Sterilization of bacteria
at low discharge voltage by using microplasma
Kazuo SHIMIZU, Yuuki KOMURO, and Marius BLAJAN
Innovation and Joint Research Center, Shizuoka University,Jyouhoku, Hamamatsu, 432-8561, Japan
E-mail：shimizu@cjr.shizuoka.ac.jp
Sterilization rate was measured with only one passprocess between the microplasma electrode.

INTRODUCTIONRESULTSIn recent years, infective diseases such as new
influenza caused by pathogenic organisms, werespread world wide. It is expected that alternatives tochemical sterilization methods such as new safety
sterilizing process using nonthermal plasma to bedeveloped.

(1) Electric Characteristics
1kV
10μs
50mA
50mA

250ns
10μs

We have investigated sterilization effect of bacteria
at low voltage by using atmospheric microplasma.

METHODSFig. 5 current-voltage characteristic(1) Microplasma Electrodes

Spike-like currents occured at the steepest slopesof the discharge voltage. This is a typical waveformof dielectric barrier discharge.
The microplasma reactor can generate atmosphericplasma at about 1 kV, since its discharge gap isnarrow (about 50 to 100 µm).

(2) Ozone Generation

Fig. 10 Sterilization rate of E.coli
The sterilization rate of E. coli versus discharge
voltages for both carrier gases of air and nitrogen
increased with discharge voltage . Sterilization ratewith air was higher than that with nitrogen.

Fig. 1 Microplasma electrode
Pair ofmetal plates covered with dielectricmaterials is used as a dielectric barrier electrode.

Fig. 6 Ozone concentration
Fig. 2 Electrode surface
The image of microplasma electrodes
discharge at discharge voltage of 1 kV.

duringGenerated ozone concentration by microplasma canbe controlled by discharge voltage. It was observedthat sprayed distilled water was lower than thatwith only air.

Fig. 11 Sterilization rate of B.subtilis
Similar sterilization rates of B. subtilis wereobtained for air as carrier gas. Sterilization rate bynitrogen plasma was lower compared to the rate ofE. coli. The thickness of the cell wall could beconsidered the sterilization effect between E. coliand B. subtilis.

(3) Emission spectrum of microplasma
15000
N2 with H2O
N2OH

Intensity (a.u.)

12000
9000
6000
3000

0303 304 305 306 307 308 309 310 311 312 313 314 315

Fig. 3 microbicidal mechanism
Each bacteria could be attacked and sterilized byatomic oxygen and nitrogen species, UV, OH radicals
and O3 generated between the electrodes.

(2) Experimental Setup

Wavelength (nm)Fig. 7 Emission spectrum of microplasma
Fig. 7 shows OH peaks observed with nitrogen andwater vapor. OH radical could be generated fromwater vapor and is well known its oxidative
characteristics. OH radicals could play a roll forsterilization process even without oxygen.

Fig. 12

Sterilization rate of E. coli with pulsedmicroplasma
Sterilization rate by pulse was lower than by neontransformer . It is expected sterilization rate could
be improved by treatment in more than one pass.

(4) Rate of sterilization

Fig. 13 SEM images of B. subtilis before(left) andafter(right) the microplasma.

Fig. 4 Experimental setup
Table. 1 Experimental condition
Discharge voltage

1.0～1.4 [kV]Gas flow rate

8.5 [L/min]

Carrier gas

Air, N2

Target bacterias

E.coli , B.subtilis

The liquid culture medium was introduced in themicroplasma reactor and sprayed at a gas flow rateof 3.5 L/min, through the electrode against a petri
dish with culture medium. Total gas flow rate was 8.5L/min. Sprayed bacteria with liquid culture medium
was sterilized between microplasma electrodes.
Discharge voltage and discharge current weremeasured by a digital oscilloscope.

Without discharge.

Vd=1.4 kV.

Fig. 8 Images of the B. subtilis treated samples by
air-plasma.

CONCLUSIONSIn this study, the following conclusions wereobtained.
1) Sterilization by microplasma was confirmed at a
relatively low discharge voltage of 1.0 kV for bothcarrier gases: air and nitrogen.
2) The sterilization rate by air plasma was higher
than that of nitrogen plasma. It could be consideredby the combination effect of high electric field, UV
radiation and other radicals.

Without discharge.

Vd=1.4 kV.

Fig. 9 Images of the B. subtilis treated samples bynitrogen-plasma.

3) The sterilization rate of E. coli was higher than
the results for B. subtilis due to thecharacteristics of the cell walls, which aredifferent in thickness and chemical composition.