Biomedical Engineering Reference
In-Depth Information
Fig. 13. Simulated IR spectra of the four models of glasses at increasing %P
2
O
5
content.
2.2.4 Future perspectives: Surface modelling
The natural subsequent step in bioactive glass simulation deals with the modeling of
surfaces. Indeed, each process of the Hench mechanism that leads to the implant integration
typically occurs at the interface between the inorganic material and the biological fluid.
Thus, the knowledge of surface properties, such as electrostatic potential and adsorptive
behavior towards simple molecules as water, becomes essential in the investigation of
bioglasses (Tilocca & Cormack, 2009).
Modeling surfaces is generally not a trivial task, particularly when the bulk material is
amorphous. For an amorphous material the identification of a particular face by
crystallographic indexes is rather arbitrary as the atomic density is statistically distributed in
space in a rather uniform way. A second difficulty is the need of breaking both ionic and
covalent bonds during the slab definition which may render the system non-neutral.
In Figure 14, the model of one of the many possible bioglass surfaces extracted from the P2.5
bulk of Figure 8b is presented. The surface was cut out from the bulk as a real 2D slab
(infinite in the two dimensions), dangling bonds were saturated with hydrogen atoms and a
full optimization run was performed. The resulting surface is very interesting per se, but
much more considering its behaviour when hydrated, since water molecules are
ubiquitously present in the biological fluids where the material is immersed. In particular, a
key issue is to see whether H
2
O will chemisorb by dissociating on the exposed Na
+
and Ca
2+
cations, a step essential in the Hench mechanism.
In our laboratory a systematic study of the several possible surfaces of the structure with the
45S5 composition is on-going. The application of different methodologies, such as
ab initio
molecular dynamics, already used in the literature (Tilocca, 2010), will be considered to fully
characterize the adsorption processes of water and even collagen occurring at the interface
between bioactive material and the biological tissue.
