On the plasma diffusion through micro-openings
Yang-Fang Li, Julia L. Zimmermann and Gregor E. Morfill
Max Planck Institute for Extraterrestrial Physics, 85741 Garching, Germany
Abstract: The cold atmospheric plasma (CAP) is considered to be a promising
source for instrumental disinfections and bacteria inactivation. Compared to the
conventional liquid disinfection methods, the plasma disinfection works at
atomic/molecular level so that it is capable to penetrate into and pass through
small openings, for example, the interior of hollow needles. We investigate the
plasma diffusion and transport properties through micro-channel plate (MCP)
that consists of collimated micro-channels with a total open area ratio of
approximately 50%. CAP is produced with a surface micro-discharge (SMD)
electrode by using ambient air. MCPs with different sizes of micro-channels and
plate thicknesses have been utilized for the experiments. It is shown that the
bactericidal effect of the SMD plasma can pass through micro-channels. In
addition, the gas plasma flow is collimated along the micro-channels and the
bactericidal efficiency is speeded up by this “channeling effect” when the target
surface area is comparable to the area of the plasma source. Using MCP will then
obviously block the side diffusion of reactive plasma species, which describes a
good method for a targeted delivery of reactive plasma species. Furthermore, UV
radiation from the CAP is significantly reduced when a MCP is placed between
the plasma source and the target surface.
Keywords: cold atmospheric plasma, micro-channel plate, channeling effect,
bacterial inactivation, plasma diffusion and transport
1. Introduction
Recent development of cold atmospheric plasma
(CAP) sources extends plasma treatment to living
tissues. [1-11] A clinical study has been started for
chronic wound treatments using a microwave torch.
[12-14] Its phase II study was recently completed
with more than 1000 treatments. [15] CAP sources
have many advantages in comparison with
conventional disinfection methods. They operate
with electricity and often use the ambient air,
therefore they are free of waste. Plasma treatment is
efficient and can be contact free with target
substances. Plasma disinfection works at
atomic/molecular level that means reactive plasma
species are capable to penetrate into and pass
through micro openings.
The skin, as the body's largest organ, is a promising
target for drug delivery. However, effective trans
dermal drug delivery needs assisted techniques to
enhance the skin permeability. Recent research
showed that the plasma treatment could increase the
permeability of the skin pores. The electrical charge
forces the opening of aqueous pathways through the
lipid bilayer of the stratum corneum which provides
a driving force for the transport of the charged drug
species such as antidepressants. [16] Thus,
investigating the effect of CAP on the skin pore
permeability would open a new possibility for the
drug delivery.
To deliver the reactive plasma species purposespecifically at the atomic/molecular level would be
crucially important for a plasma-assisted drug
delivery technology. This needs to know the
diffusion and transport properties of CAPs
especially at a microscopic level. UV emission from
CAP sources is one of the concerns for safety issues.
It is known that UV radiation plays only a minor
role for plasma bacterial inactivation. [14] However,
excessive UV exposure may cause DNA damage, so
that it is desirable to reduce the UV emission rate of
CAPs as much as possible in order to develop the
plasma treatment for a medical therapy method.
2. Experiments and results
The arrangement of the experiment is sketched in
Fig. 1. The round-shaped micro-channel plate
(MCP) with a diameter of 25 mm is mounted to
cover the through opening with a diameter of 23 mm
in the centre of a Teflon ring adapter. The interface
between the Teflon adapter and the MCP is air
tightened by an O-ring. The Teflon adapter is placed
on a SMD electrode. The gap between SMD
electrode and near side (towards the SMD electrode)
of the MCP is 2 mm, which avoids discharge on the
far side of the MCP. An agar plate with cultured
Escherichia coli (E.coli, DSM1116, density about
2×106 bacteria/cm2) is placed upside down on the far
side of the Teflon adapter. The surface dimensions
of the SMD electrode are around 270 mm × 220
mm. The agar plate has an inside diameter of ~ 85
mm. The distance from plasma to bacterial surface is
approximately 15 mm. Sinusoidal signal with a
frequency of 2 kHz and a peak-to-peak voltage of 10
kV is applied for the plasma generation.
Figure 1. (Not to scale.) Sketch of experiment arrangement with
MCPs. 1: surface plasma source, 2: MCP and 3: agar plate.
Agar plates with plasma treatments or for control (no
plasma treatment) were incubated for 16 hours at a
temperature of 35 °C and then the colony forming
units (CFUs) were counted. The bactericidal
efficiency of the plasma treatment was evaluated by
measuring the bacterial growth inhibited region on
agar plates. In the inhibited region, the bacterial
growth is significantly inhibited because of the
plasma inactivation. Around 5-log reduction of the
bacteria concentration is normally achieved and only
isolated CFUs are found in the inhibited regions.
