Full Paper
Cold Atmospheric Plasma for SurfaceDisinfection
Yang-Fang Li,* Tetsuji Shimizu, Julia L. Zimmermann, Gregor E. Morfill
A new design evolved from the surface microdischarge electrode system has been developedfor the application of surface disinfection. Having the electrode encapsulated in a dielectricsurface the plasma is produced directly on the surface of the electrode which thereforebecomes disinfected. Experiments with bacteria werecarried out and results show that the surface can bedisinfected within 30 s with typical operating conditions.Even sterilization appears possible in a reasonable time.Possible applications of this electrode design are dis-cussed for regular disinfection of work surfaces, labora-tory benches, etc.
1. Introduction
Recent progress in the development of cold atmospheric
plasmas (CAPs) has accelerated the focus on plasma
applications in health care.[1–10] CAPs are designed to
operate at normal atmospheric temperature and pressure
in air. As shown in various studies,[11–18] CAPs have
bactericidal (including antibiotic resistant bacteria, for
example, Methicillin-resistant Staphylococcus aureus
(MRSA)) and fungicidal properties. These plasma sources
have many advantages compared with conventional
disinfection methods, e.g., low cost, simple design, and
easy handling. Moreover, recent experiments with fibrous
material[19] show that the plasma disinfection and
sterilization works at the atomic/molecular level. Therefore
the reactive plasma species can reach all surfaces and can
penetrate into small openings, including the interior of
hollow syringe which is not accessible for fluid disin-
fectants.[20] In addition, plasma can be used as a prospective
Y.-F. Li, T. Shimizu, J. L. Zimmermann, G. E. MorfillMax-Planck-Institute for Extraterrestrial Physics, 85748 Garching,GermanyFax: þ49 89 30000 3950; E-mail: [email protected]
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tool for medical applications instead of or together with
current standard treatments with fluids and ointments.
In health care, CAPs can be used in two fields: hygiene and
medicine application. In the field of hygiene, disinfection
and sterilization are the main issues, i.e., to kill bacteria,
fungi, viruses, and spores. Treatment with plasma allows
efficient disinfection in a very short period of time
(seconds). Therefore, the plasma technique is very promis-
ing for the containment of infectious diseases in both public
and personal areas. In the field of medicine application,
there are several potential applications. Plasma produces
biologically reactive species (of limited life time) – charged
particles, chemically reactive species and UV light – which
affect tissue. By ‘‘designing’’ these components, CAPs can in
principle be used as a therapeutic tool, e.g., for treatment of
chronic wounds.[13,21]
Our group has developed and tested a large area plasma
dispenser based on surface microdischarge (SMD) electrode
technology.[20] This technology provides a robust and
scalable design. In this paper, we describe a modified
SMD electrode which is completely encapsulated in a thin
insulating layer. Plasma is then produced on the surface of
this insulator. Such a plasma device is then water-tight,
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Y.-F. Li, T. Shimizu, J. L. Zimmermann, G. E. Morfill
easy to clean, and free of certain chemical contaminations
and corrosion. Using this design, objects which are not in
contact with the insulating surface as well as the insulating
surface itself can be disinfected. Such electrodes (provided it
is sufficiently large) can be used as a work surface in
laboratories, the food industry, private homes, etc.
Figure 1. (a) The sketch shows the electrode arrangement of theSSS (HV: high voltage signal). The size of the large area electrodeis 9 cm in diameter. (b) The photography shows the device withplasma ignited on the Epoxy surface. The size of the discharge isapproximately 9 cm in diameter and the Epoxy board has adimension of 12 cm� 12 cm. (c) The optical emission spectrummeasured (HAMAMATSU, TM-UV/VIS: C10082CA, MiniSpectrom-eter, 200–800 nm) for the discharge. It is from the secondpositive system of Nitrogen molecules. The main peaks arelabeled with their wavelengths.
2. The Self-Sterilizing Surface
The electrode arrangement of the self-sterilizing surface
(SSS) is based on the principle of the SMD electrode as
reported by Morfill et al.[20] The SMD electrode has a
sandwich structure which consists typically of a large-area
powered electrode, a dielectric layer, and a grounded mesh
electrode. The plasma is ignited on the side of the mesh
electrode. A printed circuit board (PCB) with laminated
copper foil on both sides is utilized in our study.
The SSS device is fabricated from a commercialized PCB.
The dielectric layer of the PCB is made of Epoxy with
dimensions of 12 cm� 12 cm and has a thickness of 1.5 mm.
