Biomedical Engineering Reference
In-Depth Information
applications. Flame treatment, for example, is known
to alter the bulk mechanical properties, and other
treatments that involve high temperature may have
the same effect
[98]
. Coatings are prone to delami-
nate in aqueous environments such as the body and
the interface is often poorly bound, making handling
an issue during implantation
[99]
. Many of the
surface modifications involving incorporation of
chemical functional groups are prone to aging
effects, where the modified layers of the polymer
may reorientate into the surface, reverting to
a surface close to the unmodified polymer
[30,100,101]
. Grafted surfaces may also be unstable,
where the bonding may not withstand the handling
needed to implant medical devices. In addition, as
mentioned in Section
10.1
, for biomedical applica-
tions certain criteria must be met. Two of the major
problems for a modified surface are that it must be
able to withstand sterilization, whether that be
ethylene oxide, steam, or gamma, and the second is
that these modified surfaces must not age with time,
as most implants stay on the shelf for a substantial
amount of time before implantation, but also when
implanted, the surface should not be altered drasti-
cally. From these perspectives, we have been
working on a surface modification that is stable to
such parameters.
The focus of our research has therefore been to
identify a stable surface modification of PEEK-
OPTIMA
LT1 (PEEK; Invibio Biomaterial Solu-
tions, UK) by oxygen plasma to increase the surface
energy, with the aim to alter the cellular and tissue
interactions, for orthopedic applications
[45,107]
.
The creation of low-molecular-weight oxidized
material has been identified as one of the main
reasons for oxygen plasma treatment being unstable.
Figure 10.3
shows the effect of oxygen plasma
treatment on the wettability of PEEK.
After exposure to the oxygen plasma, some
surfaces were washed, which reduced the surface
oxygen concentration by removing the low-
molecular-weight debris
[107]
. This low-molecular-
weight debris may result from chain scission from the
plasma treatment. Therefore, the differences observed
in the washed and unwashed surfaces after plasma
treatment are likely to be due to removal of low-
molecular-weight oxidized material. There was an
increase in surface oxygen concentration with plasma
exposure and subsequent slight reduction in surface
oxygen after washing. This agrees with other reported
studies describing the formation of low-molecular-
weight oxidized material following plasma treatment
[30,31,98,105]
. The saturation level, determined from
the unwashed samples, was achieved at treatment times
>
250 s, which corresponded to a steady state between
oxygen incorporation and the loss of volatile photolysis
products from the PEEK surface
[32,83,87,98]
.This
process is accompanied by the formation of low-
molecular-weight oxidized material, which would
have a detrimental effect upon the cells and thereby
tissue interactions
[87]
, and as previously reported,
may have an adverse effect on the mechanical prop-
erties of the surface
[32]
.
The PEEK surfaces were exposed to the oxygen
plasma for various treatment times from 10 s up to
2400 s. Upon exposure to the plasma for only 10 s,
the surface oxygen increased by ~3.5 atom % for the
10.4.1 Oxygen Plasma Surface
Modification
Oxygen plasma surface modification is known to
increase cytocompatibility without altering bulk
mechanical properties to polymers
[26,76,83,
85,102
e
104]
, including PEEK
[19,34]
. However,
surface modification with oxygen plasma has been
reported to be unstable, where the surface of the
polymer reverts or rearranges with time
[30,98,
105,106]
.
Figure 10.3
Water contact angles of injection-molded PEEK: (a) untreated, (b) 600 s plasma-treated PEEK unwashed
showing autoflow due to the presence of low-molecular-weight oxidized material, (c) 600 s plasma-treated PEEK after
stabilization by washing.
