Biomedical Engineering Reference
In-Depth Information
Figure 10.1 Clinical problemdexamination
of tissue interactions with PEEK implants
shows that there is typically only limited
direct bone contact to PEEK implants,
where the majority of the tissue interacting
with the PEEK is soft/connective tissue,
indicating encapsulation. This example
shows a histological slide with Giemsa e
Kosin staining of the tissue interactions with
a PEEK plate and a titanium screw visible in
the center of the image where the hard
tissue is stained pink and soft/connective
tissue blue (Rat-Fix
,AOResearch).
albumin, which do not aid cellular adhesion, could be
more abundantly adsorbed, as albumin adsorbs more
readily to hydrophobic surfaces. PEEK is often
described in the literature as being bioinert, implying
that it does not illicit a response from a biological
environment, whether that be a positive or negative
response [17,22,23] . However, Toth et al. [24]
described that there can be some direct bone contact
to PEEK implants, although bone was not found to
form a direct chemical bond with the PEEK. An
example of this can be seen in Fig. 10.1 . These tissue
and cellular interactions with PEEK are thought to be
as a result of PEEK's intrinsic low surface energy.
The reactions of the biological surroundings to the
surface can cause a foreign body reaction that in turn
can lead to implant loosening as a result of the
implant being fibrously encapsulated. It has been well
established that surfaces with higher energy are
known to promote rapid cellular adhesion and
spreading, in contrast to surfaces with lower energy
[1,25 e 27] . By altering the surface energy of a poly-
mer, the initial reactions to the polymer surface can be
altered. This could potentially widen the application
for PEEK biomaterials to areas where good tissue
integration, for example direct bone contact, is
essential. There are a number of ways to change the
surface energy, and these can be broadly divided into
two categories of direct surface modification tech-
niques and deposition techniques. Direct surface
modification techniques include wet chemical treat-
ments [28,29] and physical treatments such as expo-
sure to high-energy species, for example plasma
[30 e 35] ,corona [36] ,orUV/ozone [37,38] . Deposi-
tion techniques include plasma coating
[22,23,39 e 41] , vacuum plasma spraying [33] ,and
laser sintering with inorganic substances such as
hydroxyapatite [42,43] or titanium [40,44] .
The ultimate aim of changing the surface of
a biomaterial is to create a surface that is optimal for
the application. There is currently a substantial
amount of research into widening the application of
PEEK not only by changing the various material
properties such as mechanical reinforcement by the
incorporation of carbon fibers but also surface
modification by oxygen incorporation, coating, or
functionalization. However, direct bone contact to
some PEEK implants is reported to be limited [20]
and some surface modification techniques are known
to increase cytocompatibility without altering bulk
mechanical properties [19,22,32 e 34,39,45] .
10.2 Surface Modification
There are several ways in which polymer surfaces
can be modified for biomedical applications. Treat-
ments fall into two main categories: physical and
chemical treatments. Both categories involve altering
the surface at the atomic and molecular scale.
Figure 10.2 shows examples of the effect of surface
modification techniques on surface chemistry and
structure.
The schematic representation shown in Fig. 10.2
gives an overview of the surface structure and
chemistry, which might result from the surface
treatments described in this chapter. Figure 10.2 a
shows the original (untreated) surface, Fig. 10.2 b
shows a surface that would result from protein
adsorption, spin coating, dip coating, or plasma
deposition, and so on, Fig. 10.2 c depicts a surface
that has been etched, Fig. 10.2 d represents a surface
that has been chemically activated by, for example,
plasma treatment, Fig. 10.2 e shows a grafted surface,
and Fig. 10.2 f depicts self-assembled monolayers on
a surface. Physical treatments include plasma, ultra-
violet (UV)/ozone, Langmuir e Blodgett films and
radiation-induced coating of a new material onto the
existing
polymer
surface.
Radiation-induced
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