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Boron doping;Low melting temperature;Degradation rate;Bioactive glass ceramics;Solgel

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properties of BGCs via low-temperature co-red process. The B2O3-free BG-B0

e glass

Journal of Non-Crystalline Solids 358 (2012) 11711179

Contents lists available at SciVerse ScienceDirect

Journal of Non-Cr

evtem (e.g., Bioglass 45S5) was able to bond to bone mineral [1]. Thebioactivity of BGs and BGCs is attributed to the formation of a bone-like hydroxyl-carbonated apatite (HCA) layer on their surface in the(simulated) biological environments, thus a strong bond can formwith the bone tissue. The typical 60SiO236CaO4P2O5 (in mol%;termed 58S) system has been studied extensively and this systemproduced by a solgel technique was more bioactive than the melt-quenching BG of the same composition [2]. SiO2CaOP2O5 systemincorporated with ZnO, MgO, and SrO as network modiers demon-strated the effect of slowing down the degradation of the material[36]. Additional drawbacks found in several families of porous

P2O5MgO system composed of crystalline wollastonite, apatite andthe residual glassy phase (Cerabone-AW) has been developedwith improved mechanical strength, whereas the over-high sinteringtemperature (>1100 C) leads to apatite production and undesiredlow degradability [13]. The liquidus temperatures of mostphosphorus-, and silica-rich phosphosilicate BGs are higher than1000 C, and it therefore suggests that the crystalline apatite in sin-tered BGCs decreases the material bioactivity, mechanical strengthand dissolution rate [10,1315]. To fabricate load-bearing BGCs it isnecessary to sintering the BGs at high temperature to full densitywhile maintaining porosity which is required for nutrient diffusionsilicate-based BG materials such as high liqbrittleness and low biodegradation ratewhere load bearing is required [710].

Corresponding author. Tel.: +86 571 8697 1782; faE-mail address: zhrgou@zju.edu.cn (Z. Gou).

0022-3093/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.jnoncrysol.2012.02.005-ceramics (BGCs) of spe-ur decades since HenchOSiO2Na2OP2O5 sys-

[11]. It is accepted that the bioactive behavior was related with theinitial composition of the BG, the phase composition after thermaltreatment and sintering process [12,13]. A well known SiO2CaOcic compositions have been studied for foet al. when they found the melt-quenching Ca1. Introduction

Bioactive glasses (BGs) and bioactivof the 20 mol% B2O3-doped BG-B20 was lowered to ~648 C and ~952 C, respectively. The BG-B20 thermallytreated at 850950 C was transformed into wollastonite and calcium borate, and crystallization decreasedthe kinetics but did not inhibit the development of hydroxyapatite on their powder and disc surface whenimmersed in simulated body uid. The in vitro degradation in Tris buffer conrmed that the degradationrate markedly increased with increasing boron content in BG-Bx. The compressive strength and exuralstrength of the 10% BG-B20-reinforced 45S5 porous BGC sintered at 850 C was nearly four times than thatof 45S5 porous constructs. These studies suggest that the boron-rich, phosphorus-low CaOSiO2P2O5B2O3system is a promising biomaterial and potential low temperature co-red aid for improving the mechanicaland biological properties of porous BGCs.

2012 Elsevier B.V. All rights reserved.

Previous studies have shown that crystallization decreases thelevel of bioactivity and can even turn a BG into an inert materialKeywords:

shrunk well at ~726 C and melted at over 1050 C, while the onset shrinking and melting temperaturesAvailable online 2 March 2012 improving the mechanicalIncorporation of B2O3 in CaO-SiO2-P2O5 bioof low-temperature co-red porous glass

Xianyan Yang a, Lei Zhang b, Xiaoyi Chen a, XiaoliangChangyou Gao a, Zhongru Gou a,a Zhejiang-California International NanoSystems Institute, Zhejiang University, Hangzhou 3b Rui'an People's Hospital & the 3rd Hospital Afliated to Wenzhou Medical College, Ruian

a b s t r a c ta r t i c l e i n f o

Article history:Received 8 November 2011Received in revised form 5 January 2012

Bioactive glass ceramics (BGon their chemical composiboron-rich, phosphorus-low

j ourna l homepage: www.e lsuidus temperature, highlimit their applications

x: +86 571 8697 1539.

