22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma coatings as diffusion barriers in aqueous environments
D. Hegemann, N.E Blanchard, M. Drabik and M. Amberg
Empa, Swiss Federal Laboratories for Materials Science and Technology, St. Gallen, Switzerland
Abstract: The diffusion through plasma coatings in aqueous environments can be
controlled thanks to their cross-linking, functional group density, stability, and film
thickness. Different plasma coatings such as metals and plasma polymers have been
investigated for their water diffusion properties, their stability in water, and their capability
to control the release of Ag ions.
Keywords: plasma deposition, water diffusion, stability, aging, silver release
1. Introduction
Water diffusion through plasma coatings and their
related stability in aqueous environments is highly
relevant for diverse applications such as drug release,
tissue engineering, implants, filtration, contact lenses,
microfluidics, adsorption/desorption, electrodes, sensors,
fuel cells, and corrosion protection [1-11]. Possible water
diffusion is mainly determined by the density of the
plasma coating, i.e. morphology and cross-linking, by its
wettability, i.e. non-polar vs. polar groups, by its
thickness, i.e. due to surface and interface effects, by
internal stress, cracks and defects, as well as by its
stability, i.e. due to chemical reactions or dissolution in
water.
Both plasma polymer films as well as metal/metal oxide
films are of interest as barrier coatings or for the
controlled water diffusion in aqueous environments. For
plasma polymers thus the cross-linking vs. functional
group density is investigated [12-14], while for dense
metal/metal oxide layers mainly the thickness is crucial
[15,16]. In this work, silver-containing base layers (as
obtained by sputtering) are used where different plasma
coatings (a-C:H, a-C:H:O, a-C:H:N, pp-HMDSO, SiO x ,
TiO x ) have been applied to adjust the Ag ion release in
aqueous conditions.
2. Experimental
Plasma polymer films have been deposited in a
capacitively coupled plasma reactor consisting of two
plane parallel electrodes. A gas shower was mounted
facing the RF-driven (13.56 MHz) electrode where
substrates have been placed. As monomers C 2 H 4 (for
a-C:H), CO 2 /C 2 H 4 , C 3 H 6 O and H 2 O/C 2 H 2 (for aC:H:O), NH 3 /C 2 H 4 (for a-C:H:N), HMDSO (for ppHMDSO), and O 2 /HMDSO (for SiO x ) have been used.
Typically, the working gas pressure was 10 Pa and the
power range 10-100 W. To obtain Ag nanocomposite
coatings (with a-C:H:O or a-C:H:N as plasma polymer
matrix), a strongly asymmetric plasma reactor with a
(smaller) silver electrode enabled Ag sputtering during
plasma polymer deposition. Ag layers as well as Ti layers
have been deposited using magnetron sputtering.
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A V/I probe was used for the measurement of the
electrical conditions (voltage, current and power
absorption) and microwave interferometry (MWI) to
measure electron densities. Mean ion energies and ion
flux can thus be calculated [17]. As a measure of the
deposited energy (per atom), the energy density during
film growth can be calculated from the energy flux (mean
ion energy multiplied by ion flux) per deposition rate and,
likewise, the momentum transfer using the square root of
the ion energies [11,12].
Chemical composition was measured by x-ray
photoelectron spectroscopy (XPS) and film morphology
by transmission electron microscopy (TEM). Neutron
scattering (PSI Villingen, Switzerland) has been used to
observe the water (D 2 O) penetration into SiO x C y H z
plasma polymer films. The silver release over different
time scales (days) into bidistilled water was detected by
ICP-OES measurements.
3. Results and discussion
3.1. a-C:H coatings
Amorphous hydrocarbon coatings can be cross-linked
depending on the deposited energy per carbon atom
[17,18]. Due to its non-polar and dense structure, a-C:H
films can be excellent barrier layers against water
diffusion (and gas permeation). Hence, Ag ion release is
already effectively suppressed by ultrathin plasma
polymer films (>25 nm).
3.2. a-C:H:O coatings
Using oxygen sources with hydrocarbon-based
monomers, oxygen-functional, polar groups are
incorporated into a-C:H:O films. The higher the amount
of (terminal) functional groups, the more water molecules
can penetrate the plasma polymer structure. Nevertheless,
stable a-C:H:O films can be obtained by adjusting crosslinking (on the expense of polar groups) showing a
permanent water contact angle around 55° [12,19,20].
