Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 5 – Alkali surface treatment effects
This conclusion is strengthened by their similar in vitro degradation, mineralization
trends and serum protein adsorption (§5.3.3., §5.3.4. and §5.3.5.), indicating that no
other factors than surface roughness were involved in inducing the different cellular
performances.
In this study, the failure of the in vitro model to predict the in vivo performance of
composites indicates that, in biomaterials science, extrapolating in vitro biological
results (e.g. cell culture or protein adsorption) into in vivo performances is a complex
issue. In vitro models reduce the intrinsic variability present in living bodies simplifying
the system. For example, as mentioned in §5.2.8., in this study we decided to culture
cells onto discs to avoid cell seeding and nutritional inhomogeneities usually
associated to three–dimensional systems such as (porous) granules. Further, when a
material is placed in cell culture, it enters in contact with a closed system having
culture medium and its composition (i.e. proteins, nutrients) carefully chosen where
pH, carbon dioxide concentration and temperature are strictly stabilized. Besides this,
cells are chosen and no other undesired lines or organisms are present to interfere
with the used ones. On the contrary, if the same material is placed in the body, many
factors that are not included in an in vitro culture system play roles on the overall
performance of the material. For example, inflammation and foreign body reaction
attract other forms of cells such as monocytes and macrophages which then release
cytokines further recruiting other kinds of cell. [217, 256, 257] During this reaction, the
material may degrade due to phagocytosis [375] and may change its properties affecting
on its own performance. Further, body fluids have more heterogeneous composition
than culture medium rendering the in vivo interaction between cells and materials
surface more complex than in vitro.
However, in this study we demonstrated that alkali treating the surface of the
composite with sodium hydroxide solutions generated differently disordered nano–
rough surfaces. In particular, the stronger treatment led to topographically disordered
surfaces with higher roughness (Figures 3 and 4, Table 5). The enhanced surface
nano–structure is likely caused by the hydrolysis of the polymer matrix [366] and by the
consequent larger exposure of apatite on the surface (as shown by thicker apatite
layers surrounding M1 and M2, Figure 2). The water contact angle of the composites
diminished with the strength of the surface treatment indicating an increase of their
polar character. This may be partially caused by the formation and exposure of polar
groups due to the polymer hydrolysis provoked by the alkaline treatment, [366] and
partly by the exposure of larger amount of hydrophilic apatite.
The composites degraded over three months as indicated by their mass loss (Table
6), which resulted similar with each other. However, higher concentrations of calcium
and phosphate ions in SPS containing M1 and M2 as compared to M0 were observed
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