Effect of Aluminum Ion Incorporation on the Bioactivity and Structure in Mesoporous Bioactive Glasses

work is dedicated to biokinetic and structural investigations of Al2O3 doping (0.5 to 15 mol %) in ordered mesoporous bioactive glasses (MBGs), based on the composition 80% SiO2-15% CaO-5% P2O5 (in mol %). The synthesis was performed with the sol gel method in combination with a structure directing agent (P-123) for the formation of mesopores. Structural investigations reveal that the incorporation of additional Al2O3 in a range of 1 to 10 mol % reduces the order of the mesostructure, whereas doping with 15 mol % Al2O3 creates well ordered mesopores again. Nitrogen adsorption-desorption isotherms show that specific surface area, pore volume, and pore diameter decrease only slightly upon incorporation of Al2O3. In vitro bioactivity tests exhibit a small decrease in bioactivity upon incorporation of small amounts and a sudden drop beyond 3 mol %. This can be related to the strong interaction of Al3+ and PO43-, which could be proven by multinuclear single and double resonance solid state nuclear magnetic resonance (NMR) spectroscopy. The number of P-O-Al linkages has been estimated by Al-27(P-31) rotational echo double resonance (REDOR) experiments. The interaction of Al3+ and PO43- and the trapping of Ca2+ required for charge compensation hamper the release of P, Ca, and Si ions and thus provide less PO43- and Ca2+ ions for the crucial formation of hydroxycarbonate apatite (HCA).