Biomedical Engineering Reference
In-Depth Information
Figure 10.2
A schematic representation of
the resulting surface from various surface
modification techniques: (a) original
surface, (b) coated surface, (c) etched
surface, (d) chemically functionalized
surface, (e) grafted surface, (f) self-
assembled monolayers. Adapted from
Ratner
[2]
.
treatments include plasma gas or corona discharge,
g
-irradiation, laser, electron and ion-beam treat-
ments. Chemical treatments include chemisorption,
acid etching, idation, and grafting.
There are specific biological and chemical criteria
that must be taken into consideration when modi-
fying surfaces for biomaterials applications. Surface
analytical techniques can be used to assess whether
the new surface meets these criteria
[36,46,47]
.
Whether a cell proliferates on a surface depends on
surface characteristics such as wettability, surface
and bulk chemistry, the ratio of hydrophobicity to
hydrophilicity, surface charge and distribution,
rigidity, and surface roughness
[46]
.
bioinert, that is, it causes a minimal inflammatory
response, or bioactive, that is, a material that allows
tissue ingrowth to provide superior attachment
[1,49]
. The implant surface, if nondegradable, should
be wear resistant to reduce the amount of debris
particles entering the blood stream. If any particulate
material from the implant is released in situ, this must
be tolerated by the biological environment and bio-
degraded or excreted, to avoid accumulation. The
implant should not swell due to the anatomical
constraints of the implant site and must also be
nontoxic. An implant will generate a tissue response
of some kind. The main response is the inflammatory
process due to surgical trauma and foreign body
reaction. The inflammation process serves to
neutralize, dilute, or enclose the affected area,
whereby cells build up an avascular collagenous wall
surrounding the implant approximately 50
e
200
m
m
thick
[11,50]
. In vitro experiments, as described in
Section
10.1
, have shown different materials to
adsorb proteins, which then allow cell attachment
and proliferation, but in vivo, the protein adsorption
is much more complex
[51]
. In vivo, protein
adsorption is less specific, whereas in vitro protein
adsorption is specific, whereby only a few proteins
are adsorbed and act as signaling agents in fixed
orientations and conformations
[5,50]
. This protein
layer can indicate that the implant is not specifically
recognized by the body. This can then lead to the
adsorption of macrophages to the protein layer,
which attempt
10.2.1 Biological Criteria
Biocompatibility may be defined as “the ability of
a material to perform with an appropriate host
response in a specific application”
[48]
. When
introducing a material or a device into biological
surroundings, the interface between the tissue and
device is very important. Materials react with their
environments and this interaction is critical in vivo.
The environment surrounding an implant is often
saline, there may be dynamic synovial fluid, and
there is typically an abundance of proteins. The
temperature will be approximately 37
C, with a pH
of approximately 7.4, depending on area of implan-
tation. A material under investigation for use in an
implant application is preferably either relatively
to digest or engulf
the implant
