Microplasma Jet Research Articles

Side-on Surface Modification of Polystyrene with an Atmospheric Pressure Microplasma Jet

An atmospheric-pressure microplasma jet operating in helium gas has been used in a side-on configuration to modify locally the surface energy of polystyrene (PS). The plasma plume and gas outflow emanating from the capillary-type discharge (8 kVp–p at 30 kHz excitation) were directed parallel to (side-on) and just above the sample surface, rather than head-on as in conventional treatment. This side-on treatment method has been shown to prevent surface damages, pitting or roughing caused by head-on treatment methods. Schlieren photography has been used to identify regions of laminar and turbulent flow in the exiting gas stream and the nature of their interaction with the substrate surface. Correlation of these observations with spatially resolved water contact angle (WCA) measurements of the treated PS, show clearly that substantial reductions in WCA (∼50–60°) occur in downstream regions where the turbulent gas mixed with air impinges the surface. In contrast, only small changes in WCA occur (∼10–20°) in regions closer to the exit orifice where the helium (He) flow is still laminar across the surface. Little or no treatment is observed for distances greater than 5 cm from the source. In addition, low levels of treatment were observed in the regions where the visible plasma glow of the jet is adjacent and parallel to the surface. The results indicate that excited air species (either mixed or entrained in the He gas flow) are the main agents causing surface covalent bond breaking leading to surface modification.

Highly Effective Inactivation of Pseudomonas sp HB1 in Water By Atmospheric Pressure Microplasma Jet Array

By using an atmospheric pressure microplasma jet array device driven by a.c. voltage with repetition rate of several kilohertz, we were able to inactivate the resistant Pseudomonas sp HB1 cells in aqueous solution. Measurements showed that all Pseudomonas sp. cells in a suspension of 80 mL with the concentration of ~108 CFU (Colony-Forming Units) were killed within a treatment time of 6 min. Rather than O radicals, OH radicals or charged species were supposed to play the most important role in the plasma inactivation process by this method. This design can provide an effective mode of killing the resistant microorganism in water.

Investigation on plasma parameters and step ionization from discharge characteristics of an atmospheric pressure Ar microplasma jet

In this communication, we report a technique to estimate the plasma parameters from the discharge characteristics of a microplasma device, operated in atmospheric pressure on the basis of homogeneous discharge model. By this technique, we investigate the plasma parameters of a microplasma jet produced by microplasma device consisting of coaxial capillary electrodes surrounded by dielectric tube. Our results suggest that the complex dependence of electrical discharge characteristics observed for microplasma device operated with Ar or it admixtures probably signify the existence of step ionization, which is well known in inductively coupled plasma.

Unexpected O and O3 production in the effluent of He/O2 microplasma jets emanating into ambient air

Microplasma jets are commonly used to treat samples in ambient air. The effect of admixing air into the effluent may severely affect the composition of the emerging species. Here, the effluent of a He/O2 microplasma jet has been analyzed in a helium and in an air atmosphere by molecular beam mass spectrometry. First, the composition of the effluent in air was recorded as a function of the distance to determine how fast air admixes into the effluent. Then, the spatial distribution of atomic oxygen and ozone in the effluent was recorded in ambient air and compared with measurements in a helium atmosphere. Additionally, a fluid model of the gas flow with reaction kinetics of reactive oxygen species in the effluent was constructed. In ambient air, the O density declines only slightly faster with distance compared with a helium atmosphere. In contrast, the O3 density in ambient air increases significantly faster with distance compared with a helium atmosphere. This unexpected behavior cannot be explained by simple recombination reactions of O atoms with O2 molecules. A reaction scheme involving the reaction of plasma-produced excited species of unknown identity with ground state O2 molecules is proposed as a possible explanation for these observations.

