115)Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Decontamination Effects of Low-temperature Plasma Generated by Corona Discharge Part I© Charles University in Prague – The Karolinum Press, Prague 2007
Decontamination Effects
of Low-temperature Plasma
Generated by Corona Discharge
Part I: an Overview
Scholtz V.1
, Julák J.2
, Kříha V.3
, Mosinger J.4
1
Department of Physics, Faculty of Electrical Engineering and Information
Technology, Slovak University of Technology, Bratislava, Slovak Republic;
2
Institute of Immunology and Microbiology, Charles University in Prague,
First Faculty of Medicine, Prague, Czech Republic;
3
Department of Physics, Faculty of Electrotechnics, Czech Technical University,
Prague, Czech Republic;
4
Department of Inorganic Chemistry, Charles University in Prague, Faculty
of Science, Prague, Czech Republic
Received April 12, 2007; Accepted May 23, 2007.
Key words: Bacteria – Corona discharge at atmospheric pressure –
Nonthermal plasma – Singlet oxygen – Sterilization – UV radiation
The financial support provided by grants MSMT ČR 0021620806
and GA ČR 202-03-H162.
Mailing Address: Associated Professor Jaroslav Julák, MSc., PhD.,
Institute of Immunology and Microbiology, First Faculty of Medicine, Studničkova 7,
128 00 Prague 2, Czech Republic; Phone: +420 224 968 461;
e-mail: jaroslav.julak@lf1.cuni.cz
116) Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Scholtz V.; Julák J.; Kříha V.; Mosinger J.
Abstract: The first part of this article is devoted to a minireview of basic terms of
plasma physics and chemistry described in a way understandable even to
nonspecialists in the field. Among the methods of generation of low-temperature
plasma, the unipolar electric corona discharge in air at atmospheric pressure is
described in more detail. Selected studies are mentioned that concern the
decontamination effects of low-temperature plasma and the currently known
mechanisms of its microbicidal action. The key part of this mechanism is the action
of UV light and both charged and uncharged particles. The most important
common mechanism seems to be different forms of reactive oxygen species and
some methods of their determination are therefore briefly described.
Introduction
The term decontamination denotes a set of methods and procedures used to
partially or completely destroy or remove microorganisms, i.e. viruses, bacteria
and eukaryotic organisms, from the given location. Although a large number of
decontamination methods is known and used, no universal method is available.
Based on their efficiency, the decontamination methods are grouped into several
categories. The highest level of decontamination is sterilization, which is required
to eliminate all viable forms on a surface or inside a given system (for more details
see, e.g., [1]).
One of the possible methods of decontamination or sterilization is the action of
electric discharges and the plasma generated by them. The method is not yet
frequently used in the practice but it is potentially important, especially for the
decontamination or sterilization of heat labile or otherwise sensitive materials. The
microbicidal properties of plasma and possibilities of their use have been the
subject of many studies of foreign [e.g. 2–9] and Czech authors [e.g. 10–13]. Their
results are discussed below.
An example of a practical application of this kind of decontamination is the
apparatus called Plasmacluster Ion Generator [14], which relatively efficiently kills
off bacteria in the air through the action of H+
and O2–
produced by
decomposition of the water vapour. Another application is the use of the Sterrad
sterilization chamber that dates from 1993 and is based on a combined action of
hydrogen peroxide vapours and the low-temperature plasma. A typical
sterilization cycle of this system includes evacuation of the chamber and a
subsequent injection of a measured amount of hydrogen peroxide whose vapours
sterilize the surface of the objects placed in the chamber. An electromagnetic
field at radio frequency 13.45 MHz is then applied on the chamber contents, in
which the hydrogen peroxide vapour breaks apart, producing a low-temperature
plasma cloud that generates ultraviolet light and free radicals. The sterilization
process takes ca. 1 hour depending on the selected regime and the temperature
does not exceed 45 °C. The process is highly efficient and cost-effective
(see, e.g., [35]).
117)Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Decontamination Effects of Low-temperature Plasma Generated by Corona Discharge Part I
Low-temperature plasma as a decontamination agent has the following advantages:
■ it does not raise the temperature of the decontaminated material,
■ purchase costs are low and operational costs are negligible,
■ no contamination of surface with chemicals occurs,
■ the decontamination need not take place in a closed chamber,
■ decontaminated objects are immediately ready to use,
■ the process does not generate any side products,
■ it presents no problems with storage of the decontamination agent.
