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SURFACE-WAVE-SUSTAINED PLASMA SOURCE FOR
BIOMEDICAL APPLICATIONS
T. Bogdanov, I. Tsonev1, M. Atanasova2, P. Marinova3, Y. Topalova4,
Y. Todorova4, I. Yotinov4, E. Benova5
Medical Faculty, Medical University – Sofia, 1 Georgi Sofiiski Blvd., 1431 Sofia,
Bulgaria
1 Faculty of Physics, Sofia University, 5 James Bourchier Blvd., 1164 Sofia, Bulgaria
2 Faculty of Mathematics and Informatics, Sofia University, 5 James Bourchier Blvd.,
1164 Sofia, Bulgaria
3 Faculty of Forest Industry, University of Forestry, 10 Kliment Ohridski Blvd., 1797
Sofia, Bulgaria
4 Faculty of Biology, Sofia University, 8 Dragan Tsankov Blvd., 1164 Sofia, Bulgaria
5 DLTIS, Sofia University, 27 Kosta Loulchev Street, 1111 Sofia, Bulgaria
Abstract. Argon plasma torch sustained by travelling electromagnetic wave excited by surfatron type wave
launcher at 2.45 GHz is studied in terms of its potential use for biomedical applications. These plasma
sources allows varying of the: geometrical parameters (length, diameter, cross section of the discharge tube),
main plasma parameters (wave power, electron and gas temperatures, concentration of charged particles and
reactive species, UV and microwave radiation), gas and gas mixture parameters (flow velocity, gas mixture
ratio). The fact that we are able to vary these parameters allows us to sustain low temperature plasma torch
(with gas temperature up to 30–37 оС) applicable for treatment of temperature sensitive materials and even
leaving tissue. Investigation is focused on the dependence of the plasma torch length and gas temperature on
the discharge conditions; the treated surface heating by the plasma; the reactive species in the torch. Some
examples of the MW plasma torch biomedical applications are also included.
1. INTRODUCTION
Cold plasma has proven to be an innovative approach for tackling important medical and
biological problems. Plasma medicine is a promising field that combines plasma physics and life
sciences. Plasma medicine is defined as the application of physical plasma for medical purposes.
This multidisciplinary field of science inspires researchers to develop suitable for the given
medical need plasma sources [1–3]. For this purpose two main concepts of plasma devices that
can vary configuration and design are most investigated: the Dielectric Barrier Discharge (DBD)
and the Atmospheric Pressure Plasma Jet’s (APPJ) [4–6]. They are both considered as Cold
Atmospheric pressure Plasmas (CAP) sources. By applying electrical energy to a noble gas (He,
Ar, N2 or their mixtures) hot electrons are produced but the ions and molecules remain with low
kinetic energy. Both concepts have a variety of designs, which follow two simple principles: are
cold (< 40 oC) at sample contact point and are stable under atmospheric conditions. The precise
mechanism of plasma interaction with different biological objects is not completely clear. It is
well known that CAP produce Reactive Oxygen Species (ROS), and Reactive Nitrogen Species
(RNS), which are then transferred to the cell through liquid phase reactions [7]. Other plasma
components that can influence biological responses are UV radiation, electric field/current,
electrons and ions. This unique combination of plasma active components provides a broad
spectrum of application in life sciences. A large variety of effects is discovered, of which a few
are well established [8]: 1) decontamination of a large number of microorganisms; 2) acceleration
of wound healing; 3) plasma induced cancer cells apoptosis. Clear evidences, such as bacteria
decontamination of microflora on skin and sterilization of live rat model wound have been
reported. Cold plasma treatments seem to be very effective for disinfection purposes eradicating
even fungal pathogens. Wound healing is another area of interest, in which a lot of progress has
been done. It is believed that plasma treatment influences the genetic pathways of cells thus
accelerating blood vessel formation and wound healing. Studies prove the elevation of tissue
generation factors after plasma treatment of wounds and show signs of reduced inflammatory
processes.
Both the DBD and APPJ are well-investigated plasma sources. Various methods of investigation
are proposed for plasma diagnostics. Mass spectrometry, TALIF spectroscopy, UV-VIS emission
and absorption spectroscopy are part of the techniques used for diagnostics. Few atoms and
molecules are of the highest interest: OH radicals and hydrogen peroxide molecules H2O2
because of their key role in biological processes; NO, which also plays a crucial role in a large
number of cellular pathways, especially regarding wound regeneration; atomic oxygen because of
the high oxidative potential. Discharges operating in pure gases (Ar, He, N2) or mixtures react
with the air to form the given molecules. Some devices operate directly in air as a working gas.
Unfortunately, for both configurations contact with the treated object is in the region of the
effluent after the active zone of the discharge.
