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

This work is dedicated to biokinetic and structural investigations of Al 2 O 3 doping (0.5 to 15 mol %) in ordered mesoporous bioactive glasses (MBGs), based on the composition 80% SiO 2 –15% CaO–5% P 2 O 5 (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 Al 2 O 3 in a range of 1 to 10 mol % reduces the order of the mesostructure, whereas doping with 15 mol % Al 2 O 3 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 Al 2 O 3 . 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 Al 3+ and PO 4 3–, 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 27 Al( 31 P) rotational echo double resonance (REDOR) experiments. The interaction of Al 3+ and PO 4 3– and the trapping of Ca 2+ required for charge compensation hamper the release of P, Ca, and Si ions and thus provide less PO 4 3– and Ca 2+ ions for the crucial formation of hydroxycarbonate apatite (HCA).