Argon metastable dynamics in a filamentary jet micro-discharge at atmospheric pressure

Space- and time-resolved measurements of Ar (3P2) metastable atoms at the exit of an atmospheric pressure radio-frequency micro-plasma jet are performed using tunable diode laser absorption spectroscopy. The discharge features a coaxial geometry with a hollow capillary as the inner electrode and a ceramic tube with a metal ring as the outer electrode. Absorption profiles of metastable atoms and optical emission measurements reveal the dynamics and the filamentary structure of the discharge. Single filaments with a diameter of 125 µm and metastable densities of about 1013 cm−3 are observed. The average spatial distribution of Ar metastables is characterized with and without a target in front of the jet, showing that the target potential and, therewith, the electric field distribution substantially change the filaments' expansion. Together with a detailed analysis of the ignition phase and the discharge's behavior under pulsed operation, the results give an insight into the excitation and de-excitation mechanisms.

The Role of Oxygen and Surface Reactions in the Deposition of Silicon Oxide like Films from HMDSO at Atmospheric Pressure

AbstractThe deposition of thin SiO2‐like films by means of atmospheric pressure microplasma jets with admixture of hexamethyldisiloxane (HMDSO) and oxygen and the role of surface reactions in film growth are investigated. Two types of microplasma jets, one with a planar electrodes and operated in helium gas and the other one with a coaxial geometry operated in argon, are used to study the deposition process. The growth rate of the film and the carbon‐content in the film are measured as a function of the O2 and HMDSO admixture in the planar jet and are compared to mass spectrometry measurements of the consumption of HMDSO. Additionally, the localized nature of the jet–substrate interaction is utilized to study surface reactions by applying two jets on a rotating substrate. The addition of oxygen into the gas mixture increases HMDSO depletion and the growth rate and results in the deposition of carbon free films. The surface reaction is responsible for the carbon removal from the growing film. Moreover, carbon free films can be deposited even without addition of oxygen, when coaxial jet operated with argon is used for the surface treatment. We hypothesize that ions or excited species (metastables) could be responsible for the observed effect. magnified image

Spatial dynamics of helium metastables in sheath or bulk dominated rf micro-plasma jets

Space resolved concentrations of helium He metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. The spatial profile of metastable atoms in the volume between the electrodes was deduced for various electrode gap distances. Density profiles reveal the sheath structure and reflect the plasma excitation distribution, as well as the dominance of the α-mode discharge. Gap width variations show the transition from a normal glow plasma to a pure sheath discharge. In order to analyse and verify the experimentally observed profiles of the metastable atoms, a two-dimensional simulation model was set up. Applying an appropriate He/N2/O2 chemistry model, the correlation between the metastable profiles and the underlying excitation mechanisms was obtained.

Arrays of Microplasma Jets Generated by Double Parabolic Microcavities in an Hourglass Configuration

Microplasma jets have been generated in double parabolic microcavities arranged in an hourglass structure with a 300-μm-diameter throat. Fabricated by micropowder blasting of nanoporous alumina grown on Al foil, the parabolic cavities are situated back-to-back and operated with a feedstock gas having a flow rate of ~ 1.5-6 standard L/min. Arrays as large as 6 × 6 jets have been demonstrated for which the jet pitch and fill factor are 1.25 mm and 17%, respectively. Images of the visible fluorescence produced by the expansion of He microplasma jets in laboratory air provide a vivid picture of the extent of the He(23S) → N2+ Penning ionization reaction zone.

Microplasma Jets Generated by Arrays of Microchannels Fabricated in Flexible Molded Plastic

Novel microplasma jet devices have been fabricated in a moldable polymer that is resilient, transparent, and flexible. With a layer-by-layer assembly process involving a mold and initially undercured polymer, arrays of plasma microjets as large as 5 × 2 have been constructed and demonstrated to date. Having a channel diameter of 340 ± 5 μm and a single gas feed 4 mm in diameter for the entire array, the microchannel jets extend ~3.6 mm into laboratory air for a He gas flow rate of ~4.6 standard liters per minute.

