Biomedical Engineering Reference
In-Depth Information
temperatures and are used for depositing hard and
passivating coatings. Due to the temperatures
reached, between 5000 and 10,000 K, this type of
treatment is often used for metallurgical operations
and melting inorganic powders
[71]
. When the
inorganic powder is melted, the plasma molten
droplets can then be sprayed onto a surface. This type
of plasma treatment has been used to coat metallic
implant materials with calcium phosphate and
hydroxyapatite, as the surface charge of calcium
phosphates strongly affects cellular adhesion
[68,72]
.
There is little application for polymers due to the
high temperatures involved
[36]
.
can be deposited so as to aid or resist protein
adsorption, and can be used to aid grafting of specific
proteins. Biomolecules such as laminin and fibro-
nectin or amino acid sequences can be covalently
bound by plasma polymerization to substrates
[3,35,74,75]
. This method of treatment can be used to
modify different types of surfaces including chemi-
cally inert ones, without affecting the bulk chemistry
[26,76]
. Another method of influencing cell adhesion
is by film deposition to aid selective protein adsorp-
tion
[77
e
79]
. Selective protein adsorption is not
easily achieved, and the exact parameters for modi-
fication have not been determined
[80]
. The treat-
ment also allows surfaces to be patterned with
a spatial resolution of less than 5
m
m
[81]
. There are
many variables in plasma treatments for applications
with mammalian cells, but many of the treatments
have been performed on silica, glass, and mica,
which have limited biomedical applications. Plasma
treatments have been applied to polymer surfaces and
show biocompatible treatments that both promote
[73,76,82,83]
and reduce
[26,84,85]
the cellular
attachment. This type of treatment is of great interest
because it can be used to coat different types of
surfaces and specific protein attachment is possible.
10.3.1.8 Cold Plasma
Plasmas formed at low temperature require low
pressure, which allows an ionized region to be formed,
leading to a much smaller percentage of the gas being
ionized, the composition of which depends on the gas
feed. The ionized region includes ions, high-energy
photons, electrons, radicals, and other excited species.
Although the electron temperature in the plasma can
be as high as 5000 K, because it occurs under vacuum
and there is a low particle density, the bulk tempera-
ture is essentially ambient
[73]
. For example, oxygen-
containing functional groups are introduced through
surface activation by low-pressure plasmas. Low-
pressure plasma can also be used to etch surfaces by
the formation of gaseous species or coat surfaces by
plasma deposition. The low pressure is maintained
by using a vacuum chamber and different gas feeds
that determine the composition of gaseous mixture.
Treatment variables involve vacuum chamber geom-
etry, gas used, gas flow rate, and electromagnetic
parameters input power
[36]
. The deposited films
are chemically bonded, but can be extremely thin,
anywhere between 200 and 2000 nm, and are
extremely resistant to delamination
[47]
. Treatment is
not limited to polymers, as films can also be deposited
on ceramics and metals. Depending on the treatment
chamber size, whole devices can be treated.
For biomedical applications, the plasma vapor-
deposited coating protects the substrate from attack
by the biological environment and also stops any
leaching from the substrate into the biological envi-
ronment. In vitro and in vivo evaluation of the
deposited films from organosilicon, acetylene, and
nitrogen-containing monomers deposited on poly-
mers has been shown to be biocompatible and the
films are sterile after deposition
[11,73]
. The films
10.3.1.9 UV/Ozone
Polymers can be degraded by exposure to sunlight,
whereby photons with short wavelengths activate
chemical reactions. The process caused by exposure
of surfaces to UV irradiation in the presence of
oxygen is termed photooxidation
[86]
. UV/ozone
treatment takes advantage of this process, causing
photon-activated cross-linking or fragmentation of
polymer surfaces. UV lamps are used, which operate
at wavelengths between 180 and 400 nm
[36]
.
UV/ozone treatment has been used successfully to
increase cellular attachment on polymer surfaces
with a range of cell types and has also been used as
the first step in grafting biomolecules, showing
increased attachment of biomolecules with increased
surface oxygen content
[27,37,38,83,87]
. Micro-
patterned surfaces can be produced by masking
during the irradiation treatment and have shown
spatially controlled attachment of cells
[81,83,88]
.
As the treatment does not require vacuum apparatus
or sophisticated gas handling equipment, it is a rela-
tively simple way to introduce controlled levels of
oxygen functionalities to polymeric substrates. Due
to the surface modification technique at atmosphere,
