Biomedical Engineering Reference
In-Depth Information
like [225-229] or needle-like [230-232] calcium orthophosphates,
as well as calcium orthophosphate fibers [49, 233].
The history of implantable polymer-calcium orthophosphate
biocomposites and hybrid biomaterials started in 1981 [234] from the
pioneering study by Prof. William Bonfield and colleagues performed
on HA/PE formulations [236, 237]. That initial study introduced a
bone-analogue concept, when proposed biocomposites comprised a
polymer ductile matrix of PE and a ceramic stiff phase of HA, and
was substantially extended and developed in further investigations
by that research group [102, 238-254]. More recent studies included
investigations on the influence of surface topography of HA/PE
composites on cell proliferation and attachment [255-261]. The
material is composed of a particular combination of HA particles at
a volume loading of ~40% uniformly dispensed in a HDPE matrix.
In addition, PP might be used instead of PE [262-264]. The idea
was to mimic bones by using a polymeric matrix that can develop
a considerable anisotropic character through adequate orientation
techniques reinforced with a bone-like bioceramics that assures
both a mechanical reinforcement and a bioactive character of the
composite. Following FDA approval in 1994, in 1995 this material
has become commercially available under the trade-name HAPEX™
(Smith and Nephew, Richards, USA), and to date it has been implanted
in over 300,000 patients with the successful results. It remains the
only clinically successful bioactive composite that appeared to be
a major step in the implant field [31, 265]. The major production
stages of HAPEX™ include blending, compounding and centrifugal
milling. A bulk material or device is then created from this powder
by compression and injection molding [63]. Besides, HA/HDPE
biocomposites might be prepared by a hot rolling technique that
facilitated uniform dispersion and blending of the reinforcements in
the matrix [266].
A mechanical interlock between the both phases of HAPEX™ is
formed by shrinkage of HDPE onto the HA particles during cooling
[102, 267]. Both HA particle size and their distribution in the HDPE
matrix were recognized as important parameters affecting the
mechanical behavior of HAPEX™ [247]. Namely, smaller HA particles
were found to lead to stiffer composites due to general increasing
of interfaces between the polymer and the ceramics; furthermore,
rigidity of HAPEX™ was found to be proportional to HA volume
fraction [239]. Furthermore, coupling agents, e.g. 3-trimethoxysiyl
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