Biomedical Engineering Reference
In-Depth Information
metals and ceramics are very high (Table 5.4). To overcome some of the limitations,
composites are developed by combining two or more components. As discussed in
Chapter 5, in a composite, one component forms the continuous phase in which
other components are embedded as discontinuous inclusions in the shape of plate-
lets, fibers, or particles.
A popular concept is using polymers as the matrix component in combination
with continuous carbon fibers to form reinforced composite materials. Polymer-
based composites possess a wide spectrum of properties, which allow them to be
used in a diverse range of medical applications. For example, titanium mesh is used
to reconstruct cranifacial defects where contours suitable to the patient have to
be formed. However, titanium mesh exhibits many sharp points when cut and the
edges can make insertion difficult. To minimize sharp edges when the implant is
cut, the titanium mesh is embedded on both sides with high-density polyethylene.
A composite of poly(ether ether ketone) (PEEK) polymer and short carbon fib-
ers is made to increase the strength of the natural unfilled polymer significantly.
The two components are processed into filaments, brought together into bundles,
and shaped into a rod. Carbon fiber loading of 30-35% by weight increases the
material's modulus from 4 to nearly 18 GPa and its tensile strength from 100 to
230 MPa. With a stiffness close to that of cortical bone, carbon-fiber-reinforced
PEEK composites are used in applications for which stress shielding may have
a critical effect on the lifespan of an implant. For example, hip stems are made
from carbon-filled PEEK compounds that demonstrate elastic properties similar
to the surrounding bone and that reduce the effects of stress shielding. Further,
the modulus of PEEK can be varied to suite to the requirement. This adaptability
reduces stress concentrations that can be transferred to the bone and stimulates
the healing process. Some of the composite biomaterials include dental composites
(acrylic polymer matrix with inclusions of inorganics such as quartz, barium glass,
and colloidal silica) and orthopedic components (high density polyethylene matrix
with inclusions of carbon fibers). While forming the composite, the process used
should produce good bonding strength between two phases. For example, when
carbon fiber-reinforced ultrahigh molecular weight polyethylene (UHMWPE) was
employed clinically in tibial components, it failed catastrophically. UHMWPE fail-
ures have been attributed to poor bonding strength between the carbon fiber and
the UHMWPE matrix.
Biodegradable composites can also be formed for use as bioactive matrices to
guide and support tissue in-growth. Composites are prepared using polyhydroxy-
butyrate (PHB), a naturally occurring
-hydroxyacid linear polyester, and as much
as 30% by volume of either hydroxyapatite (HA) or tricalcium phosphate (TCP).
One of the goals is to achieve a reasonably homogeneous distribution of the HA/
TCP particles in the PHB matrix, as this uniformity would provide an anchoring
mechanism when the materials would be employed as part of an implant. The
composites are manufactured through a compounding and compression molding
process. It is observed that microhardness increased with an increase in bioceramic
content for both the HA ad TCP compounds.
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