Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 8 – General discussion
8.1.8. Other possible factors: monomer content and polymer (semi)–crystallinity
It should be mentioned that other intrinsic characteristics of the polymer should not be
excluded from the set of possible factors influencing the properties of the composites.
For instance, it has been reported that the monomer content after extrusion varies,
presumably increases, [336, 411] facilitating the fluid uptake and thus the polymer
hydrolysis. This fact may not only affect the mechanical properties, particularly in
moistened conditions (Chapter 4), but also the instructive potential of the composites.
Another intrinsic feature is the polymer (semi)–crystallinity, which is dependent on
the processing steps, e.g. the cooling, [387] (Chapter 4). Crystallinity has been shown to
influence the mechanical performance, degradation and cell response to polymer
surfaces. [387, 412, 413] Thus, it is advisable to consider these factors as well in future
studies on composites and their performances.
8.1.9. Importance of fluid uptake in osteoinductive biomaterials
We have evaluated two classes of biomaterials, i.e. calcium phosphate ceramics and
composites, to investigate whether a common link between material properties and
osteoinduction exists. Based on our results, hydrophilicity improves the contact
between fluids and biomaterial by virtue of larger fluid uptake. As fluids carry various
molecules and ions, such improved contact would enhance biomolecule adsorption
and surface mineralization thanks to the higher probability of contact between the
biomaterial surface and such molecules. It is also proposed that it may enhance the
early cell response upon implantation, triggering macrophages to secrete cytokines
that later could induce bone induction. Further, absorbed fluids would accelerate the
polymer hydrolysis and calcium phosphate dissolution enhancing the degradation of
biomaterials and facilitating the release of calcium and phosphate ions. Degradation
of biomaterials may also generate changes at the surface, for example by exposing
more calcium phosphates or generating nano– or micro–porosity. The combination of
such phenomena triggered by fluid uptake contributed to the heterotopic bone
formation. However, we also observed that it was possible to apply this hypothesis
separately per each class of biomaterial, but it was not valid under a more general
‘biomaterial’ view (Chapter 6).
8.1.10. Closing remarks
A balance between biomaterial design complexity and feasibility (from research,
industrial and financial points of view) is necessary. We found that designing an
instructive biomaterial with proper dynamic mechanical properties, degradation rate
and osteoinductive potential is challenging because various factors are involved and
influence each other. In fact, we wanted to make a strong composite material able
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