A new sol–gel process for producing Na2O-containing bioactive glass ceramics
Introduction
Bioactive glasses have been much studied in attempts to develop suitable materials for use as implants in the human body to repair and replace diseased or damaged bone. Such implant materials need mechanical strength, but also the ability to harmlessly degrade over time to allow their gradual replacement with newly formed bone. The best characterized type is 45S5 Bioglass® (Table 1), which has been used in a number of medical devices approved by the US Food and Drug Administration (FDA) [1]. Commercially produced bioactive glasses have been made by conventional glass powder manufacturing methods, i.e. melting and quenching. Meanwhile, increasing research efforts are being invested in fabrication of bioactive glasses using the sol–gel technique [2], as it is an extremely versatile process with many advantages over melting–quenching processes. Using the sol–gel process, ceramic or glass materials can be fabricated in a variety of forms, including ultra-fine spherical powders, thin film coatings, ceramic fibres, microporous inorganic membranes, monolithic ceramics and glasses and highly porous aerogel materials [3].
Despite its advantages, the sol–gel technique has not yet been successfully applied to the production of Na2O-containing bioactive glasses or glass ceramics. All members of the 49–86S series of sol–gel derived bioactive glasses, for instance, contain SiO2, CaO and P2O5, but none contain Na2O [3], [4]. Inclusion of Na2O in a sol–gel bioactive glass represents a technical challenge due to the high hydrolytic reactivity of sodium alkoxide in water [4], which is so great that some researchers have instead turned to the use of MgO in the sol–gel fabrication of bioactive glasses, producing SiO2–CaO–P2O5–MgO glasses [5]. However, there are good reasons to believe that inclusion of Na2O in the fabrication of bioceramic materials would offer opportunities to improve mechanical strength without losing a satisfactory biodegradability. Firstly, in the glass industry Na2O is added to reduce the melting point of silica-based glasses, whereas other components such as CaO and MgO are added to stabilize theses glasses, which would otherwise be rendered water soluble. Secondly, the presence of Na2O offers advantages in relation to the crystallization treatment that is applied to improve the mechanical properties of bioceramics. Because scaffolds of amorphous bioactive glasses are very fragile, to achieve good mechanical strength bioactive glass foams have to be sintered to form crystalline phases [6]. In bioactive glasses lacking Na2O the crystalline phase is bioinert [7], which means that mechanical strength is improved, at the cost of sacrificing degradability. In contrast, sintered 45S5 Bioglass® ceramics possesses both sound mechanical strength and satisfactory biodegradability, which can be attributed to the formation of a crystalline phase, Na2Ca2Si3O9 [6]. Work on such melt-derived Na2O-containing glass ceramics [6] suggests that Na2O may be a critical component in the production of biodegradable bioceramics with enough mechanical strength to be used as scaffolding materials in bone tissue engineering.
As an alternative approach to this problem, sol–gel fabrication of SiO2–CaO–P2O5–Na2O glasses employing an organic solvent has been attempted [8], by preparing the sol of alkoxide precursors of SiO2–P2O5–CaO–Na2O in ethylene glycol solution under a nitrogen atmosphere. Although promising, the production environment employed in this protocol is inconvenient and difficult to use, and it is probably for these reasons there has been little subsequent development of this technique. Therefore, the primary objective of the present work was to develop a sol–gel based protocol for the production of Na2O-containing bioactive glass ceramics which can be employed under ambient conditions. This would potentially enable these materials to be produced to a high quality but cheaply and in commercially viable quantities. In this work, we chose 45S5, a well-known Na2O-containing composition, as a specific example to develop such a new sol–gel process, which would in principle be applicable to all Na2O-containing bioactive glass ceramics.
The characterization and evaluation of melt-derived 45S5 Bioglass® ceramics are well documented [6], [9], [10], [11], [12]. Briefly: (i) the formation of Na2Ca2Si3O9 significantly improves the mechanical properties of the material; (ii) crystallization does not inhibit bioactivity, with the bone bonding ability (indicated by the formation of hydroxyapatite) remaining in the fully crystallized ceramics; (iii) when immersed in body fluid the crystalline phase Na2Ca2Si3O9 decomposes and transits to amorphous hydroxyapatite (HA), an easily degradable mineral in vivo. Therefore, the second objective of this work was to establish that Na2O can be successfully incorporated into sol–gel 45S5 glass ceramics produced by this method. To this end, we demonstrate that the sol–gel derived 45S5 material possesses the above three features that only Na2O-containing glass ceramics have, i.e. formation of Na2Ca2Si3O9 in the sintered material and decomposition of the crystalline phase and formation of amorphous HA throughout the material in simulated body fluid (SBF). Since the new sol–gel protocol involves the use of an acidic catalyst, it is also essential to evaluate the derived 45S5 material in vitro to provide some preliminary assessment of the likely clinical utility of the product. It should be noted, however, that the excellent biocompatibility of melt-derived 45S5 materials is well documented.
