Hierarchical structures and shaped particles of bioactive glass and its in vitro bioactivity.
During the last decade, the use of mesoporous materials, which have pores ranging in size from 2 to 50 nm, was proposed in tissue engineering because their large surface area and pore volume may enhance their bioactive behavior and allow them to be loaded with the osteogenic agents used to promote new bone formation [1-4]. Mesoporous bioactive glasses (MBGs) have attracted much attention in many potential applications, such as catalysis, adsorption/separation, synthesis of nanomaterials [5-9], and, recently, also in the field of biomaterial science as bone scaffolds 10,11], bone filler , and drug delivery systems [11,13-15].
Although all of the reported MBGs show favorable bio activity, they are difficult to use as scaffolds for the regeneration of bone tissues at this stage because their mesosized pores are too small to promote cell growth. To overcome this pore size limitation, our group successfully prepared hierarchically structured three-dimensionally ordered macroporous (3DOM) by the sol-gel method using a block-copolymer and polymer colloidal crystals as dual templates, which can generate either three-dimensionally ordered macroporous structures or shaped bioactive glass nanoparticles.
2.1. General. Calcium nitrate tetrahydrate [(Ca(N[O.sub.3]).sub.2] x 4[H.sub.2] O), tetraethyl orthosilicate (TEOS), triethyl phosphate (TEP), the surfactant Brij 56 ([C.sub.16][H.sub.33] [(OC[H.sub.2]C[H.sub.2]).sub.n]OH, n~10), and polymethylmethacrylate (PMMA) colloidal crystals with 400 nm size were prepared by a published method .
2.2. Preparation of Bioactive Glasses. MBGs were synthesized by a sol-gel method. In a typical synthesis of bioactive glass nanoparticles M58S (M58SP), Brij 56 was used as a structure-directing agent for the mesostructure  and tetraethyl orthosilicate (TEOS), triethyl phosphate (TEP), and Ca[(N[O.sub.3]).sub.2]x 4[H.sub.2]O were the sources of Si, P, and Ca, respectively (Si/Ca/P = 60:36:4, molar ratio). Amounts of 0.850g of Ca[(N[O.sub.3]).sub.2]x 4[H.sub.2]O and 0.096g of citric acid were dissolved in 2mL of 1M HN[O.sub.3], and 0.073 g of TEP was added into the solution while stirring at room temperature. A solution containing 1.250 g of TEOS and 0.340 g of Brij 56 was then added to this mixture. The combined mixture was stirred at room temperature for 30 min or until the solution became clear. A monolithic piece of the PMMA colloidal crystal template was immersed into this clear solution. After complete infiltration, the samples were aged at 45[degrees]C for 48 h and dried at 45[degrees]C for 24 h in air. The as-synthesized samples were calcined at 600[degrees]C in air for 6h in order to remove the template completely. The heating rate for the calcination was fixed at 2[degrees]C/min. In the case of M80S, M70S, and M58S, bioactive glass materials were synthesized by the same method, without ageing at 45[degrees]C. The nominal compositions of the MBGs under study are listed in Table 1. The structure of the bioactive glasses was analyzed in detail by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR).
2.3. Study of Bioactivity. The in vitro bioactivity study was performed by soaking the MBGs in simulated body fluid (SBF) prepared by the method of Kokubo et al.  at 37[degrees]C for 3, 6,12, 24, and 48 h, and the surface of the samples was characterized by FTIR, SEM, and XRD.
3. Results and Discussion
3.1. Sample Characterization. Figure 1 shows the XRD patterns of MBGs after calcination at 600[degrees]C with a ramp rate 2[degrees]C/min in air. All samples show a broad peak in the 2[theta] range 15-35[degrees], which suggests that all MBGs with different chemical compositions exist as amorphous phases. The pattern of the M58S sample contained additional reflections assigned to calcium phosphate silicate and calcium phosphate (JCPDS-00-050-0905 and 00-044-0752). The EDS spectra of all MBGs showed peaks of carbon, oxygen, silicon, phosphate, and calcium with relative amounts indicated in Figure 2.
