In vitro hydroxyapatite forming ability and dissolution of tobermorite nanofibers

In vitro hydroxyapatite forming ability and dissolution of tobermorite nanofibers

Acta Biomaterialia 3 (2007) 271–276 www.actamat-journals.com Brief communication In vitro hydroxyapatite forming ability and dissolution of tobermor...

498KB Sizes 1 Downloads 129 Views

Acta Biomaterialia 3 (2007) 271–276 www.actamat-journals.com

Brief communication

In vitro hydroxyapatite forming ability and dissolution of tobermorite nanofibers Kaili Lin a

a,b,*

, Jiang Chang b, Rongming Cheng

a,*

Center of Functional Nanomaterials and Devices, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, PR China b Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 28 July 2006; received in revised form 16 October 2006; accepted 17 November 2006

Abstract In this paper, fiber-like and dispersible tobermorite (Ca5(Si6O16)(OH)2 Æ 4H2O), 80–120 nm in diameter and up to tens of micrometers in length was prepared by a hydrothermal microemulsion method. In vitro bioactivity of the nanofibers were evaluated by examing the hydroxyapatite (HAp) forming ability on the surface after soaking in simulated body fluid (SBF) for various periods. After soaking in SBF, the nanofibers were completely covered by bonelike hydroxycarbonate apatite (HCA) layers, and the nanofibers after soaking still kept stability in fibrous morphology. The dissolution of the nanofibers reached about 24.5% after soaking in SBF for 14 days. The results suggested that the tobermorite nanofibers exhibited certain desirable characteristics, including bioactivity, degradability and stability in morphology, and are a potential candidate for a reinforcement material in the development of novel bioactive and degradable composites for biomedical applications. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Tobermorite; Nanofibers; Hydroxyapatite forming ability; Dissolution

1. Introduction One-dimensional (1D) nanoscale building blocks, such as nanofibers, nanorods and nanotubes of materials, have attracted much attention because of their importance to basic scientific research and their potential technology applications [1–3]. One of the most important characteristics is that the 1D nanoscale materials possess excellent mechanical properties, and fiber-reinforced composites exhibit high tensile, flexural and fracture strengths. Therefore, advances in 1D nanoscale reinforcement are of considerable interest in improving the mechanical properties of ceramics and polymer composites [4–6]. Preparation of calcium silicate hydrate (CSH) fibers, especially tobermorite (Ca5(Si6O16)(OH)2 Æ 4H2O), has been attracting attention in recent years. This arises from *

Corresponding authors. Tel.: +86 21 52412810; fax: +86 21 52413903. E-mail addresses: [email protected] (K. Lin), [email protected]. edu.cn (R. Cheng).

the fact that these fibers are widely required in modern industry [7,8]. Tobermorite, as one of the most important CSH materials, possesses the highest mechanical strength of the CSH materials, and has been widely used in high quality cement [7,8]. Previous studies have suggested that various biomaterials containing the CaO–SiO2 component, such as Bioglass [9], CaSiO3 [10,11] and Ca2SiO4 ceramics [12], are bioactive and could induce the formation of a bonelike hydroxyapatite (HAp) layer in vivo and in vitro. This type of HAp layer plays an essential role in the formation of tight bone-bonding between the bioactive materials and the neighbouring bones, and has not been observed for non-bioactive materials [13–15]. Based on these results, it is proposed and confirmed that the essential requirement for materials to bond with living bones is the formation of bonelike apatite on the material’s surface in the living body and that this in vivo apatite formation can be reproduced in simulated body fluid (SBF). This means that in vivo bioactivity can be predicted by examing apatite formation on a material’s surface in SBF [15,16]. Therefore, we consider

