Biomedical Engineering Reference
In-Depth Information
propylmethacrylate for HA and acrylic acid for HDPE might be used
to improve bonding (by both chemical adhesion and mechanical
coupling) between HA and HDPE [268, 269]. Obviously, other calcium
orthophosphates might be used instead of HA in biocomposites with
PE [270]. Furthermore, attempts were performed to improve the
mechanical properties of HAPEX™ by incorporating other ceramic
phases into the polymer matrix, such as PSZ [271] and alumina
[272]. For example, a partial replacement of HA filler particles by
PSZ particles was found to lead to an increase in the strength and
fracture toughness of HA/HDPE biocomposites. The compressive
stress, set up by the volume expansion associated with tetragonal
to monoclinic phase transformation of PSZ, inhibits or retards the
crack propagation within the composite. This results in an enhanced
fracture toughness of the HA/ZrO
/HDPE biocomposite [271].
Various studies revealed that HAPEX™ attached directly to bones
by chemical bonding (a bioactive fixation), rather than forming
fibrous encapsulation (a morphological fixation). Initial clinical
applications of HAPEX™ came in orbital reconstruction [273] but
since 1995, the main uses of this composite have been in the shafts
of middle ear implants for the treatment of conductive hearing loss
[274, 275]. In both applications, HAPEX™ offers the advantage of
2
in
situ
shaping, so a surgeon can make final alterations to optimize the
fit of the prosthesis to the bone of a patient and subsequent activity
requires only limited mechanical loading with virtually no risk of
failure from insufficient tensile strength [102, 202]. As compared to
cortical bones, HA/PE composites have a superior fracture toughness
for HA concentrations below ~40% and similar fracture toughness in
the 45-50% range. Their Young's modulus is in the range of 1-8 GPa,
which is quite close to that of bone. The examination of the fracture
surfaces revealed that only mechanical bond occurs between HA and
PE. Unfortunately, the HA/PE composites are not biodegradable, the
available surface area of HA is low and the presence of bioinert PE
decreases the ability to bond to bones. Furthermore, HAPEX™ has
been designed with a maximized density to increase its strength
but the resulting lack of porosity limits the ingrowth of osteoblasts
when the implant is placed into the body [29, 203]. Further details
on HAPEX™ are available elsewhere [102]. Except of HAPEX™, other
types of HA/PE biocomposites are also known [276-282].
Both linear and branched PE was used as a matrix and the
biocomposites with the former were found to give a higher modulus
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