Figure 3 shows the representative SEM images of (a) M58S and (b) M58S. Bioactive glass with hierarchical porosity was first formed through the PMMA and Brij 56 dual-templating system. Without of aging step at 45[degrees]C, no structure transformation occurred and the 3DOM structure was produced (Figure 3(a)) due to no shrinkage process between the ageing step [16, 20]. After a sufficient time of aging (at least 48 h), no 3DOM structure remained and, nanocube with ~100 nm edge lengths were obtained (Figure 3(b)). Typically, an fcc array of spheres, octahedral ([O.sub.h]), and tetrahedral ([T.sub.d]) voids exist between the spheres. The bioactive glass precursor infiltrated these voids and formed an inverse replica of the template. So correspondingly, the 3DOM structure can be considered to be built up from these two basic units ([O.sub.h] and [T.sub.d]) which are interconnected through narrow necks. The disassembly occurs first by shrinkage in a polycondensation reaction during aging process until the silanol groups are no longer close enough to react with each other [16, 20]. Follow with the syneresis is defined as "spontaneous shrinkage of the gel and the resulting expulsion of liquid from the pores" . By complete disconnection of the skeleton at the narrowest connection points. Hence, the nanocubes produced from the octahedral holes. This formation mechanism is known as disassembly and occurs in order to stabilize the structure of the 3DOM material (Figure 4) 16].
3.2. Study of Bioactivity. Figure 5 shows the FTIR spectra of MBGs with different chemical compositions before and after soaking in SBF for 12 h. The FTIR spectra of series M80S, M70S, and M58S before soaking in SBF, all spectra show the characteristic absorption bands of the Si-O-Si asymmetric stretching and symmetric bending mode at 1060 [cm.sup.-1], 800 [cm.sup.-1], and 478 [cm.sup.-1], respectively, [21, 22]. The shoulder around 950 [cm.sup.-1] is related to the Si-O-Ca vibration mode . Spectra of M70S, M58S, and M58SP, after samples were soaked in SBF for 12 h, a doublet at 568 [cm.sup.-1] and 607 [cm.sup.-1] is shown, which is associated with the P-O bending vibration and at 1040 [cm.sup.-1] and 1090 [cm.sup.-1] can be attributed to the stretching P[O.sub.4] vibration [21, 23].
Additionally, SEM micrographs of the sample surfaces before and after soaking in SBF for different times are shown in Figure 6. For M80S, M70S, and M58SP after 6h of soaking, small needle-like crystal agglomerates were observed on the surface. In contrast, the surfaces of the M58S samples were needle-like after soaking in SBF within 3 h. All samples were completely covered by a needle-like crystal layer in 24 h. XRD patterns (Figure 7) confirmed that the needle-like crystal can be assigned to the bone-like apatite phase. Furthermore, the in vitro bioactivity of MBGs is dependent on the Si/Ca ratio in the network which, as a result of the higher calcium content exhibits the best in vitro bioactivity . It is clear that M58S shows faster bioctivity than M70S and M80S, and the 3DOM structure of M58S shows higher bioactivity than M58SP.
The bioactive glass skeleton with hierarchical porosity was first formed via a surfactant and polymer sphere dual-templating system, and then the three-dimensionally ordered structure was disassembled to obtain bioactive nanocubes. In addition, the study of in vitro bioactivity found that all of bioactive glass samples have the ability to induce the formation of a bone-like apatite layer on its surface when in contact with SBF within 6h and the transformation to bone-like apatite was more efficient for M58S than for M80S and M70S. Furthermore, the 3DOM structure of M58S shows higher bioactivity than M58SP.
The authors would like to thank Walailak University Research and Development Institute, Walailak University, (Grant no. WU54203) and the National Science Foundation (DMR-0704312).
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U. Boonyang, (1) F. Li, (2) and A. Stein (2)
(1) Molecular Technology Research Unit, School of Science, Walailak University, Nakhon Si Thammarat 80161, Thailand
(2) Department of Chemistry, University of Minnesota, MN 55455, USA
Correspondence should be addressed to U. Boonyang; email@example.com
Received 12 April 2013; Revised 3 June 2013; Accepted 22 July 2013
Academic Editor: Jie Huang
Table 1: Chemical compositions of MBGs (mol%). Samples Si[O.sub.2] CaO [P.sub.2] [O.sub.5] M80S 80 20 -- M70S 70 30 -- M58S, M58SP 60 36 4
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|Title Annotation:||Research Article|
|Author:||Boonyang, U.; Li, F.; Stein, A.|
|Publication:||Journal of Nanomaterials|
|Date:||Jan 1, 2013|
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