- Email: [email protected]
Atmospheric-pressure cold plasma jet for medical applications
Surface & Coatings Technology 205 (2010) S418–S421
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Atmospheric-pressure cold plasma jet for medical applications Won-Seok Kang a, Yong-Cheol Hong b, Yoo-Beom Hong a, Jae-Ho Kim a, Han Sup Uhm c,⁎ a b c
Department of Molecular Science & Technology, Ajou University, San 5, Wonchon-Dong, Youngtong-Gu, Suwon 443-749, Republic of Korea Division of Applied Technology Research, National Fusion Research Institute, 113 Gwahangno, Yuseong-Gu, Daejeon, 305-333, Republic of Korea Kwangwoon Academy of Advanced Studies, Kwangwoon University, 447-1 Wolgye-Dong, Nowon-Gu, Seoul 137-701, Republic of Korea
a r t i c l e
i n f o
Available online 6 September 2010 Keywords: Cold plasma jet Medical application Sterilization Porous alumina
a b s t r a c t An atmospheric-pressure plasma jet operated with air is presented. The plasma jet device is composed of a porous alumina dielectric element, an outer electrode, and a hollow inner electrode. Microdischarges in the porous alumina evolve to form a plasma jet that reaches lengths up to several tens of millimeters as the flow rate of the working gas increases. The discharge characteristics were investigated by measuring the voltage and current waveforms and by observing the optical emissions. Sterilization of Escherichia coli (E. coli) was carried out as an example for medical applications of the plasma jet. E. coli cells were completely removed after exposure to the air-plasma jet, even at an exposure time of less than 60 s. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Atmospheric-pressure plasmas have received much attention lately as a promising physical tool for biological decontamination and sterilization. Conventional methods of sterilization involve heat, irradiation and chemical agents. However, these methods can damage a treated substrate. In this sense, non-thermal plasmas [1–3] as a sterilization method have been used in conjunction with various microorganisms. Many plasma jet devices that produce a cold atmospheric-pressure plasma plume have been investigated for their use with thermally sensitive materials and medical applications [4–12]. For example, Nie et al. [9] presented a simple cold plasma jet with a length of several centimeters that utilized floating electrodes in a quartz tube under high voltage. In another configuration, Deng et al. [10] described a glow discharge plasma jet and its applications in protein destruction by making use of a dielectric barrier discharge arrangement. These plasma plumes ensure the stability of the plasma through the use of a noble gas at atmospheric pressure [5,9,10]. Kolb et al. [4] reported a cold atmospheric-pressure air-plasma jet in micro-hollow cathode geometry for medical applications. Their study demonstrated that an air microplasma jet can effectively treat yeast infections on skin. The utilization of atmospheric air not only reduces the complexity of the device but also enhances the production of reactive species such as hydroxyl radicals, atomic oxygen, and nitric oxide [4,9,10]. For the inactivation of Escherichia coli [13,14], RF-powered atmospheric plasma effectively reduced the number of viable cells within 2 s, which suggests that it is very effective for inactivating harmful
microorganisms. When the afterglow plume from a hollow slot microplasma device was applied to B. atrophaeus endospore [15,16] for inactivation, a ten-fold reduction was achieved within 3 min. Thus, these types of plasmas can be employed to reduce or sterilize bacteria contaminations on material surfaces [17]. The present study investigates the characteristics of an air-plasma jet from preliminary tests
Gas in Inner electrode
Porous AC high voltage alumina power supply Quartz tube
Outer electrode
Plasma jet
15 mm ⁎ Corresponding author. E-mail address: [email protected] (H.S. Uhm). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.138
Resistor
Voltage controller
Fig. 1. Schematic presentation of the atmospheric-pressure air-plasma jet device. The inset is a photograph of the 5 slm air-plasma jet.
