Structure, Dynamics, and Surface Reactions of Bioactive Glasses
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Three bioactive glasses (45S5, 55S4.3, and 60S3.8) have been investigated using atomic-scale molecular dynamics simulations in attempt to explain differences in observed macroscopic bioactivity. Bulk and surface structures and bulk dynamics have been characterized. Ion exchange and hydrolysis reactions, the first two stages in Hench’s model describing the reactions of bioactive glass surfaces in vivo, have been investigated in detail. The 45S5 composition shows a much greater network fragmentation: it is suggested that this fragmentation can play a role in at least the first two stages of Hench’s model for HCA formation on the surfaces of bioactive glasses. In terms of dynamic behavior, long-range diffusion was only observed for sodium. Calcium showed only jumps between adjacent sites, while phosphorus showed only local vibrations. Surface simulations show the distinct accumulation of sodium at the immediate surface for each composition. Surface channels are also shown to exist and are most evident for 45S5 glass. Results for a single ion exchange showed that the ion-exchange reaction is preferred (more exothermic) for Na+ ions near Si, rather than P. A range of reaction energies were found, due to a of local environments, as expected for a glass surface. The average reaction energies are not significantly different among the three glass compositions. The results for bond hydrolysis on as-created surfaces show no significant differences among the three compositions for simulations involving Si-O-Si or Si-OP. All average values are greater than zero, indicating endothermic reactions that are not favorable by themselves. However, it is shown that the hydrolysis reactions became more favorable (in fact, exothermic for 45S5 and 55S4.3) when simulated on surfaces that had already been ion-exchanged. This is significant because it gives evidence supporting Hench’s proposed reaction sequence. Perhaps even more significantly, the reaction energies for hydrolysis following ion exchange directly relate to the glass composition; the 45S5 composition is most favorable and 60S3.8 is least favorable. This correlates directly with the observed macroscopic in vivo bioactivity of these glasses.
Advisory committee members: Alexis Clare, Matthew Hall, Arun Varshneya. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University