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

INTRODUCTIONRESULTS

Recent years sterilization methods
using various kind of discharges andplasmas are investigated. But most ofthese methods require high voltage.
Therefore we investigated inactivationof bacteria by microplasma, whosedischarge voltage is very low
compared with other plasma.

(1) Electric Characteristics(5) Inactivation Rate

1kV/
div
10us/div

50mA/div
10us/
div

Without discharge.

METHODS

20mA/div

Images of the B. subtilis treatedsamples by air-plasma.

200ns/div(1) Microplasma Electrodes

Vd=1.0 kV.

Spike-like currents occur at thesteepest slopes of the discharge
voltage. This is a typical waveform of
dielectric barrier discharge.
The micro plasma reactor can
generate atmospheric plasma at
about 1 kV, since its discharge gap isvery narrow.

(2) Characteristics of Particles byNebulizer
40

Number of drops of water[×1000]

Pair of metal plates covered with
dielectric materials is used as adielectric barrier electrode. Thedischarge gap is between 0-20µm.

Without discharge.

Vd=0.96 kV.

Images of the B. subtilis treatedsamples by nitrogen-plasma.

0.3 µm
0.5 µm
1.0 µm
3.0 µm
5.0 µm35302520
151050Roomair

567Flow rate [L/min]

Living Bacteria(3) Electrical Characteristics with
Water Particles

Streamers
O3
1 kV/10μm

Excited Species
Inactivated Bacteria

Discharge current [mA]

2500.80 kV
1.00 kV

20015010050(2) Experimental Setup

5Discharge time [min]10Discharge current decrease slightly
with discharge time when water
droplets is sprayed against themicroplasma electrodes. But stable
discharge is confirmed at 5 min to10 min.

0.8

11.2

Air

100

Similarinactivation
rate
wereobtained for B. subtilis for bothcarrier gas of air and nitrogen.

CONCLUSION
In
thisstudy,
theconclusions were obtained.

following①General bacteria can be inactivatedwith microplasma.

120
Concentration of ozone [ppm]

AirN₂Discharge voltage [kV]0(4) Ozone GenerationAir or nitrogen are supplied by using
an air pump and gas cylinders as
carrier gas. Splayed bacteria withliquidculturemediumwill
beinactivatedinplasmareactor.
Discharge voltage and discharge
current are measured bydigitaloscilloscope.

10090
80
70
60
50
4030
20
1000.60Each bacteria could be attacked anddecomposed by atomic oxygen andnitrogen species, UV, and O3 generated
between the electrodes.

0.90 kV
1.12 kV

The inactivation rate of E. coli versus
discharge voltages for both carriergas of air and nitrogen increase as
the discharge voltage increase. And
the results of air–plasma is better
than the results of nitrogen-plasma.

Inactivation rate [%]The image of microplasma electrodes
during discharge at discharge voltageof 1 kV.

The diameter of water particles,
which are made by a nebulizer, are
1µm to 5µm. Since the size of colon
bacilli is about 0.5 um to 2.5 um, or E.
coli HB101 and B. subtilis JCB20036
could be in the water drops made by
the nebulizer.

②A stable discharge around 0.8-1.1
kV was obtained, although waterparticles were sprayed against theelectrodes with a nebulizer.

Air with water

8060③Air and nitrogen inactivation rateincreased as the discharge voltage
increased. And Inactivation rate of
air-plasma were better than theresults of nitrogen-plasma.

40
2000.6

0.7

0.8

0.9

11.1

Discharge voltage [kV]The concentration of ozone withsprayed distilled water was lower
than that with only air. In the casewith sprayed distilled water, flowed
gas was cooled down and dried with
iced water before measuring theconcentration.

④The inactivation rate of E. coli isbetter than the results for B. subtilis
due to the characteristics of the cellwall.