23P024-O
Mechanisms for medical sterilization and sterilization for the development ofhigh-frequency low-pressure by air plasma
低圧高周波空気プラズマを用いた医療用滅菌法の開発と滅菌機序
Yoshitaka Ishibashi1 , Nobuya Hayashi1 and Masaaki Goto2
石橋 寿崇1, 林 信哉1，後藤 昌昭2
1

Faculty of Science and Engineering, Saga University, 1Honjo-machi, Saga-shi, Saga 840-8502, Japan
佐賀大学電気電子工学科 〒840-8502 佐賀市本庄町本庄１番
2Faculty of Medicine, Saga University, 5-1-1 Nabeshima, Saga-shi, Saga 849-8501, Japan佐賀大学医学部 〒849-8501 佐賀市鍋島 5-1-1

Characteristics of medical sterilization method using the RF air plasma generated by two types antenna
under low pressure were investigated. The meandering wire antennas of semicircular and encircling
shapes were adopted to produce plasmas, and their sterilization abilities are compared. Oxygen radicals
generated by RF discharge sterilized bacillus in biological indicator during 2-4 hours.

1. Introduction
The sterilization of medical devices is needed toprevent infection. Two major methods has been used
for sterilization in the medical field. One ishigh-pressure steam heat sterilization method(autoclave). Another one is sterilization of the gaswith ethylene oxide gas (EOG). However, two
methods have problem that deterioration of heat andharmful toxic gases.
Recently plasma sterilization has attracted muchattentions. Plasma sterilization has advantages suchas low temperature and low humidity and low
toxicity. In this experiment, air plasma sterilization
changing the form of the antenna is performed toevaluate the medical sterilization performance.
2. Experimental procedure
Figure.1 shows experimental apparatus with thesemicircular type antenna. Pressure in the chamber
set at 30-60Pa by vacuum pump. The CCP typeantenna used in the experiment has the wavy form foreffective and spatially uniform generation of oxygen
radicals. When the RF power (13.56MHz) is applied
to antenna, the grow discharge plasma with highuniformity is produced below the antenna.
In order to investigate the dependence of theantenna geometry on the sterilization characteristics,
two types of RF antenna are adopted in thisexperiment. In the case of the semicircular antenna,
the antenna is placed around the upper half wall ofthe chamber. The air discharge occurs at upper regionof chamber. The encircling type antenna has a wavy
form all around along the chamber inner wall. In
this case oxygen radicals would be generated by theair-plasma and dispersed all around of the chamber.
Sterilization experiments using above two types ofantenna were carried out.

In order to evaluate the performance of air-plasma
sterilization, the generation of oxygen radicals in thechamber in measured by the emission spectroscopy
and chemical indicator (CI) method. After treatment
with air plasma, biological indicators (BI) with thebacillus were cultivated for 24h, and the color ofpigment in the culture media (successful: purple,failure: yellow) indicates the successful of theFig.1：Schematics of experimental apparatus
with semicircular type antenna.

Fig.2：Schematics of experimental apparatus
with encircling type antenna.

treatment.
The uniformity of the generated plasma using
different types antenna is confirmed using theLangmuir probe method. Obtained uniformity ofelectron densities around the axis of the chamber
have the fluctuation of several ten % within 150mm
of radical direction in the case of the encircling typeantenna, which are acceptable for the practical use.
While that of the semicircular type antenna isapproximately 15%, therefore, oxygen radicals
generated in the air plasma using the semicircular
type antenna would have enough uniformity for
medical sterilization.
3. Experimental results and discussion
3.1 Sterilization characteristics using semicircular
type antenna
The OES spectra of the air plasma indicate thegeneration of active oxygen in the whole of thetreatment region even when the semicircular typeantenna is used. Also, the chemical indicatorconfirms that the active oxygen is produced atbottom and center of the chamber. Production of theactive oxygen is larger around the center axis of thechamber than that in the bottom region of thechamber.
Table 1 shows results of sterilization experiment in
air plasma using semicircular type antenna.
Sterilization is successful for the treatment period of
the 300 min in the bottom of the chamber. In thecenter of axis, sterilization is successful for 150 min
using a BI with sterilization bag, and is successful forit 120 min without sterilization bag.

Treatment period (min)position & pressure
30 Pa at bottom
60 Pa at bottom30 Pa at center(sterilization bag)
60 Pa at center(sterilization bag)
30 Pa at center(w/o sterilization bag)
60 Pa at center(w/o sterilization bag)

90

120

150

180

240

300

×××××××××××××

○
○

××

○×

○

○

○

○

××

○

○

○

○

○

○×

○○

○

○

○

○

×

Table 1 ： BI sterilization in air plasma withsemicircular type antenna.
Treatment period (min)position & pressure
50 Pa at center(sterilization bag)
50 Pa at center(w/o sterilization bag)

150

180240

××

○×××

○○

300

Table.2 ： BI sterilization in air plasma withencircling type antenna.

3.2 Sterilization characteristics using encircling typeantenna
In order to enhance the oxygen radical generationand inactivation ability of bacillus, we created theencircling type antenna. Langmuir probe indicatesthat the electron density in the treatment region
except the antenna survives has the fluctuation below
70%. Using this result of an electrostatic probe
method, the most efficient plasma condition isdetermined.
When the RF power of 50 W and the air pressureof 50 Pa, the higher and uniform electron density isobtained. Sterilization experiments run under above
condition. Table 2 shows result of sterilization
experiment in air plasma with encircling type antenna.
Sterilization is not successful for 180 min, and is
successful for 240 min without sterilization bag.
Therefore, the encircling type antenna is not effective
for the sterilization comparing with semicircular typeantenna. This result would be due to the decrease of
the diffusion region area that is necessary for theradical generation.
4．Conclusion
Remarkable difference of sterilizing performance
between the semicircular type and encircling typeantennas is observed. Sterilizing ability is better inthe case of semicircular type antenna than theencircling type antenna. In both cases, oxygen
radicals produced from the air are considered to be asterilization factor.
References[1] N. Hayashi, W. Guan, S. Tsutsui, T. Tomari, and Y.
Hanada: Jpn. J. Appl. Phys. 45 (2006) 8358[JSAP][2] C. L. Nelson and T. J. Berger: Curr. Microbiol. 18(1989) 275.
[3] X. Lei, Z. Rui, L. Peng, D. Li-Li and Z. Ru-Juan: Chin.
Phys. 13 (2004) 913.