W.-S. Kang et al. / Surface & Coatings Technology 205 (2010) S418–S421
Fig. 2. Plot of the airflow rate vs. the plasma jet length. The inset shows pictures of plasma jets with different airflow rates.
and presents the experimental results of plasma sterilization tests with the E. coli bacteria. 2. Experimental detail Fig. 1 shows a schematic presentation of an air-plasma jet device with a porous alumina dielectric. The plasma jet system is mainly composed of electrodes, dielectrics, and a high-voltage power supply, which is a commercially available transformer for neon light operated at 20 kHz and is connected to two electrodes. The voltage controller
S419
regulates the primary voltage of the high-voltage transformer. The inner electrode is a typical injection needle made of stainless steel with an inner diameter of 1.2 mm and a thickness of 0.2 mm; it is tightly covered with a quartz tube with an outer diameter of 3.2 mm. Porous alumina 10 mm in diameter and 20 mm in length is machined for the inner electrode, through which the quartz tube is inserted. The tip of the inner electrode and the inner surface of the porous alumina are in contact. The outer electrode is fabricated from stainless steel and has a somewhat conical shape; it is centrally perforated with a hole of 1 mm through which the plasma jet is ejected to the surrounding ambient air. As shown in Fig. 1, the porous alumina with the inner electrode is installed within the outer electrode. The discharge gap, which is 2 mm in this work, is the distance between the tips of the porous alumina and the inner electrode. It can be adjusted by controlling the depth at which the inner electrode is inserted into the porous alumina. The inner surface of the outer electrode and the tip of the porous alumina are also in contact. Air is injected into the injection needle and is then ejected through the 1 mm hole in the outer electrode via the porous alumina. The alumina used in this work has approximately 30 vol.% porosity and has an average pore diameter of 100 μm. The preparation of the alumina dielectric is described in an earlier study [18]. Once air is introduced through the inner electrode and high-voltage ac power is applied, a discharge is fired in the porous alumina between the electrodes, and a long plasma jet reaching lengths of up to several tens of millimeters is ejected into the open air, as shown in the inset of Fig. 1. E. coli was used as a sample microorganism in the sterilization experiments. After attachment of the E. coli from a cover glass, the samples were directly exposed to the plasma jet. The distance between the jet exit and the cover glass was
Fig. 3. Voltage and current waveforms of the air-plasma jet at different input power levels. (a) Before discharge. (b) Vrms = 4.69 kV and Im = 0.18 mA. (c) Vrms = 3.54 kV and Im = 0.24 mA. (d) Vrms = 1.57 kV and Im = 1.6 mA.
S420
W.-S. Kang et al. / Surface & Coatings Technology 205 (2010) S418–S421
15 mm at 5 standard liters per minute (slm) of air. The plasma treatment time was varied from 0 s to 60 s. The plasma-treated samples were put into a 50 ml conical tube containing 5 ml of Tryptic Soy Broth (TSB, Difico) for 3 h, and 1 ml of the suspension was sampled for spread plate counting.
a
3. Results and discussion Fig. 2 shows the dependence of the length of the plasma jet with regard to different airflow rates at an rms voltage (Vrms) of 1.66 kV using the voltage controller shown in Fig. 1. The length of the plasma jet in Fig. 2 was measured visually. The inset displays photographs of the plasma jets taken at different air flow rates. At 1 slm of air, the plasma jet was only 8 mm long and its intensity was very weak. When the air flow rate was increased from 2 to 5 slm, the length of the plasma jet increased linearly to 15 mm. At a flow rate in excess of 5 slm, the plasma jet length was saturated at approximately 15 mm. The microdischarges in the porous alumina evolved into a plasma plume as the applied power was increased. Significant changes in the discharge voltage and current waveforms were also observed during the process of the evolution to the plasma jet. The current and voltage traces in Fig. 3 were obtained by increasing the input level of the highvoltage power supply using the voltage controller shown in Fig. 1. In this case, the airflow rate was maintained at 5 slm. Voltage and current measurements were performed using a high-voltage probe and a current probe with a digital oscilloscope (Tektronix MSO4032). Fig. 3(a) shows the typical sinusoidal voltage waveform before the discharge. The voltage was 2.62 kV in rms at this step. A slight increase in the input power in the voltage controller led to an increase in the Vrms value to 4.69 kV as well as distortion of the voltage waveform, showing pulsed waveforms and implying the generation of filamentary discharges in the porous alumina, as shown in Fig. 3(b). At this point, the average discharge current Im was 0.18 mA. The asymmetry of the positive and negative voltage waveforms in Fig. 3(b) resulted from the unsteadiness of and the random changes in the spatial and temporal distribution of the microdischarge channels. In addition, the asymmetry may have been caused by the asymmetrical electrode configuration, which consisted of an outer electrode with a hole and a hollow injection needle electrode. In Fig. 3(c), a further increase of the input power led to the generation of a large amount of pulses, a subsequent decrease of Vrms to 3.54 kV, and an increase of Im to 0.24 mA. The