Low-temperature sintering of 45S5 Bioglasss-based glass ceramics:Effect of biphasic mixing approach on the mechanical andbiological properties

Lingzhi Lin a, Lei Zhang a, Juncheng Wang a, Kailuo Xie a, Xianyan Yang b, Xiaoyi Chen b,Guojing Yang a,n, Changyou Gao b, Zhongru Gou b,nn

a Rui’an People’s Hospital & The 3rd Affiliated Hospital to Wenzhou Medical University, Rui'an 325200, Chinab Zhejiang-California International Nanosystem Institute, Zhejiang University, Hangzhou 310058, China

a r t i c l e i n f o

Article history:Received 25 November 2013Accepted 5 April 2014Available online 15 April 2014

Keywords:Mixing approachLow-temperature sinteringMechanical properties45S5 Bioglasss

Glass ceramics

a b s t r a c t

The effects of biphasic mixing approaches on the mechanical strength and in vitro biological propertiesof 45S5 BG-derived bioactive glass-ceramics (BGC) sintered at a temperature range of 860–980 1C wereinvestigated. The sol–gel-derived ZnO/B2O3-rich bioactive glass (BG-ZB) exhibited a low meltingtemperature (�860 1C) and nanoscale particle size distribution (50–670 nm). For the in situ mixing of45S5 BG particles with the precursor hydrogel of BG-ZB before drying, compressive strength (4.5-fold)and bending strength (3.1-fold) of the 45S5/BG-ZB composite significantly higher than the pure 45S5BGC were achieved at an appropriate sintering temperature of 920 1C, whereas the conventionalmechanical mixing of 45S5 BG particles with the BG-ZB nanoparticles resulted in minor strengthenhancement. in vitro immersion tests revealed some similarities in surface bioactivity in simulatedbody fluid and biodegradation in Tris buffer, between the BGC composites prepared by the two mixingapproaches. These results suggest that the distribution homogeneity of the BG-ZB phase in 45S5 matrixis mainly responsible for the mechanical strength, but does not compromise their biologically relevantperformances.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The melt-quenching silicate bioactive glasses (BGs) have beenstudied for four decades, since Hench et al. found that the CaO–SiO2–Na2O–P2O5 systems with appropriate composition (i.e. 45S5Bioglasss) have excellent bioactivity and osteoconductivity [1].Recently, many studies have demonstrated that the 45S5 BGdissolution products can cause rapid expression of genes thatregulate osteogenesis [2]. These discoveries have stimulatedextensive investigations into extending the range of applicationsof BG-based porous materials in biomedicine to include enhancingbone regeneration [3]. Although the use of such glassy materials innumerous clinical programs has exhibited favorable healing cap-ability, the most clinically used 45S5 BG is granules or powderedglass, i.e. products which do not require special heat treatmentduring manufacture. There remain some drawbacks with thesilicate-based BGs such as high melting temperature, low strength

and low biodegradation rate, which limit their uses where loadbearing is required [4,5].

It is reported that the sol–gel-derived BGs, especially the B2O3-rich systems, show low melting temperature and rapid biodissolu-tion properties [6–8]. For instance, the B-substituting-Si BG (so-called “45B5”) can be sintered below 1000 1C, and fully transformsinto hydroxyapatite in phosphate-rich fluid [9]. Our preliminarilystudies demonstrated that a B2O3-rich, P2O5-low calcium borosi-licate quaternary system is a promising low-melting-temperatureco-firer to enhance the sintering ability of bioactive glass-ceramics(BGCs) [10]. The addition of ZnO in BG systems can control thereactivity, since zinc is an trace element that has stimulatory effecton bone formation and as network modifier which can examplifythe effect of lowering melting temperature of glassy materials [11].

Herein, the objective of our study is to develop an in situ gelmixing approach to reinforce the 45S5 BG-derived BGC, in com-parison with the conventional mechanical mixing technique. TheZnO/B2O3-rich BG is designed considering the fact that the lowmelting temperature is dominated by the presence of B2O3þZnOcomponents, which is an important and positive contributionto improve the mechanical properties of (glass-) ceramics vialow temperature sintering [12]. The purpose of this study is to

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.04.0280167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author.nn Corresponding author. Tel.: þ86 571 8820 8353; fax: þ86 571 8697 1539.E-mail addresses: ygj1008@163.com (G. Yang), zhrgou@zju.edu.cn (Z. Gou).

Materials Letters 126 (2014) 154–158

investigate the effect of the two different mixing approaches onthe mechanical and biological properties of the 45S5-based BGCduring a pressureless sintering process, and to gain further knowl-edge in the design of highly bioactive, 45S5 BG-dervied (porous)materials to enhance bone tissue regeneration and repair.

