Magnetron Enhanced Plasma Polymerized Nanofilms as Antimicrobial
Coatings for Rigid Gas Permeable Contact Lenses
A. Marx1, M. Bergmann1, Y. Gier2, F. Widmer2, D. Muckenhirn2, F. Olcaytug1, G. Dame1, G.A. Urban1
1
Laboratory for Sensors, Department of Microsystems Engineering, University of Freiburg, Freiburg, Germany
2
Hecht Contactlinsen GmbH, Germany
Abstract: Biocompatibility and wettability are two key properties for contact
lenses. Nanofilms deposited by means of magnetic field enhanced plasma
polymerization have been shown to significantly improve these properties in case
of silicone-hydrogel contact lenses [1]. This work demonstrates that similar types
of coatings are applicable onto a rigid gas permeable (RGP) lens material
(fluorsilicone acrylate). For this purpose CH4-O2 mixtures at about 5 Pa were
excited in between symmetrically driven parallel plate electrodes with 15 kHz at
a power of about 50 W. The composition of different coatings was characterized
by XPS and FTIR-spectroscopy. Improvement of the wettability has been proven
by comparing uncoated and coated samples which were measured with dynamic
sessile drop method. Correlations between surface properties and the film
composition as well as deposition parameters are shown. The coatings exhibit
antimicrobial behavior in adhesion experiments carried out with E.coli GFP
(green fluorescent protein) evaluated by fluorescence microscopy.
Keywords: contact lens, coating, plasma polymerization, antimicrobial,
wettabillity
1. Introduction
2. Experimental
This work deals with glow discharge reactive
(plasma) coating of contact lenses. A rigid gas
permeable (RGP) lens material of fluorsilicone
acrylate was coated with a nanofilm of highly cross
linked hydrocarbon, the so called plasma
polymerized product of the monomer gases methane
and oxygen. Same types of coatings have been
shown to significantly improve the properties of
silicon-hydrogel contact lenses [1, 2].
The coating was deposited in a bell-jar reactor with
parallel plane electrode setup and magnetron
enhancement (Figure 1). The glow discharge was
excited in a current controlled mode with 15 kHz
and a power of 46 W. At a pressure of 5 Pa the
precursor gases methane and oxygen react due to
activation in the glow discharge to deposit the film
[3].
The purpose of this study was to investigate whether
films deposited by the technology described above
are capable to improve the surface- and biological
properties of RGP contact lens materials. Analyses
of the film composition were made by XPS and
FTIR-spectroscopy. Surface properties were
evaluated by means of contact-angle- and bacterial
adhesion measurements. We observed a steady
improvement in wettabillity on coated contact lenses
and in parallel we achieved a strong reduction of
bacterial adhesion for E.coli.
Films deposited on silicon chips were investigated
by X-ray photoelectron spectroscopy (XPS, Physical
Electronics 5600 ci) on the surface as well as in
deeper film levels after Ar sputtering, by Fourier
transform infrared spectrometry (FTIR, Bio-rad
Exalibur) and by dynamic sessile drop contact angle
measurements (Kruss DSA10-Mk2).
Surface properties were measured on uncoated as
well as on coated samples of the contact lens
material.
surface
bulk
C/O ratio
100
Figure 1: left: schematic of the plasma polymerization
equipment; right: sample holder with contact lens.
Bacterial adhesion was compared between uncoated
and coated polystyrene samples (petri dish material
of the company Greiner Bio-One GmbH) with E.coli
GFP (green fluorescent protein) by fluorescence
microscopy after an incubation time of 24 h.
10
1
∞5
1,5
1
precursor gas mass ratio [CH4:O2]
Figure 2: XPS measurements: Carbon to Oxygen ratios at the
surface and in the bulk of the coating for different precursor
mass ratios.
3. Results
With FTIR measurements it was possible to
differentiate the distribution of functional groups in
films produced with different precursor gas
mixtures. The results of these measurements are
shown in Figure 3 and they indicate an increasing
amount of OH-groups as well as a decreasing
amount of CH-, CH2- and CH3-groups for higher
oxygen concentrations in the precursor gas mixture.
