IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 7, JULY 2013 1725 Plasma Bullets Propagation Inside of Agarose Tissue Model Dayonna Park, Gregory Fridman, Alexander Fridman, and Danil Dobrynin Abstract— This paper demonstrates plasma bullets generated by microsecond pulses in He flow propagation inside of con- ductive agarose gel tubes mimicking tissue. The objective of this paper is to understand the possibility of internal diseases’ treatment (e.g., lung or intestinal cancer) using plasma jets. The propagation dynamics is studied using fast imaging technique, and production of reactive species is demonstrated both in gas phase (using optical emission spectroscopy) and inside of the agarose gel (using fluorescent dye). In addition, it is demonstrated that plasma bullets may propagate not only in a straight tubes, but also in L-shaped tubes, as well as be split in T-shaped tubes. All these facts offer an indication of possible successful application of plasma bullets for treatment of internal diseases, for example, lung cancers or intestinal diseases. Index Terms— Fast imaging, plasma bullets, plasma jet, plasma medicine, reactive oxygen species, tissue model. I. I NTRODUCTION A NUMBER of studies performed by many groups in recent years show that so-called plasma bullets gener- ated in noble gases may potentially be used for a number of biomedical applications, including, for example, cancer treatment [1]–[6]. This is because of production of a number of reactive oxygen species (e.g., hydrogen peroxide and OH radicals) that may trigger apoptotic mechanisms in cells [2], [6]–[8]. However, these antitumor effects are shown only in vitro studies–and never in vivo. This is partially because of the discharge–plasma bullets–generation methods, which is done inside of dielectric tubes (e.g., glass, quartz, plastic, and so on.) [1], [3], [5], [6]–[13]. Mechanisms of plasma bullets propagation inside of such tubes are still not clearly understood, and until now bullet propagation inside of tubes made of conductive material is not documented. We present a study that is focused on plasma jet propagating inside of an artificial tissue tubes based on agarose gel model. Previously, it was shown that agarose gel tissue model may be successfully used to mimic real tissues from the point of plasma-produced reactive species penetration [14], [15]. Manuscript received February 6, 2013; revised April 9, 2013; accepted May 16, 2013. Date of publication June 21, 2013; date of current version July 3, 2013. The authors are with the A. J. Drexel Plasma Institute, Drexel University, Camden, NJ 08103 USA (e-mail: eunju.drexel@gmail.com; greg.fridman@drexel.edu; fridman@drexel.edu; danil@drexel.edu). 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.2013.2265373 II. MATERIALS AND METHODS In our experiments, we used a dielectric barrier discharge- based reactor with a glass dielectric [Fig. 1(a)] powered with 20-kV pulses with the duration of ∼8 μs at a fre- quency of 1 kHz [Fig. 1(b)]. Plasma bullets are generated in He atmosphere (99%, Airgas) supplied at 10 L/min into a 15-cm glass capillary with 5-mm external diameter and 1-mm thick walls. The internal powered needle electrode (0.5-mm diameter, stainless steel) and external grounded electrode (1-mm diameter copper ring) are placed 3 cm apart. To monitor the discharge parameters we use P6015A high-voltage probe (75-MHz bandwidth, Tektronix) and CM-10-L current monitor (10 ns usable rise time, Ion Physics Corporation) connected to a 1-GHz DPO-4104B oscilloscope (Tektronix). The discharge visualization measurements are performed using 4Picos intensified charge-coupled device (ICCD) camera from Stanford Computer Optics. The camera has an 18-mm diameter multialkaline photocathode with a spectral response from 180 to 750 nm. The camera’s spectral response is 250–750 nm. Discharge optical emission spectrum is obtained using a fiber optic bundle (Princeton Instruments-Acton, 10 fibers–200-μm core) connected to the spectrometer (Prince- ton Instruments–Acton Research, TriVista TR555 spectrometer system with PIMAX digital ICCD camera, Trenton, NJ). III. RESULTS AND DISCUSSION Plasma bullets traveling both in air and agarose gel tube structures are studied [Fig. 1(c)]. Agarose gel is traditionally used to mimic the biological substrates, such as tissues, skin, cell layers, and so on. Although it does not represent real tissue we show that one is able to alter the gel’s buffering ability, density and fluidity to closely resemble tissue. Agarose gels of 1.5% wt are prepared using standard procedure with pure agar powder (Fisher) in either distilled H 2 O or phosphate buffered saline (PBS, Fisher). Measurements of H 2 O 2 penetra- tion into agarose gels are done using AmplexUltraRed reagent (Invitrogen, ex/em: 530/590 nm) fluorescent dye. 75 μL of PBS containing 100-μM AmplexUltraRed with 200-U/μL horseradish peroxidase (MP Biomedicals) are placed on the top surface of ∼7-mm thick 4.5 × 4-cm agar slice, spread uniformly over the agar surface and incubated for ∼15 min before the treatment to provide presence of the dye in the agar volume. Two 2 × 4-mm slices of agar with thickness of 5 mm are then placed on top of the preincubated agar piece 0093-3813/$31.00 © 2013 IEEE