Effect of magnesia on the degradability and bioactivity of sol–gel derived SiO2–CaO–MgO–P2O5 system glasses

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Abstract

Mesoporous 58SiO2–(38  x)CaO–xMgO–4P2O5 glasses (where x = 0, 5, 10 and 20 mol%) have been prepared by the sol–gel synthesis route. The effects of the substitution of MgO for CaO on glass degradation and bioactivity were studied in tris-(hydroxymethyl)-aminomethane and hydrochloric acid buffer solution (Tris–HCl) and simulated body fluid (SBF), respectively. It is observed that the synthesized glasses with various MgO contents possess the similar textural properties. The studies of in vitro degradability and bioactivity show that the rate of glass degradation gradually decreases with the increase of MgO and the formation of apatite layer on glass surface is retarded. The influences of the composition upon glass properties are explained in terms of their internal structures.

Introduction

Bioactive glasses and glass-ceramics developed during the past few decades have shown significant potential in biomedical research field as alternative materials to repair or replace parts of the skeletal system or as prosthetic coatings [1], [2], [3], [4]. At present, different types of bioactive glasses and glass-ceramics are still under development and a great effort is being put into the understanding of the mechanisms and factors governing its properties. For bioactive glasses and glass-ceramics, factors such as chemical composition, internal structure and surface morphology are believed to play important roles. In order to tailor and obtain the desired macroscopic or microscopic properties, changes in composition are primarily required, in essence, the atomic arrangement at short and intermediate range is one decisive factor for regulating the physicochemical properties of glasses and glass-ceramics.

The first reported bioactive glass and one of the most common and well-characterized is the Bioglass® 45S5 composed of SiO2, CaO, Na2O and P2O5 four components by Hench et al. in 1971 [5]. Then a variety of complex glass system was developed and the understanding of the individual function of each component in the glass system becomes necessary. Actually, typical bioactive glasses contain calcium and phosphorus, and the importance of them has been widely discussed [6], [7], [8], [9], [10]. Moreover, the effect of oxides such as MgO, B2O3, Al2O3 and Fe2O3 on glass properties has also been reported in the literature [11], [12], [13], [14], [15]. Especially for Al2O3, low (μM) concentrations of aluminum have been shown to stimulate the proliferation of osteoblasts and new bone formation in vitro and in vivo [16], [17]. However, high concentrations indicate toxic outcomes, such as deranged mineralization and decreased bone formation [18], [19]. It is notable that enamel, dentin and bone contain 0.44, 1.23 and 0.72 wt.% of magnesium, respectively [20]. And magnesium is closely associated with mineralization and indirectly influences mineral metabolism [21], [22]. The substitution of magnesium for calcium has also been conducted in biological apatite and the researches indicate that the Mg-substituted hydroxyapatite materials are expected to have excellent biocompatible and show greater osteoconductivity over the time and higher material resorption [23], [24]. However, the contribution of magnesium to the properties of bioactive glass has led to contradictory explanations, in part because the appearance of magnesium in glass structure either as a network modifier or as a network former [11], [25], [26]. In addition, it has been suggested that the presence of MgO inhibits glass bioactivity [27], [28], [29]. Vallet-Regi and co-workers [12], [30] demonstrated that the formation rate of apatite layer slows down when MgO contents is above 7 mol%. On the other hand, Ferreira and co-workers [31] indicated that Mg-bioactive glasses form a suitable substrate for human osteoblast-like cell proliferation and function. It suggests that magnesium can block apatite crystal growth but is a co-factor in a number of enzymes related to bone health. Furthermore, significant amounts of MgO are present in some Bioverit glass-ceramics, whose bioactivity has been clinically confirmed for years [32], [33].

Sol–gel technique, as one chemical method, provides one available way to synthesize bioactive glasses at lower reaction temperature and allows us to obtain glasses with high purity and homogeneity [34]. More importantly, it allows the synthesized glasses to possess high surface area and porous nature. It is recognized that the high specific surface area and silica-rich layer is the critical element necessary for the formation of an apatite layer on bioactive glasses surface [34], [35]. For the sol–gel glasses, the rate of apatite formation and index of bioactivity IB is higher [5], [36], which is attributed to a greater release of soluble silica that nucleates apatite crystals in pores of the sol–gel glasses [36]. Therefore, the amount of silica in bioactive sol–gel glasses can be much higher than in melt-derived glasses of the same composition, and these high surface area sol–gel glasses have been considered for bone graft applications.

In our previous work [37], bioactive glass containing 10 mol% of MgO was synthesized and the results indicated the formation of an apatite layer on glass surface after exposure to an in vitro solution, but a retarding formation rate of this layer was found. When considering the potential applications of MgO-containing bioactive glass compositions, this study has extended to other MgO-containing glasses in the same system and the present work endeavors to investigate the influence of magnesium contents on glass degradability and bioactivity, and understand the relationships among glass composition, structure and property.

Section snippets

Glass preparation and characterization

Four glasses 58SiO2–(38  x)CaO–xMgO–4P2O5 (x = 0, 5, 10 and 20 mol%) were prepared by sol–gel method. Briefly, glasses synthesis was carried out by hydrolysis and polycondensation of appropriate amounts of tetraethyl orthosilicate (TEOS), triethyl phosphate (TEP), calcium nitrate (Ca(NO3)2·4H2O) and magnesium nitrate (Mg(NO3)2·6H2O) in the presence of water (mole of H2O/(mole of TEOS + mole of TEP) = 10). Nitric acid (HNO3, 2 N) was used to catalyze the hydrolysis of TEOS and TEP, using a molecular

Characterization of the starting materials

Fig. 1 shows the TG/DTA curves of the gels after being dried at 130 °C. From the TG curves, two obvious weight losses are clearly observed. The first one occurs between room temperature and 151–166 °C, and it corresponds to obvious endothermic peaks around 84–97 °C. This weight loss can be attributed to the volatilization of residual water and ethanol and it associates with a weight loss of 11–24%. The second one commences from the end of the first weight loss until about 574–634 °C, and this

Discussion

According to the plots of TG/DTA, the endothermic processes around 734–827 °C are observed and they correspond to the materials glassy transitions. At this temperature, the glasses become less viscous and the porosity is highly reduced by densification. In addition, the exothermic peaks denoting the emergence of the crystallization can also be detected, the peak crystallization temperature (Tp) increases with MgO content and the Tp for 0% MgO, 5% MgO, 10% MgO and 20% MgO is 931, 934, 948 and 957 

Conclusions

Four glasses in the SiO2–CaO–MgO–P2O5 system with various MgO contents have been synthesized by the sol–gel method. All of them possess high specific surface area and present porous nature in mesoporous range. The degradability and the formation of apatite layer were studied in detail. It is clear that the incorporation of MgO slows down the rate of glass degradability and retards the formation of apatite layer on glass surface. The influences of the composition on glass properties are mainly

Acknowledgements

This work was financially sponsored by the Key Project of the National Science Foundation of Shandong Province, PR China under grant no.: Z2007F02.

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