Increasing amount of ester groups, however, results in
hydrolysis reactions and degradation [21]. The latter can
well be intended as coatings for tissue engineering, drug
release, and biodegradable packagings. A too high
amount of oxygen groups, finally, results in soluble films
1
(such as deposited at a H 2 O/C 2 H 2 ratio of 8:1) [20].
Figure 1 gives a comparison of the chemical composition
of different a-C:H:O plasma polymers which is strongly
influenced by the gas phase composition.
Fig. 1. Chemical composition of different a-C:H:O
coatings deposited with different starting monomers.
Data using C 5 H 10 O 3 (ethyl lactate) are added from
literature for comparison [21].
Films comprising lower O/C ratios (around 0.2) and
low amounts of ester/carboxylic groups were found to be
stable in aqueous environments and can thus be used to
adjust a constant water diffusion rate through the
coatings. Films with higher O/C ratios (around 0.33) and
moderate ester/carboxylic groups can be considered as
rather stable, i.e. they show some aging resulting in
increasing water contact angles with time (40° → 60°).
Plasma coatings, on the other hand, comprising a high
amount of ester groups such as deposited from ethyl
lactate undergo hydrolysis and show degradation when
sufficient oligomers are formed [14,21]. An overall high
O/C ratio and corresponding high amount of polar groups
results in soluble plasma polymers, which could
nevertheless be interesting as sacrificial layers and for
microlithography.
3.3. a-C:H:N coatings
Hydrocarbon-based plasma polymer films comprising
nitrogen-functional groups are generally not stable in
aqueous conditions due to oxidation and hydrolysis
reactions. Amino-rich films ([NH 2 ]/[C] ≥ 2%) even show
leaching of oligomers and thus film degradation. Thus,
washing or pre-incubation is required when using such
films as biointerfaces [22,23]. Nevertheless, a-C:H:N can
be used to obtain antimicrobial, yet cytocompatible Agcontaining plasma coatings, i.e. by adjusting the water
diffusion and Ag ion release [24]. Furthermore, the
nitrogen-functional films can be used for adhesion
promotion due to stabilization by chemical reactions [23].
2
3.4. pp-HMDSO
Although
plasma
films
deposited
from
hexamethyldisiloxane (HMDSO) are hydrophobic, since
CH 3 groups are maintained on a Si-O-Si backbone, (slow)
diffusion of water is enabled which was proven by FTIR
and neutron scattering as well as Ag ion release. It is
assumed that formation of silanol groups enables water
diffusion in such films [11]. Except for this hydrolysis
reaction which is limited, pp-HMDSO films are very
stable in aqueous conditions. Hence, hydrophobic
surfaces can also be used for the control of water
diffusion and drug delivery.
3.5. SiO x coatings
Dense, quartz-like films are used as barrier coatings,
e.g., for packaging foils [25]. Hence, they are also good
water barriers. If, however, some organic groups are left
in SiO x films, water diffusion is supported, probably by
formation of silanol groups [11]. Films showing good
adhesion and low internal stresses are very stable in
aqueous conditions and can thus be used to control water
diffusion.
3.6. TiO x coatings
Titanium has a high affinity towards oxidation. For the
deposition of metal Ti films, a low background pressure,
i.e. a low amount of residual oxygen sources, is thus
required. The oxidation of the Ti surface (1-2 nm of
TiO 2 ), on the other hand, is very stable and suitable as
passivation layer.
Therefore, magnetron sputtering was used to deposit
nm-thick (metallic) Ti adlayers on silver coatings (200
nm) which had been deposited by magnetron sputtering
prior to the Ti deposition. The resulting Ag ion release is
shown in Figure 2. It becomes evident that the release of
silver is sufficiently suppressed already with ultrathin
adlayers.
Fig. 2. Measured Ag release from Ag coatings (200 nm
on Petri dishes) passivated by a Ti adlayer of different
thickness. The coatings were incubated for 7 days in
bidistilled water.
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Since Ag ions are released from oxidized silver (Ag 2 O),
it is required that dissolved oxygen is transported via
water diffusion to the Ag/Ti interface. In turn, also Ag
ions must be able to diffuse through the Ti adlayer.
Nevertheless, the amount of Ag release is a good measure
to examine the water diffusion through (plasma) coatings.