pulsed voltage dropped to nearly zero, and the voltage drops occurred in conjunction with sharp current pulses. The number of pulses in a negative half cycle was two times greater than that of a positive half cycle. In terms of electrical observations, these numbers strongly depend on the input power. The generation of the voltage and current pulses during this step was steady; although the values were diverse, a stable plasma jet of approximately 15 mm in length formed. For a stable plasma jet, a power of less than approximately 2.3 W is dissipated in the discharge. Fig. 3(d) shows the waveforms when Vrms = 1.57 kV and Im = 1.6 mA, corresponding to the 5 slm plasma jet in the inset of Fig. 2. The inset in Fig. 3(d) is a magnified view of the part marked with a circle, showing the voltage waves in a sawtooth profile and the sharp current pulses. In the close-up image, the current pulse always coincides with the voltage drop. In Fig. 3(d), the current pulses were of short durations of 3–32 μs. Their repetition rates were approximately 31–334 kHz and their amplitudes reached only a few amperes. This indicates that even at a frequency as low as 20 kHz, the plasma that evolves from a large amount of microdischarge inside a porous dielectric can have characteristics that are similar to those generated at several hundreds of kilohertz. According to the electrical measurements, it is expected that not only the steady generation but also the frequency of the pulses resulting from the microdischarges in the porous dielectric play an important role in obtaining a stable plasma jet. Additionally, the effective generation of microdischarges may be possible for a specific pore size.
b
Fig. 4. (a) Plot showing gas temperatures measured at different axial positions of the plasma jet. (b) Human skin in contact with the air-plasma jet.
In this study, the temperature in the ejected plasma is mainly influenced by the applied power and airflow. Fig. 4 shows the gas temperatures of plasma jets at different airflow rates, corresponding to the input power in Fig. 3(d). The gas temperatures were measured by a thermocouple. The axial position z in Fig. 4(a) represents the distance from the outer electrode. The difference of gas temperature at z = 2 mm for the 3 and 5 slm airflow rates is approximately 30 °C. The plasma jet can be touched by the bare hand or scanned over human skin without establishing a conductive pathway, as shown in Fig. 4(b).
Fig. 5. Optical emission lines from the plasma jet at 5 slm of air.
W.-S. Kang et al. / Surface & Coatings Technology 205 (2010) S418–S421
S421
into a plasma jet in a specially designed electrode configuration through control of the applied power and gas flow. An air-plasma jet was ejected to ambient air, showing a long plasma plume of 15 mm at 5 slm of air. It was also observed that the microdischarges can have repetition pulse rates as high as several hundred kilohertz with a duration of several tens of microseconds, depending on the experimental parameters. Eventually, it is expected that highly reactive species produced from microdischarges inside the porous alumina will extend several tens of millimeters from the exit of the plasma jet, revealing a D-value of 14 s for the disinfection of E. coli. The results of this study suggest that the plasma jet can function as an effective tool in biological applications.
Acknowledgments
Fig. 6. Effect of plasma treatment on the survival of E. coli.
Fig. 5 displays an optical emission spectrum that identifies various excited plasma species produced from the air-plasma jet that evolved from the microdischarges inside the porous alumina over a wide range of wavelengths of 250 to 900 nm. The optical emission spectrum was obtained from the plasma jet at 5 slm of air. The emission spectrum in Fig. 5 was primarily dominated by the presence of atomic oxygen at 777 nm and by other reactive atomic oxygens at 616 and 844 nm [2,9]. Previous studies reported that these highly reactive species can be very effective agents in destroying cells or organic materials [19,20]. Additionally, reactive species related to nitrogen were detected. They contained N2 second (C3Пu–B3Пg) and first (B3Пg–A3П+ u ) positive systems in ranges of 300–390 nm and 610–710 nm, respectively; ionized nitrogen molecules in a range of 390–480 nm; and atomic nitrogen at 747, 822, and 868 nm [21–23]. The killing effect of the plasma jet on E. coli as a function of the treatment time is shown in Fig. 6. For plasma treatment of E. coli, 0.2 ml of cell suspension at a concentration of 105 to 106 cells per ml was deposited on a thin cover glass and then dried on a clean bench overnight at room temperature. The plasma jet effectively killed the E. coli bacteria within 15 s. The decimal reduction time (D-value), which is defined as the time necessary to reduce an initial concentration of a target microorganism by 90%, was approximately 14 s. The experimental results indicate that the E. coli were completely removed after exposure to the air-plasma jet, even at a treatment time of less than 60 s. 4. Conclusions This study showed that microdischarges inside a porous alumina dielectric element with a sinusoidal voltage wave of 20 kHz can evolve
The authors thank Mr. Kang Il Kim of the Department of Electronic Engineering at Ajou University in Suwon, Korea for obtaining the voltage and current waveforms. In addition, we thank the BK21 program of molecular science and technology at Ajou University and the SRC program of NRF at Kwangwoon University.
