Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 7 – Polymer molecular weight and instructive composites
reported for various materials, mainly sintered calcium phosphate ceramics [222–228]
but also in cements, [229, 230] coatings, [231, 232] porous BioglassTM , [233] surface–treated
titanium [234] and more recently in composites of hydroxyapatite and polymer. [235]
Numerous hypotheses have been formulated to explain the osteoinductivity of
materials. For example, when composites of apatite nano–particles and poly(D,L–
lactide) with various content ratios were intramuscularly implanted in dogs,
heterotopic bone formed only in the highest apatite containing formulation. The high
apatite presence was proposed as the main osteoinduction trigger because it
generated a rougher and disordered surface micro–topography (see Chapter 3 in
this thesis), which may have positively affected (stem) cell differentiation into the
osteogenic lineage. [195] Further, it could have promoted the adsorption of proteins
with calcium affinity such as vitronectin and bone morphogenetic protein, which
favour osteoblast adhesion and bone formation. [153, 250, 251, 346] The high apatite
content also increased calcium ion release from the composite, which may have
saturated the surrounding fluids and induced the precipitation of calcium phosphate
minerals leading to mineralized surfaces, [227, 234, 324] while the rougher surface
topography may have increased the number of nucleation sites [245] for enhanced
bio–mineralisation. In this way, a set of surface conditions for in vivo synthesis of
new bone matrix by the composites may have been met. [226, 228, 234]
Attempting to design biomaterials able to support, or even trigger, bone tissue
regeneration and to mechanically support physiological stresses, research has
striven to fully or partially mimick the bi–phasic composition of bone (i.e. collagen
matrix and carbonated nano–apatite) [21, 22] and its structure by developing composite
biomaterials. Synthetic or natural polymers, thanks to their viscoelasticity, may be
used in sites with mild mechanical stresses, such as cartilage or tendons. On the
contrary, sites when stiffness is required together with damping characteristics such
as in bone, filling polymers with inorganic particulate may be an interesting solution.
Further, the presence of inorganic filler in the polymer would enhance the biological
properties of the materials.
At the molecular level, a(n amorphous) polymer material is an entangled network of
disordered interpenetrating randomly coiled macromolecules. When the polymer is in
contact with a liquid (e.g. water), its chains can move to reach an energetic equilibrium in
the liquid–polymer contact eventually allowing more or less fluid penetration. [347, 348]
When the molecular weight is high, the polymer material is less prone to uptake fluids.
This phenomenon assumes particular importance for polyesters as they degrade
hydrolytically. After liquid uptake, they undergo random chain cleavage, which increases
with the amounts of fluids uptaken. During hydrolysis, more fluids are taken up and
catalyse the cleaving process while free carboxylic acid groups released from the
cleaved chains decrease the surrounding pH further enhancing the degradation. [349, 350]
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