1742-7061/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2006.11.003

272

K. Lin et al. / Acta Biomaterialia 3 (2007) 271–276

that tobermorite could be bioactive and that its fibers are a potential candidate for a reinforcement material. Tobermorite fibers are usually prepared from suspensions of silica and calcium hydroxide by hydrothermal treatment [17]. However, the fiber size ranges widely, from the submicrometer range to micrometers, and the aspect ratio of the obtained fibers cannot be controlled well using the normal hydrothermal method. On the other hand, it is generally difficult to obtain long CSH fibers, and experimental results are usually affected by several factors, such as composition, crystallization, size distribution of raw materials and the weight ratio of water/solid material [18–20]. At the same time, several reports have been reported in the literature concerning hydrothermal synthesis of long CSH fibers at relatively much higher temperatures (250–350 °C) and over much longer times (72–200 h), using special instruments (continuous-type or multichamber autoclave) [21,22]. A newly developed hydrothermal microemulsion technique is used to synthesize nanopowders and nanoneedles [23,24], and this method is considered to be an effective, convenient and mild synthetic methodology. The microemulsion can serve not only as nano-reactors to control the particle size and size distribution in the processing reactions, but can also inhibit the excessive agglomeration of particles, because surfactants can absorb on the particle surfaces when the particle size approaches that of the water (or oil) pool. In addition, the surfactants in the microemulsion can also serve as a versatile ‘‘soft’’ template for the synthesis of 1D nanostructural materials. Furthermore, hydrothermal treatment can effectively increase the crystallinity of the product. However, no study on the preparation, bioactivity and dissolution of CSH fibers, such as tobermorite nanofibers, has been reported, until now. In the present study, tobermorite nanofibers were synthesized by a hydrothermal microemulsion process. In vitro HAp forming ability and the dissolution of the nanofibers were evaluated by soaking in SBF. 2. Materials and methods 2.1. Synthesis and characterization of the tobermorite nanofibers Microemulsions were prepared using cetyltrimethyl ammonium bromide (CTAB) as surfactant and n-pentanol as cosurfactant. First, Ca(NO3)2 solution and Na2SiO3 solution were obtained by dissolving 1.1335 g Ca(NO3)2 Æ 4H2O and 1.3642 g Na2SiO3 Æ 9H2O in 4.0 ml distilled water, respectively, and the pH of both solutions was adjusted to 10.8 by adding ammonia solution. These aqueous solutions were used as the water phase and n-hexane was used the oil phase. In a typical experiment, a mixture of 2.60 g of CTAB, 4.0 ml of n-pentanol, 65 ml of n-hexane and 4.0 ml of Na2SiO3 aqueous solution was added to a 200 ml beaker. The mixture was stirred and ultrasonicated, and optically transparent microemulsions were obtained. Then Ca(NO3)2 solution was added dropwise into the

Na2SiO3 microemulsion solution to obtain a suspension. The suspension was transferred into a stainless steel autoclave and heated at 200 °C for 18 h, followed by cooling to room temperature naturally. After hydrothermal reaction, the obtained suspension was filtered and washed with distilled water and anhydrous ethanol three times. The resultant powders were dried at 60 °C for 48 h. The obtained powders were characterized using X-ray diffraction (XRD; Geigerflex, Rigaku Co., Japan) with monochromated Cu Ka radiation. The surface of the assynthesized powders was sputter-coated with gold and then observed by field emission scanning electron mocroscopy (FESEM) at 10 kV accelerating voltage using a JEOL (JSM-6700F, Japan). 2.2. In vitro HAp forming ability and dissolution behavior The bioactivity of the tobermorite nanofibers was evaluated from the formation of bonelike HAp on the nanofibers in SBF, which has ion concentrations similar to human blood plasma [15,25]. The solution was buffered to pH 7.4 using tris(hydroxymethyl) aminomethane and hydrochloric acid. The nanofibers were soaked in the SBF at 37.0 °C in a shaking water bath for 1, 3, 7 and 14 days, respectively, at a solid/liquid ratio of 1.50 mg ml1 without refreshing the soaking medium. After various soaking periods, the samples were filtered and gently rinsed twice with deionized water to remove SBF followed by drying in vacuum at 80 °C. The soaked powders were characterized by XRD and Fourier transform infrared spectroscopy (FTIR; Nicolet Co., USA). The surface of the soaked powders was sputter-coated with gold and then observed by FESEM at 10 kV accelerating voltage. The concentrations of Ca, P and Si soaked in the SBF solutions were determined by inductively coupled plasma atomic emission spectroscopy (Varian Co., USA). Changes in pH of the solutions were measured by a pH test-meter (pHS-2C, Jingke Leici Co., China). Based on the fact that there is no Si in SBF before soaking, the dissolution ratio (S) of the tobermorite nanofibers at different time periods was calculated by the following equation: S ¼ ðcSi  vÞ=mSi  100% where cSi, v and mSi are the Si concentration in SBF (mg ml1), volume of SBF (ml) and Si content (mg) of the samples soaked in SBF, respectively. 3. Results and discussion 3.1. Characterization of the as-synthesized nanofibers Fig. 1 shows the XRD patterns of the as-synthesized powder prepared by the hydrothermal microemulsion method. Numerous sharp peaks were observed in the