In this work we are investigating a well know microwave surface-wave-sustained discharge
(SWD) for potential biomedical applications. The plasma is sustained by 2.45 GHz
electromagnetic wave excited by a wave launcher surfatron type in Argon at atmospheric
pressure. The discharge conditions have been optimized in order to obtain a steady-state Argon
plasma torch with gas temperature less than 40 °C. High resolution optical emission spectroscopy
is used for diagnostics of reactive molecules that can activate biological responses. Determination
of the gas temperature at the treated surfaces was done by using an IR camera. Some examples of
surface-wave plasma torch bio-medical applications as decontamination of microorganisms and
enhancing wound healing in live mice models are shown.
2. EXPERIMENTAL
The experimental set-up is schematically presented in Fig. 1. Solid-state microwave generator at
2.45 GHz (Sairem, GMS 200 W) was connected by coaxial cable to the commercial
electromagnetic surface wave resonator (Sairem, SURFATRON 80). Argon discharge was
created inside a quartz tube (with real dielectric permittivity εr = 3.2541 and imaginary εi =
0.0062) with dimensions 8 mm
outer diameter to 3 mm inner
diameter. Working gas was argon
5.0 (purity of 99.999 %) at
constant mass flow 2 l/min
controlled by Omega FMA-A2408
mass flow controller. The system
axis was installed vertically with
the gas flow from top to down (see
Fig. 1).
Optical emission spectroscopy
(OES) was used for diagnostics of
the plasma torch. A quartz lens
(diameter – 25 mm, focal length –
35 mm) focused light emitted from
the discharge to a multimode
optical cable connected to a
spectrometer (TRIAX 550). A
3600 gr/mm grating (holographic
blazed for 150–450 nm) was used
for measurement of OH (A→X) 0-
0 band (306.0–310.8 nm) intensity
from which the rotational
temperature was calculated using Boltzmann plot technique. The 1200 gr/mm grating (ruled,
blazed at 550 nm) was used for registering of the Ar* spectra. From the integral intensity of
several argon lines: 603.21, 667.73, 675.28, 687.13, and 714.70 nm with constants given by NIST
the excitation temperature of Argon atoms is calculated. Experiments were conducted for series
of applied microwave power of 15 W, 20 W and 25 W.
3. RESULTS
3.1. Analysis of plasma torch temperature and length as function of the gas flow and
applied power
The Argon gas flow was fixed to 5 l/min and three discharge tubes with outer diameter of 8
mm and varying inner diameter as 2, 3 and 4 mm were used in this experiments.
Figure 1. Scheme of the experimental set-up. 1 – gas tube; 2 – mass
flow controller; 3 – quartz tube; 4 – coaxial cable; 5 – surfatron; 6 –
antenna; 7 – plasma torch; 8 – xyz movable optical line (composed
from parts 9–11); 9 – black rectangular light guide; 10 – quartz lens;
11 – yellow optical filter (optional); 12 – multimode quartz optical
fiber.
776.4 776.8 777.2
1
2
3
O intensity, a.u.
, nm
244 246 248
0.8
1.6
2.4
3.2
NO intensity, a.u.
, nm
NO-gamma 0-2 band 244.5–248.0
nm
Atomic oxygen triplet 776 nm
(a)
(b)
Figure 2. Plasma torch temperature (a) and length (b) as function of power for fixed gas flow and different discharge
tubes.
Figure 2(c). Plasma torch at lower power (left) and
formation of second filament at higher power (right).
Figure 3. Gas flow effect on the torch temperature.
The dependence between the plasma torch temperature at a fixed axial position and the applied
microwave power is presented in Fig. 2(a). It is well visible that for tubes with higher inner
diameter the temperature is significantly lower at the same wave power. The well-known almost
linear increase of the plasma length with increasing the MW power is obtained at low powers
(Fig. 2(b)). Appling higher microwave power results in deviation from the linear increase and
formation of a plateau or decreasing region. This is due to appearance of a second filament with
wave power increasing (Fig. 2(c)). At low inner diameter (2 mm) we observed appearance of the
second filament at wave power higher than 16 W; at 3 mm inner diameter of the discharge tube,
the second filament is formed at powers higher than 20 W. Configuration with the biggest inner
diameter investigated here extends linearly the length of the torch and does not forming second
filament up to wave power of 28 W. From Fig. 2 we conclude that at low power around 6–8 W
the temperature of the plasma torch is lower than 40 oC and the torch has significant length which
allows treatment of various thermo-sensitive materials. The variation of the gas temperature of
the plasma torch with the wave power at different gas flow rates is further investigated for 8/3
mm tube. The results presented in Fig. 3 show clearly decreasing of the gas temperature with
increasing the gas flow. We assume that the higher gas flow results in creation of turbulence,
which cools the plasma torch.
