Modification of the physicochemical surface properties of polymers
during the plasma-based decontamination
K. Fricke, H. Tresp, R. Bussiahn, K. Schröder, K.-D. Weltmann, Th. von Woedtke
INP Greifswald – Leibniz Institute for Plasma Science and Technology, Greifswald, Germany
Abstract: The influence of a bio-decontaminating atmospheric pressure plasma
(1.1 MHz, Ar/O2), on the physicochemical surface properties of medical polymers is
investigated. The plasma exposure results in the incorporation of oxygen which, in
turn, leads to the formation of oxygen-containing groups and in changes in surface
roughnesses. Furthermore, it is observed that the modification of the polymers
occurs over a wide area on the substrate surface whereas the plasma-based etching
process is restricted to an area where the intensities of the reactive plasma species
are highest which is validated by optical emission spectroscopy.
Keywords: atmospheric pressure plasma jet, polymers, functionalization, etching
1. Introduction
Low-temperature plasmas generated at atmospheric
pressure are widely used to activate surfaces. The
surface activation is characterized by the
modification of chemical surface properties which is
based on chain scission, the formation of functional
groups, and cross-linking on the surface [1].
Polymeric materials are widely used in various areas
of medicine (e.g. medical devices, implants, cellculture plates). By exposing polymers to gasdischarge plasmas, the previously hydrophobic
polymer surface is modified into one with improved
wettability and enhanced adhesion properties.
Among the use of plasma for modifying polymer
surfaces, in the recent years plasma sources found
expanding applications with the prospect of biodecontamination up to the level of sterilization [2].
Especially the decontamination of medical devices
comprised of polymeric materials are in the focus of
attention due to their sensitivity against heat and
chemical active reagents which in turn exclude the
use of conventional methods like wet or dry heat for
sterilization.
2. Experimental
Fig. 1: Schematic plot of the jet and a photo of the Ar plasma
flame.
For the study a RF driven (1.1 MHz) miniaturized
atmospheric pressure plasma jet was used (Fig. 1).
The jet consists of a grounded ring electrode and a
center rod electrode inside a quartz capillary (inner
diameter of 1.6 mm and outer diameter 4 mm). The
jet was operated with argon (Ar) at a flow rate of
5 slm and an admixture of 0.05 slm oxygen (1 %
O2). The overall electric power of the jet was held
constant at 65 W. All experiments were performed
by a localized jet plasma treatment (position ‘0’) at a
constant axial distance between jet-nozzle and
substrate of 5 mm. Further information has been
published elsewhere [3]. The polymers investigated
in this paper were Poly(ether ether ketone) (PEEK,
Goodfellow, UK), Poly(methyl methacrylate)
(PMMA, Goodfellow, UK), and Polycarbonate (PC,
Goodfellow, UK). The chemical composition on the
polymer surfaces were analyzed by X-ray electron
spectroscopy (XPS) (Axis, Ultra, DLD detector,
Kratos) using a monochromatic Al Kα source at
1486.6 eV. The binding state of carbon was studied
by means of a high resolution XPS analysis of the
C 1 s peak measured with high energy resolution.
The different chemical component was calculated by
deconvolution the C 1 s peak using GaussianLorentzian distribution, linear baseline and a FWHM
of maximal 1.3 eV. Water contact angle (WCA)
measurements were used to obtain information on
the wettability (sessil drop method, GBX Digidrop,
France). The surface topography was determined
with a scanning probe microscope diCP-II (Veeco,
USA) in the non-contact mode. Five AFM-images
with a scanning region of 10 x 10 µm2 were
recorded on each sample to evaluate the roughness
Ra before and after plasma treatment (SPMLab Ver.
6.0.2.,Veeco). The depth profiles of the substrates
after plasma exposure were recorded using a surface
profiler (Dektak 3ST, Veeco). The antimicrobial
effectivity was investigated using Bacillus
atrophaeus spores dried on polymer stripes. The
recovery of surviving spores on the stripes was
performed by the plate count method and is given in
CFU/object with a detection limit of 102 CFU/object,
see also [4]. A dual channel fiber optical
spectrometer (Avantes AvaSpec 2048-2-USB2) was
used for the optical emission spectroscopy (OES).
