Biomedical Engineering Reference
In-Depth Information
essential after the chemical and mechanical charac-
terizations are performed.
(a)
9.5 Substrate Analysis Method
Several competing factors must be considered
when coating PEEK implants. Mechanical and
chemical properties of the PEEK-based substrate
should remain unchanged after the coating process,
or should be at least kept within certain safety limits.
In addition, the adjustment of the plasma process
parameters to maximize coating adhesion or to
optimize roughness or porosity might negatively
affect the substrate characteristics. A compromise
among divergent inputs is therefore necessary.
Many test methods are used to investigate the
substrate physical properties, and a complete list
would go beyond the scope of this chapter. The
following list gives an example of some methods that
are helpful in examining some characteristics of
PEEK-based materials, bearing in mind that the most
important features to be investigated are those
potentially influenced by the coating process, namely
mechanical strength, fatigue strength, thermal prop-
erties, and chemical degradation.
The standards ISO 527 and ASTM D638 are
widely known methods for the determination of the
tensile properties of plastics and fiber-reinforced
plastic composites
[73,74]
, and are thus applicable to
PEEK and to carbon-fiber-reinforced PEEK. The
standards, which are technically equivalent, specify
the testing of standard dumbbell-shaped test speci-
mens, as illustrated in
Fig. 9.8
, under defined condi-
tions. The determined tensile strength (expressed in
MPa), tensile modulus (expressed in MPa or GPa),
and tensile elongation-to-break (expressed in percent)
are material-related characteristics, and do not depend
(b)
(c)
Figure 9.7
Graphical
representation of
the three
roughness parameters
R
t
. During one
measurement, the evaluation length is subdivided into
n
R
a
,
R
z
, and
sampling lengths:
R
a
and
R
z
are the average of
n
values, whereas
R
t
is a single value.
R
t
is always greater
than
R
z
, which in turn is always greater than
R
a
.
properties of PEEK compared with the common
orthopedic metallic alloys.
Before any new material is used clinically, it must
also be evaluated from the biological point of view,
for example through the methods recommended in
ASTM Practices F748 and F981 or in the set of
standards listed in ISO 10993-1
[70
e
72]
. This also
applies in the case of any newly developed coatings,
because the biological response is surely going to be
influenced by the implant surface properties. Prop-
erly designed in vitro and in vivo tests may be
Figure 9.8
Dumbbell-shaped specimen for
tensile
testing as specified in ISO 527-2 (Type 1B).