rights reserved.ctive glass system for improving strengthramics

n b, Guojing Yang b, Xingzhong Guo a, Hui Yang a,

29, China200, China

have different rates of biodegradation and mechanical properties dependings and sintering temperatures. The present study was aimed to develop theaOSiO2P2O5B2O3 bioactive glasses (BG-Bx, X=0, 10, 20) potentially for

ystalline Solids

i e r .com/ locate / jnoncryso land tissue ingrowth. However, high sintering temperatures result incoarse-grained microstructure having poor bioresorption rate faraway from the growth rate of new bone at the site of implantation.In this respect, the BGs with low liquidus temperature and highdegradation rate are preferably low-temperature co-red aids toimprove the mechanical and biological properties of the BGCs.

It is generally accepted that the crystallization, microstructure, andeven dissolution rates can be governed by adjusting the compositions

of the BGs and controlling crystallization to suit with their end appli-cations [1618]. There are several ways to reduce the sinteringtemperature of advanced ceramics, such as addition of the lowmeltingglass [19,20]. The melt-quenching calcium borosilicate glass-ceramics(CaOB2O3SiO2; CBS) with a wollastonite-type main crystallinephase have to date achieved industrial applications in wireless com-munication due to its low-temperature co-ring property [21]. Saranti

1172 X. Yang et al. / Journal of Non-Crystalline Solids 358 (2012) 11711179et al. reported that the boron in the glass network of CaOB2O3P2O5systemhas a catalytic effect at favoring bioactivity of the other calciumphosphate glasses [22]. Recently, Lee et al. found that this ternary CBSsystem was bioactive, and may be favorable as orthopaedic implantsdue to its low chemical durability in physiological uids [23,24].More recently, they have developed a high-phosphorus, low-boronCaOSiO2P2O5B2O3 BGC melted at 1550 C (with composition(wt.%): CaO 41.8, SiO2 35.8, P2O5 13.9, B2O3 0.5, CaF2 2.0 and MgO6.0). They reported that the cancellous screws coated with suchnominal quaternary BGC could bond directly to cancellous bone toimprove the bone-implant osseointegration [25], and there was notoxic effect by a subchronic intravenous administration [26]. However,the melt-quenching involves a high temperature process whichled to the volatilization of the oxides with low-melting points, suchas B2O3, it thus becomes difcult to get the desired composition ofthose oxides.

In this study, we developed a new B2O3-doping SiO2CaOP2O5system with low melting temperature via solgel process. Accordingto the literatures about the effect of incorporating boric oxide to bio-materials, boron has well-dened biological effects such as stimula-tion of wound healing in vivo, increase of the extracellular matrixturnover [27], and plays a role in bone physiology [28]. The purposeof this study is to examine the effect of B2O3 addition on the thermaland chemical properties of the SiO2CaOP2O5 system, and gain infor-mation in choosing glass compositions suitable for low-temperatureco-red BGC preparation.

2. Experimental procedure

2.1. Materials

Tetraethylorthosilicate (TEOS), triethylphosphate (TEP), boracicacid (H3BO3), ammonia (NH3H2O, ~28 wt.%), calcium nitrate(Ca(NO3)24H2O) and absolute ethanol (99.8 wt.%) were purchasedfrom Sinopharm Chemical Reagent Co. China, as starting materialsfor preparation of BG-Bx. The high-purity grade NaCl, NaHCO3, KCl,K2HPO43H2O, MgCl26H2O, CaCl2, and trishydroxymethyl amino-methane (Tris) as required materials (BBI, Canada) for preparingsimulated body uid (SBF) and Tris buffer. The melt-quenching45S5 Bioglass (with similar composition to Biolgass 45S5) has thefollowing chemical composition (wt.%): 45.0 SiO2, 24.5 CaO, 24.5Na2O, and 6 P2O5. Comminution by planetary milling until a nalparticle size of 210 m was performed.

