Influence of Calcium on the Initial Stages of the Sol-Gel Synthesis of Bioactive Glasses.

Understanding how calcium interacts with silica sources and influences their polycondensation in aqueous solutions is of central importance for the development of more effective biomaterials by sol-gel approaches. For this purpose, the atomic-scale evolutions of a calcium-containing precursor solution corresponding to a typical sol-gel bioactive glass and of a corresponding Ca-free solution were compared using reactive molecular dynamics simulations. The simulations highlight a significantly faster rate of condensation when calcium is present in the initial solution, resulting in the formation of large and ramified silica clusters within 5 ns, which are absent in the Ca-free system. This different behavior has been analyzed and interpreted in terms of the Ca-induced nanosegregation in calcium-rich and silica-rich regions, which promotes the condensation reactions within the latter. By identifying a possible mechanism behind the limited incorporation of calcium in the silica nanoclusters formed in the early stages of the sol-gel process, these results could guide further studies aimed at identifying favorable experimental conditions to enhance initial calcium incorporation and thus produce sol-gel biomaterials with improved properties.

Surface signatures of bioactivity: MD simulations of 45S and 65S silicate glasses.

The surface of a bioactive (45S) and a bioinactive (65S) glass composition has been modeled using shell-model classical molecular dynamics simulations. Direct comparison of the two structures allowed us to identify the potential role of specific surface features in the processes leading to integration of a bioglass implant with the host tissues, focusing in particular on the initial dissolution of the glass network. The simulations highlight the critical role of network fragmentation and sodium enrichment of the surface in determining the rapid hydrolysis and release of silica fragments in solution, characteristic of highly bioactive compositions. On the other hand, no correlation has been found between the surface density of small (two- and three-membered) rings and bioactivity, thus suggesting that additional factors need to be taken into account to fully understand the role of these sites in the mechanism leading to calcium phosphate deposition on the glass surface.

Modeling the water-bioglass interface by ab initio molecular dynamics simulations.

The hydration of the surface of a highly bioactive silicate glass was modeled using ab initio (Car-Parrinello) molecular dynamics (CPMD) simulations, focusing on the structural and chemical modifications taking place at the glass-water interface immediately after contact and on the way in which they can affect the bioactivity of these materials. The adsorption of a water dimer and trimer on the dry surface was studied first, followed by the extended interface between the glass and liquid water. The CPMD trajectories provide atomistic insight into the initial stages relevant to the biological activity of these materials: following contact of the glass with an aqueous (physiological) medium, the initial enrichment of the surface region in Na+ cations establishes dominant Na+-water interactions at the surface, which allow water molecules to penetrate into the open glass network and start its partial dissolution. The model of a Na/H-exchanged interface shows that Ca2+-water interactions are mainly established after the dominant fraction of Na is leached into the solution. Another critical role of modifier cations was highlighted: they provide the Lewis acidity necessary to neutralize OH(-) produced by water dissociation and protonation of nonbridging oxygen (NBO) surface sites. The CPMD simulations also highlighted an alternative, proton-hopping mechanism by which the same process can take place in the liquid water film. The main features of the bioactive glass surface immediately after contact with an aqueous medium, as emerged from the simulations, are (a) silanol groups formed by either water dissociation at undercoordinated Si sites or direct protonation of NBOs, (b) OH(-) groups generally stabilized by modifier cations and coupled with the protonated NBOs, and (c) small rings, relatively stable and unopened even after exposure to liquid water. The possible role and effect of these sites in the bioactive process are discussed.

Structural effects of phosphorus inclusion in bioactive silicate glasses.

Molecular dynamics simulations of four bioactive silicate glasses containing between 0 (P0) and 12 (P12) mol % P2O5 have been carried out in order to elucidate the structural role of phosphorus in these materials. In particular, we have focused on structural features which can have a direct role in the bioactive mechanism of dissolution and bone bonding. The higher affinity of modifier Na and Ca cations for coordinating phosphate rather than silicate, together with the formation of P-O-Si linkages, lead to increasing repolymerization of the silicate network with increasing P2O5 content, which in principle would represent a negative effect of P inclusion on the glass bioactivity. However, this effect is counterbalanced by the concomitant increase in the amount of free orthophosphate groups, whose fast release is deemed to enhance the bioactivity. The strong affinity of the orthophosphates for calcium ions leads to a clear tendency toward separation of silicate-rich and phosphate-rich phases for the P12 composition. Although this could reduce the bioactivity in the case of P12, in general, the favorable balance between the effects mentioned above should result in a positive effect of partial Si --> P substitution on the glass bioactivity.

The formation of nanoscale structures in soluble phosphosilicate glasses for biomedical applications: MD simulations.

The occurrence of chain-like fragments and rings in phosphosilicate glasses of known bioactivity was examined using classical molecular dynamics simulations, in order to reveal the possible effect of such nanostructures on the bioactive behaviour. Highly bioactive compositions display a large fraction of non-crosslinked, nonlinear chains of tetrahedra, which are not present in bio-inactive compositions. The low(er) energetic cost associated with the direct release of these silicate fragments into solution can assist the fast partial dissolution observed for bioactive glass compositions. Loss of bioactivity when the silicate content increases is accompanied by the transformation of these chains into small closed rings (3- to 8-membered), which appear to protect the silicate network from fast dissolution.

A new self-consistent empirical interatomic potential model for oxides, silicates, and silica-based glasses.

A new empirical pairwise potential model for ionic and semi-ionic oxides has been developed. Its transferability and reliability have been demonstrated by testing the potentials toward the prediction of structural and mechanical properties of a wide range of silicates of technological and geological importance. The partial ionic charge model with a Morse function is used, and it allows the modeling of the quenching of melts, silicate glasses, and inorganic crystals at high-pressure and high-temperature conditions. The results obtained by molecular dynamics and free energy calculations are discussed in relation to the prediction of structural and mechanical properties of a series of soda lime silicate glasses.