Biomedical Engineering Reference
In-Depth Information
sample and the flame. Rate of sample movement is
particularly important because it determines the
extent of surface damage and depth of penetration
into the sample. This may cause loss rather than gain
of adhesive properties due to weak boundary layer
effects. Flame treatment of polypropylene (PP) has
been shown to lead to higher surface oxygen
concentration
[62]
. The oxygen had been incorpo-
rated through hydroxyl, carbonyl, and carboxyl
functional groups, which improved wettability and
adhesion properties
[36,62]
. Flame treatments are
often unsuitable for polymers, as bulk properties may
be altered as a result of longer treatment times, to
produce even coverage
[61]
.
approximately 10 nm, increase surface roughness
due to pitting, and can also lead to the formation of
low-molecular-weight species that can affect the
adhesive properties
[36,61,64]
. This treatment is also
used to form reactive groups on the surface before
grafting, which will be described in Section
10.3.2.1
.
10.3.1.4 Electron Beam
High-energy electrons,
>
50 keV, can be used for
cross-linking and curing polymer surfaces. When low-
energy electrons,
<
25 keV, are used in a vacuum
chamber, radicals are formed. Atmospheric oxygen is
incorporated on the surface upon removal from the
vacuumchamber, improving surface adhesion
[40,63]
.
10.3.1.2 Laser
Lasers are a photon source and can therefore have
high energy with high intensity. They are often used to
cause cross-linking and chain scission in polymers.
The high energy also leads to a sintering effect of
surfaces. Lasers are very exactly controlled in both
intensity and area of irradiation. The molecular scis-
sion effects have been used for microlithography
applications
[63]
. Studies of laser treatment have
shown that oxygen-containing polymers have oxygen
removed from the surface due to themodification. This
is thought to be due to the efficient formation of stable
carbon
e
oxide molecules
[36]
. Laser treatment is also
used for cross-linking of surface coatings, and stereo-
lithography for making three-dimensional prototypes.
10.3.1.5 Ion Beams
Ion beams can be used to either directly alter the
surface or target specific treatments which are then
deposited on the surface. This latter technique is called
sputtering and will be discussed in Section
10.3.1.6
.
The ions have high energy, but due to their mass only
affect the surface of a substrate, and when used in the
presence of oxygen lead to the formation of carbonyl
and carboxyl functional groups on the surface. Ion
beams have therefore been used to improve wear and
adhesive properties and biocompatibility
[66]
.
10.3.1.6 Sputter Coating
The sputtering method also involves the use of ion
beams, where the ions are accelerated towards a target,
causing atoms to be ejected, which are then deposited
on the surface of a substrate. The treatment is used
to form inorganic coatings, although the surface
treated must be heat resistant to prevent deformation.
This limits the application to heat-resistant polymers
and metal implants
[36,67,68]
. This treatment leads
to good adhesion of the coating to the substrate
and has been used to improve biocompatibility
[23,33,40,69,70]
.
10.3.1.3 Corona
The Corona effect occurs when high-energy
electromagnetic fields are formed close to the
charged thin wires or electrodes, causing ions to be
formed, even at atmospheric pressure or low
temperatures. In the region of ionization, excited
species such as radicals, ions, electrons, and mole-
cules are present. The excited species can cause
surface modification by electron formation and
elimination of a weak boundary layer, which leads to
increased surface roughness due to pitting. The
Corona effect results in the formation of polar groups
that usually lead to the incorporation of oxygen-
containing functional groups
[64,65]
. The radicals
formed through the treatment have a relatively short
lifetime due to reactions with atmospheric oxygen, so
that the surfaces change with aging
[47]
. Corona
treatment is known to affect polymers to a depth of
10.3.1.7 Hot Plasma
Plasma is often described as the fourth state of
matter and plasmas are formed when energy is
supplied, leading gases to become ionized, whereby
electrons are disassociated from their atoms within
a gas. Hot plasma refers to the high temperatures and
the high degree of ionization within the gas. Plasmas
generated at atmospheric pressures reach very high