2.2. Preparation of BG-Bx

The SiO2CaOP2O5B2O3 systems were prepared using the solgel process. Samples were labeled using the following convention:BGs named BG-Bx were composed of SiO2, CaO, P2O5, and B2O3,respectively, with x=0, 10, and 20 mol%. The compositions of thequaternary systems were listed in Table 1. The content of B2O3 was

Table 1Compositions (mol%) of the as-prepared BG-Bx in the experiments.

Series B2O3 SiO2 CaO P2O5

BG-B0 0 60.68 35.83 3.49BG-B10 9.98 49.02 39.46 1.54BG-B20 20.00 29.87 48.43 1.70limited at a maximum value of 20 mol%, since it is detrimental tothe biocompatibility of the highly biodegradable BG. In a typical pro-cedure for preparing BG-B10, the 0.041 g H3BO3, 2.07 mL TEOS, and0.22 mL TEP were added to 20 mL of ethanol and stirred for 20 min.Then, 1.404 g Ca(NO3)24H2O and 1.0 mL NH3H2O were added2.0 mL of deionized water and mixed with 23.0 mL absolute ethanolunder continuous magnetic stirring for 20 min. The two kinds ofsolutions were mixed and the suspension was aged at 80 C for 12 hand nally calcined at 600 C for 90 min. To investigate the effect ofB2O3 content on thermal and chemical properties of BG, the BG-B20was prepared in the presence of B2O3 while the other conditionsremained the same. Similarly, as a control, the boron-free CaOSiO2P2O5 (BG-B0) was prepared in the absence of H3BO3 by thesame method.

2.3. Sintering characteristic evaluation of BG-Bx

In order to understand the effects of glass compositions on thesintering characteristic, dilatometric analyses were performed using adilatometer (DIL 402PC, NETZSCH) with a heating rate of 5 Cmin1

in air to characterize the shrinkage of the disc-shaped BG-Bx compactswith respect to temperature. The discs were prepared using a uniaxialpressure of 12 MPa to yield the nal dimension of 6 mm in diameterand 4 mm in thickness, and then pre-heated at 560 C for 2 h. Thethermogravimetric and differential thermal analysis (TG/DTA) wascarried out on TG/DTA6200 of TA Instruments with a 10 Cmin1

heating rate under an air atmosphere. According to the thermal analy-sis, powders were subsequently sintered by electric furnace at 850 Cand 950 C for 2 h, respectively.

2.4. Biological evaluation of BG-Bx in vitro

In vitro apatite formation test was carried out by soaking BG-Bxpowders and discs in SBF at nal concentration of 1 mgmL1 andmonitoring the formation of HCA on the sample surface at 37 Cfor different time intervals (3 h14 d). The SBF was prepared bydissolving inorganic salts reagents in deionized water and bufferedat pH=7.40 with Tris and 1.0 molL1 HCl at 37 C, according toKokubo's recipe [29]. The circular BG-Bx discs ( 6 mm2 mm)were prepared using a uniaxial pressure of 4 MPa and sintered at850 C and 950 C for 2 h, respectively. After soaking, the discs wereultrasonically washed in the fresh SBF solution for 3 min, and thepowder samples were ltered, rinsed with deionized water, anddried in an oven at 60 C for 24 h before analysis. All the solutionsafter soaking powder samples were saved for inductively coupledplasma (ICP; Varian Co., USA) analysis of B, Si, Ca and P to measureionic concentrations.

2.5. In vitro degradation test

Degradation tests were conducted in two different simulatedphysiological conditions following the ISO 10993 Biological evalua-tion of biomedical devices Part 14: Identication and quanticationof degradation products from ceramics using the as-prepared BG-Bxpowders. The tests were performed at 37 C in SBF, and in0.05 molL1 Tris buffer at pH 7.4, simulating the body's pH as afunction of immersion time (up to 28 d). A ratio of 0.50 g powdersto 50.0 mL solution was used, and the aqueous media were replacedevery three days. Weight changes were measured by separating thepowders from the solutions, washing with deionized water, anddrying at 95 C.