References [1] M. Laroussi, F. Leipold, Int. J. Mass Spectrom. 233 (2004) 81. [2] M. Moisan, J. Barbeau, S. Moreau, J. Pelletier, M. Tabrizian, L.H. Yahia, Int. J. Pharm. 226 (2001) 1. [3] M. Laroussi, IEEE Trans. Plasma Sci. 30 (2002) 1409. [4] J.F. Kolb, A.-A.H. Mohamed, R.O. Price, R.J. Swanson, A. Bowman, R.L. Chiavarini, M. Stacey, K.H. Schonenbach, Appl. Phys. Lett. 92 (2008) 241502. [5] X.-P. Lu, Z.-H. Jiang, Q. Xiong, Z.-Y. Tang, X.-W. Hu, Y. Pan, Appl. Phys. Lett. 92 (2008) 081502. [6] Y.C. Hong, H.S. Uhm, Appl. Phys. Lett. 89 (2006) 221504. [7] X. Zhang, M. Li, R. Zhou, K. Feng, S. Yang, Appl. Phys. Lett. 93 (2008) 021502. [8] Y.C. Hong, H.S. Uhm, W.J. Yi, Appl. Phys. Lett. 93 (2008) 051504. [9] Q.-Y. Nie, C.-S. Ren, D.-Z. Wang, J.-L. Zhang, Appl. Phys. Lett. 93 (2008) 011503. [10] X.T. Deng, J.J. Shi, M.G. Kong, J. Appl. Phys. 101 (2007) 074701. [11] Y.C. Hong, S.C. Cho, J.H. Kim, H.S. Uhm, Phys. Plasmas 14 (2007) 074502. [12] Y.C. Hong, S.C. Cho, H.S. Uhm, Appl. Phys. Lett. 90 (2007) 141501. [13] T. Sato, T. Miyahara, A. Doi, S. Ochiai, T. Urayama, T. Nakatani, Appl. Phys. Lett. 89 (2006) 073902. [14] T. Sato, K. Fujioka, R. Ramasamy, T. Urayama, S. Fujii, IEEE Trans. Ind. Appl. 42 (2006) 399. [15] R.C. Hockney, Trends Biotechnol. 12 (1994) 456. [16] T. Ohshima, K. Okuyama, M. Sato, J. Electrostat. 55 (2002) 227. [17] R. Rahul, O. Stan, A. Rahman, E. Littlefield, K. Hoshimiya, A.P. Yalin, A. Sharma, A. Pruden, C.A. Moore, Z. Yu, G.J. Collins, J. Phys. D Appl. Phys. 38 (2005) 1750. [18] I.G. Koo, M.Y. Choi, J.H. Kim, J.H. Cho, W.M. Lee, Jpn. J. Appl. Phys. 47 (2008) 4705. [19] Q.S. Yu, F.H. Hsieh, H. Huff, Y. Duan, Appl. Phys. Lett. 88 (2006) 013903. [20] M. Laroussi, IEEE Trans. Plasma Sci. 30 (2002) 1409. [21] C.O. Laux, T.G. Spence, C.H. Kruger, R.N. Zare, Plasma Sources Sci. Technol. 12 (2003) 125. [22] X.J. Huang, Y. Xin, Q.H. Yuan, Z.Y. Ning, Phys. Plasmas 15 (2008) 073501. [23] R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, Wiley, New York, 1950, p. 169.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Atmospheric-pressure cold plasma jet for medical applications Won-Seok Kang a, Yong-Cheol Hong b, Yoo-Beom Hong a, Jae-Ho Kim a, Han Sup Uhm c,⁎ a b c
Department of Molecular Science & Technology, Ajou University, San 5, Wonchon-Dong, Youngtong-Gu, Suwon 443-749, Republic of Korea Division of Applied Technology Research, National Fusion Research Institute, 113 Gwahangno, Yuseong-Gu, Daejeon, 305-333, Republic of Korea Kwangwoon Academy of Advanced Studies, Kwangwoon University, 447-1 Wolgye-Dong, Nowon-Gu, Seoul 137-701, Republic of Korea
a r t i c l e
i n f o
Available online 6 September 2010 Keywords: Cold plasma jet Medical application Sterilization Porous alumina
