Biomedical Engineering Reference
In-Depth Information
orthopedic grade PEEK, combined with an increase
in surface energy (discussed in Section
8.3.2
), does
not increase S. aureus or S. epidermidis adhesion. In
addition, the same treatment has been shown to have
a positive effect on human primary osteoblasts in
vitro (see Chapter 10), providing a potential method
for improving the integration of PEEK in vivo.
interactions of the implant with the diverse molecules
present in the extracellular matrix of the implant site.
When any biomaterial enters a biological system,
water molecules interact with the surface within
nanoseconds
[43]
. The orientation and strength of
interaction between the water molecules and the
surface is dependent on the exact surface chemistry
of the surface. The bonds formed depend on the
polarity, or wettability, of the functional groups
present and the physical surface structure. In turn, the
subsequent interactions of proteins and organic
molecules with the surface are influenced by the
orientation of the water molecules
[43,89,90]
. The
importance of the interactions formed between the
surface, water, and proteins is highly apparent when
the number and diversity of physiological proteins is
considered. Blood plasma contains an array of over
200 proteins that may potentially interact with
biomaterials and therefore mediate the response of
any cells, bacteria, or host tissues that come into
contact with the conditioned surface. Therefore, the
surface chemistry of a biomaterial is integral in
controlling biological responses including wound
healing, inflammation, cytotoxicity, and importantly
in the context of this chapter: bacterial adhesion. The
initial protein interactions occur within the micro-
and millisecond time frames and continue for much
longer periods of time as the proteins present and the
conditions around a biomaterial change.
There are many studies in the literature which
focus on the relationship between surface hydro-
phobicity, a fundamental feature of surface chem-
istry, and bacterial adhesion
[91
e
98]
. Both the
hydrophobicity of the bacteria and the surface can
influence adhesion. The role of bacterial wettability
has been broadly investigated and in general, from
a range of wettabilities, more hydrophobic bacterial
species adhere more readily to hydrophobic surfaces,
such as polystyrene
[91,98]
. Additionally, the same
trend of hydrophobic bacteria adhering more readily
to hydrophobic surfaces has also been observed for
individual strains of the same species
[99]
. As PEEK
is a relatively hydrophobic surface, one would expect
this trend to be true for PEEK as well, although this
has yet to be shown.
In addition to the effect of bacterial wettability on
adhesion, the effect of biomaterial wettability has also
been similarly characterized. However, it has been
observed that surfaces with very high and very low
hydrophobicity generally reduce bacterial adhesion.
Furthermore, controlled efforts to influence bacterial
8.3.2 The Influence of Surface
Chemistry on Bacterial Adhesion
8.3.2.1 Surface Chemistry and Protein
Interactions
Proteins are complex molecules containing many
different functionalities; therefore, it is difficult to
predict precise protein interactions with surfaces.
Many proteins contain multiple nonpolar, polar, and
charged residues, and additionally the orientation and
structure of proteins can change depending on the
surrounding environment
[85
e
88]
.Inpa icula ,
surfaces that are neither extremely hydrophilic nor
hydrophobic are thought to bind eukaryotic adhesion
proteins well, permitting reorientation dependent on
the surrounding environment to promote eukaryotic
cell adhesion
[85]
. Fibronectin, for example, is an
important protein for eukaryotic cell adhesion, which
readily adheres to highly hydrophobic surfaces.
However, when adhered, the molecule is bound so
strongly that it is not capable of reorientating to aid cell
adhesion. When fibronectin binds to less hydrophobic
surfaces, the molecule can reorientate and actively aid
eukaryotic cell adhesion
[85,87]
. This is a clear
example of how surface chemistry influences biolog-
ical interactions through surface-adsorbed proteins. As
described in Section
8.2
, bacteria adhere to surfaces
using both specific and nonspecific mechanisms, both
of which are mediated by the adsorbed layer of
proteins in vivo. The adhesion of proteins is impor-
tantly influenced by surface chemistry, which is
dependent on the biomaterial used, for example PEEK,
and any extra surface modification applied.
8.3.2.2 Surface Chemistry and Bacterial
Interactions
The study of the relationship between pure surface
chemistry and bacterial adhesion provides a good
foundation to understand bacterial adhesion.
However, the models used may not be fully relevant
for the clinical application of biomaterials due to the