The studies published so far on the subject are difficult to compare and reproduce
since they use different types of discharges that generate plasma under different
conditions, and make use of a variety of experimental parameters. In addition, the
decontamination properties of the plasma or the plasma-generating discharges are
rarely addressed in a sufficiently complex manner and are usually demonstrated on
a single or a low number of tested microorganisms; the decontamination
properties of the given method are often stated only qualitatively. Model organisms
were frequently only vegetative forms of bacteria; when the much more resistant
spores were tested; no comparison is often given with the effectiveness of the
treatment in the vegetative forms. Elimination of resistant spores, especially bacilli
and clostridia, represents a specific problem in decontamination. The possible
mechanisms and causes of the decontamination action of different discharges have
not yet been fully clarified despite intensive research, due to the complexity of
physical and chemical processes taking place in the discharges. Among the
potential mechanisms is the action of UV radiation, charged particles, free radicals
or reactive oxygen species. The relative shares of these mechanisms in the final
effect, their details or possible synergistic actions are not yet fully clear.
Our study aims at contributing to a complex characterization of the
decontamination properties of low-temperature plasma generated by a corona
discharge at atmospheric pressure. Its first part brings a concise overview of basic
terms relevant in the field and some earlier results. The second part brings more
recent results on the action of the corona discharge on a set of model
microorganisms on different surfaces. The attendant problems and results are
described in a way that should be comprehensible even to readers unfamiliar with
plasma physics and chemistry. A more detailed treatise of the data gathered here
can be found in [15].
Plasma
Plasma is by definition [16] a quasineutral gas formed by neutral as well as charged
particles (ions and electrons) that exhibit collective behaviour. Ionization occurs
when an electron gains sufficient energy for overcoming the attractive force of
atom nuclei. Electron can gain this energy by, e.g., heating of a neutral gas to
sufficiently high temperature of at least several thousand Kelvins. The plasma so
118) Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Scholtz V.; Julák J.; Kříha V.; Mosinger J.
formed, in which both electrons and ions have about the same temperature, is
called thermal or isothermic plasma. The radiation emitted by thermal plasma has
a continuous heat radiation spectrum. This plasma is found, e.g., inside stars, in the
initial stages of a nuclear explosion, in an electric arc or in a lightning channel. In
terms of sterilization, it represents an ultimate solution in which the contaminated
object is incinerated at a very high temperature.
Ionization can occur also by other mechanisms, in particular by collisions of
electrons in electron shell with free accelerated electrons or as a result of an
external photoelectric effect. These processes give rise to plasma in which the
temperature of ions and neutral atoms is considerably lower than that of electrons.
This plasma is then called low-temperature, nonthermal or nonisothermic.
Accelerated electrons can be supplied to the plasma from outside in the form of an
electron beam, or the electrons can be accelerated directly inside the plasma
volume by an electric field or electromagnetic radiation. The photons necessary to
produce the photoelectric effect can also be provided from the outside or they
can be emitted by the discharge itself. Nonthermal plasma emits the light at
discrete wavelengths that correspond to the transitions of electrons in atom or
molecule shells in both the visible and the ultraviolet (UV) part of the spectrum.
Due to the low heat capacity of the electron component, the application of
nonthermal plasma on surfaces does not produce any considerable heating of the
surface. High-energy electrons in a relatively cold gas produce a number of
nonequilibrium chemical reactions called plasmachemical reactions. Even in a
simple mixture of oxygen and nitrogen, the plasma is the site of hundreds of
chemical reactions. The plasma chemical reactions proceed under the direct
participation of free electrons and photons of the radiation emitted by the
discharge.
In addition to the emission of UV radiation, an important property of the
low-temperature plasma is the presence of high-energy electrons, which are highly
reactive and cause numerous chemical and physical processes such as oxidation,
excitation of atoms and molecules, production of free radicals and other reactive
particles. This kind of plasma is most often generated by electrical gas discharge,
especially microwave discharge, dielectric barrier discharge and corona discharge.
