S3-2

Application of Microwave Plasma to Medical Sterilization
マイクロ波プラズマの滅菌応用
Masaaki Nagatsu, Fumie Terashita, Hiroyuki Nonaka, Lei Xu, Toshi Nagata*,
Yukio Koide*, Shohei Nanko** and Ichiro Kurawaki***
永津 雅章, 寺下 文恵, 野中 雄之, 徐 蕾, 永田 年*,
小出 幸夫*, 南光 正平**, 倉脇 一郎***
Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan
静岡大学工学部，〒432-8561 浜松市城北3-5-1*Hamamatsu University School of Medicine, 1-20-1 Handayama, Hamamatsu 431-3192, Japan
*
浜松医科大学，〒431-3192 浜松市半田山1-20-1**Nissin Inc., 10-7 Kamei-cho, Takarazuka 665-0047, Japan**
株式会社ニッシン，〒665-0047 宝塚市亀井町10-7***
GMA Co. Ltd., 3898-1 Asaba-cho, Iwata-D.C. 437-1101, Japan***
ジーマ株式会社，〒437-1101 磐田郡浅羽町 3898-1

In this paper, we present recent experimental results on low-temperature sterilization using surface-wave
plasma (SWP) and volume-wave plasma (VWP).
Characteristics of plasma sterilization were presented
for both the cases of SWP and VWP. In the SWP sterilization, the sterilization of 106 spores wasconfirmed for 3 min oxygen discharge.
We also investigated the internal sterilization using
microwave-excited VWP for application to the sterilization of the medical instruments wrapped withperforated plastic materials. The spores of population of 106 were irradiated during 10 min plasma
discharge of He and O2 mixture gas. The preliminary results showed a successful sterilization for 10 min.
plasma treatment at a relatively low temperature of 70 °C.

1. Introduction
Recently it has been noticed to utilize the plasma
discharges driven by DC[1-2], RF[3] ormicrowave[4-6] in the sterilization of medical
instruments or in disinfect of food or drink
packages. Conventionally, sterilization of medical
instruments has been performed using a dry-heat or
hot-steam technique for heat-resistant materials, orthe ethylene-oxide gas (EOG) for heat-sensitive
materials.
In the former method, however,sterilized materials are limited to the metal or glass
usable under high-temperature circumstances.
Furthermore, in the latter case, there remains a big
issue of handling the dangerous ethylene-oxide gas
in the sterilization procedure. Recently, it isbelieved that EOG itself is referred to as carcinogen,and its toxicity and serious effects on environment
are concerned. In the EOG sterilization, it might
be also a big problem that it takes one week or more
to reduce the toxic ethylene-oxide residuals to thesafety level after the EOG sterilization procedure.
Therefore, the novel sterilization method, which canbe done safely, environmentally friendly andrapidly at low temperature, is strongly demanded.
Hence, we have been aiming at developing thelow-temperature plasma sterilization techniqueusing SWP and VWP. So far, we have shown that
rapid sterilization could be accomplished for 3 min.
oxygen SWP discharges[6]. Furthermore, in orderto sterilize inside the materials, such as catheter
wrapped with perforated plastic packing, we
proposed the novel technique using VWP for theinternal sterilization.
2. Experiment
Illustrations for production schemes of SWPand VWP are shown in Fig. 1. In the SWP,
plasma was locally produced with the strong
microwave launched from the microwave launcher,especially below the quartz window. Then plasma
diffuses downward toward the medical equipment,so that it simply leads to the surface sterilization.
Microwave

Microwave
Quartzplate
SWP

Surface
Sterilization

VWP
Internal
Sterilization

Medicalequipment
(a)

(b)

Fig. 1 Illustrations of (a)SWP and (b)VWP in thesterilization experiments.

On the other hand, in the case of VWP, the plasma
was produced in the region having stronger fields
distributed inside the chamber. We employed the40 cm-diam. SWP device and 25 cm-diam. VWP
device in the present experiments. The details of
the experimental setups are described in ref. [6,7].
3. Results and discussion
In the SWP sterilization experiments, we havefound that the spores with a population of 1.5x106
Bacillus stearothermophilus were killed byirradiation during 3 minutes of oxygen plasma dueto the etching effect by oxygen radicals[6]. In
addition to oxygen gas, we tested the sterilization
experiments using the other gases, such as Ar andhydrogen. Figure 2 show the results of survival
curves of spores by counting the numbers of colony
forming units after incubation procedure.

(a)Number of spores

107

(a)

106B. stearothermophilus

105population 1.5x10 6104
103

D value~15-25 s102

100

100102468Plasma irradiation period (min.)107106
Number of spores(b)

Fig.3 SEM images of the spores irradiated by theoxygen plasmas for (a) 2 min (not sterilized) and(b) 8min. (sterilized).

101(b)

sec, 50 sec or 40 sec, respectively.
In the VWP sterilization, we aimed atconfirming the internal sterilization, so that we used
the biological indicators without unpacking sample
from the glassine envelope. We changed theplasma irradiation period up to 10 min. Afterincubation procedure, we confirmed that B.I.
having spores of 105 and 106 were sterilized for 10min He/O2 mixture gas plasma. From themeasurement using thermal label, it was found thattemperature of Petri dish was about 70°C or less.

Ar plasma
H2 plasma

5104
1031021011000246810
Plasma irradiation period (min.)Fig. 2 Survival curves of spores by counting thenumbers of colony forming units in the cases of(a) oxygen and (b) Ar and hydrogen plasma.

In the case of oxygen plasma, we found roughly
linear reduction of spore numbers with a D-value ofroughly D=15~25 sec, as shown in Fig. 2(a).
However, when Ar or hydrogen gas was used,sterilization was not confirmed even for 10 min
plasma discharges. Next, by increasing the cw
incident power, we found the sterilization can beaccomplished more rapidly. When the incident
power was Pi=700W, 1 kW or 1.5 kW, sterilization
could be confirmed for oxygen plasma during 180

Figure 3 shows the SEM images of spores treatedfor 2 min- and 8 min -oxygen plasmas discharges.
It is apparently seen that the spore sizes after 8 min
plasma irradiation are significantly reduced due tothe chemical etching by oxygen plasma sterilization,
similarly to the previous results using oxygensurface-wave plasma[6].
Acknowledgments
This work has been partly supported by
Grant-in-Aid for Scientific Research from JSPS,
regional new consortium projects from METI,
Grant from the RSP program of the Shizuoka
Organization for the Creation of Industries andpartly by Faculty of Engineering of Shizuoka
University as one of advanced research projects.
References[1] I. A. Soloshenko, V.V. Tsiolko, V. A. Khomich, et al:Plasma Phys. Report, 26 (2000) 792.
[2] M. Laroussi, I. Alexeff, W. L. Kang, IEEE Trans.
Plasma Sci., 28 (2000) 184.
[3] P. T. Jacobs and S. M. Lin: Disinfection, Sterilization
and Preservation, 5th ed. Lippincortt Williams &Wilkins, (2000), Chap. 38, pp 747-763.
[4] M. Moisan, J. Barbeau, S. Moreau, J. Pelletier, et al,Int. J. Pharmaceutics 226 (2001) 1.
[5] S. Lerouge, M. R. Wertheimer, R. Marchand, et al. J.
Biomed. Mater. Res., 51 (2000) 128.
[6] M. Nagatsu, F. Terashita, Y. Koide, Jpn. J. Appl. Phys.,
42 (2003) L856.
[7] A. Ogino, M. Nagatsu,et al., submitted to Jpn. J. Appl.
Phys. (2004).