MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 386

can be assigned to Si atoms present in an ortho-silicate group, SiO 4− . Most of the
literature references of chemically-synthesised Si-HA show that since the SiO 4−
groups occupy PO 3− sites, the commonly agreed mechanism is a subsequent sub-
stitution of PO 3− by SiO 4− groups [Gibson, 1999; Arcos, 2004].
On the other hand, there is evidence that when CO 2− groups are present in
the HA structure, it is more accurate to talk about a competition between the
CO 2− and SiO 4− groups for the PO 3− sites, rather than about a 1 : 1 substitution of
phosphate by silicate [Marques, 2001]. Given that the percentage of P does not
vary importantly with the Si substitution, and that FTIR results show an impor-
tant decrease of the CO 2− groups in the coatings as the Si is incorporated, it can
be stated that in this case and with the deposition technique, SiO 4− groups are
preferentially displacing the CO 2− rather than the PO 3− groups, although substitu-
tion of phosphate by silicate may occur at some extent.
In fact, when the Ca and P values remain very stable from the HA pure to the
HASi75 coating, the HASi100 sample has slightly lower values for these atoms,
indicating that the phosphate by silicate substitution is gaining importance in this
case. Another indicator that is very frequently used in the fi eld of bioactive
calcium phosphates is the Ca to P ratio (Ca/P), and its purpose is to be compared
with the Ca/P ratio that is found in the mineral part of the bone (1.67). In the
study of Si-HA this factor is replaced by the Ca/P+Si ratio [Gibson, 1999], and it
is usually intended to remain the closest to the Ca/P ratio of the bone. On the
other hand, it has been found that with this technique silicate groups are princi-
pally replacing carbonate instead of phosphate groups, and consequently the Ca/
P+Si ratio grows with the Si incorporation as the quantity of P is not being altered.
11.4.5 Physico-Chemical Properties of the Bioactive Glass Coatings
Comprehensive works on bioactive glass coatings grown by Pulsed Laser Deposi-
tion (PLD) have been reported [D ' Alessio, 1999 ; Serra, 2001 ; Liste, 2004 a, 2004 b].
The physico-chemical properties of the coatings should be carefully tuned by
changing the processing conditions in order to obtain fi lms with optimized prop-
erties and adequate biological response. To illustrate this fact, in this section, the
dependence of the composition of the bulk glasses used as ablation targets on the
properties and bioactivity of the bioactive glass coatings is reported.
Different compositions of bulk glasses (Table 11.4) in the system SiO 2 - Na 2 O -
K 2 O - CaO - MgO - P 2 O 5 - B 2 O 3 have been used to grow bioactive coatings. Although
the PLD coating technique allows the congruent transfer of the bulk glass com-
position to the coatings [Liste, 2004a, 2004b], FTIR and XPS analyses show
important differences between the bulk and the fi lm bonding confi guration.
Figure 11.21 shows the FTIR spectra of a typical bioactive glass target and the
corresponding coating. In both cases, the main peaks of bioactive glass [González,
2002 ; Serra, 2002 ] are observed:
1. a band at 1000 - 1200 cm − 1 assigned to the Si-O-Si asymmetric stretching
vibration,