The last objective of this work was to demonstrate that incorporation of Na2O results in a material with a mechanically sound and yet biodegradable crystalline phase. To this end, the sol–gel derived 45S5 glass ceramic material and a Na2O-free (S70C30 composition, Table 1) bioceramics were compared for the following aspects: crystalline phases, degradation kinetics and formation of HA.
Section snippets
Materials
The following chemicals were used as precursors for the synthesis of the sol–gel 45S5 and S70C30 materials: tetraethyl orthosilicate (TEOS) (Aldrich, 99%), triethyl phosphate (TEP) (Eastman, 99.8%), sodium nitrate (Sigma–Aldrich, 99%) and calcium nitrate tetrahydrate (Sigma–Aldrich, 99%).
Sol–gel process
The process and a flowchart are provided in Table 2 and Fig. 1, respectively. In brief, the molar ratios of TEOS, TEP, NaNO3 and Ca(NO3)2·4H2O were designed according to the molar ratio of SiO2, P2O5, Na2O and
Particle size of the sol–gel derived 45S5 powders
As the molar concentration of HNO3 was decreased from 1 to 0.1 M the average particle size in the powder increased from approximately 1–5 μm, as estimated by SEM observation (Fig. 2). However, the fine particles made using the 0.5 and 1 M HNO3 solutions were severely agglomerated, such that the bulk samples had to be mechanically ground to break down the agglomerated particles. In contrast, the microspheres of the powders produced in the 0.1 and 0.25 M HNO3 solutions were loosely packed, resulting
Conclusions
Fine powders of Na2O-containing glass ceramics have been successfully synthesized using the sol–gel technique in aqueous solution under ambient conditions. Microspheres of size ∼5 μm can be easily achieved at low cost, eliminating the time-consuming processes, i.e. grinding and sieving. The sol–gel derived and sintered 45S5 glass ceramic materials possess the essential features of Na2O-containing bioactive materials, namely the formation of crystalline phase Na2Ca2Si3O9 during sintering, a
Acknowledgement
Q.-Z.C. would like to acknowledge support from the New Staff Research Grants and Small Research Grants of the Faculty of Engineering at Monash University.
References (30)
- et al.
Bioceramics: past, present and for the future
J Eur Ceram Soc
(2008) New trends in bioactive scaffolds: the importance of nanostructure
J Eur Ceram Soc
(2009)Sol–gel materials for bioceramic applications
Curr Opin Solid State Mater Sci
(1997)- et al.
45S5 Bioglass (R)-derived glass–ceramic scaffolds for bone tissue engineering
Biomaterials
(2006) - et al.
Effect of electrically inert particulate filler on electrical resistivity of polymer/multi-walled carbon nanotube composites
Polymer
(2008) - et al.
Highly bioactive P2O5–Na2O–CaO–SiO2 glass–ceramics
J Non-Cryst Solids
(2001) - et al.
Bioactivity of tape cast and sintered bioactive glass–ceramic in simulated body fluid
Biomaterials
(2002) - et al.
A kinetic and mechanistic study of the thermal decomposition of calcium nitrate
Thermochim Acta
(1996) - et al.
Crystallization kinetics of tape cast bioactive glass 45S5
J Non-Cryst Solids
(2003) - et al.
Structural transformations of bioactive glass 45S5 with thermal treatments
Acta Mater
(2007)
Surface functionalization of bioglass((R))-derived porous scaffolds
Acta Biomater
Synthesis routes for bioactive sol–gel glasses: alkoxides versus nitrates
Chem Mater
Sol gel derived SiO2–CaO–MgO–P2O5 bioglass system-preparation and in vitro characterization
J Biomed Mater Res Part B Appl Biomater
Sol–gel synthesis of the P2O5–CaO–Na2O–SiO2 system as a novel bioresorbable glass
J Mater Chem
Effect of crystallization on apatite-layer formation of bioactive glass 45S5
J Biomed Mater Res
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