Synthesis of ceramics in MO_n/2-SiO₂ systems through sol-gel processing under coexistence of polyethylene glycol and in vitro evaluation of their bioactivity

Abstract

Sol-gel processing from tetraethoxysilane (TEOS) under coexistence of polyethylene glycol (PEG) allows controlled morphology of the formed silica because of the phase separation during the polycondensation of the hydrated sol. The presence of PEG controlled the morphology effective in preparation of the ceramics, even in the CaO-SiO₂ binary system, starting from TEOS and Ca(NO₃)₂. In the present study, some ceramics were prepared using the MO_n/2-SiO₂ binary systems, where M is given as Na⁺ (n=1), Mg²⁺ (n=2) and Al³⁺ (n=3), through sol-gel processing in the presence of PEG, to compare the compositional changes in the morphology of the ceramics. In the case of NaO_1/2-SiO₂, the framework size increased remarkably with increasing amounts of Na⁺ in the starting compositions up to 30 mol%, while continuous pores of approximately 10 μm were observed at 5 mol% of Na⁺. In the case of MgO-SiO₂, the framework size increased with increasing amounts of Mg²⁺ up to 20 mol% in the starting composition. Aggregation of particles with approximately 3 μm in size was observed at 25 mol% and more. The trend in morphological change with addition of Mg²⁺ was quite similar to that with Ca²⁺. With AlO_3/2-SiO₂, particles were obtained when the starting composition was more than 10 mol% Al³⁺. The size of the particles appeared to decrease with increasing amounts of Al³⁺. The morphology and phase separation behavior are strongly affected by the charge value of the metal ions, because the number of non-bridging oxygens coordinated to the metal ion increased with increasing charge value in the sol-gel processing. A specimen prepared with starting composition 20NaO_1/2•80SiO₂ and heated at 600°C formed hydroxyapatite on the particle surfaces within 7 d after soaking in a simulated body fluid (SBF), suggesting a potential for bioactivity on application of bone repair.