22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma sterilization of poly(tetrafluoroethylene) films
L. Benterrouche1, S. Sahli1, A. Bellel2, N. Kacem Chaouch3, M. T. Benabbas1 and S. Benhassine3
1
University of Frères Mentouri, Laboratory of Microsystems and Instrumentation (LMI), Electronic department,
Faculty of Technologies Sciences, Constantine, Algeria
2
University of Frères Mentouri, Laboratoire d'étude des Matériaux Electroniques pour Applications Médicales
(LEMEAMed), Constantine, Algeria
3
University of Frères Mentouri, Laboratory of Mycology, Biotechnology and Microbial Activity (LaMyBAM),
Microbiology department, Nature and Life Sciences Faculty, Constantine, Algeria
Abstract: Poly(tetrafluoroethylene) (PTFE) surface films contaminated with Escherichia
coli (E. coli) bacteria, have been sterilized by a pulsed dielectric barrier discharge (DBD)
plasma generated in air at atmospheric pressure in a controlled chamber using a homemade
high voltage power supply. Effects of time treatment variations and DBD plasma treatment
mode as well as the surface state of PTFE substrates on microorganism’s inactivation
efficiency have been investigated.
Keywords: atmospheric air plasma, DBD, PTFE, bacteria, E. coli, sterilization
1. Introduction
Because of their ambient working conditions
(temperature and pressure), great interests have been
focused these last years on the use of Dielectric Barrier
Discharges (DBDs) created at atmospheric pressure in
many biomedical applications [1, 2]. Among this type of
cold plasmas applications, the developing of new
sterilization processes, especially for heat sensitive
polymer materials, is the most invested [3–5]. Due to their
safety for both the operator and the patient, the plasma
sterilization process becomes an alternative promising
technique to other conventional sterilization methods such
as gamma ray irradiation and ethylene oxide (EtO) gas.
Generated at room temperature and atmospheric pressure,
many sorts of reactive species created with DBD
atmospheric plasmas discharges, such as radicals, UV
photons, atoms, electrons, positive and/or negative ions
[6, 7], can directly or indirectly interact with
microorganisms and thus lead to the death of not only
pathogenic bacteria, but also the highly resistant
microorganism [8].
In this work, dielectric barrier discharge plasma was
generated at atmospheric air in controlled homemade
reactor using a homemade-pulsed high voltage power
supply. The plasma is used to inactivate E. coli bacteria
spread out on the surface of transparent medical poly
(tetrafluoroethylene) (PTFE) films. Effects of plasma
treatment time, plasma treatment mode and the initial
substrate surface state on the sterilization efficiency are
studied using optical microscopy observations, loss mass
and contact angle measurements.
2. Materials and methods
Figure 1 shows the experimental set up used for the
sterilization process. It consists of a DBD reactor
constituted of two plane-parallel metallic electrodes with
P-III-10-2
80 mm of diameter, spaced by a gap varying from 1 to 5
mm.
Fig. 1. Schematic diagram of the DBD plasma reactor.
The lower electrode used as a substrate holder was
grounded and the upper one was covered with a glass
layer of 1.3 mm of thickness and 110 mm of diameter.
This electrode was connected to a pulsed high voltage
power supply to generate the DBD plasma in the
atmospheric air gap. The DBD plasma reactor was
mounted in a homemade test chamber with the
dimensions of 140 mm (width) x 140 mm (depth) x 100
mm (height).
In
this
study,
medical
transparent
poly
(tetrafluoroethylene) (PTFE) films of 50 μm in thickness
were used. First, the films were cut into sections of
dimensions 20 x 20 mm2, each one was washed
successively in Bleach, methanol and twice-distilled
water and then dried naturally at room temperature. Few
of these washed substrates were then treated by air
atmospheric pressure DBD plasma during 5 min (pretreated films). 100 μl of E. coli bacteria were transferred
1
and spread out on the surface of the only washed and on
the plasma pre-treated PTFE films and then dried at room
temperature for 1 h. After that, these contaminated PTFE
samples were exposed to air DBD plasma in direct or
remote mode. Samples treated in remote plasma mode
were placed outside the plasma discharge zone, far from it
of an arbitrary distance of 20 mm (Fig. 1). For each
treatment time, triplicate samples were prepared for
different measurements and observations. All experiments
were carried out using air as working gas; the applied
high voltage as well as its frequency and the electrode
discharge gap were fixed to 9 kV, 0.2 kHz and 3 mm
respectively.
The surface wettability of the PTFE films and its
evolution as function of their DBD plasma treatment time
in atmospheric air was characterized by contact angle
technique. All the measurements were carried out at 26°C
and 42% RH. A distilled water drop of 5 μl was delivered
by a micro syringe onto the films surface immediately
after plasma treatment experiments. The contact angles
were measured at least three different locations on the
treated samples and a maximum error less than ± 2° had
been recorded.
The effect of the DBD plasma treatment on the
sterilization process has been investigated by optical
microscopy and by measuring the weight loss of the E.
coli contaminated layers spread out on the PTFE films
surface. The weight of each sample was measured before
and immediately after the plasma treatment, using a
microbalance (Adventurer OHAUS, AR0640). The mass
loss induced by the DBD plasma treatment was calculated
using the following expression [9]:
Mass Loss = (
M0 − Mt
M0
) 100%
Fig. 2. Optical microscope photograph (1000 x
magnification) of E. coli distribution on the surface of
PTFE films before plasma treatments.
a)
b)
(1)
Where M 0 and M t are the weight of sample before and
after plasma treatment, respectively.