Fig. 2 shows one example of the treated agar plates
and the black circle describes the inhibited region by
the plasma treatment.
Figure 2. It is shown one bacterial sample after 40 seconds
plasma treatment using a capillary plate with thickness of 1 mm
and micro-channel size of 5 µm. The black circle shows the
bacteria growth inhibited region after the plasma treatment. A
few isolated CFUs exist and around 5-log reduction rate is
achieved in this region.
In Fig. 3, diameter of the bacterial growth inhibited
region by the plasma inactivation is plotted with
respect to the plasma treatment time. Plasma
treatments were repeated three times and the error
bars were calculated to present the standard
deviations. All the MCPs have a same thickness of 1
mm but different sizes of micro-channels. The result
indicates different micro-channel sizes do not
change obviously the bactericidal efficiency by the
plasma treatment. Time dependence of the bacterial
growth inhibited region, the inhibition curve, shows
a three-phase structure. The first phase is limited
within first five seconds, as no obvious inhibition is
found on the agar surface. It is followed by the
second phase that lasts until in total 10 seconds of
plasma treatment. The inhibition diameter increases
almost linearly with the plasma treatment time. After
first two phases, the inhibited region on agar has
approximately the same size as the ϕ23 mm opening
in the Teflon adapter. Afterwards, in the third phase,
increasing of the inhibited region is slowed down
and the inhibition curves suggest the tendency to
saturation.
Figure 3. The inhibition curves of the bacterial treatment by the
SMD plasma when using capillary plates with thickness of 1
mm and sizes of the micro-channels 50 µm (the squares), 10 µm
(the diamonds) and 5 µm (the triangles), and when no capillary
plate was used (the diamonds connected by solid lines.
In addition, the same treatments were also conducted
without MCP, i.e., the ø23 mm opening was left
open. The inhibition curve is also presented in Fig.
3. It is shown that the bactericidal inactivation by the
plasma treatment is actually accelerated by the
micro-channels when the target surface area is
comparable to the area of the plasma source.
3. Block of UV radiation
Figure 4 shows light emission spectrum of the SMD
plasma in the wavelength range of 190 nm and 450
nm measured by an Avantes spectroscopy
(AvaSpec-2048). The detector with collimator was
placed about 3 cm away from the electrode. The
measurement was first made without MCP as it is
presented by the dotted line. Then the spectrum was
measured after three 1 mm thick capillary plates
with different micro-channel sizes: 5 µm for the
solid line, 10 µm for the dash-dotted line and 50 µm
for the dashed line. It is shown that the UV emission
is significantly reduced because of the shielding by
the MCPs. Plates with smaller micro-channel sizes
show higher reduction rate of UV radiation.
Therefore, the MCPs can be also applied to
effectively reduce the UV radiation from CAPs.
Figure 4. UV emission spectrum of SMD plasma in a fully open
situation (dotted line) and when it is blocked by a 1 mm thick 5
µm (solid line), 10 µm (dash-dotted line) or 50 µm (dashed line)
MCP.
4. Reduction of charged particles
Estimated by an air ions counter, it indicates that the
concentrations of both the negative and positive ions
are significantly eliminated when using MCPs.
When no MCP is placed between the plasma and the
target, the volume density of positive ions measured
at the distance for the bacteria treatment is
approximately 106 cm-3, while it is about 103 cm-3
when a MCP is used. The negative ions
concentration is, respectively, 5×103 cm-3 and less
than 102 cm-3 without and with MCP. It turns out
that the significant reduction of the charged particles
concentration does not lead to an obvious difference
of the bactericidal effect. This result indicates that
the charged particles do not play a major role for the
bacterial inactivation by CAPs. The other possible
effective factors for the bacteria inactivation by
CAPS include the electric field, the energetic
photons and reactive neutral species (reactive atoms
such as oxygen, fluorine, ozone, nitrogen oxides,
excited states atoms, and reactive molecular
fragments).
5. Summary
It is shown that the MCPs can obviously increase the
bactericidal efficiency caused by CAPs. The
“channeling” effect will obviously enhance the gas
plasma flow along the micro channels so that
reactive species are more effectively transported to
the targets. When the target area is comparable to the
area of the plasma source, using MCP can speed up
the bacterial treatment in comparison with the open,
unhindered situation. Furthermore, MCP also
significantly reduces the UV radiation reaching the
target. Therefore, MCP would be an ideal tool to
improve the bactericidal efficiency and also for an
accurate location control of plasma treatments.
6. Acknowledgement
This work was carried out in the frame of the plasma
health care project, a collaboration initiated by the
Max Planck Institute for Extraterrestrial Physics. We
would like to thank Hans-Ulrich Schmidt for
providing us the stock cultures of E.coli. YFL would
like to thank M. G. Kong, G. Isbary, T. Shimizu and
T. Klämpfl for discussions.
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