Plasma sterilization in low-pressure RF discharge
The present paper clarifies the conditions under which the process of plasma sterilization of medical tools may be efficiently performed in the RF capacitive gas discharge of low pressure in air. Experiments were performed with a number of gram-positive and gram-negative bacteria as well as with fun...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2000
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| Cite this: | Plasma sterilization in low-pressure RF discharge / V.A. Lisovskiy, S.D. Yakovin, V.D. Yegorenkov, A.G. Terent’eva // Вопросы атомной науки и техники. — 2000. — № 1. — С. 77-81. — Бібліогр.: 14 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859603463864844288 |
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| author | Lisovskiy, V.A. Yakovin, S.D. Yegorenkov, V.D. Terent’eva, A.G. |
| author_facet | Lisovskiy, V.A. Yakovin, S.D. Yegorenkov, V.D. Terent’eva, A.G. |
| citation_txt | Plasma sterilization in low-pressure RF discharge / V.A. Lisovskiy, S.D. Yakovin, V.D. Yegorenkov, A.G. Terent’eva // Вопросы атомной науки и техники. — 2000. — № 1. — С. 77-81. — Бібліогр.: 14 назв. — англ. |
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| container_title | Вопросы атомной науки и техники |
| description | The present paper clarifies the conditions under which the process of plasma sterilization of medical tools may be efficiently performed in the RF capacitive gas discharge of low pressure in air. Experiments were performed with a number of gram-positive and gram-negative bacteria as well as with fungi. The process of sterilization in the RF discharge is shown to possess a threshold pattern. Probably the bombardment of bacteria with positive ions and hot molecules of the neutral gas is the main sterilizing factor in the low pressure RF discharge, and the UV radiation of plasma plays the auxiliary role.
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| first_indexed | 2025-11-28T01:57:54Z |
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ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ 2000. №1.
Серия: Плазменная электроника и новые методы ускорения (2), с. 77-81.
77
UDK 533.9
PLASMA STERILIZATION IN LOW-PRESSURE RF DISCHARGE
V.A. Lisovskiy, S.D. Yakovin, V.D. Yegorenkov, A.G. Terent’eva
Kharkov National University, Kharkov,Ukraine
lisovskiy@ftf.univer.kipt.kharkov.ua
The present paper clarifies the conditions under which the process of plasma sterilization of medical tools may be
efficiently performed in the RF capacitive gas discharge of low pressure in air. Experiments were performed with a
number of gram-positive and gram-negative bacteria as well as with fungi. The process of sterilization in the RF dis-
charge is shown to possess a threshold pattern. Probably the bombardment of bacteria with positive ions and hot
molecules of the neutral gas is the main sterilizing factor in the low pressure RF discharge, and the UV radiation of
plasma plays the auxiliary role.
1. Introduction
STERILIZATION in microbiology and medicine
stands for total destruction of micro-organisms and their
spores with the help of physical and chemical means.
Most frequently one uses such physical means as tem-
perature, ultraviolet rays, high-energy radiation, ultra-
sound and filtration.
Methods of contemporary sterilization:
• High-temperature method [1]:
1) over the flame, 2) through boiling, 3) with dry heat,
4) with flowing vapour, 5) with vapour under pressure.
• Low-temperature sterilization [2]:
1) with ionising radiation, 2) with liquid chemical sub-
stances, 3) with hydrogen peroxide, 4) with ozone, 5)
plasma sterilization.
Plasma methods of sterilization:
• In the plasma of a chemical mixture [2].
• In the low-temperature plasma of hydrogen perox-
ide [2 – 5].
• In the gas discharge under ambient pressure
[6 − 11].
• In the DC glow discharge under low gas pressure
[12].
•
Plasma sterilization is the “youngest” and the most
promising way to disinfect medical tools. The surge of
the developments in the plasma sterilizer design in vari-
ous countries indicates the vital nature of this topic.
However, as a rule one applies in plasma sterilizers the
hydrogen peroxide vapour and some costly mixtures of
gases and vapours thus making the operation costs of
such a sterilizer higher. Therefore this paper devotes the
main attention to the feasibility of using the RF capaci-
tive low-pressure gas discharge in air for sterilizing
medical tools.
The theme of this paper lies at the interface between
the gas discharge physics and medicine. The limited
number of publications on this theme has made it neces-
sary to perform a number of experiments with the cul-
tures of various bacteria. The experiments have resulted
in establishing the conditions under which the RF ca-
pacitive low-pressure discharge in air may be applied for
sterilizing medical tools. We have also obtained a num-
ber of new results, i.e., establishing the almost threshold
pattern of the plasma sterilization process as well as the
parameters of the RF capacitive discharge in the pres-
ence of a gauze spanning inside the discharge volume.