3. Results and Discussion
Figure 2 shows an optical microscope photograph of E.
coli bacteria spread out on the PTFE films before any
plasma treatment. A high bacterial concentration is
observed on the surface of the PTFE substrate films. This
bacterial concentration decreases significantly after the
exposition of the contaminated PTFE surface to a DBD
plasma discharge during 15 min (Fig. 3(a)). This
concentration decrease is induced by the interaction of a
large amount of plasma reactive species created in the air
(such as hydroxyl radicals (OH), ozone (O 3 ), UV
radiation, charged particles and other energetic species)
with the bacterial structure. These plasma species interact
directly with Escherichia coli bacteria structure leading to
an ablation process of their membrane constituents [10,
11]. An etching effect of the bacterial structure induced
by the charged and the energetic plasma species [12] can
also contribute to the removal of the bacteria membranes.
However, this behaviour of the bacterial concentration is
2
dependent of the location of the samples in the plasma
chamber.
c)
Fig. 3. Optical microscope photographs (1000 x
magnification) of E. coli distribution after 15 min of (a)
direct plasma treatment; (b) remote plasma; (c) direct
plasma of pre-treated PTFE films.
The large disappearance of the E. coli cadavers
observed on the contaminated film substrates placed on
the lower electrode and exposed directly to the plasma
discharge (Fig. 3 (a)) is less pronounced in the case of
samples treated far away from this plasma discharge zone
(remote plasma mode) (Fig. 3 (b)). The concentration of
the survival and/or bacteria cadavers is more important on
contaminated surface films treated in this mode than on
those treated in the direct plasma discharge mode. As in
the remote plasma species are less energetic and/or less
reactive than those found in the discharge mode, this
difference in the sterilization efficacy is due to an
exposition of the microorganisms to less UV radiations
and to less reactive and/or energetic plasma species [2].
Figure 3 (c) shows that the efficacy of the DBD plasma
treatment on the microorganisms inactivation is more
P-III-10-2
120
Contact angle (°)
105 Untreated
90
75
60
45
30
0
5
10
15
20
Treatment time (min)
Fig. 4. Effect of plasma treatment time in direct plasma
mode, on the contact angle of PTFE films.
On figure 5 is reported the variation of the loss mass of
the contaminated pre-treated and untreated PTFE
substrates as function of the DBD plasma treatment time.
The loss mass increases with the increase of the
exposition time to the air DBD plasma and is dependent
of the plasma treatment mode and the surface state of the
polymer. It was found more significant for films treated in
direct plasma mode and more pronounced for
contaminated pre-treated films. This behaviour of the loss
mass with the DBD plasma treatment time, the location of
the samples in the reactor and the surface state of the
PTFE films can be explained by an ablation effect of the
P-III-10-2
bacteria membrane structure during their interaction with
the reactive plasma species and by an etching effect of
these membranes by the energetic plasma species. This
result confirms the optical microscopy observations
presented on Fig. 3.
2,0
Loss Mass (%)
pronounced for pre-treated PTFE substrates. A quasi-total
disappearance of the E. coli cadavers was obtained on
these samples after their exposition in a direct plasma
mode, to the air DBD plasma discharge during 15 min.
This improvement of the microorganism inactivation
efficiency is due to the difference of the wettability
between the untreated and pre-treated PTFE substrates.
Figure 4 shows that the contact angle of PTFE surface
films decreased significantly with the increase of plasma
treatment time. From about 105° for untreated PTFE
substrates (control), the contact angle decreases to 65° for
substrates treated during 5 min by an air atmospheric
DBD discharge prior to the bacteria spreading process.
The films surface wettability increase allows a better
microorganisms culture spreading and then, a more
homogeneousness thickness of the culture layer is
obtained on the PTFE plasma pre-treated substrates. In
contrary, the hydrophobicity of the untreated PTFE
surface makes difficult the spreading process of the
bacteria on the polymer surface, leading to the formation
of nonhomogeneous islands of bacteria culture. The
thickness of these islands-like is more important than that
of the homogenous microorganism’s layer obtained on
pre-treated substrates. Because of the difference in their
thickness, the bacteria culture spread out over the
untreated surface takes more time to be removal than that
spread out over the pre-treated surface.
1,5
1,0
Direct Plasma
Remote Plasma
Pretreatment
0,5
5
10
15
Treatment time (min)
Fig. 5. Effect of plasma treatment time on loss mass of E.
coli contaminated PTFE films.
4. Conclusion
Using a homemade-pulsed high voltage power supply,
dielectric barrier discharge plasma was generated in air at
atmospheric pressure in controlled chamber to inactivate
E. coli bacteria spread out on the surface of medical poly
(tetrafluoroethylene) (PTFE) films. The effects of plasma
treatment time, plasma treatment mode and the initial
surface state of the sterilized substrates on the sterilization
efficiency were investigated. Results show less
pronounced sterilization efficiency for samples treated by
remote plasma mode and slightly more effective in the
case of plasma discharge mode (direct plasma mode).
However, contaminated samples previously treated by air
atmospheric pressure DBD plasma show the best
sterilization efficiency effect.
5. Acknowledgment
This work was supported by the Algerian Thematic
Agency of Research in Sciences and Technology
(ATRST).
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