Biomedical Engineering Reference
In-Depth Information
3.2 What Is a Composite Material?
metal. This is illustrated in
Fig. 3.3
, which compares
two CT images: the one on the left was made with an
implant constructed from “image contrast” grade of
PEEK-OPTIMA
from Invibio
with a moderate
ability to absorb X-rays and the one on the right was
made with a metallic implant that is completely
radiopaque. It is clearly seen that the image made
with a metallic implant contains significant artifacts
and suffers from a type of “starburst” pattern of lines
radiating from the implant site.
In terms of device manufacture, devices can be
fabricated from compounds and composites using
a variety of techniques including injection molding,
different types of compression/flow molding, extru-
sion, pultrusion, and filament winding, as will be
described later.
In summary, for reasons of biocompatibility,
tailored mechanical properties, and compatibility
with imaging technologies, as well as processing
considerations for device manufacture, PEEK is
becoming increasingly important as a commercial
and medical grade material for implantable devices.
This chapter focuses on the form and function of
CFR PEEK materials in relation to processing and
applications, specifically short carbon fiber
compounds and continuous fiber composites,
although the important range of X-ray contrast
grades of PEEK, based on the use of powdered
radiopaque fillers, as described earlier, will also be
discussed. Some general background information on
basic composite technology will be provided,
although the reader will be directed to further reading
for more in-depth theoretical treatments, wherever
applicable.
When two or more substances such as polymer,
fibers, or powder are combined at a microscopic
level, the resulting material may demonstrate
macroscopic physical properties that are
superior
to
those of either of the constituent parts. Each
component making up the material contributes its
unique physical property such that, when combined
with the second material, there is some beneficial
outcome. Such combinations may be termed as
composite materials (“composites” for short) or
compounds. The term “composites” is usually used
when the reinforcing component comprises long, or
continuous, fibers and the term “compound” when
the additive is in the form of discrete particles, such
as powder, flakes, or short fibers. The respective
manufacturing routes for each of these classes of
material and their resulting physical properties are
quite different, as will be explained in this chapter.
Classical examples of naturally occurring fibrous
composites
include wood and bone. It is well known
that wood is a combination of cellulose fibers in
a lignin polymer “matrix” and that bone is a combi-
nation of inorganic hydroxyapatite (calcium phos-
phate compound) and organic Type 1 collagen
(protein) fiber, but the point here is that, for each of
these materials, it is the specific combination and
interaction of their constituent parts that together
impart the observed desirable physical and mechan-
ical properties. Lignin alone cannot carry the
substantial physical load of mighty oak without the
support of cellulose fibers and bones would be poorly
adapted for their role in animal skeletal protection,
Figure 3.3
“Starburst” artifacts in
a CT image with metallic implant
(right) compared with PEEK-
OPTIMA image contrast grade
(left). Photo courtesy of Invibio.