2.6. Fabrication of 45S5/BG-B20 composite BGCs

The 45S5 Bioglass powder was mixed with BG-B20 powder in

two weight proportions (95:5, 90:10) in liquid media (ethanol) by

of thermal decomposition of the nitrate used in the experiment.Then little weight loss took place up to 800 C. In general, the BG-Bxmanifested a Tg and at heating crystallized within the temperatureregion of 800900 C and mostly in two stages, corresponding to cal-cium borate and calcium silicate crystallization (Fig. 2c, d). The princi-pal characteristic difference was the presence of a Tm at ~956 C forBG-B10, and ~952 C for BG-B20. The liquidus peaks (Tl) were fullyshifted toward lower temperature in comparison with that of BG-B0(>1050 C), which did not appear in the curve (Fig. 2b). Accordingto the phase diagram, the Ca-B-O system demonstrated a full liquidphase below 980 C [31]. It is noted that the temperature of endother-mic peaks (~650750 C) of BG-B10 and BG-B20 coincided with thetemperature range for the rapid densication of BG-B10 and BG-B20in Fig. 1b. This coincidence indicated that endothermic peaks wereattributed to the densication by liquid phase. Meanwhile, withincreasing B2O3 content, both Tg and the onset of crystallization peakTc shifted toward lower temperatures. This clearly corresponds tothe theory that the network addition is charge balanced resulting inpolymerization of silicate network and also decrease of the Tg [19,32].

3.3. Phase transformation analysis of the BG-Bx

XRD analysis results for the powders before and after sintering at850 and 950 C were shown in Fig. 3. The amorphous phases at 600 Cgave an indication of the formation of homogeneous silica networkin the BG-B0, but the gels with different B2O3 addition after calciningat 600 C contained trivial crystallites, represented by a set of low

Fig. 1. Size distribution of the as-prepared BG-Bx particles (a) and sintering shrinkingcurves for the as-prepared BG-Bx discs (b).

1173X. Yang et al. / Journal of Non-Crystalline Solids 358 (2012) 11711179planetary milling for 2 h in an agate jar with agate balls. After dryingthe 45S5/BG-B20 mixture was then mixed homogeneously withparafn spheres of ~500 m in diameter. The cylindrical and cuboidBGC green bodieswere prepared using two kinds of stainless steel dies( 6 mm20 mm or 8 mm10 mm40 mm) and densied with4 MPa pressure. The green samples were sintered at 700950 C for2 h, respectively. To understand the mechanical strengths of the45S5 and BG-B20 individuals, their porous materials were preparedas controls at the same conditions.

2.7. Sample characterization

The particle size distributionwas determined by laser granulometryon a netasizer nano (Malvern, S90). The sintered BGCs and powdersamples before and after high heat treatment, and after soaking inSBF and Tris buffers were observed using scanning electric microscopy(SEM; JEM-6700F, Japan), with energy dispersive X-ray (EDX) analysis.Prior to examination, the samples were coated with a thin layerof gold. Additionally, the phase of samples was examined in a RigakuD/max-rA (Geigerex) X-ray diffractometer (XRD) using Cu Karadiation with a scanning rate of 0.02min1. The compressivestrengths of the cylindrical porous BGC samples sintered at 850 Cwere measured using a universal testing machine (Instron, Canton,MA). The compressive strength and three-point bending strengthof the long rod-like porous BGCs were determined using a universaltesting machine (Instron, Canton, MA) with a crosshead speed of0.5 mmmin1.

3. Results

3.1. Primary characterizations of the BG-Bx powders

In the solgel process, using ethanol/water as phase separationsolvent, the BG-Bx with nanoscale feature was synthesized. Accordingto the ICP analysis (Table 1), BG-B0 was a boron-free, silicon-high BGwith compositions similar to 58S, BG-B20 represented a new boron-rich, silicon-low BG, but BG-B10 was a boron-containing, moderate-silicon BG. The size distribution of the BG-Bx was shown in Fig. 1a.The BG-B0 showed a particle size ranging between 600 and 1250 nm,and an average equivalent diameter of ~930 nm. The BG-B10 showeda particle size distribution ranging from 230 and 1000 nm, the averageequivalent diameter being equal to ~580 nm. The BG-B20 showed anarrow particle size distribution from 160 to 580 nm, and an averageequivalent diameter of ~330 nm. It suggests that increasing B2O3/SiO2 ratio result in particle size decrease. Dilatometeric resultsfor various BG-Bx discs, which indicated the shrinkage proles versustemperature, were shown in Fig. 1b. The onset of softening tempera-ture (Ts) was dependent on the addition of B2O3. The silica-richphosphosilicate BG-B0 caused a rapid shrinkage of over 10% after726 C. This rapid densication of BG-B0 powder compacts mightresult from viscous sintering which was observed in silicate glasses[30]. As B2O3 content increased, the onset of shrinkage moved towardthe lower temperature. Both BG-B10 and BG-B20 had low Ts at 648664 C, which produced a rapid densication accompanied with79% shrinkage. This change undoubtedly corresponded to thechange in the nature of bonding in the structural network.

