Compounds and Composite Materials - PEEK Biomaterials - page 31

Figure 3.11 X-ray of metallic and
composite screws with and
without markers implanted in
phantom. Photo courtesy of

Icotec.

them visible radiographically. This will be described
in more detail in Section 3.5.2.2 , although it is,
perhaps, appropriate to show a radiograph in this
section of this form of PEEK composite material.
Figure 3.11 compares the image contrast achieved
with CFR PEEK (containing wire markers) with that
for titanium and CFR PEEK (with no markers) under
X-ray inspection.

point is important because a material is of little use if it
cannot be processed to make the final device.
There are, then, several options available to the
materials technologist based on the design criteria for
the mechanical strength, or stiffness, of the material
in relation to its performance as a device and the
processing route selected to manufacture the device.
For implantable fiber-reinforced materials, these

options include the following:

Choice of fiber type (carbon, PAN or pitch)
Fiber length (short < 1 mm, long > 1mm < 6 mm,
continuous >> 6 mm)
Fiber volume fraction (low < 30% or high > 55%)
Processing (injection molding, hot pressing, etc.)

3.3 Additive Geometry, Volume,
and Orientation Effects

The reinforcing efficiency of any additive is
related to how the applied loads become redistributed
within the bulk of the material between the compliant
polymer matrix and stiff additive. Fibers are more
efficient than regular (spherical) particles at taking up
the load and the longer the fiber (greater length/
diameter ratio) the more capable the fiber is of taking
on more of the applied load. Continuous fibers,
therefore, have a greater reinforcing ability than short
fibers (length < 1 mm) and stiff fibers are more effi-
cient than compliant fibers. The theory of fiber
reinforcement is quite mature and numerous reviews
are available. A detailed treatment of stress analysis
in composites is available for the more ambitious
reader [30] .
The volume fraction of filler is also an important
determinant. High concentrations of filler (high-
volume fraction, or V f ) have a greater reinforcing effect
than low concentrations of filler. The objective of the
materials technologist is to achieve an appropriate
concentration of an efficient reinforcing additive while
still enabling the material to be processed into useful
artifacts by a convenient processing route. The latter

These are strongly interrelated, as illustrated in
Fig. 3.12 , which shows that for maximum mechan-
ical performance, such as may be needed for struc-
tural metallic replacement, it is desirable to use
materials with long/continuous fibers in relatively
high-volume concentrations (60 e 65%). Materials
with lower (but still useful) properties can be made
using shorter fibers in lower concentrations of around
20 e 30% and their usefulness extends to their ability
to be compounded and injection molded, with rela-
tive ease, using conventional polymer processing
equipment. A more detailed description of form,
function, and processing will follow.

3.3.1 Short CFR PEEK
As the name suggests, the fibers in these materials
are discontinuous. They exist as discrete entities
distributed throughout the polymer matrix and have
a distribution of length and orientation characterized