The spectra were measured end-on with a cosine
corrector behind a quartz glass. They were relativly
calibrated and normalized to the exposure time.
chosen due to their extensive application in medical
devices and medical goods (e.g. implants).
The water contact angle measurements provide
information on the changes in the wettability.
Especially in this study the data are indicative of the
variation of the WCA across the whole polymer
surface depending on the radial distance (Fig. 3).
3. Results and Discussion
CFU/object
The biocidal and sporocidal effect of plasma is based
on plasma generated components mainly VUV/UV
radiation, charged particles (ions), and radicals [5].
In Fig. 2 the antimicrobial treatment of localized
contaminated polyethylene strips depending on the
operating gas and treatment time is shown. The
triangles show the spread between minimum () and
maximum () number of viable spores.
10
6
10
5
10
4
10
3
10
2
10
1
Ar plasma
Ar/O2 plasma
detection limit
Fig. 3: Radial distribution of the WCA after 60 s Ar (filled
symbols) and Ar/O2 plasma treatment (unfilled symbols).
As expected the wettability is improved after plasma
treatment. Moreover, despite the localized plasma
treatment a considerably larger area of the polymer
is modified (about 40 mm diameter). When oxygen
is added to the plasma, a further dip is observed in
the center of the treated surface except for PMMA
which exhibits an increase of the WCA in the center
of the polymer surface after Ar/O2 plasma exposure.
The changes in the chemical surface properties after
60 s Ar plasma were examined using XPS and are
depicted in Fig. 4.
35
0
60
120
180
240
300
treatment time [s]
O/C ratio [%]
Fig. 2: Inactivation of B. atrophaeus spores after Ar () and
Ar/O2-plasma (). n = 3.
30
PMMA
25
20
It can be seen, that the number of viable spores is
reduced
with
increasing
treatment
time.
Furthermore, adding oxygen results in an increase of
the lethal effect with a maximum reduction of viable
spores of 4 orders of magnitude after 180 s plasma
exposure. Consequently, the action of reactive
oxygen species is crucial for the biodecontamination.
Fig. 4: XPS O/C ratio of PMMA, PEEK, and PC after 60 s Ar
plasma exposure.
During the plasma-assisted bio-decontamination the
polymer is also affected by the plasma. Hence, the
influence of the plasma treatment on the chemical
and morphological surface properties will be
discussed in detail. PEEK, PMMA, and PC were
According to the improved wettability the plasma
treatment results in an increase of the O/C ratio with
its maximum beyond the local plasma treatment. At
the position where the plasma jet directly impinged
the polymer surface (position ‘0’) the O/C ratio is
PEEK
15
PC
10
-40 -30 -20 -10
0
10
20
30
40
radial distance [mm]
reduced. The enhanced incorporation of oxygen
adjacent to position ‘0’ might be due to the
admixture of oxygen-containing ambient air. The
O/C ratio of Ar/O2 plasma treated PMMA, PEEK,
and PC showed an increase of the O/C ratio for all
polymers at position ‘0’ which implies the reason of
the enhanced wettability of PEEK and PC when
oxygen is admixed to Ar plasma (data not shown).
The high resolution C 1 s XPS spectra after 60 s Ar
plasma treated PC and PEEK presented in Fig. 5
provide further information. Non-treated PC
displays the C1 peak at 285.0 eV (composed of C-C
aromatic and C-C aliphatic / C-H), C2 at 286.3 eV
(C-O groups), C5 at 290.6 eV (O(C=O)O groups),
and C6 at 291.6 eV (π→π* shake up) [6]. After
plasma treatment two further peaks can be observed
labeled as C3 at 287.9 eV (C=O groups) and C4 at
289.3 eV (O-C=O groups). Non-treated PEEK
shows the following sub peaks: C1 (C-C aromatic),
C2, C3, and C6 [6]. The plasma treatment of PEEK
results in the generation of a distinctive peak C4 at
289.0 eV assigned to O-C=O groups.
Fig. 6: 3D AFM images of non-treated and Ar plasma treated
(60 s) PMMA, PC, and PEEK.
Fig. 5: XPS highly resolved C 1 s peak of PC (left) and PEEK
(right) after 60 s Ar plasma treatment.
