Biomedical Engineering Reference
In-Depth Information
positions, a melt-and-quench procedure was simulated: heating at 6000 K and then
equilibrating for 100 ps. Next, continuously cooling from 6000 to 300 K in 1140 ps with a
nominal cooling rate of 5 K/ps was performed. The temperature was decreased by 0.01 K
every time step using Nose-Hoover thermostat with the time constant parameter for the
frictional coefficient set to 0.1 ps (Hoover, 1985). Simulations were carried out in the
constant volume NVT ensemble and 100 ps of equilibration at constant volume and 50 ps of
data production were run at 300 K. On the derived structures, static energy minimizations
were carried out at constant pressure and volume and the most representative model was
chosen for
ab initio
calculations.
The final candidate structure was minimized both in terms of internal coordinates and
lattice parameters, performing full relaxation runs. In Figure 7a the quantum-mechanical
optimized structure of the selected Bioglass model is displayed. In the unit cell, which has
become triclinic due to the lattice parameter deformation, two phosphate groups are
present: one isolated (orthophosphate) and the other linked to one silicon atom. The
structural analysis was followed by the simulation of the IR spectrum. Figure 7b reports the
comparison between experimental (Lusvardi et al., 2008b) and computed spectra, which
shows a very good agreement between the two spectra. The punctual assignment of each
peak in the simulated case has been published in a previous paper (Corno et al., 2008),
where the 45S5 model has been compared with an amorphous silica structure, to investigate
the role of network modifier cations and phosphate groups in structural and vibrational
properties of a pure SiO
2
framework. Hence, the reliability of the chosen multi-scale strategy
has been proved.
a.
b.
Fig. 7. 45S5 Bioglass model: a. best view of the optimized structure (Na
12
Ca
7
P
2
Si
13
O
44
composition), colour coding: silicon light blue, oxygen red, sodium pink, calcium dark blue
and phosphorous yellow; cell parameters drawn in red for
a
, in green for
b
and in blue for
c
,
while cell borders are in black; b. experimental (red line) and B3LYP (black line) IR spectra.
(Corno et al., 2008; Lusvardi et al., 2008b)
2.2.2 Simulation of bioactive glasses with different P
2
O
5
content
More recently, the influence of P
2
O
5
content on the structure of bioactive glass compositions
has been object of investigation (O'Donnell et al., 2009; O'Donnell et al., 2008a; O'Donnell et