Phantom model of human skin, pigskin, and agar plates were used for further temperature
investigation. IR images of the different objects during plasma treatment further confirm the idea
that higher gas flows cool the plasma torch.
The plasma torch is sustained with applied power of 9 W and gas flow of 6 l/min in 8/4 mm
discharge tube. The thermal camera images of agar plate are presented in Fig. 4. Figure 4(a)
shows the temperature of the agar before plasma treatment and its maximum is about 24 °C. The
IR images during 30 s plasma treatment (Fig. 4(b)) and immediately (up to 15 s) after treatment
stopping (Fig. 4(c)) show that two spots are formed under the torch: a hot spot denoted as EI1 and
a cold one EI2. The cold spot is formed at the region of the treated surface which is touched by
the plasma torch. Its maximum temperature is even lower than the agar temperature before
treatment being about 22 °C during the treatment and decreasing very fast to about 19 °C after
stopping it. One not yet understood problem is the appearance of the hot spot by hot Argon flow
on the side where the plasma torch touches the dielectric wall. Its temperature is much higher
than the plasma torch temperature at the same conditions reaching about 35 °C and 30 °C after
stopping it.
The results are consistent with obtained during treatments of pig skin. Patches of pig skin 5×5
cm2 were treated for 10 s with 10 W of applied power for different gas flows. Increasing the
Argon flow rate from 2 l/min to 4 l/min and then to 6 l/min we obtain decrease of the maximum
temperature of pigskin from 68.9 °C to 41.2 °C and then to 32.6 °C at the highest flow rate.
A phantom model with the same properties as the human skin was treated by the plasma torch
produced in 8/3 mm discharge tube with applied power of 9 W and Argon gas flow 8 l/min. The
phantom has a cubic form
and the discharge was
positioned to treat the
center area of the cube.
Five minutes treatment
time was enough to obtain
a well distinguished hot
spot with temperature
29.4 °C (Fig. 5(b)) while
the initial temperature is
21.1 °C (Fig. 5(a)). The
(a)
(b)
(c)
Figure 4. Petri dish with agar: (a) before treatment; (b) during treatment; (c) after treatment.
(a)
(b)
Figure 5. Phantom temperature: (a) before and (b) after plasma treatment.
position of the spot is at the cube edge suggesting that the plasma column itself is cold but the Ar
gas, which flows between the plasma and the dielectric tube is hot. Increasing the gas flow leads
to lower heat transfer because of the turbulence. In that case the plasma column is moving inside
the tube and is in contact with different parts of the tube and Argon gas thus cannot heat only a
small part of the tube and the Argon to very high temperatures.
3.2. Spectroscopic investigation of the active species in the plasma torch, electron excitation
and rotational temperatures
The dissipation of the electromagnetic wave sustaining the torch leads to its axial inhomogeneity.
The axial distribution of the electron excitation temperature as function of the applied wave
power for given gas flow is presented in Fig. 6.
(a)
(b)
(c)
Figure 6. Electron excitation temperature variation with the wave power at: (a) 2 l/min; (b) 4 l/min; (c) 6 l/min.
Nearly liner decrease in the electron excitation temperature along the plasma torch was observed
in all of the experimental conditions. Increasing the applied MW power also increases the
electron excitation temperature.
(a)
(b)
(c)
Figure 7. Integral intensity of NO-gamma 0-2 in the 244.5–248.0 nm band at: (a) 2 l/min; (b) 4 l/min; (c) 6 l/min.
The dependence of the integral intensity of NO–gamma 0-2 as function of power for given gas
flow is presented иn Fig. 7. One can see that at the lowest gas flow of 2 l/min the intensity
increases at the tip of the plasma torch while at higher flow rates (Fig. 7(b), (c)) it remains almost
constant. At flow of 2 l/min the plasma torch looks to be a relatively homogenous needle and
because of that the interactions with nitrogen from the air are with higher probability at low
flows. Increasing the flow creates turbulences and the plasma no longer had needle-like structure,
which leads to less nitrogen reactions at the tip of the torch.
(a)
(b)
(c)
Figure 8. Variation of integral О 776 nm line intensity with wave power at different gas flow: (a) 2 l/min; (b) 4 l/min;
(c) 6 l/min.
Atomic Oxygen intensity variation with wave power for given gas flow is presented in Fig. 8.
Increase in the intensity at higher wave power is observed at all gas flow rates. Generally, the
atomic Oxygen intensity is higher at lower Argon flow and the reason can be the same as in the
case of NO, keeping in mind that it is produced by reactions of the Argon plasma torch with the
Oxygen in the ambient air.
(a)
(b)
(c)
Figure 9. Integral intensity of OH(A→X) 0-0 in the 306.0–310.8 nm band variation with the wave power at Argon
gas flow: (a) 2 l/min; (b) 4 l/min; (c) 6 l/min.
