical applications [9,10]. Increasing the specific surface area and pore volume of BGs may greatly accelerate the kinetic deposition process of hydroxycarbonate apatite and therefore enhance their bone-forming bioactivity. Zhao et al. successfully synthesized highly hexagonally ordered MBGs by ⁎ Corresponding author. Tel.: +82 55 280 3351; fax: +82 55 280 3399. E-mail address: [email protected] (H.-S. Yun). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.02.075
templating with a triblock copolymer, EO20PO70EO20 (P123) and demonstrated their superior bone-forming bioactivities in vitro compared to normal BG [8,11,12]. Although several other studies related to the synthesis of MBGs have since been carried out, all of the synthetic methods involved employed the same template polymer, P123, which generally leads to 2D hexagonal symmetry [13–16]. MBGs with 3D cubic pore structures have different accessibilities [17] and may facilitate the transportation of bioactive agents such as protein and nutrients from their hexagonal pore structures. MBGs with a tunable pore size and pore structure may greatly influence the protein adsorption and nutrient delivery behavior, thereby allowing them to have better in vivo bioactivity. A 3D cubic structure can be produced using F127 as a template. Zhao et al. reported using F127 to form a 3D cubic structure, but they only obtained a wormlike pore structure [8]. The successful control of the mesostructure of MBGs adds a new dimension to the art of the designed synthesis of MBGs. Herein, we demonstrate the successful synthesis of highly 3D cubic ordered MBGs with a large BET surface area and good bioactivity in vitro by templating with F127. 2. Experimental 2.1. Preparation of MBG In a typical synthesis, 2.88 g of F127 is dissolved in 18.1 ml of ethanol (EtOH). Stock solutions, which were prepared by mixing 1.36 g of calcium nitrate tetrahydrate (CaNT), 0.26 ml of triethyl
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gel films were easily separated from the vessel after aging [18] and were calcined at 600 °C for 6 h in air to remove the template. In addition, a series of MBGs with different amounts of F127 (denoted as MBGF6, MBGF12, MBGF24, MBGF36, and MBGF48 according to the weight percent of F127 to TEOS) were prepared to study the formation of the MBG. 2.2. Characterizations
Fig. 1. XRD patterns of calcined MBGF6 (a), MBGF12 (b), MBGF24 (c), MBGF36 (d) and MBGF48 (e).
Structural characterization was carried out using X-ray diffraction (XRD; θ–2θ scanning, Philips-X'pert MPD 3040), using Cu Kα (λ = 1.5406 Å) radiation (40 kV-40 mA), transmission electron microscopy (TEM; JEOL-JEM2100F) with field emission gun at 200 kV, and field emission scanning electron microscopy (FE-SEM; Hitachi-S5500 and JEOL-5800) at an accelerating voltage of 1–5 kV. The specific surface area was measured by the N2-gas adsorption method using a BET apparatus (BelJapan-Belsorp mini II). 2.3. In vitro bioactivity test
phosphate (TEP), 6 ml of tetraethyl orthosilicate (TEOS), 0.95 ml of HCl (1 M), 7.62 ml of EtOH and 2.86 ml of H2O, were added to this solution after stirring them separately for 1 h, and were vigorously stirred together for another 4 h at 40 °C. The molar composition was TEOS:CaNT:TEP:F127 = 1:0.2:0.05:0.008 in this case. The reactant solution was transferred to a polystyrene vessel without a cap, and aged at 40 °C, 40 RH% for 48 h without stirring. Gel films were obtained on evaporating the solvent. These
The MBG films were ground and sieved (d b 25 μm). The assessment of the in vitro bioactivity of the MBGs was carried out in SBF at 37 °C. Before immersing MBGs in SBF, they were treated using an alternate soaking process [19]: The MBGs were soaked in 300 ml of calcium chloride (CaCl2, 200 mM) for 10 s and then rinsed with excess water. The MBGs were subsequently soaked in a solution of potassium hydrogen
Fig. 2. TEM images of calcined MBGF6 (a), MBGF12 (b), MBGF24 (c), and MBGF48 (d). The inset shows the Fourier transform patterns of (d).
H.-S. Yun et al. / Materials Letters 61 (2007) 4569–4572
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Fig. 3. FE-SEM images (a)–(d) and optical images (e)–(h) of as-synthesized MBGF6 (a), (e), MBGF12 (b), (f), MBGF24 (c), (g) and MBGF48 (d), (h).
phosphate trihydrate (K2HPO4·3H2O, 200 mM) and then rinsed with excess water. These steps were repeated 3 times and MBGs rinsed well. The MBGs were immersed in SBF after drying. The SBF contained 142.0 mM Na+ , 5 mM K+ ,1.5 mM Mg2+ , 2.5 mM Ca2+ , 147.8 mM HCl− , 4.2 mM HCO3− , 1.0 mM HPO42− , and 0.5 mM SO42− [20]. Its chemical composition was similar to that of human plasma. The solution had a pH of 7.4 and was kept at 37 °C before use. 3. Results and discussion The XRD patterns of the MBGs produced using five different amounts of polymer template (MBGF6–MBGF48) calcined at 600 °C are shown in Fig. 1. The weight ratio of the triblock copolymer, F127, to TEOS significantly affected the formation of the mesostructure. MBGF6 and MBGF12 present no diffraction peaks (Fig. 1a and b). XRD peaks, indicating the formation of a mesostructure, can be detected from the samples with an amount of F127 greater than 24% (MBGF24; Fig. 1c). The intensity of the peaks increased with increasing amount of F127, corresponding to a better and longer range mesostructure arrangement. MBGF48 exhibits three diffraction peaks in the small-angle regime (2θ = 0.95, 1.3, 1.61°), which can be indexed to the (110), (200), and (211) diffractions of a 3D Im3m lattice, respectively (Fig. 1e) (d100 = 13.1 nm). The TEM observations of these samples also confirm this conclusion as shown in Fig. 2. No mesostructure was observed in the case of the MBGF6 sample. MBGF12 has a wormhole-like mesostructure, while MBGF24 has an ordered mesostructure. The TEM image along the [001] direction of MBGF48 reveals a highly ordered cubic arrangement (Fig. 2d). Fig. 3 shows the FE-SEM and optical images of the as-synthesized films. Micrometer sized pores with heterogeneous size distribution were observed in the case of MBGF6 (Fig. 3a). These pores apparently resulted from the aggregation of F127, owing to the heterogeneous reactions between F127 and the inorganic species, which led to the sample having inferior optical properties (Fig 3e). The large pores became increasingly homogeneous in size and gradually disappeared with increasing amount of F127 (Fig. 3b–d). As a result, the films evolved to become transparent, continuous, and flexible (Fig. 3f–h).
