Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 5 – Alkali surface treatment effects
(Figure 5; Table 6). Consistently to these observations, the bulk of all three samples
was similar (i.e. dense at BSEM, Figure 5) and the polymer phase hydrolysis was
similar (i.e. comparable decreases in intrinsic viscosity, Table 2). At the same time,
similar decreases in the apatite layer thickness around M1 and M2 were seen (Table
4). All these observations indicate that the more pronounced ion release from the
treated materials is likely caused by the dissolution of the surface apatite particles,
which were exposed in larger quantities in M1 and M2. Summarizing, the alkali
treatment influenced on the ion release as it allowed exposure of apatite particles on
the surface and their direct contact with fluids.
It has been suggested that calcium and phosphate ions also favor surface
mineralization, and it is well known that the presence on the surface of some
functional groups, particularly those polar such as carboxyl and hydroxyl, renders the
surface more hydrophilic and positively influences on the spontaneous apatite
precipitation onto the surfaces. [245] Hydrophilic surfaces were reported to favor surface
mineralization because they enable better coverage of the surface by the body fluids
and significantly accelerate the nucleation and precipitation resulting in deposited
mineral. [410] Consistently, we observed that M0, as compared to the more hydrophilic
M1 and M2, had lower mineralization potential. After one week in SBF, significant
increases in the thickness of apatite layers surrounding granules were observed for all
the three materials (Table 4). We also saw that in vitro mineralization occurred at
similar rate for M1 and M2 indicating that increasing the alkali treatment strength or
the surface hydrophilicity did not have any role on the in vitro surface mineralizing
potential. XRD and CR highlighted the presence of apatite also on M0 surface even if
no clear layers were seen at BSEM (Figure 2). Most likely, apatite was partially
embedded in the polymer at the surface of the composite. Considering this fact, we
conclude that the sole (full or partial) exposure of apatite particles on the surface is
sufficient to trigger in vitro mineralization in SBF.
Interestingly, we observed that the surface mineralization rate of the composites is
decreased when the samples were placed in SBF containing serum. This can be
correlated with the fact that the apatite nucleation sites, in this situation, are also
binding sites for proteins. Besides adsorbing onto the exposed apatite initially present
on the composites surface, serum proteins may have adsorbed also on the growing
mineralized layers. When proteins bound to the apatite nucleation sites (i.e. the
starting exposed apatite and/or the precipitated apatite), they may have reduced their
availability for further apatite precipitation and growth. This fact was already reported
in literature for hydroxyapatite and biphasic calcium phosphate ceramics. [376, 377] In
particular, it was observed that albumin, which is the most abundant protein in serum,
plays a crucial role in this inhibitory process. [376, 377] This protein–controlled
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