3.2. Thermal analysis of the BG-Bx

Fig. 2 illustrated the TG/DTA curves for the dried gels. The glasstransition temperature (Tg), onset of exothermic crystallization peak(Tc), and onset of endothermic melting peak (Tm) were determined.On TG curves (Fig. 2a), different stages were found with the changeof oxide contents, ascribing to the residual water and ethanol elimina-tion in the systems below 200 C. As the heating process proceeded,

a steep weight loss stage occurred at 200600 C, which was because intensity XRD peaks distributed in the glass matrix (Fig. 3a). The

1174 X. Yang et al. / Journal of Non-Crystalline Solids 358 (2012) 11711179sample pattern was in agreement with the standard XRD pattern forwollastonite (-CaSiO3, PDF# 42-0550) in the BG-B0 when sinteringtemperature risen to 850950 C (Fig. 3b, c). However, BG-B20 was

Fig. 2. TG and DTA curves for the as-prepared BG-Bx precursors. (a) TG f

Fig. 3. XRD patterns of the BG-Bx before (atransformed into BGC with two crystalline phases after nucleationand crystallization at 850 or 950 C (Fig. 3d, e). The main phase was-CaSiO3 and the characteristic diffraction peaks of calcium borate

or BG-Bx, (b) DTA for BG-B0, (c) DTA for BG-B10, (d) DTA for BG-B20.

) and after (b-e) sintering treatment.

(CaB2O4, PDF# 32-0155) began to appear. It must be noted that nocrystalline phase containing phosphorus in the BGC composition canbe detected by XRD, mainly due to the limited amount of phosphateexisting in the glass matrix. We compared the as-sintered powdersto the documented SiO2CaOB2O3 system prepared by the solid-state reaction method [33] and detected that there was no cristobaliteor quartz (SiO2) phase in the sintered BG-B20 (Fig. 3d, e). It is probablethat powder prepared by solgel route is more uniform in the micro-scopic eld, resulting in a smaller space amount molecule adjustedduring crystallization. In addition, the crystallization peaks ofCaB2O4 became more obvious with the increase of thermal treatmenttemperature. That is the reason why there are two exothermic peaksat 800900 C in the DTA curves.

3.4. Formation of HCA on the particle surface

The bioactivity study was performed with the samples calcined at600 C and those sintered at 850 and 950 C, respectively, becausethese samples were representative of the different phase composi-tions of the BGs and BGCs. The unsoaked BG-B0 and BG-B20 powderswere composed of aggregates of several hundreds of nanometers(Fig. 4a, g). In contrast, the BG-B0 particles softened and fused afterthermal treatment at 850950 C (Fig. 4c, e). The surface morphologyof the as-sintered BG-B20 possessed more ceramics nature withsmoother surface, which contained CaSiO3/CaB2O4 binary crystallites(Fig. 4i, l). After 168 h the surfaces of BG-B0 and BG-B20 exhibitedsimilar morphology to those of unsoaked samples, though the prima-

after sintering at 950 C. Since the crystallization (XRD patterns) andsurface microstructures (SEM images) of BG-B10 before and aftersintering and soaking treatment were similar to those of BG-B20,data of BG-B10 were not shown.

