Biomedical Engineering Reference
In-Depth Information
Other surface-grafting or surface-modification
technologies are highly dependent upon the
chemical nature of the substrate.
4. They exhibit good adhesion to the substrate. The
energetic nature of the gas-phase species in the
plasma reaction environment can induce mixing,
implantation, penetration, and reaction between
the overlayer film and the substrate.
5. Unique film chemistries can be produced. The
chemical structure of the polymeric overlayer films
generated from the plasma environment usually
cannot be synthesized by conventional chemical
methods.
6. They can serve as excellent barrier films because of
their pinhole-free and dense, cross-linked nature.
7. Plasma-deposited layers generally show low levels
of leachables. Because they are highly cross-linked,
plasma-deposited films contain negligible amounts
of low-molecular-weight components that might
lead to an adverse biological reaction. They can also
prevent leaching of low-molecular-weight material
from the substrate.
8. These films are easily prepared. Once the apparatus
is set up and optimized for a specific deposition,
treatment of additional substrates is rapid and
simple.
9. The production of plasma depositions is a mature
technology. The microelectronics industry has made
extensive use of inorganic plasma-deposited films
for many years (Sawin and Reif, 1983; Nguyen,
1986).
10. Plasma surface modifications, although they are
chemically complex, can be characterized by IR
(Inagaki et al. , 1983; Haque and Ratner, 1988;
Krishnamurthy et al. , 1989), NMR (Kaplan and
Dilks, 1981), electron spectroscopy for chemical
analysis (ESCA) (Chilkoti et al. , 1991a), chemical
derivatization studies (Everhart and Reilley, 1981;
Gombotz and Hoffman, 1988; Griesser and
Chatelier, 1990; Chilkoti et al. , 1991a), and
static secondary ion mass spectrometry (SIMS)
(Chilkoti et al. , 1991b, 1992; Johnston and
Ratner, 1996).
11. Plasma-treated surfaces are sterile when removed
from the reactor, offering an additional advantage
for cost-efficient production of medical devices.
It would be inappropriate to cite all these advantages
without also discussing some of the disadvantages of
plasma deposition and treatment for surface modifica-
tion. First, the chemistry produced on a surface is often
ill-defined ( Fig. 3.2.14-3 ). For example, if tetrafluoro-
ethylene gas is introduced into the reactor, PTFE will
Table 3.2.14-3 Biomedical applications of glow discharge
plasma-induced surface modification processes
A. Plasma treatment (etching)
1. Clean
2. Sterilize
3. Cross-link surface molecules
B. Plasma treatment (etching) and plasma deposition
1. Form barrier films
a. Protective coating
b. Electrically insulating coating
c. Reduce absorption of material from the environment
d. Inhibit release of leachables
e. Control drug delivery rate
2. Modify cell and protein reactions
a. Improve biocompatibility
b. Promote selective protein adsorption
c. Enhance cell adhesion
d. Improve cell growth
e. Form nonfouling surfaces
f. Increase lubricity
3. Provide reactive sites
a. For grafting or polymerizing polymers
b. For immobilizing biomolecules
The application of RFGD plasma surface modification
in biomaterials development is steadily increasing.
Because such coatings and treatments have special
promise for improved biomaterials, they will be em-
phasized in this section. The specific advantages of
plasma-deposited films (and to some extent, plasma-
treated surfaces) for biomedical applications are:
1. They are conformal. Because of the penetrating
nature of a low-pressure gaseous environment in
which mass transport is governed by both molecular
(line-of-sight) diffusion and convective diffusion,
complex geometric shapes can be treated.
2. They are free of voids and pinholes. This continuous
barrier structure is suggested by transport studies
and electrical property studies (Charlson et al. ,
1984).
3. Plasma-deposited polymeric films can be placed
upon almost any solid substrate, including
metals, ceramics, and semiconductors.
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