Sterilization process of bacteria by using atmospheric microplasma
Y. Komuro, S. Tatematsu, M. Blajan, K. 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 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
10μs50mA50mA250ns
10μs

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

METHODS

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

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 contributed to lower
than ozone concentration with only air.

Fig. 11

Sterilization rate of E. coli with pulsed
microplasma

Sterilization rate by pulse voltage application waslower thanby AC voltage application. It is
expected sterilization rate could be improved bytreatment in more than one pass.

(3) Rate of sterilization

Without discharge.

Fig. 3 microbicidal mechanism

Vd=1.4 kV.

untreated(×1600)

treated (×1800)

Fig. 7 Images of the B. subtilis treated samples byair-plasma.

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

(2) Experimental Setup
treated (×2300)

Without discharge.

Vd=1.4 kV.

Fig. 8 Images of the B. subtilis treated samples bynitrogen-plasma.
Sterilization rate was measured with only one passprocess between the microplasma electrode.

treated(×4500)

Fig. 12 FE-SEM images of B. subtilis before and after
the microplasma.

The photos of B. subtilis before and after
treatment with nitrogen gas were taken by a SEM.
B. subtilis bacterial could be identified as thecylindrical shape on the untreated photo which
after the sterilization process transforms in awider flat shape due to the effect of microplasma.

Fig. 4 Experimental setup

CONCLUSIONS

Table. 1 Experimental condition

In this study, the following conclusions wereobtained.

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 8.5
L/min. Sprayed bacteria with liquid culture medium
was sterilized between microplasma electrodes.
Discharge voltage and discharge current weremeasured by a digital oscilloscope.

1) Sterilization by microplasma was confirmed at a
relatively low discharge voltage of 1.0 kV for bothcarrier gases: air and nitrogen.

Fig. 9 Sterilization rate of E.coli by AC voltage
application
The sterilization rate of E. coli versus dischargevoltages for both carrier gases of air and nitrogen
increased with discharge voltage . Sterilization ratewith air was higher than with nitrogen plasma.

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.
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.