To take into account the possibility of spontaneous HCA precipita-tion in SBF at physiological temperature, the BGC discs were alsosuspended and immersed in SBF for 37 days. It can be seen thatthe as-sintered discs exhibited tough surface with the primary parti-cle cements due to incomplete sintering (Fig. 5ad). After soakingfor 3 days, the nanoscale heterogeneous plates or noduses were de-posited on the surface of the ultrasonically washed discs (Fig. 5eh).With the increase of soaking time up to 7 days, a continuous coatinglayer thought to be HCA completely covered the discs (Fig. 5il).The face-scanning EDX analyses (Fig. 5 il, insets) conrmed thebiomimetic Ca-decient apatitic characteristic as observed on theother BG and BGC surface reported previously [35,36].

3.5. Ionic concentration changes of SBF solutions

Examining the compositional change of SBF with time may pro-vide some insight into the mechanisms of enhanced bioactivity andbiodegradability of the BGs and BGCs. Fig. 5 showed the changes inSi, Ca, B, and P concentrations of the SBF as a function of soakingtime. As for the calcined BG-B0, the Si and Ca concentrations in SBFincreased rapidly during the rst 6 h and then Ca concentration de-creased (Fig. 6a, b). Phosphorus (P) concentration decreased abruptlyrstly and kept stable after 72 h (Fig. 6c). In contrast, as for the

1175X. Yang et al. / Journal of Non-Crystalline Solids 358 (2012) 11711179ry irregular particle individuals in the aggregates converted to ovoidnodules. In the case of the sintered BG-B0 at 950 C, no signs of forma-tion of biomimetic HCA could be found. Only the microstructures con-sisted of small particles on the crystal surface (Fig. 4f). The surfacesof the sintered BG-B20 particles were covered by nanoparticles. Ithas been reported that excessive crystallinity in BG might defer theformation of HCA when exposed to SBF [34,35], and thus lead tolower bioactivity. However, the EDX analysis (Fig. 4, insets) indicatedthat the surface layers were essentially composed of calcium, phos-phorus, carbon, and oxygen, with Ca/P molar ratio of 1.521.56 afterimmersion in SBF, indicating that the material maintained bioactivityFig. 4. SEM images of the as-prepared BG-B0 (a-f) and BG-B22sintered BG-B0 samples at 850 and 950 C, the Si and Ca concentra-tions of the SBFs soaking gradually increased during the initial 72 hand P concentration gradually decreased. Appreciable differencesfor Si and Ca concentrations were observed at 72 h. For the BG-B20samples, the concentrations of B and Si had similar trends showinga high increase during the initial 2 h, followed by a slowly decreaseto 168 h (Fig. 6d, e). Calcium concentration also increased rstlyand then decreased (Fig. 6f). Obviously, the increase in B, Si and Caconcentrations in the initial stage and the decrease of P concentrationwere attributed to the dissolution of particles and the formationof HCA on the surface of the particles. The concentration of P in the(g-m) particles before and after soaking in SBF for 7 days.

BG-B20 glasses soaking in SBF experienced a decreased and stabilizedat a certain value after 72 h, similar to that of BG-B0 (data not shown).

particles. As a result, the powders gradually degraded themselvesand HCA precipitated on the surfaces of the particles, which may dis-

Fig. 5. SEM images of the as-sintered BG-B0 and BG-B22 discs before (ad) and after (el) soaking in SBF for 3 and 7 days, respectively.

1176 X. Yang et al. / Journal of Non-Crystalline Solids 358 (2012) 11711179It must be mentioned that the dissolution in SBF of the BG-B20 with-out and with sintering treatment caused the supersaturation of Caions for HCA formation, and the dissolution of particles hydrated thesurface to provide the HCA nucleation sites. Through this dissolutionand enhanced HCA formation processes, the BGC in the CaOSiO2P2O5B2O3 system become highly bioactive as well as biodegradable.