∞
1,5
1
CH4:O2 by mass
CH, CH2, CH3
transmission [%]
Film composition evaluated on silicon substrates:
XPS measurements gave information about the
atomic composition of the films. In this case the
elements carbon and oxygen especially their ratios
are of interest. We observed lower C/O ratios on the
film surface than in the bulk of the film. Precursor
gas mixtures with higher oxygen content result in
lower C/O ratios on the film surface as well as in the
bulk of the film. These relations are shown in
Figure 2.
O-H
3700
3200
2700
2200
1700
1200
wavenumber[cm-1]
Figure 3: FTIR measurements of 50 nm films on silicon
produced with different precursor mass ratios [CH4:O2].
Dynamic contact angle measurements with deionized water showed an increase in wettabillity for
coatings produced with higher oxygen ratios in the
precursor gas mixture. There is a noticeable contact
angle hysteresis for surfaces which were produced
with oxygen in the precursor gas mixture. These
results are shown in Figure 4.
advancing
bio-fouling. In context of contact lenses, low
bacterial adhesion will decrease the risk of infection
of the user’s eye. To show the antimicrobial
potential of the presented coatings bacterial adhesion
was compared between uncoated and coated
polystyrene surfaces. Figure 6 shows a partially
coated polystyrene surface that was incubated for
24 h with E.coli GFP. On the coated left hand side as
the figure shows, the bacterial adhesion was
significantly reduced.
receding
dynamic contact angle [°]
100
80
60
40
20
0
5
∞
1.5
1
precursor gas mass ratio [CH4:O2]
Figure 4: dynamic contact angle measurements performed
immediately after coating on surfaces produced with different
gas mass ratios in the precursor gas mixture.
Surface properties of fluorsilicone acrylate
contact lens material: Contact angle measurements
were carried out on uncoated and coated samples in
dry- and swollen states. The uncoated RGP lens
material showed hydrophobic behavior (contact
angle > 90°) in dry state but contact angles < 60° in
swollen state. In re-dried state the contact angle was
significantly higher. Coated samples showed lower
contact angles in all stages (Figure 5).
dry state
swollen
swollen + redried
100
advancing contact angle [°]
90
80
70
60
50
40
30
uncoated
coated
Figure 5: Dynamic contact angle measurements on RGP contact
lens material in different swelling states.
Bacterial adhesion is an important issue for many
kinds of medical products since it occurs as an initial
step and triggers subsequent bio-film formation and
Figure 6: Partially coated polystyrene surface with adherent
bacteria (E.Coli GFP), left: coated, right: uncoated.
4. Conclusion
We characterized the composition of plasma
polymer films deposited by a special mode of
PECVD called also magnetron enhanced LCVD
"Luminous Chemical Vapor Deposition". The results
for elemental composition correlate with the
identified functional groups. Surface properties, in
particular wettabillity verify the obtained results:
Higher oxygen concentrations in the precursor gas
mixture result in higher oxygen contents of the
coatings. Oxygen will form OH-groups among
others and therefore provide more hydrophilic
surfaces.
The biological experiments give an outlook about
the antibacterial potential of the coating which is
achieved without silver and therefore is suitable for
medical products like contact lenses.
We showed the possibility to coat a rigid gas
permeable lens material (here fluorsilicone acrylate)
in a way that provides improved wettabillity as well
as antibacterial behavior in a firm way.
Acknowledgments
This
work
was
supported
by
the
“Bundesministerium für Wirtschaft und Technologie
(BMWi)” in context of a ZIM project coordinated by
the AiF.
References
[1] H. Yasuda, “Biocompatibility of NanofilmEncapsulated Silicone and Silicone-Hydrogel
Contact Lenses” Macromolecular Bioscience 6 (2):
121-138 (2006).
[2] C. McKenny, N. Becker, S. Thomas, C. CastilloKrevolin, T. Grant, American Academy of
Optometry, Annual Meeting Optometry and Vision
Science 75 (12), 276 (1998).
[3] L. Ledernez, F. Olcaytug, G.A. Urban, H.K.
Yasuda, „Magnetically Enhanced 15 kHz Glow
Discharge of Methane“ J. Plasma Chemistry and
Plasma Processing 27 (6): 659-667 (2007).