Biomedical Engineering Reference
In-Depth Information
Chapter 6
Chemical and Radiation Stability of PEEK
Steven M. Kurtz Ph.D.
6.1 Introduction to Chemical
Stability
susceptible to hydrolysis, concerns have been raised
that the interface between the polymer and rein-
forcements, such as carbon fiber, may be vulnerable
to fluid environments in vivo [4] . In theory, water can
infiltrate polymer composites by two potential
mechanisms, diffusion and wicking [5] . With PEEK
and its carbon fiber composites, water absorption
follows an exponential relationship over time,
consistent with a Type I Fickian diffusion process [3] .
As water infiltrates the composite, differential
swelling of the fiber and matrix may occur, poten-
tially leading to the slight changes within the
The structure of polyaryletherketones, including
polyaryletheretherketone (PEEK), confers out-
standing chemical resistance (Fig. 1.3). The aryl
rings in PEEK are interconnected via ketone and
ether groups located at opposite ends of the ring
(referred to in chemistry as the “para” position). The
resonance-stabilized chemical structure of PEEK
results in delocalization of higher orbital electrons
along the entire macromolecule, making it extremely
unreactive and inherently resistant to chemical,
thermal, and postirradiation degradation. We have
already noted in Chapter 2 that PEEK cannot be
damaged by exposure to solvents except for
concentrated sulfuric acid. The inherent inertness of
PEEK's chemical structure also explains its
biocompatibility, which will be more fully discussed
in Chapter 7.
PEEK implants must be sterilized prior to
implantation and must also withstand years of
exposure to aqueous environments at body tempera-
ture. This chapter summarizes the literature as it
relates to the stability of PEEK in water-based
environments. We also review the thermal and radi-
ation stability data for PEEK and discuss the rami-
fications of
inter-
phase
region of the matrix within a few micrometers
adjacent to the reinforcing fiber [5] . Boinard and
coworkers [3] have found evidence that water
absorption may slightly reduce the crystallinity of
PEEK.
Although water sorption has negligible effects on
the bulk behavior of PEEK [1 e 3] , it has nonetheless
been considered important to account for fluid
exposure in biomechanical testing of PEEK
composite materials for implants [6 e 8] , especially if
slight changes in weight are going to be used as
a marker for material loss, as in a wear experiment.
Presoaking specimens for 30 days has been estimated
to account for 98% of fluid absorption by PEEK prior
to conducting a long-term mechanical test [8] . Cyclic
compression fatigue experiments conducted on
carbon fiber-reinforced PEEK (CFR-PEEK) compos-
ites in saline at temperatures ranging between 37
and 95 C have shown no significant change in
compressive modulus, Poisson's ratio, and compres-
sive strength after 5000 h of testing [8] . Several other
studies have similarly observed that no significant
changes occur to flexural mechanical properties in
PEEK composites after exposure to high-temperature
saline environments [6,7] .
the material's
stability for
typical
sterilization techniques for medical devices.
6.2 Water Solubility
PEEK has a water solubility of 0.5% w/w but as
mentioned earlier is not chemically damaged by
long-term water exposure, even at temperatures of up
to 260 C [1 e 3] . Although PEEK itself
is not
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