3.6. Degradation in vitro

During the immersion in SBF, the powder particles were attackedby the surround environment, and the ions were leached from theFig. 6. Changes in ionic concentrations in SBF during soaking BG-B0 (ac) and BG-B20 (turb the weight loss measurement. Thus, the weight variation in theTris buffer would be more sensitive to detect the degradation. Fig. 7showed the weight changes of the BG samples, normalized to thestarting weight of the powders. Then, the weight loss increased sig-nicantly with the increase of the B content in the BG-Bx systems inthe Tris buffer (Fig. 7a). In contrast, the weight of the BG-Bx increasedwhen immersion in SBF, and BG-B0 showed the highest increase by~45%, due to HCA precipitation from SBF (Fig. 7b). These results sug-gested that the B-doping in the CaOSiO2P2O5 system was favorableto improve its degradation rate. More importantly, it was found fromSEM images and EDX analysis that the soaked samples were richdf) with and without sintering treatment. (a) Si, (b) Ca, (c) P, (d) B, (e) Si, (f) Ca.

Fig. 7. Weight variation and SEM images of the soaked BG-Bx samples in 0.05 molL1 Tris buffer (a, c, d, e) and SBF (b, f, g, h), respectively. Insets represent face-scanning EDXspectra of the samples after soaked in Tris buffer and SBF. (c, f) BG-B0, (d, g) BG-B10, (e, h) BG-B20.

1177X. Yang et al. / Journal of Non-Crystalline Solids 358 (2012) 11711179of silicon (>20 at.%) after soaking in Tris buffer (Fig. 7ce) and ofCaP (>4.5 at.%) in SBF (Fig. 7fh), which was the essential featurefor BG-Bx dissolution in Tris buffer and apatite deposition in SBF.

3.7. Microstructure and strength of BGCs

Fig. 8 showed the SEM images of the cross-section of the BG-B20-reinforced 45S5 BGCs porous materials; for comparison, SEM imagesof constructs prepared by 45S5 Bioglass were also shown. Generally,there was some similarity on the macropores with spherical mor-phology, resulting from the thermolysis of parafn microspheres.The interconnective pores and the small pores in the strut wallscould be observed by SEM. Thus, after low temperature co-ring pro-cess, the microstructure consisted of solid grains with solidied liquidnetwork, and possibly residual pores. To regenerate bone tissue in situFig. 8. SEM images and high-magnication of the cross-section of BG-B20-reinforced 45S5 BB20=95:5; (c) 45S5/BG-B20=90:10.a partially or fully open porous network is more desirable, for vascu-lar inltration of the articial scaffold [37]. Moreover, these highlyporous constructs would be advantageous for degradation in thebody.

As shown in Fig. 9a, the compressive strengths of 5% and 10% BG-B20-reinforced 45S5 BGC were nearly 1.43-fold and 4.15-fold thanthat of unreinforced porous 45S5 BGC sintered at 850 C. The primaryreason for this is that the high temperature softens the BG-B20 parti-cles, further assisting densication. It can be seen from the fractureface of the porous samples sintered at 700 C that the nanoscale BG-B20 particles (~250 nm in size) were embedded by the 45S5 particles(28 m in size), but those were confused with the large 45S5 parti-cles after sintering at 850 C (Fig. 9a, insert). It suggests the BG-B20nanoparticles turn into a viscous liquid to wet and coalesce the45S5 particles, while there is progressive microstructure coarseningGC porous materials sintered at 850 C for 2 h. (a) 45S5/BG-B20=100:0; (b) 45S5/BG-

ced(b)

1178 X. Yang et al. / Journal of Non-Crystalline Solids 358 (2012) 11711179and bonding to increase rigidity. This densication can be also con-rmed from the decreased porosity, from ~69.62.2% to 64.22.9%for the porous constructs with increasing BG-B20 content. Thedecrease of porosity occurred in 10% BG-B20-reinforced 45S5 BGC,in part due to annihilation of small pores, as shown in the high-magnication SEM image of Fig. 8. Similarly, the exural strength ofthe BGCs composed of 45S5 and BG-B20 showed an increase comparedwith 45S5 without doping BG-B20 (Fig. 9b). The long, rod-like 10%BG-B20-reinforced 45S5 BGC exhibited considerable bending strength(~3.21 MPa) which was more than four-fold that of single 45S5 BGC(0.67 MPa). It could be deduced that the addition of a small amountof low-melting point BG caused a signicant improvement of themechanical properties of 45S5 BGC.

