Structural Fingerprint of Crystallization in Mixed-Alkali Bioactive Glasses

Hench-type bioactive glasses such as 45S5 exhibit excellent biological and therapeutic performance, including osteogenesis, angiogenesis, bactericidal activity, and anti-inflammation properties [1, 2]. However, the pronounced devitrification tendency significantly reduces the processing window, which limits their clinical use [3]. In this study, we aim to decipher the underlying structural fingerprint correlated with the crystallization propensity of such glasses. To this end, the atomic-scale arrangements of mixed-alkali bioactive (MAB, 46.1SiO2-2.6P2O5-26.0CaO-(24.2-x)Na2O-xLi2O) glasses were determined using high energy synchrotron X-ray diffraction, reverse Monte Carlo simulation, Raman and solid-state nuclear magnetic resonance spectroscopy. The glasses were prepared by two quenching protocols with different cooling rates. The MAB glasses formed through rapid cooling (containerless aerodynamic levitation quenching) show much better stability (higher glass transition Tg and crystallization Tc temperatures) and processability (∆T = Tc-Tg) compared to the slowly cooled glasses (conventional melt quenching). Moreover, these thermal properties exhibit significant composition dependence with the Li:Na ratio. Overall, Tg shows a nonlinear negative deviation, while ∆T displays a parabolic-like tendency consistent with the mixing entropy. Variations of Tg and ∆T are intricately correlated with the hierarchical-scale network connectivity prompted by rapid cooling and mixed alkali effects, including but not limited to the flexible Si-O-P linkages that were validated in the latest simulations [4]. A physics-based structural fingerprint is then developed, where the contours of topological constraints and local configurational entropy projected on individual network-formers (Si, P) are associated with the barriers of potential nucleation. We show that the enhancement of crystalline resistance is linked to the decrease of possible nucleation sites.