Biomedical Engineering Reference
In-Depth Information
not be deposited on the surface. Rather, a complex,
branched fluorocarbon polymer will be produced. This
scrambling of monomer structure has been addressed in
studies dealing with retention of monomer structure in
the final film (Lopez and Ratner, 1991; Lopez
et al.
,
1993; Panchalingam
et al.
, 1993). Second, the apparatus
used to produce plasma depositions can be expensive. A
good laboratory-scale reactor will cost $10,000-30,000,
and a production reactor can cost $100,000 or more.
Third, uniform reaction within long, narrow pores can be
difficult to achieve. Finally, contamination can be
a problem and care must be exercised to prevent extra-
neous gases and pump oils from entering the reaction
zone. However, the advantages of plasma reactions out-
weigh these potential disadvantages for many types of
modifications that cannot be accomplished by other
methods.
is placed affect the final outcome of the plasma de-
position process (
Fig. 3.2.14-2
). A diagram of a typical
inductively coupled radio frequency plasma reactor is
presented in
Fig. 3.2.14-2
. The major subsystems that
make up this apparatus are a gas introduction system
(control of gas mixing, flow rate, and mass of gas entering
the reactor), a vacuum system (measurement and control
of reactor pressure and inhibition of backstreaming of
molecules from the pumps), an energizing system to ef-
ficiently couple energy into the gas phase within the re-
actor, and a reactor zone in which the samples are treated.
Radio-frequency, acoustic, or microwave energy can be
coupled to the gas phase. Devices for monitoring the
molecular weight of the gas-phase species (mass spec-
trometers), the optical emission from the glowing plasma
(spectrometers), and the deposited film thickness
(ellipsometers, vibrating quartz crystal microbalances)
are also commonly found on plasma reactors. Technology
has been developed permitting atmospheric-pressure
plasma deposition (Massines
et al.
, 2000; Klages
et al.
,
2000). Another important development is ''reel-to-reel''
(continuous) plasma processing, opening the way to
low-cost treatment of films, fibers, and tubes.
The nature of the plasma environment
Plasmas are atomically and molecularly dissociated gas-
eous environments. A plasma environment contains
positive ions, negative ions, free radicals, electrons,
atoms, molecules, and photons (visible and UV). Typical
conditions within the plasma include an electron energy
of 1-10 eV, a gas temperature of 25-60
C, an electron
density of 10
9
-10
12
/cm
3
, and an operating pressure of
0.025-1.0 torr.
A number of processes can occur on the substrate
surface that lead to the observed surface modification or
deposition. First, a competition takes place between
deposition and etching by the high-energy gaseous spe-
cies (ablation) (Yasuda, 1979). When ablation is more
rapid than deposition, no deposition will be observed.
Because of its energetic nature, the ablation or etching
process can result in substantial chemical and morpho-
logical changes to the substrate.
A number of mechanisms have been postulated for the
deposition process. The reactive gaseous environment
and UV emission may create free radical and other re-
active species on the substrate surface that react with and
polymerize molecules from the gas phase. Alternately,
reactive small molecules in the gas phase could combine
to form higher-molecular-weight units or particulates
that may settle or precipitate onto the surface. Most
likely, the depositions observed are formed by some
combination of these two processes.
RFGD plasmas for the immobilization
of molecules
Plasmas have often been used to introduce organic
functional groups (e.g., amine, hydroxyl) on a surface
that can be activated to attach biomolecules (see Section
3.2.16). Certain reactive gas environments can also be
used for directly immobilizing organic molecules such as
surfactants. For example, a PEG-
n
-alkyl surfactant will
adsorb to PE via the propylene glycol block. If the PE
surface with the adsorbed surfactant is briefly exposed to
an argon plasma, the
n
-alkyl chain will be cross-linked,
thereby leading to the covalent attachment of pendant
PEG chains (Sheu
et al.
, 1992).
High-temperature and high-energy
plasma treatments
The plasma environments described above are of rela-
tively low energy and low temperature. Consequently,
they can be used to deposit organic layers on polymeric or
inorganic substrates. Under higher energy conditions,
plasmas can effect unique and important inorganic sur-
face modifications on inorganic substrates. For example,
flame-spray deposition involves injecting a high-purity,
relatively finely divided (w100 mesh) metal powder into
a high-velocity plasma or flame. The melted or partially
melted particles impact the surface and rapidly solidify
Production of plasma environments
for deposition
Many experimental variables relating both to reaction
conditions and to the substrate onto which the deposition
