Biomedical Engineering Reference
In-Depth Information
yet commercially significant for implantable devices
and will not be considered further here.
For implantable materials, the most significant
fiber additive is carbon, because this class of mate-
rial, above others, is considered more appropriate for
introduction into living tissue. Indeed, a good deal of
reported work supports the biocompatibility of CFR
PEEK materials, as will be described in
Section 3.6
.
elastic modulus (stiffness). PAN-based carbon fibers
show a good balance of strength and stiffness, but
they are generally categorized as “high-strength”
fibers (up to around 6400 MPa) or “high-stiffness”
fibers (around 590 GPa).
Figure 3.5
illustrates in the
case of PAN fibers the wide variety of properties that
can be found for commercial products from various
suppliers. Most “general purpose” fibers are in the
3500 MPa strength/230 GPa stiffness range.
Pitch-based fibers can be very stiff (up to around
900 GPa) but have comparatively low strength, as
shown in
Table 3.1
. This makes them very difficult to
handle as continuous fibers because they can break
relatively easily during the various stages of
composite production. Some of the lowest strength
fibers with least stiffness are derived from pitch,
although these can have other attributes, such as good
wear resistance.
Carbon fiber-filled PEEK has been studied in some
detail as a bearing surface in prosthetic hip and knee
applications
[14
e
22]
and has been shown to be an
excellent alternative to ultrahigh-molecular-weight
polyethylene (UHMWPE). Invibio has commercial-
ized specific grades of PEEK-OPTIMA (under the
MOTIS
3.2.4 Carbon Fibers
Carbon fibers are made by two principal routes:
one route uses a synthetic organic polymer fiber as
the starting material (polyacrylonitrile, PAN, fiber)
and the other route uses coal tar pitch. The physical
arrangement of carbon atoms and the mechanical
properties of the resulting fibers from each of these
routes are somewhat different. Even for any one
specific manufacturing route the fiber properties such
as tensile strength and stiffness may be significantly
different for different products. These are set
according to the specific processing parameters
employed in their production, such as the source
material, carbonization temperature, and fiber
tension at specific stages in the manufacturing
process, because these factors influence the structure
of the carbonized material and degree of perfection in
the aligned graphite sheets.
Table 3.1
lists some typical values for PAN and
pitch-based carbon fibers from leading suppliers.
Fibers can be classified according to strength and
trade name) for such applications. Invibio
data in
Fig. 3.6
illustrate for a hip prosthesis the low
wear rate for MOTIS compared with UHMWPE
tested against ceramic and cobalt chrome counter
faces.
It is beyond the scope of this chapter to describe
science and technology behind carbon fibers and
their production methods. Indeed, there are good
reviews in the literature for those interested in
reading further. For example, Pierson provides an in-
depth account of carbon in all of its forms, including
fibers
[23]
.
Table 3.1
Carbon Fiber Properties for PAN and
Pitch Fibers
Tensile
Strength
(MPa)
Tensile
Modulus
(GPa)
Carbon
Fiber Type
3.2.5 Role of the Fiber/Matrix
Interface
The interface enables load transfer between fibers
and matrix. It is particularly important in deter-
mining the mechanical response of short fiber rein-
forced materials, because the discontinuous fibers
exist as more or less isolated entities within the
matrix that are disconnected with the outside world
except via the matrix and interface. Loads applied
from the outside environment must be “transferred”
to the individual fibers by some mechanism and
analysis shows that under tension this process occurs
as a result of shear stresses at a microscopic scale in
PAN (high strength)
3500e6400
230e300
PAN (high
elastic modulus)
4000e5500
370e590
Pitch (low
elastic modulus)
1100e2400
50e150
Pitch (high and
ultrahigh modulus)
3400e3600
620e900
PAN data sourcedTorayca.
Pitch data sourcedNippon Graphite Fiber Corporation.
