Page 374 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMEDICAL COMPOSITES 351

friction in a moving part, such as in orthopedic or dental composites, can cause abrasion of the
matrix and produce new voids at the interface, exposing the reinforcing material to the host.
The interaction of materials at the interface is integral to composite performance, and this can be
affected by the tissue response in various ways, such as filling and swelling of interfacial voids with
fluid and fibrous tissue, altering the interfacial adhesion strength between matrix and reinforcement,
and delamination of laminate composites. Such effects can lead to failure of a composite, particularly
in structural applications.
Thermosetting polymers, although uncommon in biomedical implants, may contain unreacted
monomer and cross-linking agents, particularly in laminated composites made from prepreg layers.
In both thermosetting and thermoplastic polymer composites, the sizing applied to glass and carbon
fibers is another compound that may be present, and some residual solvents may also leach from the
matrix if they are not completely removed during processing. These trace amounts may not be an
issue if the application is external to the body, as in prosthetic limbs.

14.8 BIOMEDICAL APPLICATIONS

The use of composite materials in biomedical implants and devices is illustrated by the following
examples of structural applications.

14.8.1 Orthopedic
Composite materials have found wide use in orthopedic applications, as summarized by Evans, 2
particularly in bone fixation plates, hip joint replacement, bone cement, and bone grafts. In total hip
replacement, common materials for the femoral-stem component such as 316L stainless steel,
Co-Cr alloys, and Ti-6A1-4V titanium alloy have very high stiffness compared with the bone they
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replace. Cortical bone has a stiffness of 15 GPa and tensile strength of 90 MPa. Corresponding
values for titanium are 110 GPa and 800 MPa, which are clearly very high. This produces adverse
bone remodeling and stress shielding, which over the long term leads to reduction in bone mass and
implant loosening, specially in the proximal region. Fiber composites can be tailored to match the
specific mechanical properties of the adjacent bone. Carbon-fiber composites in PEEK or polysul-
fone matrices can be fabricated with stiffness in the range 1 to 170 GPa and tensile strength from
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70 to 900 MPa. Examples are press-fit femoral stems made from laminated unidirectional carbon
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fibers in PEEK, polysulfone, liquid crystalline polymer (LCP), and polyetherimide (PEI). These
composites are difficult to fabricate and have not had very encouraging durability, but they continue
to be developed for the inherent advantages of tailorability, flexibility, noncorrosiveness, and
radiolucency. 12 Problems with biocompatibility due to particulate carbon debris from these compos-
ites have been addressed by polishing and coating with hydroxyapatite (Fig. 14.6) or carbon-titanium
alloy. 13
For fracture fixation, a fully resorbable bone plate is desirable to avoid the need for a second
operation to remove the implant after healing. This in the form of a tailored low-stiffness composite
also avoids the problem of stress shielding described earlier. The rate of degradation must be
controlled to maintain the mechanical properties such that strength loss in the implant mirrors
strength increase in the healing. In addition, the degradation by-products must be nontoxic. A sum-
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mary of the design of adsorbable fixation devices is provided by Pietrzak. Examples of composite
bone plates include laminated continuous carbon fiber in a polylactide (PLA) matrix, which is
partially adsorbable, and calcium-phosphate glass fibers also in PLA, which is fully resorbable. 15
Continuous poly (L-lactide) fibers in a PLA matrix also produced a fully resorbable composite. 16
These composites, however, did not have adequate mechanical properties and degraded quite
rapidly. Nonresorbable carbon-epoxy bone plates with sufficient strength and fatigue properties are
available from such manufacturers as Orthodesign, Ltd.