OH integral intensity presented in Fig. 9 shows strong dependence on the applied power for
higher gas flows (Fig. 9(b), (c)). At low gas flow increasing the applied microwave power does
not change the intensity of the band emission (Fig. 9(a))..
Above results show that because of Argon microwave plasma torch interaction with ambient air
we can detect the same molecules that are controlling basic biological processes inside the human
body and nature. Higher gas flow reduces the temperature of the treated object at the contact
point between the plasma and the samples.
We have also investigated the variation of the same species intensities with the gas flow rates at
fixed microwave power of 15 W. The results at 2, 4 and 6 l/min Argon flow are presented in Fig.
10. We have observed increasing of the OH band intensity at gas flow of 6 l/min (Fig. 10(a))
while the intensity at 4 l/min is even lower than at 2 l/min. Further investigations are needed for
understanding the mechanism of OH radical creation by the MW torch. Atomic oxygen intensity
is not significantly influenced by the gas flow change (Fig. 10(b)). The same is observed also for
the electron excitation temperature shown in Fig. 10(c). The intensity of NO-gamma band
intensity increases around the tip of the discharge at lower gas flows, and remains almost
constant at 6 l/min, which can be explained by the plasma interaction with the atmosphere.
The axial distribution of the rotational temperature obtained from the OH (A→X) 0-0 band
(306.0–310.8 nm) intensity is presented in Fig. 11. One can see that with increasing the gas flow
the rotational temperature decreases. Even so
the temperature is far from being close to the
electron temperature or the gas temperature
measured by the IR camera. This means that
the microwave plasma torch is strongly non-
equilibrium. Conventional method of using the
OH rotational temperature to determine the gas
temperature of CAP cannot be applied to our
discharge. For precise determination of the gas
temperature using optical emission
spectroscopy a self-consistent model is needed
taking into account all of the processes in the
discharge kinetics and the electrodynamics of
electromagnetic wave propagation and
sustaining the plasma torch.
(a)
(b)
(c)
(d)
Figure 10. Variation of: (a) OH (A-X) 0-0 band integral intensity; (b) atomic oxygen О integral intensity; (c) electron
excitation energy of Ar;, (d) integral intensity of NO-gamma band with Argon gas flow at applied power of 15 W.
Figure 11. OH rotational temperature variation with gas
flow
3.3. Effects of microwave plasma torch treatment on selected examples of biological systems
The plasma torch is applied for direct treatment of (i) bacteria for investigation of its bactericidal
effect; (ii) mouse wound for the possible effect in wound healing.
(i) A bacterial strain Pseudomonas sp. AP-9 was used as a suitable model of pathogenic Gram
negative bacteria. Thick layers of Pseudomonas with density from 2×107 to 6×109 cells/ml in
agar plate were treated directly
by the plasma torch. The
sterilization effect dependence
on the discharge conditions was
studied by varying the wave
power from 14 W to 22 W and
the treatment time from 3 s to
20 s. In Fig. 12 well-presented
completely sterilized zones can
be seen (dark zones on the
right). For comparison the non-
treated control without such
zones is presented on the left
photo. The sterilized zones
diameter depends on the wave power and treatment time which is shown in Fig. 13. The diameter
of the plasma torch is about 2 mm and that of sterilized zones can be more than 1 cm with no
movement in radial direction during the treatment. The sterilized zones diameter increases with
increasing the treatment time but does not depend significantly on the wave power and is bigger
at lower bacteria concentration (compare Fig. 13 (a) and (b)). The sterilization was complete,
without any survived colonies and stable (confirmed by more than 168 hours monitoring).
Figure 12. Control (left) and plasma treated agar plates (right) inoculated
with Pseudomonas sp. AP-9.
(a)
(b)
Figure 13. Diameter of sterilization area at various wave power and treatment time at Pseudomonas
concentration: (a) 6×109 cells/ml and (b) 2×107 cells/ml.
(ii) Some preliminary results of mice wound healing by MW plasma torch treatment are obtained.
The wave power was fixed to 9 W and Argon flow was 7 l/min. The treatment was organized in
series: first day 45 s, next three days 15 s each with 24 h interval between treatments. In Fig. 14
the effect on wound closure is presented. The left wound is treated by the plasma torch and the
right one is untreated control. The time necessary for wound closure is normally about 3 weeks.
As one can see, after a week the treated wound is almost closed. Wound treatment in series with
short treatment time accelerates wound closing with days. This effect needs further investigation
with various different schemes of the series but the very positive results shows the applicability
of the microwave plasma torch for such applications.
(a)
(b)
(c)
Figure 14. Effect of mouse wound treatment by MW plasma torch: (a) Day 1; (b) Day 4; (c) Day 7.
Acknowledgments
This work was supported by Bulgarian National Science Fund under Grant No. DN08/8, 2016.
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