The copper foils which are laminated on both sides of the
dielectric layer are 0.1 mm thick and with size of 9 cm in
diameter. The sketch in Figure 1 shows the electrode
arrangement of the SSS used in this study. Below the Epoxy
board the copper foil with a large area functions as the
powered electrode. Above the Epoxy board the copper foil is
fabricated into two electrically isolated wire electrodes, i.e.,
‘‘wire electrode 1’’ and ‘‘wire electrode 2’’ in the sketch. The
distance between two wire electrodes is 2 mm. The wire
electrodes are covered by a thin layer of glass Epoxy
(https://www.epotek.com/SSCDocs/datasheets/920-
FL.PDF) with a thickness of approximately 0.2 mm. The large
area electrode and one of the two wire electrodes are
powered. The other wire electrode is grounded. The plasma
is ignited on the surface of the glass Epoxy and the height of
the plasma glow above the Epoxy glass is <1 mm. The
device is presented as the lower image in Figure 1 when the
plasma was ignited on the Epoxy surface.
The voltage signal is provided by a Voltcraft 7202 Sweep/
Function Generator and amplified by a TREK PM04015 High
Voltage AC/DC Generator. A square waveform was used
for igniting the plasma. Compared with sinusoidal and
triangular waveforms the voltage for igniting plasma was
lower when a square waveform was used.
3. Results and Discussion
In our study, Escherichia coli (E. coli, DSM1116) were used.
Number of bacteria on agar plate, which is about 90 mm in
diameter, was estimated by counting the colony forming
units (CFUs) after 16-h incubation at a temperature of 35 8C.
A voltage signal of 2 kHz with peak-to-peak amplitude of
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18 kV was applied to the electrode. Figure 1c shows the
optical emission spectrum of the discharge measured by a
spectrometer (HAMAMATSU, TM-UV/VIS: C10082CA) with
integration time of 0.1 s. The emission is from the second
positive system of Nitrogen molecules. No detectable
signals from NO were found. The possible reason can be
that the discharge produced large amount of O3 and the
lifetime of NO was too short to be detected because of
oxidation by O3. No emission signals from the first positive
system of the Nitrogen molecules were detected. The B3Pg
state of N2 may have interacted with other species instead
of decaying to the meta-stable state. The UV emission
measured by a UV power meter (HAMAMATSU, C8026) at a
DOI: 10.1002/ppap.201100090
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Figure 3. Surviving number of CFUs on the agar samples byapplying plasma treatment with different plasma-on times (firstnumber in the horizontal coordinate) and afterglow time (secondnumber in the horizontal coordinate). The control shows thenumber of CFUs on the agar plates without applying any plasmatreatment. The numbers inserted above each column (withplasma treatment) refer to the log-reductions.
Figure 4. Survival ratio of bacteria after plasma treatment by theSSS with respect to different treatment times and for differentbacteria dilutions.
Figure 2. The voltage and the current waveforms for the dischargeare shown with light gray line and the black line, respectively, andthe power density absorbed by the discharge is about 1 W � cm�2.
The Self-Sterilizing Surface
distance of 1 cm from the discharge was approximately
0.5 mW � cm�2. The current as well as the voltage waveforms
were measured and presented in Figure 2. To get the current
waveform, a pure resistor of 3.3 V was connected to the
discharge circuit in serial and the voltage signal on this
resistor was detected with a LeCroy active voltage probe
(Model: HFP2500, Bandwidth: 2.5 GHz). Both the voltage
and the current waveforms were recorded by a LeCroy
WavePro 725Zi oscilloscope (Bandwidth: 2.5 GHz) with a
sampling rate of 20 G � s�1. The power density absorbed by
the discharge is about 1 W � cm�2. Bacterial sample was
placed upside down on the SSS. The distance between the
surface where the plasma was ignited and the bacterial
surface was 7 mm. We first did a series of experiments to
check the effect of afterglow and the result is presented in
Figure 3. Every treatment was repeated three times and also
three plates were used for the control. The error bars shown
in the plot were calculated as the standard deviations. The
ambient temperature and relative humidity for the after-
glow experiment are 24 8C and 60%, respectively and the
cell density on the agar plates before the plasma treatment
was about 3.3� 105 cm�2. The first four treatments have the
same total treatment time, including the plasma-on time
and the afterglow time. It shows that, by including 28 s of
afterglow effect, even 2 s plasma production can achieve
approximately 5-log reduction. Given the same total
treatment time, increasing the plasma-on time until 10 s
does not significantly increase the log-reduction of the
bacterial killing. In addition, when the plasma-on time for
the treatments is the same (15 s, the last two treatments in
Figure 2), including 15 s afterglow time can obviously
increase the bacterial killing rate.
