Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 5 – Alkali surface treatment effects
mineralization process has been suggested to occur in living organisms. [378] Thus,
alkali surface treatment improved the surface mineralising potential of the composites,
which decreases in presence of proteins.
Significant more serum proteins adsorbed onto the treated composites as compared
to the non–treated one. The better performances in protein adsorption may be
assigned to the larger exposure of apatite particles and/or to the improved surface
nano–texture of the surface–etched materials. However, despite its larger nano–
roughness M2 adsorbed less serum proteins than M1. This may be connected to the
larger surface hydrophilicity of M2 compared to M1. It has been suggested that the
formation of a layer of water on hydrophilic surfaces generates an energy barrier for
the adsorption of most proteins [379, 380, 381] because they need to change their
conformation to reach an energetic equilibrium with the surface. However, some
proteins such as albumin are reported to be conformationally ‘flexible’ protein able to
adsorb also on hydrophilic surfaces. [382]
As discussed earlier, surface mineralization occurred in SBF/FBS solution generating
apatite nano–textured surfaces which offered more adsorption sites to proteins for
their adsorption. At the same time surfaces in contact with serum alone did not
mineralize and thus had less adsorption sites available (i.e. only the apatite initially
exposed on their surfaces). Consequently, surface mineralization significantly
increased the adsorption of serum proteins and albumin in all the considered
materials. This may be related to the apatite based chemistry and nano–structure of
the precipitated layers.This fact is strengthened by the reporting that proteins, such as
fibronectin and vitronectin, adsorb in larger amounts onto rougher surfaces, [383, 384]
and they are calcium–affine. [153, 250, 251, 346]
5.5. Conclusion
Summarizing, the roughest material (i.e. M2) adsorbed proteins, generated a
mineralised surface and was able to trigger osteogenic differentiation. In view of these
properties we expected it would have at least initiated bone formation but it did not.
This study showed that surface roughness is a parameter that should be kept in mind
when designing biomaterials as it affects the final material properties and can have
implications on its biological performances, both in vitro and in vivo. However, it is
necessary to carefully consider the fact that in vitro biological systems (e.g. cell
culture, protein adsorption) used to study the biomaterial may lead far from the real in
vivo performance of the material.
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