Microwave discharge is produced either in the absence of electrodes by ionizing
electrons by external electromagnetic field, or by using metal electrodes inside
waveguides or resonators. The amplitude of the electric component of the
microwave radiation has to be sufficient to cause breakdown. The frequency of
the driving electromagnetic field is in the gigahertz range. In view of the high
frequency of the external field, no spark channels are formed.
The properties of different kinds of discharges and types of plasma are described
in more detail, e.g., in [17, 18, 19]. The properties of the plasma produced by a
unipolar corona discharge, which is the main subject of this study, are briefly
described below.
119)Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Decontamination Effects of Low-temperature Plasma Generated by Corona Discharge Part I
Corona discharge
Corona discharge can be observed only in an inhomogeneous electrical field
formed between two or more electrodes. At least one of the electrodes has to
have a small curvature diameter (the active electrode or coronizing electrode).
In an experiment, this electrode is usually realized as a point electrode (using
e.g. a needle) or a thin wire. Around this electrode is formed the light-emitting
part of the discharge which, in the case of a point electrode, has the form of
a crown, giving the name to the discharge (the Latin corona = crown). This
phenomenon, called St. Elmo’s fire, has long been known to sailors since it has
appeared at the end of ship’s masts and spars during storms. The other
electrode can be a plane electrode in the form of any planar conductor, or a
conductor with an order-of-magnitude larger curvature radius than the
corona-forming electrode. In this case, the discharge is called a unipolar corona
discharge; it can be positive or negative depending on the polarity of the
corona-forming electrode. The electrode system can also feature small
curvature diameters of both electrodes; in this case, a bipolar corona is formed
around both the cathode and the anode. On switching on the voltage,
a high-intensity electric field is formed in the close vicinity of the electrode with
a small curvature diameter. When this field attains sufficient intensity, local
electron avalanches and a local breakdown are formed within it; they however
cannot spread into the whole inter-electrode space. This close vicinity of the
electrode, in which ionization takes place, is called the ionization region. It can
be observed as a weakly lucent space around the point electrode (corona in the
stricter sense of the word). The ionization region is surrounded by a dark outer
region. The current in the outer region of a unipolar corona discharge consists
predominantly of ions with the same charge.
Our work concerns solely the direct-current negative point-to-plane corona
discharge whose typical configuration is schematically illustrated in Fig. 1.
As shown in the figure, a small ionization region is formed around the point
electrode; the high-intensity electric field triggers here avalanche ionization.
Apart from the ionization region, the space between the electrodes contains a
relatively large drift region with a low intensity of the electric field. The
current in this region is conducted by ionized gas molecules; electrons and
ions drift here and interact with neutral particles but lack sufficient energy to
cause their ionization. The density of charged particles in this region is
relatively low so that their mutual interactions are negligible. With the current
of the order of microamperes and higher, the negative corona burns in a
stable way. Negative coronas in an electronegative gas such as the
oxygen-containing air are characterized by the appearance of current pulses
with a frequency of hundreds to thousands kilohertz (named the Trichel
pulses); this frequency is so high that the ion flux and the drift regions appear
to be continuous.
120) Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Scholtz V.; Julák J.; Kříha V.; Mosinger J.
The distribution of current density in a corona discharge is described by the
Wartburg law (see e.g. [17, 18]) that was determined empirically and is
mathematically described as follows:
j(r,d) = j0
cos5
1; 1 < 65°;
j0
» I / 2d2
where j(r,d) is current density, j0
is corona axis current density, Q is the apex angle
defined by the equation Q = r / d, I is the total corona current, r and d are radial
and axial distances from the tip of the point electrode, respectively. The meaning
of the symbols is also apparent from Figure 1.
The corona discharge burning at atmospheric pressure in air is one of the
simplest, and thus well-reproducible, sources of low-temperature plasma. In
contrast to other kinds of discharges, which usually require a construction of a
complex device, it can be generated in a simple device with low first expenses and
negligible operating costs. It is therefore suitable as a model for experimental
research into the microbicidal properties of electric discharges.
Possible mechanisms of the microbicidal action of low-temperature plasma
Although the killing of microorganisms by low-temperature plasma is known (see
e.g. [20]), it has not yet been satisfactorily studied and elucidated. The current
explanations include the action of
■ UV light and/or
■ reactive particles, which can carry electric charge.