K. Lin et al. / Acta Biomaterialia 3 (2007) 271–276

Fig. 1. XRD patterns of the as-synthesized powders.

273

which facilitates the homogeneous mixing of the raw materials. Second, CTAB is suggested to inhibit the excess aggregation of nanofibers because the surfactants adsorb onto the surfaces of the nanofibers. The most important role, like for other surfactants, is that CTAB can serve as a soft template to synthesize 1D nanorods or nanofibers [26]. CTAB is a cationic surfactant and its critical micelle concentration (CMC) is 0.03% (0.9–1.0 mM) [27]. Above the CMC, rod-like micelles are formed and the size of micelles increases with increasing CTAB concentration, eventually resulting in long, flexible wormlike micelles [28]. In our system, with a CTAB concentration of around 5%, CTAB could easily form rod-like micelles. According to the studies by Xiong and Yao [29,30], a probable mechanism for the formation of tobermorite might be explained as follows: first, the CTAB formed rod-like micelles in which water phase solution-containing 2n ½SiO3 n was enwrapped. Because of the concentration difference between the inside (water phase) and outside (oil 2n phase) of the rod-like micelles, the ½SiO3 n transferred 2n to the surface of the micelles and CTAB–½SiO3 n rod-like 2+ micelles were formed. When the Ca -containing solutions were added into the ½SiO3 n2n -containing microemulsion solutions, CHS clusters were preferentially condensed on the rod-like micellar surface. The micelles acted as nucleating sites for the growth of tobermorite crystals. During the hydrothermal stage, CTAB-tobermorite complexes formed and coalesced to form a stable 1D nanofiber structure (see Fig. 3). 3.2. In vitro HAp forming ability and dissolution behavior

Fig. 2. FESEM images of the as-synthesized tobermorite powders.

XRD patterns and were identified as tobermorite (JCPDS card: No. 45-1480). The sharp diffraction peaks indicates that the tobermorite formed are well crystallized. Fig. 2 shows the FESEM micrographs of the as-synthesized tobermorite particles. The formed tobermorite particles have a typical nanofiber-like morphology, are 80–120 nm in diameter and up to tens of micrometers in length, and have a smooth surface. The nanofibers exhibited a high level of dispersibility with less aggregation, which was favorable to fabricate composites. From the results of the XRD and FESEM analyses, the hydrothermal microemulsion method is found to be an effective way to obtain tobermorite nanofibers. The size distribution (diameter) of the nanofibers is much narrower than that of the fibers prepared by the normal hydrothermal method [17]. The microemulsion method is effective in controlling the particle size and its distribution, and inhibits the excess aggregation of the tobermorite nanofibers by serving as nano-reactors. The CTAB is thought to perform multiple roles in the synthesis. First, it enhanced emulsification the n-pentanol/n-hexane/water system,

Fig. 4 shows the XRD patterns of the tobermorite nanofibers after soaking in SBF for various periods. Most of the peaks of tobermorite disappeared and the peaks of HAp appeared after soaking in SBF for 3 days. After prolonged soaking for 7 and 14 days, the peaks of tobermorite disappeared and the intensity of the HAp peaks increased, indicating the increased amount of HAp. In addition, the broad peaks of HAp imply that the deposited HAp had a low crystallinity structure. Fig. 5 presents the FTIR spectra of the samples soaked in SBF for 0, 3, 7 and 14 days. As observed, the absorption

Fig. 3. Schematic illustration of the formation mechanism of tobermorite nanowires in microemulsion. ‘‘A’’ stands for the reactants of ½SiO3 2n n .