A recent report showed that the bactericidal efficiency by
CAPs also depends on initial bacteria concentration of
cultured samples.[22] Therefore we performed the bacterial
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experiments with different initial concentrations of E. coli
on the agar plates. In this experiment, the afterglow was not
used (The bacterial samples were immediately removed
from the SSS electrode after the plasma treatment). The
ambient temperature and relative humidity for this
experiment were 24 8C and 55%, respectively. To improve
the statistics, every treatment was repeated three times
and standard deviation of the CFUs was calculated. The
obtained results are presented in Figure 4. Before plasma
treatment, approximately 107 E. coli bacteria were cultured
on the agar surface. The survival curves for different
dilutions of this basic culture show a bi-phasic structure
similar to that found in ref. [23], although the survival ratios
of bacteria are different for a given treatment time. The first
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Figure 5. Direct bacteria inactivation experiment on the SSS usingmembrane filters. Image (a) and (b), respectively, show the CFUson the membrane filter without and with plasma treatment. Thearea covered by the CFUs (normalized by the filter area) is plottedin (c).
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Y.-F. Li, T. Shimizu, J. L. Zimmermann, G. E. Morfill
phase takes place in first 2 s where a very high rate of
bacteria inactivation is obtained. For longer treatment
times, the inactivation rate increases comparatively slowly.
For the master suspension a 4-log reduction of the bacteria
number is achieved within 2 s. This result indicates that the
decimal time (D-value, time to reduce the number of
survivors by one log) of the plasma is much <1 s. Although
not presented here, it was found all bacteria were killed
after 30 s plasma exposure. Figure 4 shows higher
inactivation rates for higher initial bacteria concentrations.
The explanation can be straightforward. Even for the
highest initial concentration, the agar surface was not
covered totally by E. coli. Given that bacteria were
homogeneously distributed on the agar surface, the
shielding and stacking effects of bacteria played no
role. The size of the SMD plasma was larger than the size
of the agar plate. The plasma treatment was applied to
the whole agar plate and each bacterium had the same
probability to receive the same dosage of plasma treatment
from the beginning of the plasma treatment. Therefore the
plasma treatment had high probability to kill more bacteria
when the initial concentration was higher. The experiment
presented in ref. [22] has a major difference from ours. In
ref. [22], a plasma jet was used and size of plasma (cross
section of the plasma plume) was much smaller than the
surface area of the bacterial samples. The plasma inactiva-
tion started from the center of the bacterial samples and
then dispersed to larger sample area when longer plasma
treatment was applied. Most of the bacteria were shielded
from the plasma treatment by the bacteria closer to the
sample center. The shielding effect played a major role and
their results showed higher bacterial inactivation rate for
lower initial bacteria concentration.
In order to test for bacteria inactivation directly on the
SSS, bacterial experiment was conducted by using mem-
brane filters: [24] In this case, the master bacteria suspension
was applied directly to the SSS. After a drying time of
10 min, the plasma was switched on for a chosen treatment
time and subsequently the bacteria on the SSS were
collected using a membrane filter (analogous to the
technique used in ref. [13]). The result is shown in
Figure 5. Without plasma treatment, more than 75% of
the filter area is covered by overlapping CFUs. After a
plasma treatment time of 30 s only isolated CFUs were
found on the filter and the CFU covered area was <10% of
the filter area. We are therefore able to show that the
bacteria concentration is significantly decreased after 30 s
of plasma exposure. This experiment was also repeated
three times. The results led to a good reproducibility and the
standard deviation is presented in Figure 5c. Although the
electrode surface was kept drying for 10 min, the humidity
in the ambient air (ambient conditions for the experiment:
relative humidity of 60% and temperature of 25 8C) was
sufficient to keep the bacteria alive at least for short period.
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This was confirmed by the non-exposed sample presented
in Figure 5a.
4. Conclusion
It could be shown that a fully encapsulated SMD electrode –
the SSS – has high bactericidal effect directly on the electrode
surface. The design of the SSS can be easily improved by
optimizing the distance between the electrodes, the thick-
ness of the surface and the material for the dielectric layer.
Thus the SSS has a potentially large range of application
areas, such as in food hygiene, biological laboratories,
domestic kitchens etc. – i.e., in any surface which is
intensively used and therefore needs a regular disinfection.
Acknowledgements: This work was carried out in the frame of theplasma health care project, a collaboration initiated by the MaxPlanck Institute for Extraterrestrial Physics. We would like tothank Hans-Ulrich Schmidt for providing us the stock cultures ofE. coli and Georg Isbary for the membrane filters.
Received: May 24, 2011; Revised: October 5, 2011; Accepted:October 20, 2011; DOI: 10.1002/ppap.201100090
DOI: 10.1002/ppap.201100090
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The Self-Sterilizing Surface
Keywords: atmospheric pressure glow discharge; self-steriliza-tion; surface disinfection; surface micro-discharge
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