The most important among these particles are apparently various reactive oxygen
species. Different assessments have been offered for the role of individual plasma
components in the ultimate decontamination effect and the share and significance
of individual mechanisms of this action.
Figure 1 – Configuration of a direct-current
negative point-to-plane type corona discharge.
1
121)Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Decontamination Effects of Low-temperature Plasma Generated by Corona Discharge Part I
Action of UV light
UV light is not considered an overly effective decontamination agent although its
bactericidal properties are unquestionable. They are most apparent in the
wavelength range of 250 to 280 nm, with a maximum effect around 260 nm. UV
light acts only on irradiated surfaces. The mechanisms of its effect are taken to
include its propensity to damage DNA and prevent thereby cell reproduction,
cause damage to proteins, generate hydrogen peroxide in aqueous solutions
including intracellular environment, and generate ozone in air. However, its
efficiency and the contribution to the overall microbicidal effect of the plasma are
the matter of contention. Some studies take UV light to be the prime cause of cell
death; others assume its contribution to the microbicidal effect to be marginal.
Thus, study [2] assumes that UV light is a dominant component in the cell-killing
effect, and the same conclusion, at least for some microorganisms, was made in
study [8]. On the other hand, comparison of the effect of UV light produced by an
atmospheric pressure glow discharge with the same dose of UV light generated by
another source, performed in [21], did not reveal any significant contribution of
UV light to the microbicidal effects of low-temperature plasma. Study [22]
documented the killing of some bacterial spores by a sufficient dose of UV light
while according to [36] Bacillus subtilis spores are not inactivated even by a UV
light intensity of 100 J cm–2
.
Attempts at explaining these discrepancies included examination of the effect of
composition of the atmosphere around the decontaminated objects. The results
were again ambiguous. According to study [2], the suitable atmosphere is oxygen
O2
but not nitrogen N2
which, in contrast to oxygen, does not emit photons with
wavelengths shorter than 230 nm. Likewise, study [10] in which plasma was
generated by a microwave device called surfatron stated that the sterilization
effects were detectable after a shorter time period in an oxygen-containing
atmosphere than in an oxygen-free atmosphere. Also study [6] showed that the
decontamination efficiency significantly rises even at low oxygen concentrations in
the discharge atmosphere. In contrast, experiments with cascaded dielectric
barrier discharge [8] showed only a weak decontamination effect in an oxygencontaining
atmosphere; a 15 s exposure of Bacillus subtilis spores in an atmosphere
of N2
:O2
= 8:2 reduced their count by an order of magnitude whereas in a pure
nitrogen atmosphere the drop was 4 orders of magnitude with the same exposure.
With the exception of this last study, the differences in the results of individual
studies can be associated with the action of reactive particles described below.
Action of reactive particles
The important role of reactive particles in decontamination is indicated by the
effect of the ambient atmosphere, especially the effect of oxygen. Study [6]
documented a significant increase in decontamination efficiency when even low
concentrations of oxygen were present in the discharge atmosphere. Significant
122) Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Scholtz V.; Julák J.; Kříha V.; Mosinger J.
can be also the action of other reactive atoms or molecules, e.g. NOX
. According
to [23] the killing of microorganisms is strongly affected by charged particles/ions,
whose charge concentrates on the surface of cell membranes and causes their
disturbance or disruption. This effect should be especially strong with Gramnegative
bacteria. Also study [9] attributed the bactericidal effect of the corona
discharge in a nitrogen atmosphere to the action of unspecified ions. Particles
considered to be most important for the microbicidal effect are mainly reactive
oxygen species.
Oxygen is commonly found in the form of stable O2
molecules. The reactivity of
these molecules is not negligible, but it is not sufficiently high to permit measurable
rates of spontaneous oxidation of most biological materials under normal life
conditions. Since oxygen is a major component of air, all aerobic organisms are
appropriately adapted to its presence. The factor of fundamental importance for
the existence of life is the anomaly in the electron structure of the O2
molecule,
which in its basic low-energy state contains in the highest antibonding orbital two
unpaired electrons with parallel spins. The spin multiplicity is therefore 3 (triplet),
usually denoted as 3
O2
. This contrasts with the basic state of the majority of other
substances, which is a singlet state in which all electrons are paired and, according
to the spin conservation law, their reactions with the triplet structure of oxygen
are spin-forbidden. The reactions of 3
O2
with singlet molecules therefore have
very large activation energy and proceed at a measurable rate only when
conditions for bypassing the spin law, are met. The spin prohibition in fact forms
the basis of our life in the current form since without it all organic matter would
be rapidly oxidized by atmospheric oxygen to carbon dioxide and water.