274

K. Lin et al. / Acta Biomaterialia 3 (2007) 271–276

Fig. 4. XRD patterns of the tobermorite nanofibers soaked in SBF for various periods.

Fig. 5. FTIR spectra of the tobermorite nanofibers soaked in SBF for various periods.

bands of silicate group were evident in the spectra of the tobermorite nanofibers before soaking (0 day). The intense band at about 960 cm1 was assigned to the Si–O–Si asymmetric stretch, the bands at 1014 and 1072 cm1 to the Si–O–Si bending stretch, the band at around 900 and 1120 cm1 to the Si–O symmetric stretch, the band at about 800 cm1 to the Si–O–Si bending vibration and the band at 460 cm1 to the Si–O–Si vibrational mode of bending [9]. After soaking for 3 days, the FTIR spectra were analogous to that of the hydroxycarbonate apatite (HCA), where the bands at 1090, 650 and 567 cm1 could be assigned to the phosphate group ðPO3 4 Þ, those at 1427 and 875 cm1 to the carbonate group ðCO2 3 Þ and that at 1630 cm1 to the OH absorption [31]. Further increasing the soaking periods, the intensity of silicate absorption bands decreased sharply and almost disappeared at 14 day. The FTIR spectra further confirmed that the bonelike HCA covered the tobermorite nanofiber surfaces after soaking in SBF, suggesting the excellent HAp forming ability of the tobermorite nanofibers in vitro.

Fig. 6 shows the FESEM micrograph of the tobermorite nanofibers after soaking in SBF for 3 days. It was obvious that a layer of a new phase with a nanosheet-like arrangement grew over the surface of the tobermorite nanofibers, and the tobermorite nanofibers were also preserved after soaking in SBF. The XRD and FTIR results have shown that the newly formed phase was bonelike HCA. It is obvious from the results described above that the tobermorite nanofibers could develop a bonelike HCA layer on their surface when soaked in SBF. The bonelike HCA layer can be reproduced in SBF, and previous studies have shown that CaO–SiO2 based ceramic powders, such as Bioglass, A-W bioactive glass, CaSiO3 and Ca2SiO4, could induce HCA deposition when soaking in SBF, and could be used as bioactive components for the preparation of bioactive composites [15,32–35]. However, a much higher calcination temperature (800–1300 °C) is required to prepare these materials [9–12]. The present study suggests that, as a CSH, tobermorite nanofibers could induce the bonelike HCA deposition in SBF, at a preparation temperature that is much lower than that of Bioglass, A-W bioactive glass, CaSiO3, Ca2SiO4 and other CaO–SiO2based bioactive ceramic powders. Therefore, tobermorite nanofibers might be a more economic reinforcement material for the preparation of bioactive composites. Fig. 7 shows the changes of pH and concentrations of Ca, P and Si of SBF solutions after soaking for various periods. It is clear from the figure that the concentration of phosphorus in the SBF solution decreased very steeply to a level that is only 17.7% of the starting concentration after 1 day, and then continuously decreased at a slower rate up to 14 days. The concentration of calcium ions and silicate ions increased with increasing soaking time. There was also a simultaneous rise in pH from 7.25 to 8.88. In addition, during the soaking period from 1 to 14 days, the calcium and silicate ion concentrations increased slightly slower than in the early stage but phosphorus concentration decreased drastically. The increase in the calcium and silicate ion con-

Fig. 6. FESEM micrographs of tobermorite nanofibers soaked in SBF for 3 days.

