Effect of boron incorporation on the bioactivity, structure, and mechanical properties of ordered mesoporous bioactive glasses

Abstract

B₂O₃ doped (0.5–15 mol%) ordered mesoporous bioactive glasses (MBG) with the composition 80% SiO₂–15% CaO–5% P₂O₅ were synthesized via a sol–gel based evaporation-induced self-assembly process using the block-copolymer P123 as a structure directing agent and characterized by biokinetic, mechanical and structural investigations. Nitrogen physisorption isotherms and electron microscopy indicate no detrimental effect of B₂O₃ on the ordered hexagonal pore structure. Boron incorporation increases both the bulk modulus and hardness of the glasses. In vitro bioactivity tests reveal a rapid initial release of Ca²⁺ and PO₄³⁻ ions, followed by formation of hydroxyapatite carbonate within a few hours. Contrary to the tight incorporation of Al in Al-doped MBGs, the rapid release of borate species into simulated-body-fluid suggests loosely bound species localized at the internal surfaces of the mesopores. ²⁹Si, ¹¹B, ³¹P, and ¹H solid state NMR spectroscopy reveal that the majority of the borate is present as anionic BO_4/2⁻ species. The need for charge compensation leads to an increase in the average degree of polymerization of the phosphate species for high boron contents. ¹¹B{³¹P} rotational echo double resonance NMR results reveal the absence of B–O–P linkages. This structural model explains the rapid release of borate and the enhanced dissolution kinetics of the Ca²⁺ and phosphate species.