23P017
High-speed Sterilization for Medical Equipment
Using Microwave Air Torch Plasma
マイクロ波空気トーチプラズマを用いた医療機材のハイスピード滅菌特性
Takayuki Ayabe, Takaomi Nakashima, Nobuya Hayashi and Akira Yonesu
綾部 孝之1), 中島 崇臣1)，林 信哉1), 米須 章2)
1) Faculty of Science and Engineering, Saga University, 1, Honjo-machi, Saga-shi, Saga 840-8502, Japan
佐賀大学 〒840-8502 佐賀市本庄町１番地
2) Faculty of Science and Engineering University of Ryukyus, 1 Chihara, Nishihara-machi,
Nakagami-gun, Okinawa 903-0213, Japan
琉球大学 〒903-0213 沖縄県中頭郡西原町千原１番地
Characteristics of the sterilization of medical equipments were studied using the air torch plasma producedby microwave discharge. Silicone points which is used in dental treatment was used as medical equipment.
A factor of sterilization is NOx related particles produced in the plasma. Bacilli spores that adhere tosilicone points were sterilized, and the inactivation period of bacilli on silicone points with population of
104 require 60 minutes.

2. Experimental Procedure
The glass pipe with inner diameter 11 mm
wrapped by a stainless wire spirally is inserted into
the stainless chamber with the dimension of 72 cmin length and 18cm in inner diameter. The chamber
and the inside of the glass pipe are evacuated, andthen the air flows into the inside of the glass pipe.
When the microwave of 2.45GHz is absorbed to thestainless wire, air plasma is produced in the glasspipe. The radicals with high sterilization ability isemitted from the glass pipe opening edge which isseveral cm apart from the plasma generation area in
the glass pipe.
The sample microorganism used in thisexperiment is the spore of Geobacillus
stearothermophilus, and is put on silicone point thatis the small dental equipments with various
populations. After the treatment, spores are

Microwave power
150 W
200 W
250 W

4000
Intensity (arb.unit)

3500
3000
2500
2000
1500
1000
500
0400 500 600 700 800 900 1000
Wavelength (nm)Fig. 1. Typical emission spectra of down-stream region at the plasma torch.
10

Number of bachilli

1. Introduction
High-speed sterilization of medical equipments
has been performed by the high-pressure steam(autoclave) and the ethylene oxide gas. However,
above methods have some problems concerning
damage to the equipment and residual toxic gas.
Recently, plasma sterilization using an RFdischarge has been studied as a safe andpollution-free method. Typical treatment periodrequired for the sterilization is more than 3 hours.
Reduction of sterilization time is necessary forthe high-speed and safety sterilization for medicalequipment. In this experiment, characteristics ofthe microwave plasma sterilization using air has
been studied in order to realize a rapid
sterilization method for medical devices.

010-1

10-2

10-3

10-4

10-510-601020 30 40 50 60
Treatment time (min)70Fig. 2. Population curve of bacillus adheredon silicone point.

Table I. Period required for sterilization withdifferent sample material of treatment time.

Population
5BI
10Silicone point 104

Sterilization period [min]2550

fluctuations on the surface of silicone points. The
detailed observation of the surface indicates thatbacilli stack in the microstructure. Therefore,sterilization of tiny dental equipments is found to bedifficult.
In the case of the RF plasma using air, theinactivation of bacilli on silicon points requires overthan 3 hours. The sterilization period of silicone
points with the population of 104 requires 60 min inthis experiment. This fact suggests that themicrowave torch plasma is suitable for thesterilization of dental equipments with high-speed.
4. Conclusion
The high-speed sterilization characteristics havebeen studied using bacilli on dental equipments aswell as a biological indicator. The sterilization
period, that is, sterilization ability depends strongly
on the surface structure and materials of samples tobe sterilized.

Fig. 3. SEM image of surface of silicon point.

cultivated in culture medium for 24 hours, and thensuccessful of the sterilization is confirmed bypigment in the medium.
3. Results and Discussion
Typical emission spectra of the plasma torch
downstream are shown in Fig. 1. This broad peakwould be attributed to nitrogen oxide radical, andits height is maximum in the case of the mixture gasof N2 : O2 = 1 : 1. Preliminary experiment implies
that the peak height and the sterilization period
have a linear correlation. Therefore, the sterilization
factor would be the nitrogen oxide radical with longlifetime.
Inactivation period of bacilli on silicone points
with various populations is determined. Bacilli withthe population of 101, 102, 103, 104 are inactivatedwith the treatment period of 30, 36, 50, 60 minutes,
respectively, as shown in Fig. 2. The decimal
reduction value (D value) evaluated from thesurvival curve of Fig. 2 is 9.6 min. Obtained Dvalue is relatively large, which would be due to thesurface roughness of the silicone points.
Dependence of sterilization period on sample
materials are shown Table I. Sterilization of avial-type biological indicator with bacilli with thepopulation of 105 requires 25 min. On the otherhand, the sterilization period of bacilli on silicone
points with population of 104 is 60 min. The SEM
image of surface of silicone point is shown in Fig. 3.
Silicone gum and SiC form a microscopic

References[1] H. Okada, Y. Ishida, H. Noguchi, H. Oikawa, K.
Nagayama, Y. Kiyoura: Jpn. Ohu University Dental
Journal, 227-232 (2003).
[2] M. Nagatsu: Plasma Sterilization, Jpn. J. Plasma
Fusion Res. 83, No.7, 601-606 (2007).
[3] F. Terashita, M. Nagatsu: Jpn. Soc. Plasma Sci. andNuclear Fusion Research, 163 (2002).