Modeling of Bioactive Glass Structures
To meet the increasing medical and socioeconomic challenges of our aging population, substantial efforts have been devoted for finding improved bone/tooth implants with the ability to bond to both soft and hard tissues. One option, in clinical use since the 1980s, is to use melt-derived bioactive glasses (BGs) comprising Na, Ca, P and Si. On their exposure to body fluids, a bio-mimetic surface layer of hydroxycarbonate apatite grows and forms a strong bone-glass interface. Boron-bearing borosphosphosilicate (BPS) BGs were introduced a decade ago by substituting a part of the silicon reservoir by boron. This improves the glass solubility and thereby also the bioactive properties. However, the structures of Na–Ca–B–P–Si–O glasses, notably so the important structural roles of P and B, remain essentially unexplored. One part of our ongoing VR-funded project aims at improving the structural insight into BPS glasses by combining results from solid-state NMR experiments and atomistic molecular dynamics (MD) simulations. The glass models are obtained by simulating the melt-quench procedure. Although no MD simulations are reported for BPS glasses, several studies exist for the limiting borosilicate system, where a well-known problem is the quantitative prediction of the relative populations of the two co-existing B coordinations, i.e., trigonal (BO3) and tetrahedral (BO4) groups. These populations depend strongly on the precise glass composition, but they are only captured moderately well by previous MD simulations that generally invoked rigid-ion force fields. By accounting for polarization effects with the polarizable core-shell model that splits each O species into two harmonic-oscillator coupled “core” and “shell” portions, we hope to improve the modeling of the B speciation. We have already initiated the development of the B–O pair-potential parameters suitable for the complex BPS glass matrix, but resources at SNIC are urgently required to complete this task, as well as for performing the subsequent modeling of a series of BPS glasses with variable compositions. Recently, we have gained a sound experience in implementing atomistic MD modeling of various glass systems (reflected in 12 publications since 2012), as well as in developing new force fields for such simulations. First, the interatomic potential parameters will be validated against various experimental data on the short-range glass structures obtained from 11B and 31P NMR on ~20 BPS glasses [Yu & Edén, RSC Adv., 2016, 6, p101288]. Once the force-field developments are completed, the MD-derived glass models will assist the interpretation of already existing experimental results from advanced NMR techniques that inform about the intermediate-range glass structure (0.3–0.6 nm), notably so the preferential association of the various silicate, borate and phosphate network building blocks, as well as their potential dependencies on the glass composition. Moreover, MD simulations yield information not easily accessible experimentally, such as the distributions of interpolyhedral bond-angles, the sets of calcium and sodium coordination numbers, and their preferences for distributing around the silicon, borate, and phosphate groups.