In the case of plasma treated PMMA no further
peaks were observed compared to non-treated
PMMA. Only the concentration of the C-O-C and
C=O groups was increased whereas the percentage
of the carbon atoms in C-C groups was reduced at
the same time after plasma exposure. It can be
summarized that new functional groups were
generated on the PC and PEEK surface and that the
concentration of already existing oxygen-containing
groups on all investigated polymers was increased
after plasma exposure. Furthermore, it was observed
that the relative intensities of the functional groups
on PEEK slightly increase at longer treatment times.
However, additionally to surface functionalization
reactive plasma species can lead to surface etching.
Therefore changes in the surface topography
affected by Ar plasma can be observed (Fig. 6).The
roughness Ra of the PMMA surface increases from
5.1 ± 1.4 nm to 54 ± 1.3 nm , for PC from
6.4 ± 0.8 nm to 26.6 ± 2.9 nm, for PEEK from initial
12 ± 2.4 nm to 17.9 ± 1.1 nm, respectively.
Moreover, on PMMA and PC a uniform spike-like
and hilly surface is generated after plasma treatment
whereas the PEEK surface displays the initial
granular structure but with additional surface grains.
The differences in surface roughening among the
polymers are mainly based on their chemical
structure. It is well known that aliphatic polymers
are easier to etch than polymers that consist of
aromatic C-C groups due to their stabilization by
plasma-generated aromatic functional groups like
phenolic compounds [7]. However, admixing
oxygen to argon influences the changes in surface
topography more strongly. This is based on
dominating etch processes resulting from the
generation of reactive oxygen species, particularly at
long treatment times (> 60 s) [8]. But in contrast to
the modification process, it can be observed that
etching processes are restricted to a smaller area of
the plasma exposed polymer. Figure 7 shows the
radial dependency of Ra on PEEK after 180 s Ar/O2
plasma treatment. It can be seen that with increasing
radial distance to position ‘0’ where the localized
plasma treatment was performed the Ra values
decrease. After a radial distance of 4.5 mm the Ra
value is similar to the Ra value of non-treated PEEK.
40
before plasma
after plasma
35
Ra [nm]
30
25
20
15
10
0.0
1.5
3.0
4.5
6.0
radial distance [mm]
Fig. 7: Radial development of Ra on PEEK after 180 s Ar/O2
plasma treatment.
The local limitation of the etch processes is also
displayed by Fig. 8 which shows the etching profiles
of PMMA (---) and PEEK (
). After 180 s Ar/O2
plasma treatment hollows of 6 µm on PEEK and 22
µm on PMMA, respectively, are shown. Once again,
it can be observed that the etch process is restricted
to an area of less than 6-7 mm in diameter.
4. Summary
The influence of an atmospheric pressure plasma jet,
applied for bio-decontamination, on the chemical
und morphological surface properties of bio-relevant
polymers has been investigated. It has been shown
that the O/C ratio of the polymers increases after
plasma treatment which, in turn, results in an
increased wettability. The creation of new oxygencontaining bindings as well as the percentage
increase of already present polar groups might be
responsible for the improved wettability. Additional
to surface modification the organic polymers
undergo degradation reactions. A significant change
in surface topography, in particular a roughening of
the surface, was observed as shown by AFM. The
admixture of oxygen to the Ar plasma leads to an
increased etching of the surface resulting in the
formation of etching depths of several micrometers.
Furthermore, it was shown, that the area of the
polymer which is affected by plasma etching is
considerably smaller compared to the area that was
functionalized by plasma. This is due to the decrease
of plasma-generated species, particularly oxygen,
with increasing radial distance verified by OES.
References
[1]
[2]
[3]
Fig. 8: top left: Radial distribution of emission lines of Ar, O,
OH and N2 of Ar/O2 plasma. Bottom left: Etching profiles of
PMMA and PEEK after 180 s Ar/O2 plasma exposure Right: 3D
picture of the etched hollow on PEEK (15 min Ar/O2 plasma).
Deduced from the OES measurements performed at
the distance of 5 mm to the Ar/O2 plasma effluent
(Fig. 8 bottom left) it can be observed that the range
of the highest intensity of the excited plasma species
(Ar: 751 nm, O: 844.6 nm; OH: 308 nm N2: 337 nm)
is coincident with the area on the polymer where the
etch process mainly occurs. That means that
decreasing intensities of excited plasma species
result in a reduction of etching reactions on the
polymer surface.
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