a b s t r a c t An atmospheric-pressure plasma jet operated with air is presented. The plasma jet device is composed of a porous alumina dielectric element, an outer electrode, and a hollow inner electrode. Microdischarges in the porous alumina evolve to form a plasma jet that reaches lengths up to several tens of millimeters as the flow rate of the working gas increases. The discharge characteristics were investigated by measuring the voltage and current waveforms and by observing the optical emissions. Sterilization of Escherichia coli (E. coli) was carried out as an example for medical applications of the plasma jet. E. coli cells were completely removed after exposure to the air-plasma jet, even at an exposure time of less than 60 s. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Atmospheric-pressure plasmas have received much attention lately as a promising physical tool for biological decontamination and sterilization. Conventional methods of sterilization involve heat, irradiation and chemical agents. However, these methods can damage a treated substrate. In this sense, non-thermal plasmas [1–3] as a sterilization method have been used in conjunction with various microorganisms. Many plasma jet devices that produce a cold atmospheric-pressure plasma plume have been investigated for their use with thermally sensitive materials and medical applications [4–12]. For example, Nie et al. [9] presented a simple cold plasma jet with a length of several centimeters that utilized floating electrodes in a quartz tube under high voltage. In another configuration, Deng et al. [10] described a glow discharge plasma jet and its applications in protein destruction by making use of a dielectric barrier discharge arrangement. These plasma plumes ensure the stability of the plasma through the use of a noble gas at atmospheric pressure [5,9,10]. Kolb et al. [4] reported a cold atmospheric-pressure air-plasma jet in micro-hollow cathode geometry for medical applications. Their study demonstrated that an air microplasma jet can effectively treat yeast infections on skin. The utilization of atmospheric air not only reduces the complexity of the device but also enhances the production of reactive species such as hydroxyl radicals, atomic oxygen, and nitric oxide [4,9,10]. For the inactivation of Escherichia coli [13,14], RF-powered atmospheric plasma effectively reduced the number of viable cells within 2 s, which suggests that it is very effective for inactivating harmful
microorganisms. When the afterglow plume from a hollow slot microplasma device was applied to B. atrophaeus endospore [15,16] for inactivation, a ten-fold reduction was achieved within 3 min. Thus, these types of plasmas can be employed to reduce or sterilize bacteria contaminations on material surfaces [17]. The present study investigates the characteristics of an air-plasma jet from preliminary tests
Gas in Inner electrode
Porous AC high voltage alumina power supply Quartz tube
Outer electrode
Plasma jet
15 mm ⁎ Corresponding author. E-mail address: [email protected] (H.S. Uhm). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.138
Resistor
Voltage controller
Fig. 1. Schematic presentation of the atmospheric-pressure air-plasma jet device. The inset is a photograph of the 5 slm air-plasma jet.
W.-S. Kang et al. / Surface & Coatings Technology 205 (2010) S418–S421
Fig. 2. Plot of the airflow rate vs. the plasma jet length. The inset shows pictures of plasma jets with different airflow rates.