2. Materials and methods

The 45S5 Bioglasss (US Biomat Co.) has been comminuted byplanetary milling until a final particle size of 0.6–4 μm. Thechemical reagents were purchased from Sinopharm ChemicalReagent Co. China, as starting materials for sol–gel preparationof ZnO/B2O3-rich BG with optimal composition (30SiO2–28CaO–2P2O5–30B2O3–10ZnO, in mol%; hereafter refered as BG-ZB)described previously [13]. Briefly, 9.5 ml HNO3, 1.09 ml triethyl-phosphate, 5.56 g H3BO3, 9.36 ml tetraethylorthosilicate, 9.86 g Ca(NO3)2 �4H2O and 4.46 g Zn(NO3)3 �6H2O were added to 150 mldeionized water. The sol-phase solution was aged at 60 1C for 16 huntil turned into gel-phase solution, then aged at 80 1C for 48 hand calcined at 560 1C for 2 h. The particle size distribution of theBG-ZB (50–670 nm) was measured by the laser diffraction method.

In order to investigate the effects of mixing approaches on themechanical properties of composites, the gel precursor of BG-ZBwas firstly added into 45S5 BG powder in ethanol medium andfollowed by milling for 30 min in an agate jar with agate balls, andsubsequently kept in 80 1C for 48 h, followed by calcining at 560 1Cfor 2 h. In the mechanical mixing approach, the as-calcined BG-ZBpowders were mixed with 45S5 BG in an agate jar with agate balls.The composite powders with 2 or 4 wt% (x) of BG-ZB prepared byin situ gel mixing and mechanical mixing were labeled as 45S5/BG-ZBxg and 45S5/BG-ZBxm, respectively. Differential thermalanalysis (DTA) was performed up to 1000 1C using 15 mg powdersusing the heating rate of 10 1C/min.

The cylindrical (∅ 6�15 mm2, ∅ 6�2 mm2) and cuboid(45�10�5 mm3) samples were prepared using stainless steeldies and densified with a pressure of 8.0 MPa. The powdercompacts were sintered at 860 1C, 920 1C and 980 1C for 2 h,respectively. Compressive strength and three-point bendingstrength were determined using a universal testing machine(Instron) with a rate of 0.02 mm/s.

The discs (∅ 6�2 mm2) were washed ultrasonically andsoaked in the simulated body fluid (SBF) with an surface area-to-volume ratio of 0.1 mm�1 at 37 1C waterbath for different timeintervals (1–3 d). The standard SBF was prepared by dissolvinghigh purity-grade inorganic salts reagents in deionized water andbuffered at pH¼7.40 with tris(hydroxymethyl)aminomethane(Tris) and 1.0 mol l�1 HCl at 37 1C [14]. The assessment ofin vitro biodegradation was conducted at 37 1C by immersing thesamples (W0; n¼6) in 0.05 mol l�1 Tris solution buffered with HClat pH 7.4 with an surface area-to-volume ratio of 0.1 mm�1,following the ASTM standard. The immersion medium wasreplaced with the fresh Tris buffer by 20% every 2 d (up to 28 d).The samples were removed from the medium at different timestages (t) and abundantly rinsed with absolute ethanol andfollowed by drying in vacuum and weighing (Wt). Weight loss(WL%) were calculated according to equation: WL%¼(W0�Wt)/W0�100%. The sintered discs before and after soaking in SBF, andthe fracture surface of the specimens after bending strength testwere observed using scanning electric microscopy (SEM; HITACHI)with energy disperse spectroscopy (EDS).

3. Results and discussion

DTA curves of the powders are shown in Fig. 1. The character-istic temperatures (Tg, Tp, and Tm) for the different samples are

shown in Table S1 (Supporting information (SI)). All curves for the45S5-containing powders presented a large exothermic peakbetween 600 and 900 1C (Fig. 1b–d), while the crystallizationand melting process resulted in the occurrence of both exothermicand endothermic peaks in this temperature range for the pureBG-ZB4g sample (Fig. 1a). After cooling to room temperature, onlythe BG-ZB sample had a part glassy aspect, indicating that melting(Tm, �860 1C) had occurred during the DTA measurement.A strong exothermic peak at 585 1C for 45S5/BG-ZB4g was possiblyattributed to the nitrate residual in the samples. However, themelting temperature of the BG-ZB component could not beidentified from DTA analysis possibly due to low content (4 wt%)in the 45S5/BG-ZB composites. In the TG curves, the weight loss(3–5%) mainly occurred below 400 1C and only the 45S5/BG-ZBcomposites showed significant weight loss in 400–600 1C (Fig. S1; SI).These results suggest that the sol–gel-derived, ZnO/B2O3-rich BGshows a much lower melting temperature (�860 1C) than that(�1260 1C) of the melt-derived 45S5 BG.