Under certain conditions, oxygen can give rise to reactive forms that have a
much higher reactivity than 3
O2
.
1) Atomic oxygen O. This form of oxygen is found in the stratosphere where it is
formed as a transient side product of ozone or peroxides splitting by UV light. It
is also formed in electric discharges during the interaction of high-energy ions
or electrons with oxygen molecules, or by decomposition of ozone. It is highly
reactive and oxidizes rapidly other materials or reacts with other oxygen
molecules to yield O2
or ozone O3
.
2) Singlet oxygen 1
O2
. Unlike the common 3
O2
, this molecule is in an electron
excited state in which all electrons are paired and spin multiplicity is 1 (singlet).
It exists in two electron states denoted O2
(1
)g
) and O2
(1
Eg
). The lifetime of
the longer-lived O2
(1
)g
) in water is ca. 4 µs, in other solvents it can persist up
to milliseconds. Singlet oxygen is generated in a number of chemical,
photochemical or photosensitized reactions; one of the physical processes
yielding 1
O2
is the microwave discharge in oxygen atmosphere [24]. Singlet
oxygen is highly reactive species since its reaction with most substances does
not contradict the spin prohibition.
123)Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Decontamination Effects of Low-temperature Plasma Generated by Corona Discharge Part I
3) Ozone O3
is a stable allotropic modification of oxygen. It is commonly found
in the stratosphere in concentrations of 1 to 100 ppm at 15–50 km altitudes.
It arises in electrical discharges in an oxygen-containing atmosphere – in
nature due to lightning, in the laboratory it is usually produced by silent
electrical discharges in so-called ozonators. Ozone is also generated by
photochemical reactions (near germicide UV lamps, in photooxidative smog).
It is a strong oxidant; in higher concentration, it is toxic, and an 8-hour
exposure to a concentration of 0.1 ppm ozone is deleterious to humans. Its
microbicidal properties are well known and ozone is sometimes used for
water disinfection. In high concentrations (up to 1500 ppm) and with longer
exposures (up to 4 hours) it is at least partially efficient even against bacterial
spores [25, 26, 27].
4) Superoxide anion O2
●–
arises by reduction of triplet oxygen. The lifetime of O2
●–
does not exceed 5 µs. It is not too reactive but it can react with other atoms
and molecules to produce the considerably more reactive hydroxyl radicals and
singlet oxygen. A reaction of two superoxide anions and hydrogen yields
hydrogen peroxide. It is formed in the cells by numerous biochemical reactions
and aerobic organisms can eliminate it by the action of superoxide dismutase.
5) Hydroxyl radical ●
OH is formed in the so-called Haber-Weiss reaction of
hydrogen peroxide with superoxide. It is a very reactive particle with a lifetime
of ca. 10–9
s. It is very dangerous for live cells, which cannot eliminate it
enzymatically in contrast to the superoxide anion.
Among these particles, the role of ozone in the killing of microorganisms by plasma
or electric discharge is largely indisputable, as stressed also in [20]. As shown e.g.
in [3], another probable cause of the microbicidal action of the plasma is singlet
oxygen 1
O2
. Assertions about the potential role of other particles often lack
experimental evidence.
Figure 2 – Photograph of Bacillus subtilis
spores from a scanning electron microscope.
Mechanical damage to spore surface after
exposure to a barrier discharge, caused by
reactive particles, is denoted by arrow.
Adapted from [8].
124) Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Scholtz V.; Julák J.; Kříha V.; Mosinger J.
Various proposals of the mechanisms of bactericidal action of reactive particles
can be found in the literature. According to [2] and [8], these particles most
probably damage biological membranes, the bacterial cell wall and other biological
structures by a mechanism similar to that occurring in the plasma etching of
semiconductors. The picture from a scanning electron microscope given in
Figure 2 shows Bacillus subtilis spores after an exposure to a barrier discharge. It
clearly illustrates a mechanical damage to the spore surface caused by reactive
particles and preventing further spore survival.