The nitrogen adsorption and desorption isotherms were also changed when the amount of F127 was increased. MBGF6 shows H3-type hysteresis loop (corresponding to slit-like pores), as shown in Fig. 4a. That is, it has no mesostructure. It is observed that the samples, MBGF12–MBGF48, exhibit similar type IV isotherms typical of mesoporous materials and that increasing the amount of template results in a progressive augmentation in their nitrogen sorption capacity and mesopore volume (Fig. 4b–d). In particular, the mesopore volume of MBGF48 was considerably increased. MBGF48 has a type IV isotherm with a sharp capillary condensation step at high relative pressures and a broad H2 hysteresis loop that is indicative of large uniform cage-type pores [21]. It has a BET surface area of 520 m2 g− 1, a total pore volume of 0.51 cm3 g− 1, and a pore size of 5.4 nm. The bone-forming activity of MBGF48 in vitro was tested in SBF to monitor the formation of hydroxyl apatite (HA) on the surface of the MBG over time. The FE-SEM image of MBGF48 before soaking shows a smooth and homogeneous surface (Fig. 5a). The FE-SEM image reveals that the surface of the MBG undergoes important changes
Fig. 4. N2 adsorption–desorption isotherms of MBGF6 (a), MBGF12 (b), MBGF24 (c), and MBGF48 (d).
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Fig. 5. SEM images of MBGF48 after immersing in SBF for 0 h (a) and 24 h (b).
during the reaction with SBF. The growth of HA-like nanoparticles is observed after soaking the sample for 1 h, and HA with a rod-like morphology composed of rods with a length of 100–200 nm, which is similar to the morphology of HA in human bones, is subsequently observed after 24 h (Fig. 5b).
4. Conclusions Highly 3D cubic ordered MBG with large specific surface area and pore volume, and superior bone-forming bioactivity in vitro were obtained by means of the EISA method, using a nonionic triblock copolymer, F127, as the structure-directing agent. The formation of a long-ranged cubic mesostructure largely depends on the concentration of F127. These cubic ordered MBGs as a new family of biomaterials, may find potential applications in drug delivery systems, implant coating materials, and functional bone tissue engineering. References [1] T. Yanagisama, Z. Shimizu, K. Kuroda, Bull. Chem. Soc. Jpn. 63 (1990) 988. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [3] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403.
[4] M. Hartmann, Chem. Mater. 17 (2005) 4577. [5] M. Vallet-Regí, Chem. Eur. J. 12 (2006) 5934. [6] M. Vallet-Regí, L. Ruiz-González, I. Izquierdo-barba, J.M. GonzálezCalbet, J. Mater. Chem. 16 (2006) 26. [7] I. Izquierdo-Barba, L. Ruiz-González, J.C. Doadrio, J.M. GonzálezCalbet, M. Vallet-Regí, Solid State Sci. 7 (2005) 983. [8] X. Yan, C. Yu, X. Zhou, J. Tang, D. Zhao, Angew. Chem., Int. Ed. 43 (2004) 5980. [9] L.L. Hench, R.J. Splinter, W.C. Allen, T.K. Greenlee, J. Biomed. Mater. Res. 2 (1971) 117. [10] L.L. Hench, J. Am. Ceram. Soc. 81 (1998) 1705. [11] X.X. Yan, H.X. Deng, X.H. Huang, G.Q. Lu, S.Z. Quio, D.Y. Zhao, C.Z. Yu, J. Non-Cryst. Solid 351 (2005) 3209. [12] X. Yan, X. Huang, C. Yu, H. Deng, Y. Wang, Z. Zhang, S. Qiao, G. Lu, D. Zhao, Biomaterials (2006) 3396. [13] A. López-Noriega, D. Arcos, I. Izquierdo-Barba, Y. Sakamoto, O. Terasaki, M. Vallet-Regi, Chem. Mater. 18 (2006) 3137. [14] Q. Shi, J. Wang, J. Zhang, J. Fan, G. Stucky, Adv. Mater. 18 (2006) 1038. [15] W. Xia, J. Chang, J. Control. Release 110 (2006) 522. [16] T.A. Ostomel, Q. Shi, C.K. Tsung, H. Liang, G.D. Stucky, Small 2 (2006) 1261. [17] T. Yamada, H. Zhou, H. Uchida, M. Tomita, Y. Ueno, T. Ichino, I. Honma, K. Asai, T. Katsube, Adv. Mater. 14 (2002) 812. [18] H.S. Yun, H. Zhou, I. Honma, Chem. Commun. (2004) 2836. [19] T. Taguchi, Y. Muraoka, H. Matsuyada, A. Kishida, M. Akashi, Biomaterials 22 (2001) 53. [20] T. Kokubo, H. Takadama, Biomaterials 27 (2006) 2907. [21] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169.