2. Experimental details
Experiments on plasma sterilization were performed
in the discharge chamber of 100 mm in diameter and the
interelectrode gap of 54 mm. A RF capacitive gas dis-
charge in air was ignited in the chamber within the pres-
sure range р = 0.1-1 Torr. The RF voltage with the
13.56 MHz operation frequency was supplied from the
generator to one of the electrodes. Another electrode
was grounded. Gauze with 0.5-cm mesh size was in-
stalled between electrodes occupying all the cross-
section of the discharge tube. Samples under study with
bacterial cultures were put on this gauze (we used nee-
dles for blood tests as samples). The chamber was
evacuated via a preliminary vacuum pump down to pres-
sures below 0.1 Torr, then air was puffed up to the oper-
ating pressure and the RF discharge was ignited.
The following cultures were put on the needles:
gram-positive (S. aureus, S. epidermidis, Str. mitis) and
gram-negative (K. pneumoniae, P. mirabilis, E. cloacca,
E. coli) bacteria, as well as fungi (Candida). First we
sterilized the samples (needles) in the RF discharge
during 5 min to remove possible contamination of sam-
ples with environmental microbes.
3. Experimental results
Figure 1 shows the air pressure dependence of the
number of K. pneumoniae left after sterilization at the
RF voltage Urf = 500 V and the processing period
t = 1 min. One sees from this figure that the pressure
range p > 0.4 Torr is the most advantageous for per-
forming sterilization.
78
Fig.1. Air pressure dependence of the number of K.
pneumoniae left after sterilization at the RF voltage
Urf = 500 V and the processing period of t = 1 min
Figure 2 shows the ratio of the number of microbes
left after sterilizing the sample to the initial number of
K. pneumoniae against the Urf voltage value for the
processing period of t = 0.5 min. This figure exhibits the
threshold-like pattern of the sterilization process. For the
given processing period of t = 0.5 min the RF voltage
values required for sterilizing are Urf > 350 V. At lesser
RF voltage values these microbes remain alive, their
number being approximately equal to the initial number
of microbes put on the sample.
Fig.2. Ratio of the number of microbes left after
sterilizing the sample to the initial number of K. pneu-
moniae against the Urf voltage value for the processing
period of t = 0.5 min and the air pressure of p = 0.6
Torr
Figure 3 depicts the ratio of the number of S. aureus
microbes N left after sterilization to the initial number
N0 against processing period at three different RF power
levels. Figure 3 also indicates the threshold-like pattern
of the N/N0 versus processing period. The more is the
RF discharge power level, the less is the time needed for
sample sterilizing.
Fig. 3. Ratio of the number of S. aureus microbes
N left after sterilization to the initial number N0
against the processing period at three different RF
power levels W = 20 W (Urf = 195 V), W = 60 W
(Urf = 350 V) and W = 100 W (Urf = 450 V) and the
air pressure p = 0.6 Torr
Figure 4 depicts the dependence of the ratio N/N0
of K. pneumoniae and E. coli microbes against proc-
essing period at the RF power value W = 100 W. We
see from the figure that the quantity N/N0 for these
microbes sharply decreases with time at t ≥ 0.5 min,
and the pattern is also threshold-like.
Fig.4. Ratio N/N0 of K. pneumoniae and E. coli
microbes against processing period at the RF power
value W = 100 W (Urf = 450 V) and the air pressure
p = 0.6 Torr.
Figure 5 shows the dependence of the ratio N/N0
for the Candida fungus culture against processing pe-
riod at RF power levels W = 20 W and W = 100 W. It
follows from the figure that at the RF power level
W = 20 W the sample becomes sterile with the proc-
essing period t ≥ 2 min, whereas with the RF power
W = 100 W it is sufficient to burn the discharge for 10
seconds to sterilize the sample.
79
Fig.5. Ratio N/N0 for the Candida fungus culture
against processing period at RF power levels W = 20 W
(Urf = 195 V) and W = 100 W (Urf = 450 V) and the air
pressure p = 0.6 Torr.
