Toward a Rational Design of Bioactive Glasses with Optimal Structural Features: Composition–Structure Correlations Unveiled by Solid-State NMR and MD Simulations

The physiological responses of silicate-based bioactive glasses (BGs) are known to depend critically on both the P content (n(P)) of the glass and its silicate network connectivity (N(BO)(Si)). However, while the bioactivity generally displays a nonmonotonic dependence on nP itself, recent work suggest that it is merely the net orthophosphate content that directly links to the bioactivity. We exploit molecular dynamics (MD) simulations combined with ³¹P and ²⁹Si solid-state nuclear magnetic resonance (NMR) spectroscopy to explore the quantitative relationships between N(BO)(Si), n(P), and the silicate and phosphate speciations in a series of Na₂O-CaO-SiO₂-P₂O₅ glasses spanning 2.1 ≤ N(BO)(Si) ≤ 2.9 and variable P₂O₅ contents up to 6.0 mol %. The fractional population of the orthophosphate groups remains independent of n(P) at a fixed N(BO)(Si)-value, but is reduced slightly as N(BO)(Si) increases. Nevertheless, P remains predominantly as readily released orthophosphate ions, whose content may be altered essentially independently of the network connectivity, thereby offering a route to optimize the glass bioactivity. We discuss the observed composition-structure links in relation to known composition-bioactivity correlations, and define how Na₂O-CaO-SiO₂-P₂O₅ compositions exhibiting an optimal bioactivity can be designed by simultaneously altering three key parameters: the silicate network connectivity, the (ortho)phosphate content, and the n(Na)/n(Ca) molar ratio.