and presents the experimental results of plasma sterilization tests with the E. coli bacteria. 2. Experimental detail Fig. 1 shows a schematic presentation of an air-plasma jet device with a porous alumina dielectric. The plasma jet system is mainly composed of electrodes, dielectrics, and a high-voltage power supply, which is a commercially available transformer for neon light operated at 20 kHz and is connected to two electrodes. The voltage controller
S419
regulates the primary voltage of the high-voltage transformer. The inner electrode is a typical injection needle made of stainless steel with an inner diameter of 1.2 mm and a thickness of 0.2 mm; it is tightly covered with a quartz tube with an outer diameter of 3.2 mm. Porous alumina 10 mm in diameter and 20 mm in length is machined for the inner electrode, through which the quartz tube is inserted. The tip of the inner electrode and the inner surface of the porous alumina are in contact. The outer electrode is fabricated from stainless steel and has a somewhat conical shape; it is centrally perforated with a hole of 1 mm through which the plasma jet is ejected to the surrounding ambient air. As shown in Fig. 1, the porous alumina with the inner electrode is installed within the outer electrode. The discharge gap, which is 2 mm in this work, is the distance between the tips of the porous alumina and the inner electrode. It can be adjusted by controlling the depth at which the inner electrode is inserted into the porous alumina. The inner surface of the outer electrode and the tip of the porous alumina are also in contact. Air is injected into the injection needle and is then ejected through the 1 mm hole in the outer electrode via the porous alumina. The alumina used in this work has approximately 30 vol.% porosity and has an average pore diameter of 100 μm. The preparation of the alumina dielectric is described in an earlier study [18]. Once air is introduced through the inner electrode and high-voltage ac power is applied, a discharge is fired in the porous alumina between the electrodes, and a long plasma jet reaching lengths of up to several tens of millimeters is ejected into the open air, as shown in the inset of Fig. 1. E. coli was used as a sample microorganism in the sterilization experiments. After attachment of the E. coli from a cover glass, the samples were directly exposed to the plasma jet. The distance between the jet exit and the cover glass was
Fig. 3. Voltage and current waveforms of the air-plasma jet at different input power levels. (a) Before discharge. (b) Vrms = 4.69 kV and Im = 0.18 mA. (c) Vrms = 3.54 kV and Im = 0.24 mA. (d) Vrms = 1.57 kV and Im = 1.6 mA.
S420
W.-S. Kang et al. / Surface & Coatings Technology 205 (2010) S418–S421
15 mm at 5 standard liters per minute (slm) of air. The plasma treatment time was varied from 0 s to 60 s. The plasma-treated samples were put into a 50 ml conical tube containing 5 ml of Tryptic Soy Broth (TSB, Difico) for 3 h, and 1 ml of the suspension was sampled for spread plate counting.
a
3. Results and discussion Fig. 2 shows the dependence of the length of the plasma jet with regard to different airflow rates at an rms voltage (Vrms) of 1.66 kV using the voltage controller shown in Fig. 1. The length of the plasma jet in Fig. 2 was measured visually. The inset displays photographs of the plasma jets taken at different air flow rates. At 1 slm of air, the plasma jet was only 8 mm long and its intensity was very weak. When the air flow rate was increased from 2 to 5 slm, the length of the plasma jet increased linearly to 15 mm. At a flow rate in excess of 5 slm, the plasma jet length was saturated at approximately 15 mm. The microdischarges in the porous alumina evolved into a plasma plume as the applied power was increased. Significant changes in the discharge voltage and current waveforms were also observed during the process of the evolution to the plasma jet. The current and voltage traces in Fig. 3 were obtained by increasing the input level of the highvoltage power supply using the voltage controller shown in Fig. 1. In this case, the airflow rate was maintained at 5 slm. Voltage and current measurements were performed using a high-voltage probe and a current probe with a digital oscilloscope (Tektronix MSO4032). Fig. 3(a) shows the typical sinusoidal voltage waveform before the discharge. The voltage was 2.62 kV in rms at this step. A slight increase in the input power in the voltage controller led to an increase in the Vrms value to 4.69 kV as well as distortion of the voltage waveform, showing pulsed waveforms and implying the generation of filamentary discharges in the porous alumina, as shown in Fig. 3(b). At this point, the average discharge current Im was 0.18 mA. The asymmetry of the positive and negative voltage waveforms in Fig. 3(b) resulted from the unsteadiness of and the random changes in the spatial and temporal distribution of the microdischarge channels. In addition, the asymmetry may have been caused by the asymmetrical electrode configuration, which consisted of an outer electrode with a hole and a hollow injection needle electrode. In Fig. 3(c), a further increase of the input power led to the generation of a large amount of pulses, a subsequent decrease of Vrms to 3.54 kV, and an increase of Im to 0.24 mA. The pulsed voltage dropped to nearly zero, and the voltage drops occurred in conjunction with sharp current pulses. The number of pulses in a negative half cycle was two times greater than that of a positive half cycle. In terms of electrical observations, these numbers strongly depend on the input power. The generation of the voltage and current pulses during this step was steady; although the values were diverse, a stable plasma jet of approximately 15 mm in length formed. For a stable plasma jet, a power of less than approximately 2.3 W is dissipated in the discharge. Fig. 3(d) shows the waveforms when Vrms = 1.57 kV and Im = 1.6 mA, corresponding to the 5 slm plasma jet in the inset of Fig. 2. The inset in Fig. 3(d) is a magnified view of the part marked with a circle, showing the voltage waves in a sawtooth profile and the sharp current pulses. In the close-up image, the current pulse always coincides with the voltage drop. In Fig. 3(d), the current pulses were of short durations of 3–32 μs. Their repetition rates were approximately 31–334 kHz and their amplitudes reached only a few amperes. This indicates that even at a frequency as low as 20 kHz, the plasma that evolves from a large amount of microdischarge inside a porous dielectric can have characteristics that are similar to those generated at several hundreds of kilohertz. According to the electrical measurements, it is expected that not only the steady generation but also the frequency of the pulses resulting from the microdischarges in the porous dielectric play an important role in obtaining a stable plasma jet. Additionally, the effective generation of microdischarges may be possible for a specific pore size.