Typical microstructures of the BGC composites sintered atdifferent temperatures are shown in Fig. 2. With the temperaturerising from 860 1C to 920 1C, the pure 45S5 BG particles weresoftened and densified, and were also bonded tightly withoutsignificant grain growth at 980 1C (Fig. 2a–c). Pits or holes with asubmicrometre size were observed inside the 45S5/BG-ZB2m and45S5/BG-ZB4m composites by sintering of the powder compacts at860 1C (Fig. 2d and g). BG-ZB phase was heterogeneously distrib-uted inside the 45S5 matrix, and thus abnormal grain growth withincreasing the sintering temperature up to 920 and 980 1C wascaused by the heterogeneous distribution of the two componentsof glass materials (Fig. 2e, f, h, and i). As the temperature becamehigher than the melting temperature (�860 1C) of BG-ZB, theaggregated glass started to melt and spread to over some 45S5matrix. In this case, first the grains inter-grew, and then the BG-ZBphase covered the 45S5 grains and the system became two-layergrains. DTA analysis has demonstrated this phenomenon for theBG-ZB phase melted at 860 1C, so that some grains maintained aspherical morphology after sintering at 920 1C (Fig. 2h). In con-trast, the gel-mixing way resulted in a decrease in the number ofpores, and the 45S5/BG-ZBg composites exhibited slow crystalgrowth (Fig. 2j–o). Although a small amount of BG-ZB phase (2–4%) were embedded in the 45S5 matrix when sintering below920 1C, they were equally distributed in the 45S5 matrix. Mean-while, the addition of less BG-ZB (i.e. 2%) was favorable for a lowtemperature (860–920 1C) sintering by bonding the 45S5 particlesand retarding grain growth (Fig. 2j–l). These results suggest thataddition of gel-phase precursor is better than that of as-preparedparticles to distribute homogenously within the microstructure.

Fig. 1. DTA analysis of the pure BG-ZB (a), pure 45S5 BG (b), 45S5/BG-ZB4g (c), and45S5/BG-ZB4m (d). The deep-gray bar and light-gray bar respectively representingendothermic and exothermal peak.

L. Lin et al. / Materials Letters 126 (2014) 154–158 155

As shown in Fig. 3a, the compressive strength of 45S5/BG-ZB2g(�70 MPa) is nearly 1.5-fold of that of 45S5/ BG-ZB2m, and over3-fold of unreinforced 45S5 (�20 MPa) after sintered at the range of860–920 1C. Basing on the microstructure observation and analysisabove, the main reason should be that the added BG-ZB phasecoalesced with the 45S5 BG particles, leading to a progressivemicrostructure coarsening, and bonding, to increase the rigidity.The performance between 45S5/BG-ZB2g and 45S5/BG-ZB4g orbetween 45S5/BG-ZB2m and 45S5/BG-ZB4m both showed littledifference, indicating that the mechanical strength was enhancedlittle with the mixing ratio of BG-ZB increased from 2% to 4%.Similarly, it can be seen from Fig. 3b that the 45S5/BG-ZB2gsintered at 920 1C exhibited considerable bending strength(�15.8 MPa), more than 1.8-fold that of 45S5/BG-ZB2m, and nearly

3-fold that of 45S5 (�5.3 MPa). Compared with mechanicalmixing, addition directly to the gel-phase resulted in more homo-geneous distribution, and less defects and pits occured in themicrostructures of cross-section (Fig. S2; SI).

The surface bioactivity of the 45S5 BG-derived BGCs sintered at920 1C was tested in SBF (see Fig. S3, SI). With the increase ofimmersing time from 1 to 3 days, a continuous coating layerthought to be carbonated hydroxyapatite (CHA) completely cov-ered the 45S5 disc, indicating that the nanoscale particles werenucleated on the surface of the disc leading to the formationof apatite. This nucleated formation of apatite is consideredresponsible for in-situ/ in-vivo bone bonding, and a crucialcriterion to judge the bioactivity of biomaterials. The surface ofthe 45S5/ BG-ZB2g and 45S5/BG-ZB2m discs developed similar

Fig. 2. Low- (bar: 5 μm) and high-magnification (inset; bar: 1 μm) of the integral microstructure (SEM observation) of the 45S5-derived BGCs in the absence (a–c) andpresence (d–o) of BG-ZB sintered at different temperatures.

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biomimetic apatite precipitates, indicating that the BG-ZB-doped45S5-based BGCs exhibited a relatively well bioactivity evensintered at 920 1C. In addition, the face-scanning EDX analysesindicated that the surface layers were essentially composedof Ca, P, and O, and the Ca/P molar ratio declined noticablyafter immersion in SBF, owing to the release of calcium ionsand deposition of phosphate through apatite formation on theirsurface.

