Structural origin of high bioactivity in zirconia containing bioactive glasses

ZrO₂ containing phosphosilicate bioactive glasses with high in vitro bioactivity and mechanical strength are developed and the roles of ZrO₂ and P₂O₅ on structure, dissolution behavior, and in vitro bioactivity are elucidated through a integrated experimental and simulation study. In comparison to conventional bioactive glasses such as 45S5, these zirconia containing bioactive glasses exhibit an intriguing phenomenon of slow initial dissolution rates yet overall high ion releases in longer term reactions. This is found to originate from the inhibited reorganization of gel layer structures due to the formation of Zr-O-Si linkages, similar to what previously observed in zirconia-containing nuclear waste glasses. The suppression effect is evidenced by the progressive increase in gel layer thickness, reaching its maximum at 4 mol% ZrO₂ incorporation, as a function of ZrO₂ concentration. Additionally, transmission electron microscopy imaging reveals formation of nano sized pores within the gel layer and a depletion zone between the gel and pristine glass, further highlighting the critical role of ZrO₂ in modulating dissolution and enhancing bioactivity. Molecular dynamics simulation results confirm a mixed network structure consisting sixfold-coordinated [ZrO₆] and fourfold-coordinated [SiO₄] units in the porous gel layer with interconnected porosity that facilitates solution diffusion and glass dissolution. This work thus identifies a new class of ZrO₂ containing bioactive glasses and provides insights on the relationships between structure, dissolution behavior, and bioactivity, greatly expanding the composition and design space of advanced bioactive materials. STATEMENT OF SIGNIFICANCE: Our findings show that ZrO₂ containing bioactive glasses have unusual dissolution behaviors and high bioactivity. By using a range of characterization methods and molecular dynamics computer simulations, the short and medium range glass structure were revealed and their relation to the dissolution behavior and in vitro bioactivity were elucidated. We found that although zirconia decreases initial glass dissolution rate, it hinders the gel layer reorganization hence increases the overall dissolution and leaching of alkali and alkali earth ions that lead to improved bioactivity. This behavior increases the design space of bioactive glass compositions and enables design of glasses that have simultaneously high modulus, low thermal expansion, high thermal stability and high in vitro bioactivity.