b
Fig. 4. (a) Plot showing gas temperatures measured at different axial positions of the plasma jet. (b) Human skin in contact with the air-plasma jet.
In this study, the temperature in the ejected plasma is mainly influenced by the applied power and airflow. Fig. 4 shows the gas temperatures of plasma jets at different airflow rates, corresponding to the input power in Fig. 3(d). The gas temperatures were measured by a thermocouple. The axial position z in Fig. 4(a) represents the distance from the outer electrode. The difference of gas temperature at z = 2 mm for the 3 and 5 slm airflow rates is approximately 30 °C. The plasma jet can be touched by the bare hand or scanned over human skin without establishing a conductive pathway, as shown in Fig. 4(b).
Fig. 5. Optical emission lines from the plasma jet at 5 slm of air.
W.-S. Kang et al. / Surface & Coatings Technology 205 (2010) S418–S421
S421
into a plasma jet in a specially designed electrode configuration through control of the applied power and gas flow. An air-plasma jet was ejected to ambient air, showing a long plasma plume of 15 mm at 5 slm of air. It was also observed that the microdischarges can have repetition pulse rates as high as several hundred kilohertz with a duration of several tens of microseconds, depending on the experimental parameters. Eventually, it is expected that highly reactive species produced from microdischarges inside the porous alumina will extend several tens of millimeters from the exit of the plasma jet, revealing a D-value of 14 s for the disinfection of E. coli. The results of this study suggest that the plasma jet can function as an effective tool in biological applications.
Acknowledgments
Fig. 6. Effect of plasma treatment on the survival of E. coli.
Fig. 5 displays an optical emission spectrum that identifies various excited plasma species produced from the air-plasma jet that evolved from the microdischarges inside the porous alumina over a wide range of wavelengths of 250 to 900 nm. The optical emission spectrum was obtained from the plasma jet at 5 slm of air. The emission spectrum in Fig. 5 was primarily dominated by the presence of atomic oxygen at 777 nm and by other reactive atomic oxygens at 616 and 844 nm [2,9]. Previous studies reported that these highly reactive species can be very effective agents in destroying cells or organic materials [19,20]. Additionally, reactive species related to nitrogen were detected. They contained N2 second (C3Пu–B3Пg) and first (B3Пg–A3П+ u ) positive systems in ranges of 300–390 nm and 610–710 nm, respectively; ionized nitrogen molecules in a range of 390–480 nm; and atomic nitrogen at 747, 822, and 868 nm [21–23]. The killing effect of the plasma jet on E. coli as a function of the treatment time is shown in Fig. 6. For plasma treatment of E. coli, 0.2 ml of cell suspension at a concentration of 105 to 106 cells per ml was deposited on a thin cover glass and then dried on a clean bench overnight at room temperature. The plasma jet effectively killed the E. coli bacteria within 15 s. The decimal reduction time (D-value), which is defined as the time necessary to reduce an initial concentration of a target microorganism by 90%, was approximately 14 s. The experimental results indicate that the E. coli were completely removed after exposure to the air-plasma jet, even at a treatment time of less than 60 s. 4. Conclusions This study showed that microdischarges inside a porous alumina dielectric element with a sinusoidal voltage wave of 20 kHz can evolve
The authors thank Mr. Kang Il Kim of the Department of Electronic Engineering at Ajou University in Suwon, Korea for obtaining the voltage and current waveforms. In addition, we thank the BK21 program of molecular science and technology at Ajou University and the SRC program of NRF at Kwangwoon University.
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