IEEE TRANSACTIONS ON PLASMASCIENCE, VOL. 37, NO. 1, JANUARY 2009 121 The Interaction of a Direct-Current Cold Atmospheric-Pressure Air Plasma With Bacteria Hongqing Feng, Peng Sun, Yufeng Chai, Guohua Tong, Jue Zhang, Weidong Zhu, and Jing Fang Abstract—A direct-current cold atmospheric-pressure air plasma microjet (PMJ) based on the microhollow cathode dis- charge design is used to inactivate six types of bacteria within a small well-defined area on a large petri dish. We show that the PMJ is very effective in inactivating bacteria in their vegetative state as well as in the spore state within the area of plasma exposure. We also observed that bacteria in their vegetative state were inactivated efficiently outside the area of direct plasma ex- posure. Different bacteria responded differently to an increase in the plasma exposure (dose). Lastly, we observed two types of colony forming unit (CFU) distributions after plasma treatment; one distribution is diffusionlike with a gradual increase of the surviving CFU as one moves radially away from the area of direct plasma exposure, and the other distribution shows an essentially uniform reduction in surviving CFU across the entire petri dish. Index Terms—Atmospheric pressure, colony forming units (CFUs), direct current (dc), plasma jet. I. I NTRODUCTION N ONTHERMAL (“cold”) plasmas are susceptible to in- stabilities when operated in air at atmospheric pressure. Confining the plasmas to small dimensions (with at least one dimension at or below 1 mm) has been shown to improve the stability of atmospheric-pressure air plasmas, and several such “microplasma” designs have been reported in the liter- ature, e.g., the plasma needle [1], the atmospheric-pressure plasma jet [2], various corona discharges [3], [4], the split- ring resonator microplasma [5], and the microhollow cathode discharge (MHCD) [6]. These microplasmas can be generated by direct-current (dc), alternating-current (ac including RF and microwave), or pulsed excitation. Stable atmospheric-pressure air plasmas can also be generated using ac or pulsed power in Manuscript received May 19, 2008; revised August 19, 2008, September 7, 2008 and October 21, 2008. Current version published January 8, 2009. This work was supported in part by Bioelectrics Inc. (USA) and in part by the National Basic Research Program of China 2007CB935602. H. Feng and Y. Chai are with the Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China. P. Sun and G. Tong are with the Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, China. J. Zhang is with the Laboratory of Biomedical Signal and Image Studies, Department of Biomedical Engineering, College of Engineering, Peking Uni- versity, Beijing 100871, China (e-mail: zhangjue@pku.edu.cn). W. Zhu is with the Department of Applied Science and Technology, Saint Peter’s College, Jersey City, NJ 07031 USA (e-mail: wzhu@spc.edu). J. Fang is with the Academy for Advanced Interdisciplinary Studies and the Department of Biomedical Engineering, Peking University, Beijing 100871, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2008.2008438 an arrangement where at least one electrode is covered with an insulating (or highly resistive) material (dielectric barrier dis- charges (DBDs) [7], floating-electrode DBDs [8], and resistive barrier discharges [9]). Applications of these plasmas span a wide range from UV/VUV radiation sources [4], [10] or radia- tion detector [11] to microdischarge plasma reactors [12] to the remediation of volatile organic compounds [13], [14]. The use of atmospheric-pressure cold plasmas in biomedical application has attracted considerable attention recently due to possible applications in wound healing [8], [15], sterilization of heat- sensitive reusable medical instruments [16], [17], or the sur- face modification of biocompatible materials [18], [19]. Both atmospheric- and low-pressure plasmas have been used exten- sively in the inactivation of various microorganisms [20], [21]. In this paper, we report results of the interaction of a dc atmospheric-pressure cold air plasma microjet (PMJ) based on the MHCD concept with six types of bacteria. The samples after preparation and characterization were placed in a large petri dish. Only a well-defined small area were subjected to various doses of plasma exposure. The number of colony forming units (CFU count) before and after plasma treatment was examined inside the area of direct plasma exposure as well as outside. Both bacteria in their vegetative state and spores were subjected to plasma inactivation. II. EXPERIMENTAL DETAILS A. Generation of the Atmospheric-Pressure PMJ The PMJ used in the present study is based on the MHCD concept [22]. The MHCD structure comprises a cathode with a microhollow structure and an anode (with a similar micro- hollow structure, which is aligned with that in the cathode) separated by a dielectric layer of dimensions below 1 mm. When the operating gas (here, air or nitrogen) is pushed through the opening of this structure and dc power is applied, a spatially well-defined atmospheric-pressure PMJ can be sustained in ambient air [23]. The device used in this study was originally developed at the Frank Reidy Research Center for Bioelectrics at Old Dominion University by Schoenbach et al. [24] and was slightly modified from its original design here. A schematic diagram of the device is shown in Fig. 1(a). Two metal electrodes are separated from each other by a dielectric layer of ∼ 0.5-mm thickness. The openings in the two elec- trodes are ∼ 0.8 mm in diameter. The high-voltage electrode is completely embedded in the device and powered by a dc power supply (Matsusada AU5R120). The outer electrode is grounded for safety considerations. Although both positive and negative 0093-3813/$25.00 © 2008 IEEE