Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 4 – Control of mechanical and degradation properties in composites
Remarks
This Chapter will introduce to composite materials comprising apatite particles in
polymer, which were prepared with extrusion that will be used throughout the next
Chapters of this thesis. The extrusion process was optimized during pilot studies (not
reported in this thesis) according to some requirements. The main requirements were:
1) maximize the homogeneity of apatite distribution in the polymer matrix, 2) minimize
the degradation due to processing of the polymer phase, and 3) facilitate the
manufacturing steps (e.g. the green body should flush out of the extruder in a solid
state, and not as quasi–liquid). To reach an equilibrium amongst these requirements,
the extrusion temperature, the screw rotatory speed and the extrusion duration were
the optimized parameters. A range of various polymers was used and, on the basis of
the results, some were chosen to manufacture the composites in this thesis.
4.1. Introduction
Ancient people already treated bone fractures by realigning the parts and joining them
with metal sticks inserted into the medullary canal. An example is the Egyptian
mummy Usermontu (656–525 BC), which was found with a metallic leg prosthesis in
its femora. [170] During the last century metals and their alloys have been used in
fixation devices and supporting structures for fracture healing and joint
replacement, [328, 329] such as stems and acetabulum cups of various commercial hip
prostheses. It is well–known that bone, in response to the surrounding mechanical
stimuli, adapts its anatomical structure through natural growth and resorption
processes. [330] Therefore, by virtue of their higher stiffness than bone tissue, metal–
based implants cause mechanical stress shielding in bone provoking its resorption,
ultimately leading to prosthesis loosening and/or osteolysis. [331]
To avoid mechanically–induced bone resorption, scientists have striven to develop
biomaterials fully or partially mimicking the bi–phasic composition of bone and its
structure. In general, bone tissue comprises a collagenous matrix reinforced by nano–
particles of carbonated apatite. [21, 22] Synthetic or natural polymers have viscoelastic
characteristics and may be used alone in sites where mild mechanical stresses exist,
such as in soft tissues like cartilage or tendons. On the contrary, in sites where
stiffness is required together with damping abilities such as bone, filling polymers with
inorganic particulate may be an interesting solution. Some scientists replicated in vitro
the process of collagen mineralization and obtained materials chemically and
hierarchically mimicking bone tissue, [213] while others synthesized fibrous silk macro–
porous blocks containing calcium phosphate. [214] Micro– or nano– (hydroxy–)apatite
has been added to synthetic polymers generating materials not only with mechanical
characteristics comparable to those of bone, [239, 240] but also capable to act as
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