Study of bacteria sterilization
at low discharge voltage by using microplasma
Yuuki KOMURO, Marius BLAJAN, and Kazuo SHIMIZU
Innovation and Joint Research Center, Shizuoka University,Jyouhoku, Hamamatsu, 432-8561, Japan
E-mail：shimizu@cjr.shizuoka.ac.jp

INTRODUCTIONRESULTSIn recent years, infective diseases suchas new influenza caused by pathogenicorganisms, were spread world wide. It is
expected that alternatives to chemicalsterilization methods such as new safety
sterilizing process using nonthermal
plasma to be developed. Therefore we
investigated sterilization of bacteria at low
voltage by using microplasma.

(1) Electric Characteristics

METHODS(1) Microplasma Electrodes

Spike-like currents occur at the steepest
slopes of the discharge voltage. This isa typical waveform of dielectric barrierdischarge.
The microplasma reactor can generate
atmospheric plasma at about 1 kV,
since its discharge gap is narrow (about50 µm).

(2) Characteristics of Particles byNebulizer

The sterilization rate of E. coli versus
discharge voltages for both carrier gasesof air and nitrogen increased as thedischarge
voltageincreased.
Sterilization rate with air was higher thanthat with nitrogen.

Pair ofmetal plates covered with
dielectric materials is used as a dielectricbarrier electrode.

The image of microplasma electrodes
during discharge at discharge voltage of 1kV.

Similar sterilization rates were obtainedfor B. subtilis for both carrier gases, airand nitrogen.
The diameter of water particles, which
are made by a nebulizer, are 1µm to5µm. The size of colon bacilli is about
0.5 um to 2.5 um, thus E. coli HB101
and B. subtilis JCB20036 could be in
the water drops made by the nebulizer.

(4) Ozone Generation

Image of B. subtilis treated sample by SEM.

CONCLUSION
In this study, the following conclusions
were obtained.
Each bacteria could be attacked anddecomposed by atomic oxygen andnitrogen species, UV, OH radicals and O3
generated between the electrodes.

(2) Experimental Setup

Concentration of ozone with sprayeddistilled water was lower than that with
only air. In the case with sprayeddistilled water, flowed gas was cooled
down and dried with iced water before
measuring the concentration.
(5) Rate of sterilization

1) Sterilization by microplasma wasconfirmed at a relatively low discharge
voltage of 1.0 kV for both carrier gases:
air and nitrogen.
2) The sterilization rate by air plasma
was higher than the results by nitrogenplasma due to the combination effect ofozone, high electric field, UV radiationand other radicals.
3) The sterilization rate of E. coli washigher than the results for B. subtilis dueto the characteristics of the cell walls,
which are different in thickness andchemical composition.

References
Discharge Voltage

1.0～1.4 [kV]Flowing Quantity

8.5 [L/min]

Carrier Gas

Air, N2

Processing Object

E.coli , B.subtilis

The liquid culture medium was introducedin the microplasma reactor and sprayed
at a gas flow rate of 3.5 L/min, throughthe electrode against a petri dish with
culture medium. Gas flow rate was 5L/min. Sprayed bacteria with liquid culture
medium was sterilized in plasma reactor.
Discharge voltage and discharge current
were measured by a digital oscilloscope.

Without discharge.
Vd=1.4 kV.
Images of the B. subtilis treatedsamples by air-plasma.

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Koide, Jpn. J. Appl. Phys, 42, No. 7B,856-859 (2003).
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[3] R. Stainer, J. Ingraham, M. Wheelis,and P. Painter, The microbial world. 5thedition, (Prentice Hall, New York, 1986).

Without discharge.
Vd=1.4 kV.
Images of the B. subtilis treatedsamples by nitrogen-plasma.