Charged particles, i.e. positive and negative ions, can mechanically damage cell
envelopes. An experimental evidence of this mechanism is given in study [14],
which documents protein cleavage in bacterial membrane under the action of H+
and O2–
ions generated by water vapour decomposition. The result of this action is
illustrated in the electron microscopic picture in Figure 3, which shows a bulge on
the bacterial envelope caused by damaged cell membrane.
A sophisticated proposal of a combined mechanism of the microbicidal effect of
plasma is described in [3]. It consists in a synergistic action of active oxygen and
UV radiation, which occurs in three phases:
1. Destruction of the genetic material of the microorganisms by UV light.
2. Erosion of microbial surface atom-by-atom or by etching caused by reactive
particles and supported synergistically by UV light.
3. Further destruction of unprotected genetic material by UV light.
These hypotheses notwithstanding, the mechanism of microbicidal action of plasma
has not yet been completely elucidated. The methodology of the experiments that
Damaged membrane
200 nm
Figure 3 – Damage to cell envelopes
by H+
and O2–
ions from water vapour.
Arrows denote damage caused by cleavage
of bacterial membrane proteins. Adapted
from [14], experimental details are not given.
125)Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Decontamination Effects of Low-temperature Plasma Generated by Corona Discharge Part I
form the basis of the above hypotheses is not standardized and different
mechanisms can therefore play different roles under various sets of experimental
conditions. The importance of different mechanisms can also vary depending on
the microorganism in question.
Detection methods for reactive oxygen species
Any hypotheses on the participation of individual reactive particles in the ultimate
decontamination effect have to be based on the determination of the presence and
amount of these particles. The detection and proof of reactive oxygen species,
which are obviously the most important for the microbicidal effect, make use
of sophisticated methods such as electron paramagnetic resonance (EPR),
photoaccoustic calorimetry, IR spectroscopy, luminescence methods, etc.
An alternative is represented by simple methods based on the reactions of the
reactive species in liquid phase with compounds forming characteristic primary or
secondary products. The choice of suitable reactions is complicated e.g. by the
short lifetime of most particles and the selectivity of the requisite reactions. These
reactions should be sufficiently selective for different reactive oxygen species and
permit their differentiation despite the fact that the chemical properties of
individual reactive oxygen species are very similar.
One of the possibilities is the so-called iodide method used mostly for the
detection of 1
O2
and other reactive oxygen species [28]. The method is based on
the reaction of reactive oxygen species with iodide I_
in the presence of
ammonium molybdate (NH4
)2
MoO4
as catalyst. The reaction product is the
triiodide anion I3
_
whose amount is proportional to the amount of reactive oxygen
species and its yellow colour with an absorbance maximum at 287 or 351 nm
permits a quantitative spectrophotometric determination. The method is very
sensitive but its disadvantage is a low selectivity.
The diagnostics of oxidizing particles makes use of several reagents such as D2
O
or NaN3
in the iodide detection reagent for 1
O2
. The lifetime of 1
O2
in D2
O is
about 16 times longer than in H2
O [24, 29, 30] whereas NaN3
is an important
physical quencher of 1
O2
[31, 32]. The presence of D2
O instead of H2
O in the
detection reagent increases the production of I3
_
whereas the presence of NaN3
suppresses it. Other more selective organic substrates for 1
O2
oxidation that can
be used as detection and diagnostic reagents include e.g. uric acid [33, 34]. It
should be noted, however, that the high selectivity is compensated by an order of
magnitude lower sensitivity as compared with the iodide reagent.
Conclusion
Low-temperature plasma can be generated by electric discharges of different
types in gases of different composition. The simplest generation appears to be
its production by means of a unipolar corona discharge in air at atmospheric
pressure. Plasma has inhibitory to killing effects on different microorganisms.
126) Prague Medical Report / Vol. 108 (2007) No. 2, p. 115–127
Scholtz V.; Julák J.; Kříha V.; Mosinger J.
This effect involves the participation of UV light and different uncharged and
charged particles among which reactive oxygen species appear to be the most
important. The decontamination effects of low-temperature plasma are well
known but the mechanism or mechanisms of its action is/are not sufficiently
known and the technological parameters of its practical use have not yet been
sufficiently elaborated. Further research in this field is therefore useful and
necessary.
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