It follows from our results that among the gram-
negative bacteria the K. pneumoniae happened to be the
most resistant to the action of the RF discharge. It has
not only the outer casing (all bacteria have it) but a cap-
sule protecting it from the effect of unfavourable condi-
tions. Other gram-negative bacteria (P. mirabilis, э E.
cloacca, E. coli) are less resistant to the action of the RF
discharge. Therefore one requires less time for a com-
plete sterilization of a specimen. Among the gram-
positive bacteria S. Aureus is the most resistant, whereas
other microbes we have studied (S. epidermidis, Str.
mitis) have perished quickly in the RF discharge. The
fungi (Candida) are killed easily with a burning RF dis-
charge even at low RF power levels.
We also obtained the dependencies of the gas tem-
perature and plasma density against air pressure near the
gauze surface (Fig.6). The gas temperature was meas-
ured with a thermocouple located at the 2-mm distance
from the gauze surface. The plasma density was meas-
ured simultaneously with a cylindrical probe.
The temperature values measured with a thermocou-
ple represent not only the gas temperature, because the
thermocouple is heated in the discharge by the mole-
cules of the neutral gas hitting its surface as well as by
the flows of ions, electrons and radiation (infrared, visi-
ble and ultra-violet light always present in the radiation
spectrum of the RF capacitive discharge). The thermo-
couple was at the floating potential; thus its heating by
the flows of charged particles was reduced. Actually the
temperature values obtained corresponded to the tem-
perature an isolated body can acquire in the discharge.
In what follows, we will call it a “gas temperature”.
As is seen from Fig. 5, under low pressures and mod-
erate values of the applied RF voltage (Urf = 200-300 V)
the gas temperature is comparatively low (T ≈ 50÷60
оC), but on increasing the RF voltage, the gas tempera-
ture grows and approaches the values T ≈ 100÷150 оC.
Increasing the pressure (up to 0.6-0.7 Тоrr) leads to
the monotonous growth of the gas temperature, but at
higher pressure values and small RF voltages the gas
temperature decreases. The increase of the RF voltage
at all pressure values was accompanied by the increase
of the gas temperature. Within the pressure range up to
0.4 Тоrr and with RF voltages up to 450 V the plasma
density did not actually depend on air pressure, by the
subsequent increase of pressure led to the decrease in
plasma density. In Fig. 6 the first three curves
(Urf = 210-450 V) relate to the weak-current mode of
the RF discharge burning. At Urf = 600 V and p > 0.7
Torr the RF discharge assumes the strong-current
mode accompanied by a sharp increase of the gas tem-
perature as well as of the plasma density near the
gauze surface.
Fig.6. Gas temperature (a) and plasma density (b)
near the gauze surface against air pressure at differ-
ent RF voltages.
The abrupt decrease of the gas temperature and
plasma density at p > 0.7 Тоrr and low RF voltage
values may be attributed to approaching the RF dis-
charge quenching. Therefore the ohmic power con-
sumed by the RF discharge decreases. Consequently,
at the constant RF voltage Urf = 210 V the ohmic dis-
charge current decreases leading to the decrease in the
gas heating and plasma density lowering. Further in-
crease of the air pressure at Urf = 210 V leads to
quenching the RF discharge.
The electron temperature and plasma density vaues
80
near the gauze surface shown in Fig. 7 have been deter-
mined from the current-voltage characteristics of the
cylindrical probe (5 mm in length and 0.18 mm in di-
ameter). One sees from this figure that the plasma den-
sity near the gauze surface is approximately proportional
to the RF voltage applied in the weak-current regime.
After the transition of the RF discharge to the strong-
current regime the growth rate of the plasma density
increases abruptly. The electron temperature near the
gauze surface is practically constant in the weak-current
regime (Te = 3.6 eV). On increasing the RF voltage the
electron temperature incrases up to 5 eV before the
quenching of the discharge. The transition of the RF
discharge to the strong-current regime of burning leads
to the decrease of the electron temperature [12-14].
Fig.7. The electron temperature and plasma density
near the gauze surface against the RF voltage at the air
pressure p = 0.6 Torr.
Figure 8 shows the axial profiles of the plasma den-
sity ni(z) at various values of the RF voltage applied. In
this figure the broken line shows the position of the
gauze on which the samples we have put. The axial pro-
files possess clearly pronounced maxima in the vicinity
of the boundaries of near-electrode layers even in the
weak-current regime of the RF discharge burning. One
also observes a not very large maximum near the gauze.
On increasing the RF voltage, the plasma density in-
creases within the whole discharge gap. First, at low RF
voltage values the discharge exhibits the approximately
uniform luminosity in the whole volume of the chamber
with a slight increase of luminosity near the boundaries
of the near-electrode layers and near the gauze surface.