4. Discussion

Sintering is usually used to form materials which have a highmelting point. However, this method requires high temperature andlong duration for heating in order to bond the junctions betweenground material particles by congelation or diffusion. Boron-dopedlow-melting co-ring aid, employed in BGCs, makes it possible toprocess the sintering at lower temperature and short duration.Some complex systems such as SiO2CaOP2O5Na2OMgOK2OB O and SiO CaOP O Na OB O Al O have also been investi-

Fig. 9. Compressive strength, porosity (a) and exural strength (b) of the BG-B20-reinfor(a) Insets represent SEM images of the porous materials sintered at 700 C and 850 C.2 3 2 2 5 2 2 3 2 3

gated [38,39] for the possibility of manufacturing machinable high-strength BGC into dental implants. However, high magnesium oraluminum composition in such systems is still inferior to those biode-gradable quaternary bioglasses in bone repare already in existence.Manupriya et al. investigated that P2O5 played an important rolein controlling chemical durability and bioactivity of a silica-freeborate glass system (B2O3CaOP2O5Na2O) [40]. O'Donnell et al.also found that the crystalline phase of biomimetic HCA occurredrapidly in SBF as the phosphate content increased in the soda-lime-phosphosilicate glass system. [41] However, it is known that increas-ing P2O5 contents in BGs increased the apatite phase in the BGCs aftersintering treatment, and this would reduce the resorption rate in theBGCs [4244]. Recently, some investigations has demonstrated thatthe bioactivity and resorption rate of -CaSiO3 ceramics and -CaSiO3/CaB2O4 BGCs were much higher than that of -tricalciumphosphate and apatite, and these porous materials may signicantlyenhance the spinal fusion or calvarial defect repair in rabbit model[23,45]. It suggests that, the boron-rich, phosphorus-low CaOSiO2P2O5B2O3 system (e.g. BG-B20) may not only be appropriate biode-gradable materials for bone repair, but also has signicant implica-tion as low-temperature co-ring aid with respect to the other BGCs.5. Conclusions

In this study, the boron-rich, phosphorus-low CaOSiO2P2O5B2O3 was prepared and characterized by the solgel method at con-siderably lower temperatures than required for conventional meltingmethod. The BG-Bx-derived BGC materials can be sintered at notmore than 950 C, and at sintering temperature above 800 C crystal-lization occurred and glass-ceramics with wollastonite and calciumborate were formed. The melting temperatures of the BG-Bx werelowered to ~950 C by doping B, and the size of the BG-Bx particlescould be controlled in the range of 200900 nm. Crystallizationdecreased the kinetics but did not inhibit the development of HCAlayer, even in fully crystallized BG-Bx, and the biodegradation rateof the BG-Bx was based on the boron content. Such improved thermaland biological properties of boron-rich, phosphorus-low quaternarysystem should be helpful for lowering the sintering temperature ofBGC porous constructs potentially for improving bone regenerationand defect repair in situ.

Acknowledgments

The authors would like to acknowledge the Zhejiang ProvincialNatural Science Foundation of China (No. Y2090009), theHealth Bureau

45S5 BG porous materials with different 45S5/BG-B20 ratio thermally treated at 850 C.Insets represent the cuboid samples and three-point bending test images.of Zhejiang Province Foundation (2010SSA005 and 2011C33049) andthe Science and Technology Bureau of Wenzhou City (H20080039 andH20100076).

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Incorporation of B2O3 in CaO-SiO2-P2O5 bioactive glass system for improving strength of low-temperature co-fired porous glass ceramics1. Introduction2. Experimental procedure2.1. Materials2.2. Preparation of BG-Bx2.3. Sintering characteristic evaluation of BG-Bx2.4. Biological evaluation of BG-Bx in vitro2.5. In vitro degradation test2.6. Fabrication of 45S5/BG-B20 composite BGCs2.7. Sample characterization

3. Results3.1. Primary characterizations of the BG-Bx powders3.2. Thermal analysis of the BG-Bx3.3. Phase transformation analysis of the BG-Bx3.4. Formation of HCA on the particle surface3.5. Ionic concentration changes of SBF solutions3.6. Degradation in vitro3.7. Microstructure and strength of BGCs

4. Discussion5. ConclusionsAcknowledgmentsReferences