Biomedical Engineering Reference
In-Depth Information
greater than its cross-sectional dimensions. Particle-
reinforced composites are sometimes referred to as
particulate composites.
Fiber-reinforced composites are,
understandably, called
fibrous composites. Laminates
are
composite structures made by stacking laminae of fiber
composites oriented to produce a structural element.
Characteristics, number, and orientation of laminae are
such as to match specific design requirements.
Fibers are much more mechanically effective than
particles, and polymer-fiber composites can reach stiff-
ness and strength comparable to those of metals and even
higher.
Moreover, whereas particle-reinforced composites are
isotropic, fiber-reinforced composites are basically an-
isotropic. Properties in different directions can be in
most cases designed to match specific requirements.
At the molecular and microstructural level, tissues
such as bone and tendon or vessels are certainly com-
posites with a number of levels of hierarchy. Their
properties are highly anisotropic, and the only possibility
to mimic them is to use composites.
Failure of a composite material implant can expose
fibers or particles to the surrounding biological environ-
ment. In many cases failure in composites is preceded by
the failure of the interface between filler and matrix, this
being due to idrothermal aging or stresses exceeding the
interface strength. Sterilization methods or conditions
can play an important role.
As with all biomaterials, the question of biocompati-
bility (tissue response to the composite) is paramount.
Being composed of two or more materials, composites
provide enhanced probability of causing adverse tissue
reactions. Also, the fact that one constituent (the rein-
forcement) usually has dimensions on the cellular scale
always leaves open the possibility of cellular ingestion of
particulate debris that can result in either the production
of tissue-lysing enzymes or transport into the lymph system.
Although durability and biocompatibility can be con-
sidered major issues in a composite medical device,
composites offer unique advantages in terms of design
ability and fabrication. These advantages can be used to
construct isocompliant arterial prostheses (
Gershon
et al.
, 1990, 1992
), intervertebral disks duplicating the
natural structure (
Ambrosio
et al.
, 1996
), or fixation
plates and nails with controlled stiffness (Veerabagu
et al
., 2003).
For some applications, moreover, radiolucency is con-
sidered to be a further potential advantage. An example is
external or internal fracture fixation devices not shielding
the bone fracture site from the X-ray radiography.
Design flexibility, strength, and lightweight have made
polymeric composite materials, mostly carbon fiber
reinforced, the ideal materials also for orthotic aids able
to return walking and even athletic performances to
impaired people (
Dawson, 2000
).
Reinforcing systems
The main reinforcing materials that have been used in
biomedical composites are carbon fibers, polymer
fibers, ceramics, and glasses. Depending upon the ap-
plication, the reinforcements have been either inert or
absorbable.
Carbon fiber
Carbon fiber is a lightweight, flexible, high-strength,
high-tensile-modulus material produced by the pyrolysis
of organic precursor fibers, such as rayon, poly-
acrylonitrile (PAN), and pitch in an inert environment.
The term carbon is often indifferently interchanged
with the term graphite, but carbon and graphite fibers
differ in the temperature of fabrication, thermal treat-
ment, and the content of carbon (93-95% for carbon
fibers and more than 99% for graphite fibers). Because of
their low density (depending on the precursor from 1.7
to 2.1 g/cm
3
) and high mechanical properties (elastic
modulus up to 900 GPa and strength up to 4.5 GPa,
depending on the precursor and on the fabrication
processdhence they can be much stiffer and stronger
than steel!) these fibers are used in composites in a vari-
ety of applications demanding lightness and high me-
chanical properties. Their disadvantage is that carbon
fibers have poor shear strength.
In medicine, several commercial products have used
carbon fibers. Some of the first devices, however, have
experienced
severe
negative
effects
and
have
been
recalled from the market. Two examples are:
Short carbon fiber reinforced UHMWPE for
orthopedic applications. The assumption was that
increase of strength and decrease of creep would
increase the bearing longevity. The favorable
indications of the laboratory wear tests contrasted
with the
in vivo
results: Many patients presented
with osteolysis and failure of the tibial inserts (
Kurtz
et al.
, 1999
).
In the 1980s carbon fibers were used to develop
a scaffolding device to induce tendon or ligament
repair. The low shear strength of fibers caused fiber
breakage and the formation of harmful debris.
A resorbable polymeric coating was somewhat
successful in preventing carbon fiber breakage and
localizing debris. However, because of poor
performance and permanent wear debris in the joint,
the carbon fiber device was not approved by the FDA
for ACL reconstruction (
Dunn, 1998
).
In spite of these early failures, however, carbon fibers
display unique properties for the fabrication of load-
bearing medical devices.