Ti-passivated silver surfaces enable electrically
conductive films that are exceptionally stable in aqueous
environments [16]. Since only the outermost Ti surface
becomes insulating, the contact resistance remains
unaffected. Moreover, adjusted Ti/Ag surfaces pass the
cytotoxicity test (ISO norm 10993-5).
3.7. Nanocomposite coatings
Pure silver surfaces typically show an initial burst
release of Ag ions when exposed to aqueous
environments which might yield cytotoxic conditions.
The Ag ion release can be better controlled using
nanocomposite coatings. Co-sputtering of a metal (here
Ag) during plasma polymerization enables the embedding
of metal nanoparticles into the growing plasma polymer
film [26]. Water diffusion through the plasma polymer
matrix and the surface area of the metal nanoparticles
(exposed to the penetrating water molecules) determine
the Ag ion release. Different plasma films with different
functional group densities and cross-linking (a-C:H,
a-C:H:O, a-C:H:N, pp-HMDSO, SiO x ) can thus be used
to adjust the release properties.
TEM investigations on Ag-poor (0.5 at% Ag) and Agrich (5 at% Ag) nanocomposites with an a-C:H:O plasma
polymer matrix reveal modifications within the
nanocomposite structure after immersion in bidistilled
water (Figure 3).
It is revealed that most of the roughly 5 nm large Ag
nanoparticles within Ag-poor coatings are dissolved after
immersion for one day in water (Figure 4). Hence, such
films might be used on implant surfaces to avoid early
stage adhesion of bacteria before cell attachment. The
latter can even be supported by the functional plasma
polymer matrix. Ag-rich coatings, on the other hand,
show Oswald ripening of initially small nanoparticles
(growing from 6 to about 30 nm in size) which limits the
effective Ag ion release. This situation might result in an
initial burst release and later Ag nanoparticle leaching and
should thus be avoided. Note that nanocomposites with a
filling factor close to percolation threshold can become
stable in aqueous conditions, again, due to a cross-linked
matrix and nanoparticles of same size and distance which
suppress Oswald ripening. Such coatings can, e.g., be
used as sensors for water penetration [27].
An elegant way to control the Ag ion release is given by
applying adlayers or gradient structures. As shown in
Figure 4, a steady-state release can be reached already by
a bilayer structure with the higher Ag content in the
bottom layer (reservoir). Coatings that act as barriers for
water diffusion such as a-C:H can be used to suppress Ag
ion release. Furthermore, degradable plasma polymers
might be used to delay drug release.
Fig. 4. Ag release depending on incubation time for two
single layer and two bi-layer structures containing Ag
nanoparticles as Ag ion releasing reservoir.
Fig. 3. TEM images of Ag nanocomposites: a) Ag-poor,
as-deposited, b) Ag-rich, as-deposited, c) Ag-poor, after
immersion in water for 24 h, and d) Ag-rich, after
immersion in water for 24 h.
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4. Conclusions
The film properties of plasma-deposited coatings
depend on the gas phase conditions, i.e. the nature of
film-forming species, and on the surface conditions, i.e.
the energy influx during film growth. Both influence the
structure of the plasma coating (film density, crosslinking, internal stress, functional group density, etc.) and
thus the capability of water diffusion through the plasma
coating as well as its stability in aqueous environments.
Since increased functional group density, i.e.
incorporation of numerous terminal groups, is on the
expense of cross-linking, the stability of plasma coatings
3
in aqueous environments can be classified as depicted in
Figure 5.
Fig. 5. Overview showing stability ranges of different
plasma coatings depending on their functional group
density in aqueous environments.
While siloxane and silicone-like coatings are stable in
aqueous conditions due to their Si-O-Si backbone,
oxygen-functional hydrocarbon films tend to become
soluble (or degradable) with increasing functionality.
Nitrogen-functional films readily undergo oxidation and
hydrolysis reactions in water and in humid air and are
thus less stable. The stability of Ag nanocomposites,
either used as antibacterial coatings or as sensors can be
adjusted over a broad range. Small Ag nanoparticles (<20
nm) dissolve easily in water, while Ag surfaces show
initial burst release followed by saturation due to a change
in the oxidation stage forming higher oxides. Silver
surfaces, however, can be effectively passivated by thin
Ti adlayers.
The water diffusion and stability of plasma coatings can
thus be controlled by the plasma process and the related
film structure enabling various applications for coated
materials used in aqueous environments.
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