Biomedical Engineering Reference
In-Depth Information
used in the development of tibial bearing components
in a total knee arthroplasty device. Seventeen of 35
implants failed
[24]
. These failures were categorized
into three modes: breakout of mainly the posterior
lips, delamination, and
poor fiber
/
matrix adhesion
.
The latter point is important and it is worth
comparing laboratory measurements of the interfa-
cial strengths of PEEK and UHMWPE coupled with
carbon under different conditions and by different
methods.
The strength of the fiber/matrix interface
between carbon fibers and UHMWPE has been
studied under both dry and wet (saline) conditions
using single fiber pull-out tests
[25]
. The results by
this method indicate that the bond strength is
extremely low (7.1 MPa) in the dry condition and
even lower (4.5 MPa) in a saline environment. In
comparison, the interfacial shear strength of CFR
PEEK polymer has been measured to be 72.8 MPa
using an identical fiber pull-out method
[26]
.
Further, it has been reported to be 85
Figure 3.8
Electron micrograph of fractured barium
sulfate PEEK compound illustrating the uniform distri-
bution of powder.
The effect on the level of X-ray absorption (as
evidenced by increasing amounts of observable image
contrast) of adding greater amounts of barium sulfate
to PEEK can be seen in a standard configuration
shown in
Fig. 3.9
. Here the PEEK compounds con-
taining 4%, 6%, and 20% by weight of barium sulfate
powder have been molded into rectangular pieces at
different thicknesses ranging from 2 to 10 mm in
2 mm increments and assembled in the order shown.
Next to these is a machined aluminum “stepped
wedge” of the same dimensions by way of compar-
ison. It can be seen that the compound containing 20%
barium sulfate matches very closely the radiopacity of
the metal at like-for-like thicknesses. By selecting
29 MPa
using a more accepted transverse tensile test
method
[27]
, and between 74 and 112 MPa for CFR
PEEK polymer quenched after annealing
[28]
.
Other studies report even higher interfacial shear
strengths for CFR PEEK polymer (202 MPa)
determined from the tensile stress/strain behavior of
short CFR PEEK polymer
[29]
.
Taking the lesser of these values, the interfacial
strength between carbon and PEEK polymer is, on
average, at least an order of magnitude stronger than
that between carbon fibers and UHMWPE. This is an
important conclusion that supports the use of PEEK
in preference to UHMWPE in combination with
carbon fibers
for applications
such as bearing
surfaces.
3.2.6 Powder Additive for Image
Contrast
Barium sulfate (chemical formula BaSO
4
)
powder is efficient as an X-ray absorber. It is added
as a means of producing X-ray contrast, so that
normally radiolucent (X-ray invisible) PEEK arti-
facts are rendered some visibility under X-ray
inspection in vivo. It is introduced to PEEK poly-
mer by extrusion compounding (see
Section 3.4.1
for a description of this process) to achieve an even
distribution of particles, as illustrated in the elec-
tron micrograph of a fracture surface shown in
Fig. 3.8
.
Figure 3.9
PEEK-OPTIMA X-ray contrast grades
compared with aluminum-stepped wedge. Courtesy
of Invibio.
