Deciphering the role of aluminum in silicate-based bioactive glasses: Atomic-scale structural evolution and network connectivity using molecular dynamics simulations

The incorporation of aluminum into bioactive glasses (BGs) remains controversial because Al 3+ can simultaneously improve mechanical properties while potentially suppressing bioactivity by increasing network connectivity (NC). In this study, the structural evolution and predicted dissolution behavior of Al 2 O 3 -substituted, silicate-based BGs (60SiO 2 –(36−x)CaO–4P 2 O 5 –xAl 2 O 3 , mol%, with Al 2 O 3 replacing CaO) were investigated using molecular dynamics (MD) simulations. Eight compositions containing 0–20 mol% Al 2 O 3 substitution were generated using Buckingham-type interatomic potentials and a melt–quench procedure. Structural analysis revealed that Al incorporation produced only minor variations in local bond lengths. The average Si–O bond length increased from 1.609 Å to 1.611 Å, while P–O and Al–O bond lengths varied from 1.491–1.494 Å and 1.780–1.786 Å, respectively. The coordination numbers (CNs) remained approximately constant at 4.0 for Si–O, P–O, and Al–O species, whereas Ca–O CN ranged between 6.1 and 6.2. Increasing Al 2 O 3 content substantially altered oxygen speciation, with Si–O–Al linkages increasing from 0 to 36.85% and Al–O–Al bonds increasing from 0 to 11.81%, accompanied by a pronounced reduction in non-bridging oxygens (NBOs) from 45.33% to 8.84%. The density of network-forming oxygen bonds increased concurrently, while the oxygen number density rose from 45.43 to 46.87 nm −3 . Medium-range structural analysis demonstrated a redistribution of silicate Q n species toward lower polymerization states (Q 0 –Q 2 ) and a decrease in Q 3 –Q 4 species. Because the Q n and NC of silicon count only Si–O–Si and Si–O–P bridges, this indicates depolymerization of the silicate-only subnetwork (Si-only NC decreased from 3.01 to 2.35); when Al–O–Si and Al–O–Al bridges are included, the overall NC of the whole network increased from 2.83 to 3.65. These findings indicate that aluminum predominantly acts as a network former in this BGs, which, according to the structural descriptors, is expected to reduce the dissolution tendency at elevated concentrations. Overall, the structural descriptors predict that limited Al 2 O 3 incorporation (≤3 mol%) retains an NBO-rich network (NBO 36.9% vs. 45.3% in the Al-free glass), whereas higher Al 2 O 3 concentrations (>8 mol%) strongly reduce the NBO fraction and are expected to lower the dissolution tendency and bioactivity. These limits are model-based predictions that were verified using MD simulations, which can be considered as a new base for designing new generation BG compositions.