While soaking in Tris buffer, the grains in surface layer wereattacked by the surrounding solution and gradually degraded asa result. Weight loss is widely used to monitor the degradationof the soaked samples, as in this work of the weight variationof 45S5, 45S5/BG-ZB2m and 45S5/BG-ZB2g plotted in Fig. 4. The45S5, sintered at 860 1C, degraded nearly 23% after soaking for28 d, lower than that of 45S5/BG-ZB2g and 45S5/BG-ZB2mwhich both degraded nearly 28%, and the performance was evenworse when sintered at 920 and 980 1C. According to previousreports by other researchers [15,16], it is assumed that an apatitelayer can deposit on the silica-rich layer in such Tris-bufferedmedium due to migration of calcium and phosphate ions from the45S5 BGC, and the dissolution of this silica-rich layer controlledthe overall weight loss of 45S5 BGC. The relatively dense reactionlayer on 45S5 BGC may hinder the transport of ions between theunreacted BGC core and the solution, causing the reaction todecelerate. On the other hand, it is reasonable to postulate that thepromotion of degradation in 45S5/BG-ZB composites correspondsto the theory that with the borate BG addition the grain interfacenetwork becomes a facile breakage and leads to a relatively fast

degradation. However, the release of a minor amount of zinc ionfrom BGC composites may increase the chemical stability of themedium, and decrease the formation rate of apatite [17]. Thus,addition of ZnO/B2O3-rich BG may provide 45S5/BG-ZB compositeswith the capacity for appropriate biodegradation.

It is known that the crystallization, microstructure, and evenbiodegradation of BG-derived BGCs can be governed by adjustingthe thermal treatment temperature to suit with their end applica-tions. Conventionally, it is recognized that obtaining ceramicswith enhanced strength necessitates the extensive densificationin the solid structure, which would otherwise be made of looselybonded particles and thus be too fragile to handle. Our investiga-tion confirms that the 45S5-derived BGCs pressureless sinteredat 920–980 1C maintain highly dense microstructure as reportedin document but keeps considerably low compressive strength.This means the crystallization and tight bonding of 45S5 crystals isnot enough to contribute the significant improvement of mechan-ical strength. Conventional pressureless sintering is usually used tocreate materials which have a high melting temperature. However,this method requires high temperature and long heating times inorder to bond the junctions between ground material particlesby congelation or diffusion. It is also generally agreed upon that45S5 BG possesses high bioactivity but the low thermal treatment(o1000 1C) results in too fragile to suitable for load-bearingcondition. This raises the question as to what preparation pro-cesses produce bioglass with the optimal balance betweenmechanical and physicochemical properties. The low meltingtemperature glass-assisting approach can effectively reduce thesintering temperature of advanced electric capacitors [18]. There-fore, the ZnO/B2O3-rich BG sintering aids, employed in 45S5BG-derived BGCs, makes it possible to process the sintering atlower temperature and short duration. On the other hand, it maybe assumed that although the BGC composites show a fasterbiodegradation rate, the apatite re-mineralization due to theirhigh bioactivity likely contributes to strength retention in vivo[19], and the strength retention also have other biological sig-nificance, as preconditioning treatment of BGs or BGCs result inpositive biological responsess [20]. It has also been reported thatthe porous BGC of 45S5/BG-ZB composite, free of defects in struts,may be artificially fabricated using direct ink writing [21] orlithography-based additive manufacturing [22], followed by themore advanced solid-state assisted sintering techniques, and suchbiomaterials with superior mechanical properties can be used toresolve the bone regeneration and repair in load-bearing boneconditions.

Fig. 3. Compressive strength (a) and bending (b) strength of the 45S5-derived BGCs.

Fig. 4. Weight loss of the immersed 45S5-derived BGCs sintered at differenttemperatures in 0.05 M Tris buffer.

L. Lin et al. / Materials Letters 126 (2014) 154–158 157

4. Conclusions

In this work, the mixing approach of the sol-gel-derived BG, asa reinforcing aid to improve the mechanical properties of melt-quenching BG, has been explored. The 45S5 BG was reinforced byadding sol-gel prepared bioglass, containing boron oxide and zincoxide, in mechanical-mixing and gel-mixing by two methods. The45S5-based BGC exhibits remarkable improvement of mechanical,as well as the biological activity and biodegradation in vitro, withthe addition of nanoscale BG-ZB particles, which can be homo-genously embedded within 45S5 particles. Thus, this study pro-vides a new strategy of low-temperature sintering which canenhance the mechanical strength, without affecting the bioactivityof 45S5 BG-derived BGCs, with respect to the conventional high-temperature sintering technique.

Acknowledgment

This work was supported by the National Science Foundation ofChina (81271956, 51102211, and 51372218), the FundamentalResearch Funds for the Central University (2012QN81001), theZhejiang Provincial Natural Science Foundation of China (No.Z14E020002, Q14H060011). The authors also acknowledge Dr.Michael Palmer for his aid in the language help.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2014.04.028.

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