K. Lin et al. / Acta Biomaterialia 3 (2007) 271–276

275

Fig. 8. The dissolution ratio of tobermorite nanofibers in SBF. Fig. 7. Changes of Ca, Si and P concentrations and pH value in SBF after soaking the tobermorite nanofibers for various periods.

centrations was attributed to the dissolution of calcium and silicate ions from the tobermorite nanofibers, and the decrease of the phosphorus was attributed to the formation of bonelike HCA. Although the formation of an HCA layer consumed some calcium ions, more calcium ions were dissolved from the nanofibers than were consumed, and the ionic exchange between calcium ions in the nanofibers and H+ in the SBF solution resulted in the increase in pH of the resultant SBF solution. According to the pH increasing of the materials after degradation in solution, the tobermorite nanofibers might be used as reagents to neutralize the acidic degradation products of the biopolymers. It is well known that synthetic biodegradable polyesters like polyglycolic acid, polylactide acid and their copolymer poly(lactic acid–coglycolic acid) have been widely used as scaffolds in tissue engineering [36,37]. These materials have many advantages, such as being biodegradable, biocompatible and easily processed into the expected configuration. However, a number of problems have been encountered regarding the use of these polymers in tissue engineering applications. One problem is the release of acidic degradation by-products, which can lead to a decrease in pH in the vicinity of the scaffolds and inflammatory responses [38,39]. Several recent studies have reported adverse effects because of the local decrease in pH around the implants [40,41]. One method to avoid this shortcoming is to fabricate the composites by combining with alkaline biodegradability materials [42]. Tobermorite nanofibers might be used as the alkaline biodegradability materials in such fabrications, which might not only contribute to the improved bioactivity of those acid biodegradability polymer materials, but also neutralize the acidic degradation products of the polymers. Degradability is an important requirement for the biomaterials used in tissue engineering applications [43,44]. Corresponding to the Si concentration in SBF after soaking the samples, the quantitative dissolutions of tobermorite nanofibers were calculated and the results are shown in Fig. 8. It is clear from the figure that the increase in soaking

time resulted in an obvious increase in the weight loss of the tobermorite nanofibers. With the increase of soaking time from 0 to 3 day, the weight loss increased sharply, reached about 17.4%. With a further increase in soaking time, the weight loss increased more slowly, reaching 19.6% at 7 day and 24.5% at 14 day. The excellent degradability was mainly attributed to the large surface area afforded by the nanoscale fibers, which resulted in faster supersaturation of the medium with respect to the HCA crystal nucleation and led to faster apatite forming in SBF. The studies of Kim et al. have also shown that the degradability and bioactivity of the bioactive glass nanofibers were much higher than those of the normal micrometer-sized bioactive glass [45]. On the other hand, in vitro cell culture studies have also revealed that the fibers could accelerate the cell attachment and proliferation [46]. Therefore, when the tobermorite nanofibers are used as bone substitutes, they could be expected to quickly provide a favorable environment owing to the rapid formation of bonelike minerals on the surface, thereby exhibiting excellent responses for adhesive proteins and cells and consequently resulting in improved bone formation [45,47]. The in vitro bioactivity and degradation results indicated that the tobermorite nanofibers might be suitable for preparation of bioactive and degradable composite scaffolds for bone tissue engineering applications. 4. Conclusions Easily dispersible tobermorite nanofibers 80–120 nm in diameter and up to tens of micrometers in length were prepared by a hydrothermal microemulsion method. The tobermorite nanofibers, as a novel CSH, exhibit excellent in vitro bioactivity and can develop a bonelike HCA layer in SBF, and show considerable degradability. The results suggest that tobermorite nanofibers possess certain desirable characteristics, including bioactivity, degradability and morphological stability in SBF, and could be a potential candidate for a reinforcement material in the preparation of bioactive and degradable composites.