The increase of the RF voltage leads to the picture when
the sharp maxima of luminosity are observed near the
layers and the gauze whereas in all other regions the
discharge luminosity is weak. In the weak-current re-
gime of the discharge burning the characteristics of the
luminosity near the gauze are similar to those of the
positive column of the DC glow discharge.
The following main factors may affect the micro-
organisms in the RF discharge in air at low pressures:
• flows of charged particles (electrons, ions);
Fig. 8. Axial profiles of the plasma density at vari-
ous values of the RF voltage applied Urf and the air
pressure p = 0.6 Torr.
• heated neutral gas;
• UV radiation;
• low pressure (vacuum).
Our studies show that storing samples in the vac-
uum chamber with the inner air pressure order of 0.1 –
10 Torr during 12 hours does not decrease the quantity
of microbes put on the sample initially. Therefore vac-
uum has no noticeable effect on bacterial cultures
stored in it. Some samples with the cultures on them
were processed only with the discharge radiation be-
cause we had put them outside the chamber but close
to the quartz wall in the region of maximum radiation
at W = 400 W during 5 min. Such a processing tech-
nique also did not affect the initial number of bacteria
on the sample. For comparison it may be stated that
under the same conditions the samples inside the
chamber were sterilized already after 5 s after switch-
ing on the discharge. Therefore the discharge UV ra-
diation itself does not kill bacteria, and some other
factors are also needed for the sample sterilization.
This somewhat unexpected result may be explained
as follows. In order to sterilize with the UV radiation,
one requires the time period order of some ten minutes
[12], i.e., this process is comparatively slow. In our
case the required period of sterilization was in the
range of ten seconds to some minutes. Therefore other
faster mechanisms were at work under conditions of
our experiments
As the gauze with the samples on it was under the
floating potential, the decelerating potential of 10-15
V stopped the bulk of electrons, and only a small por-
tion of the fastest electrons could reach the samples.
The gauze with samples was located sufficiently far
from the boundaries of near-electrode layers, and ster-
ilizing was performed most frequently in the weak-
current mode of the discharge. Therefore the beams of
fast electrons generated by the ions hitting the elec-
trodes and gaining energy in the near-electrode layers
as well as the electrons gaining energy under stochas-
tic heating near the layer boundaries actually did not
reach the sample surface.
81
Consequently, the contribution of the electron flow to
the sterilization process could not be substantial.
A flow of positive ions was also incident on the
samples. For the ions this voltage drop 10-15 V was
accelerating. The accelerated ions bombarded the sur-
face of bacteria demolishing their protective outer
shells. If one measures the flux of positive ions on the
probe at the RF voltage values given in Figs 2-5 and
multiplies the threshold time ts, at which one observes
the step-like decrease of the number of bacteria on the
sample by the ion current value Ii, then the product
D = ts Ii remains approximately constant. For example,
for the cylindrical probe applied (0.18 mm in diameter,
5 mm in length) we have obtained the value D = 770-
800 µА⋅s. The quantity D is the dose of ion irradiation.
Probably for a bacterium to be killed it should obtain a
certain dose of ion irradiation, i.e., its shell should expe-
rience a certain amount of collisions with accelerated
ions.
Hot gas molecules may cause heat damage to the
outer shells of bacteria involving their death. At the air
pressure p = 0.6 Torr even at the lowest RF voltage of
RF discharge burning Urf = 210 V the neutral gas tem-
perature is approximately 100 оС (Fig.6), and at
Urf = 600 V the gas temperature is already 200 оС. For
the instantaneous death of S. aureus bacteria one should
heat them to the temperature of 100 оС. The processing
occurs at the lowered gas pressure, then for heating
bacteria one requires more time than in air at atmos-
pheric pressure. The more is the temperature of the neu-
tral gas, the less time is required for heating bacteria to
death temperatures.
Consequently, bombardment of bacteria with the
flows of positive ions and hot molecules of the neutral
gas is the main sterilizing factor in the low-pressure RF
discharge. Probably the UV radiation of plasma plays
the auxiliary role.
4. Conclusions
Thus the present paper clarifies the conditions under
which the process of plasma sterilization of medical
tools may be efficiently performed in the RF capacitive
gas discharge of low pressure in air. Experiments were
performed with a number of gram-positive and gram-
negative bacteria as well as with fungi. The process of
sterilization in the RF discharge is shown to possess a
threshold pattern. Probably the bombardment of bacteria
with positive ions and hot molecules of the neutral gas is
the main sterilizing factor in the low pressure RF dis-
charge, and the UV radiation of plasma plays the auxil-
iary role.
