Biomedical Engineering Reference
In-Depth Information
vibrates and rotates due to thermal energy or in
response to an externally applied deformation. The
PEEK molecule is relatively stiff because of the
presence of the aromatic (benzene) rings along its
backbone (
Fig. 1.3
); however, the molecule does
have the freedom to rotate axially about the ether
(
e
O
e
) bonds and ketone-carbon bonds (
e
CO
e
).
When cooled slowly from the molten state, the
molecular chain can rotate upon itself to form chain
folds and to organize into ordered domains, known as
crystals. PEEK crystals are embedded within amor-
phous (disordered) regions and form a two-phase
microstructure (
Fig. 1.6
).
PEEK conforms well to the conceptual model of
a two-phase semicrystalline polymer, consisting of
an amorphous phase and a crystalline phase. Similar
to many semicrystalline polymers, including ultra-
high-molecular-weight polyethylene (UHMWPE),
the crystalline content of PEEK varies depending
upon its thermal processing history. The crystal-
linity of injection-molded PEEK in implants typi-
cally ranges from 30% to 35%
[9]
. By adjusting the
cooling rate during fabrication of films, crystalli-
zation of PEEK can be greatly reduced, resulting in
a nearly completely amorphous material. More
details about the crystallinity, microstructure, and
physical properties of PEEK can be found in
Chapter 4 of this
Handbook
.
Figure 1.5
Powder, pellets, extruded rods, and film of
unfilled “neat” PEEK.
applications, such as aircraft and turbine blades; for
example, see Ref.
[27,28]
. Its stability, biocompati-
bility, radiolucency, and mechanical properties make
PEEK a suitable biomaterial for orthopedic and
spine implants.
Unfilled “neat” PEEK is available as tan pellets or
powder (
Fig. 1.5
), which can be converted into
implant parts by standard polymer processing tech-
niques, such as injection molding. PEEK implants
are also fabricated by machining from extruded rods
or compression molded sheets.
Although neat PEEK has a tan appearance, when
PEEK powder is blended with carbon fibers for added
strength, the resulting material is black. For those
interested in additional details about manufacturing,
we explain the processing of PEEK for implants in
Chapter 2 of this
Handbook
.
1.5 Thermal Transitions
As described in a previous section of this chapter,
an important distinguishing feature of polymers is the
temperature dependence of their properties. In
general, upon heating many polymers undergo three
major thermal transitions: the glass transition
temperature (
T
g
), the melt temperature (
T
m
), and the
1.4 Crystallinity and PEEK
The molecular chain of PEEK may be visualized
as a tangled strand of spaghetti that is hundreds of
meters long. The molecular chain is not static but
Figure 1.6
Schematic representation
of PEEK microstructure, consisting of
amorphous and crystalline regions.