276

K. Lin et al. / Acta Biomaterialia 3 (2007) 271–276

Acknowledgements This work was supported by grants from the Science and Technology Commission of Shanghai Municipality (Grant No. 05DJ14005) and the National Basic Research Program (h973i Program) of PR China (Grant No. 2005CB522704). References [1] Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Gates B, et al. Onedimensional nanostructures: synthesis, characterization, and applications. Adv Mater 2003;15:353–89. [2] Wang ZL. Nanobelts, nanowires and nanodiskettes of semiconducting oxides – from materials to nanodevices. Adv Mater 2003;15:432–6. [3] Zhong ZH, Qian F, Wang DL, Lieber CM. Synthesis of p-type gallium nitride nanowires or electronic and photonic nanodevices. Nano Lett 2003;3:343–6. [4] Kobayashi S, Kawai W. Development of carbon nanofiber reinforced hydroxyapatite with enhanced mechanical properties. Compos Part A 2007;38:114–23. [5] Yang W, Araki H, Kohyama A, Thaveethavorn S, Suzuki H, Noda T. Fabrication in-situ SiC nanowires/SiC matrix composite by chemical vapour infiltration process. Mater Lett 2004;58:3145–8. [6] Gillett N, Brown SA, Dumbleton JH, Pool RP. The use of short carbon fibers reinforced thermo-plastic plates for fracture fixation. Biomaterials 1985;6:113–21. [7] Roy DM, Johnson AM. Autoclaved calcium silicate building products. London: Society of Chemical Industry; 1965. [8] Taylor HFW. Cement chemistry. New York: Academic Press; 1990. [9] Hench LL. Bioceramics: from concept to clinic. J Am Ceram Soc 1991;74:1487–510. [10] Siriphannon P, Hayashi S, Yasumori A, Okada K. Preparation and sintering of CaSiO3 from coprecipitated powder using NaOH as precipitant and its apatite formation in simulated body fluid solution. J Mater Res 1999;82:529–35. [11] De Aza PN, Luklinska ZB, Anseau MR, Guitian F, De Aza S. Bioactivity of pseudowollastonite in human saliva. J Dent 1999;27: 107–13. [12] Gou Z, Chang J. Synthesis and in vitro bioactivity of dicalcium silicate powders. J Eur Ceram Soc 2004;24:93–9. [13] Hench LL, Wilson J. An introduction to bioceramics. Singapore: World Scientific; 1993. [14] Hench LL, Paschall HA. Histo-chemical responses at a biomaterials interface. J Biomed Mater Res 1974;5:49–64. [15] Kokubo T, Takadama H. How useful is SBF in predicting in vivo bone bioactivity. Biomaterials 2006;27:2907–15. [16] Mu¨ller L, Mu¨ller FA. Preparation of SBF with different HCO 3 content and its influence on the composition of biomimetic apatites. Acta Biomater 2006;2:181–9. [17] Huang X, Jiang DL, Tan SH. Novel hydrothermal synthesis method for tobemorite fibers and investigation on their thermal stability. Mater Res Bull 2002;37:1885–92. [18] Shibahara K, Kubo K, Takahashi A. Effect of silica raw materials on the texture of synthesized xonotlite. Gypsum Lime 1986;202:170–6. [19] Shibahara K, Kubo K, Takahashi A. Effect of raw lime materials on the textures of synthesized xonotlite. Gypsum Lime 1986;203:229–34. [20] Arai Y, Yasue T, Aoki S, Kokumai A, Kijima Y, Kiso M, et al. Crystal shape and size controls of xonotlite. Gypsum Lime 1994;248:17–25. [21] Kunugida K, Tsukiyama K, Teramura S, Yuda S, Isu N, Shoji T, et al. Direct formation of xonotlite fibber with continuous-type autoclave. Gypsum Lime 1988;216:288–94. [22] Yanagisawa K, Feng Q, Yamasaki N. Hydrothermal synthesis of xonotlite whiskers by ion diffusion. J Mater Sci Lett 1997;16:889–91. [23] Chen DL, Gao L, Zhang P. Synthesis of nickel sulfide via hydrothermal microemulsion process: nanosheet to naoneedle. Chem Lett 2003;32:996–7.