References
1. V.I. Vashkov Sredstva i metody sterilizatsii, prime-
nyaemye v meditsine. M.: "Meditsina", 1973 (In Rus-
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3. P.T. Jacobs // J. Healthcare Materiel Management,
1989, vol. 7, p. 49.
4. K.B. Frey // Surgical Technology, 1994, p. 8 .
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new technology for instrument sterilization. // Ad-
vanced Sterilization Products, 1994.
6. Y. Ku, C. Brickman, K. Wintenberg, T.C. Montie,
P. Tsai, L. Wadsworth, J.R. Roth // Record-Abstracts
of IEEE Intern. Conf. On Plasma Sci., 1996, Boston,
USA, p. 2IP15.
7. M. Laroussi // IEEE Trans. Plasma Science, 1996,
vol. 23, p. 1188.
8. A.K. Carr, J.R. Roth, C. Brickman, K. Kelly-
Wintenberg, T.C. Montie, P. Tsai, L. Wadsworth //
Record-Abstracts of IEEE Intern. Conf. On Plasma
Sci., 1997, San Diego, USA, p. 4Q21.
9. D.M. Sherman, R.B. Gadri, F. Karakaya, Z.Che,
T.C. Montie, K. Kelly-Wintenberg, P.P.Y. Tsai, J.R.
Roth // Record-Abstracts of IEEE Intern. Conf. On
Plasma Sci., 1999, Monterey, USA, p. 2B08.
10. K. Kelly-Wintenberg, T.C. Montie, J.R. Roth, Z.
Chen // Record-Abstracts of IEEE Intern. Conf. On
Plasma Sci., 1999, Monterey, USA, p. 4P09.
11. E. Garate, O. Gornostaeva, I. Alexeff // Record-
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Monterey, USA, p. 4P10.
12. V.A. Khomich, I.L. Mikhno, I.A. Soloshenko,
V.V. Tsiolko // Abstracts of 25th EPS Conf. on Con-
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p. 209.
12. Yu.P. Raizer Gas Discharge Physics. Berlin:
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4. Conclusions
|
| id | nasplib_isofts_kiev_ua-123456789-81613 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-11-28T01:57:54Z |
| publishDate | 2000 |
| publisher | Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| record_format | dspace |
| spelling | Lisovskiy, V.A. Yakovin, S.D. Yegorenkov, V.D. Terent’eva, A.G. 2015-05-18T12:48:38Z 2015-05-18T12:48:38Z 2000 Plasma sterilization in low-pressure RF discharge / V.A. Lisovskiy, S.D. Yakovin, V.D. Yegorenkov, A.G. Terent’eva // Вопросы атомной науки и техники. — 2000. — № 1. — С. 77-81. — Бібліогр.: 14 назв. — англ. 1562-6016 https://nasplib.isofts.kiev.ua/handle/123456789/81613 533.9 The present paper clarifies the conditions under which the process of plasma sterilization of medical tools may be efficiently performed in the RF capacitive gas discharge of low pressure in air. Experiments were performed with a number of gram-positive and gram-negative bacteria as well as with fungi. The process of sterilization in the RF discharge is shown to possess a threshold pattern. Probably the bombardment of bacteria with positive ions and hot molecules of the neutral gas is the main sterilizing factor in the low pressure RF discharge, and the UV radiation of plasma plays the auxiliary role. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Газовый рaзряд, ППР и их применения Plasma sterilization in low-pressure RF discharge Article published earlier |
| spellingShingle | Plasma sterilization in low-pressure RF discharge Lisovskiy, V.A. Yakovin, S.D. Yegorenkov, V.D. Terent’eva, A.G. Газовый рaзряд, ППР и их применения |
| title | Plasma sterilization in low-pressure RF discharge |
| title_full | Plasma sterilization in low-pressure RF discharge |
| title_fullStr | Plasma sterilization in low-pressure RF discharge |
| title_full_unstemmed | Plasma sterilization in low-pressure RF discharge |
| title_short | Plasma sterilization in low-pressure RF discharge |
| title_sort | plasma sterilization in low-pressure rf discharge |
| topic | Газовый рaзряд, ППР и их применения |
| topic_facet | Газовый рaзряд, ППР и их применения |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/81613 |
| work_keys_str_mv | AT lisovskiyva plasmasterilizationinlowpressurerfdischarge AT yakovinsd plasmasterilizationinlowpressurerfdischarge AT yegorenkovvd plasmasterilizationinlowpressurerfdischarge AT terentevaag plasmasterilizationinlowpressurerfdischarge |