Biomedical Engineering Reference
In-Depth Information
polymers in bone tissue engineering include polyethylene (PE), polyam-
ides (PAs), poly(methyl methacrylate) (PMMA), poly(ether ether ketone)
(PEEK), polypropylene (PP), and selected polyurethanes (PUs). These poly-
mers are also known to be biostable in the human body and employed in a
wide range of biomedical applications. For example, ultra-high-molecular-
weight polyethylene (UHMWPE) for acetabular cups [24-26]. However, it
is to be noted that each polymer material has its own characteristic advan-
tages and disadvantages. Composites and nanocomposites can offer a
suitable set of properties which often show an excellent balance between
strength and toughness, and usually possess improved characteristics com-
pared to their individual components. The desired advances are primarily
related to improving their biocompatibility and performance, both of which
are already remarkable in terms of actual clinical applications. Since natural
bone is an organic/inorganic hybrid material, made of collagen and apatite,
composites consisting of a polymer matrix and apatite-based nanoparticles
seem to be suitable candidates for bone tissue engineering applications.
5.3.1 PolyethyleneNanocomposites
Polyethylenes (PE), particularly in its high-density (HDPE) or UHMWPE
(ultra-high-molecular-weight) form are used as polymer matrices for the
preparation of nanocomposites with HAp. UHMWPEs are linear chains with
very high molecular weight in the range of between (2-10) × 10
6
Da. These
have very high wear resistance, chemical resistance and low coeffi cient of
friction, and are self-lubricating, and can be processed by either sintering,
compression molding or by extrusion [27]. However, the shorter lifetime of
UHMWPE is one of the major drawbacks which limit its application for total
hip replacement. Numerous attempts have been made to improve UHMWPE-
based devices' life-time, as well as incorporation of ceramic nanoparticles for
improvements in other parameters such as reinforcements, high-temperature
recrystallization and crosslinking [28]. PE allows large amounts of bioceramic
particles to be incorporated into the matrix via melt-processing using cur-
rent technologies. Particulate HAp-reinforced HDPE composites have been
developed since the early 1980s [29] for bone replacement and commercial-
ized (HAPEX) by Smith & Nephew. They were the fi rst bioactive ceramic/
polymer composites designed for mimicking the structure and properties
of bone, and have supported research and development of other bioactive
composites using the same rationale [24]. The close elastic modulus match-
ing of HDPE/HAp composite to bone shows promise in solving the prob-
lem of bone resorption that has been encountered with the use of implants
made up of conventional materials, such as metals and ceramics, which
possess much higher modulus values than human cortical bone [30].
In another approach, synthetic HAp whiskers were utilized as rein-
forcement for orthopaedic biomaterials [31]. High-density polyethylene
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