[24] Zhang P, Gao L. Cadmium sulfide nanocrystals via two-step hydrothermal process in microemulsions: synthesis and characterization. J Colloid Interf Sci 2003;266:457–60. [25] Cho SB, Nakanishi K, Kokubo T, Soga N, Ohtsuki C. Dependence of apatite formation on silica-gel on its structure – effect of heattreatment. J Am Ceram Soc 1995;78:1769–74. [26] Meldrum FC, Kotov NA, Fendler JH. Preparation of particulate mono- and multilayers from surfactant-stabilized, nanosized magnetite crystallites. J Phys Chem 1994;98:4506–10. [27] Delsannti M, Moussaid A, Munch JP. Effect of electric charges on the growth process of micelles. J Colloid Interf Sci 1993;157:285–90. [28] Han SH, Hou WG, Dang WX, Xu J, Hu JF, Li DQ. Synthesis of rodlike mesoporous silica using mixed surfactants of cetyltrimethylammonium bromide and cetyltrimethylammonium chloride as templates. Mater Lett 2003;57:4520–4. [29] Xiong YJ, Xie Y, Yang J, Zhong R, Wu CZ, Du GA. In situ micelle– template-interface reaction route to CdS nnanotubes and nanowires. J Mater Chem 2002;12:3712–6. [30] Yao J, Tjandra W, Chen YZ, Tam KC, Ma J, Soh V. Hydroxyapatite nanostructure material derived using cationic surfactant as a template. J Mater Chem 2003;13:3053–7. [31] Fowler BO. Infrared studies of apatite. I: Vibrational assignments for calcium, strontium, and barium hydroxyapatites utilizing isotopic substitution. Inorg Chem 1974;13:194–214. [32] Blaker JJ, Maquet V, Je´roˆme R, Boccaccini AR, Nazhat SN. Mechanical properties of highly porous PDLLA/BioglassÒ composite foams as scaffolds for bone tissue engineering. Acta Biomater 2005;1:643–52. [33] Li HY, Chang J. Fabrication and characterizationof bioactive wollastonite/PHBV composite scaffolds. Biomaterials 2004;25:5473–80. [34] Li HY, Du RL, Chang J. Fabrication, characterization, and in vitro degradation of composite scaffolds based on PHBV and bioactive glass. J Biomater Appl 2005;20:137–55. [35] Cheng W, Chang J. Fabrication and characterization of polysulfone/ dicalcium silicate composite films. J Biomater Appl 2006;20:361–76. [36] Lin HR, Kuo CJ, Yang CY, Shaw SY, Wu YJ. Preparation of macroporous biodegradable PLGA scaffolds for cell attachment with the use of mixed salts as porogen additives. J Biomed Mater Res 2002;63:271–9. [37] Cai Q, Yang J, Bei JZ, Wang SG. A novel porous scaffold made of polylactide-dextran blend by combining phase-separation and particle-leaching techniques. Biomaterials 2002;23:4483–92. [38] Agrawal CM, Athanasiou KA. Technique to control pH in vicinity of biodegrading PLA–PGA implants. J Biomed Mater Res 1997;38: 105–14. [39] Ara M, Imai Y. Effect of blending calcium compounds on hydrolytic degradation of poly (DL-lactic acid-co-glycolic acid). Biomaterials 2002;23:2479–83. [40] Martin C, Winet H, Bao JY. Acidity near eroding polylactidepolyglycolide in vitro and in vivo in rabbit tibial bone chambers. Biomaterials 1996;17:2373–80. [41] Lu L, Peter SJ, Lyman MD, Lai HL, Leite SM, Tamada JA. In vitro and in vivo degradation of porous poly (DL-lactic-coglycolic acid) foams. Biomaterials 2000;21:1837–45. [42] Li HY, Chang J. pH-compensation effect of bioactive inorganic fillers on the degradation of PLGA. Compos Sci Technol 2005;65:2226–32. [43] Langer R, Vacanti JP. Tissue engineering. Science 1993;260:920–6. [44] Gunatillake PA, Adhikari R. Biodegradable synthetic polymers for tissue engineering. Eur Cells Mater 2003;5:1–16. [45] Kim HW, Kim HE, Knowles JC. Production and potential of bioactive glass nanofibers as a next-generation biomaterial. Adv Funct Mater 2006;16:1529–35. [46] Clupper DC, Gough J, Embanga PM, Notingher IN, Hench LL, Hall MM. Bioactive evaluation of 45S5 bioactive glass fibres and preliminary study of human osteoblast attachment. J Mater Sci Med 2004;15:803–8. [47] Hench LL, Polak JM. Third-generation biomedical materials. Science 2002;295:1014.