Apoptosis Experiments of Cultured Tumor Cells Treated With 200- $\mu\hbox{m}$-Sized Flexible Microplasma Jet

A flexible atmospheric pressure microplasma jet device utilizing a 200-μm hollow-core optical fiber was developed in order to trigger the apoptosis of cultured tumor cells and determine the feasibility of plasma endoscopy. As determined by in situ apoptosis assay, the TUNEL assay, this flexible microplasma jet induced cultured murine tumor cells to undergo apoptosis. In addition, the apoptosis was induced to a greater extent in B16F0 tumor cells than in CL.7 cells for the same plasma dose.

Schlieren Photography of the Outflow From a Plasma Jet

Using Schlieren photography, the helium outflow configuration from a fine capillary-based microplasma jet discharge has been captured for free-stream conditions. The transition from laminar to turbulent flow is clearly identified with and without operation of the plasma. At a flow rate of 2.3 L·min <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> with no plasma operating, this transition occurs 54 mm from the nozzle; however, with plasma struck (peak voltages of 8 kV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p-p</sub> ), this reduces to 40 mm.

Atmospheric-Pressure Microplasma Jets From Linear Arrays of Hollow-Core Optical Fibers for Biomedical Applications

Atmospheric-pressure microplasma jets from a linear array of five hollow-core optical fibers are described. All plasma plumes from the array were quite narrow but long enough to apply the direct treatments to a very small object. The optical intensities from five individual plasma jets were also stable and had a good uniformity. The variation of the optical intensities of individual plasma plumes was within 15%. This plurality of the small-sized plasma jets can be utilized to target some groups of different cells simultaneously in an in vitro biomedical application.

Influence of gas flow on argon microwave plasma jet at atmospheric pressure

Many kinds of an atmospheric-pressure plasma jet have been developed and used for widespread applications such as a surface treatment and modified. This study focused on the argon atmospheric-pressure microplasma jet generated by discharging of RF power of 2.45 GHz microwave. The plasma jet shows sensitivity to surrounding environment: pressure, temperature and gaseous species. It is therefore absolutely imperative that a nature of atmospheric-pressure plasma jet should be understood from a point of fluid dynamics. This study, therefore, focused on the interrelationship between the plasma jet and the working gas. Motion of the plasma jet and the working gas was evaluated by velocity measurement and fast photography. As a result, the unsteady sinusoidal waving motion in the radial direction of a torch was observed. Advection velocity of the plasma in just downstream region of the torch exit increases with the supplying flow rate, and the velocity ratio is in the range of 0.75–0.87.

Development of a cold atmospheric pressure microplasma jet for freeform cell printing

An atmospheric pressure non-thermal microplasma jet (Ø 50 μm) was developed for localized functionalization of various substrates, including polymers, to allow maskless freeform cell printing. The applied microplasma jet power ranged from 0.1 to 0.2 W without causing any damage to the polyethylene substrate. The surface characterization results demonstrate that the microplasma treatment locally changes the surface roughness and the concentration of oxygen-containing functional groups on the polyethylene surface. The biological characterization confirms that the osteoblast cells attach and survive on the plasma activated line while untreated surfaces show almost no attachment and viability.

Time-resolved mass spectroscopic studies of an atmospheric-pressure helium microplasma jet

Using molecular beam mass spectroscopy, time-resolved measurements of the ionic species in the plasma plume of an atmospheric-pressure helium microplasma jet are made for a range of excitation frequencies (5, 10 and 25 kHz) and source–instrument orifice distances (1, 7 and 11 mm). Ionic species can only be observed in the visible plasma plume, with the main positive species being (65.26%) and (21.11%), and a few percentages of N+, O+, NO+and He+. For the negative ions, the majority species are (22.68%), (H2O) (10.49%) and a large range of minority species observed, namely , , , and clusters, , , (CO3) and . The flux of ions created from air species, such as NO+ and , is seen to be maximized at a distance of several millimeters from the nozzle, whereas the He+ concentration continually decreases with distance from the exit orifice. The time-resolved measurements (time resolution down to 2 µs) show that positive ions appear twice in one full period of the voltage waveform, correlated directly with positive and negative current peaks. The rise and fall times of the positive ions are typically tens of μs. In contrast, the appearance of negative ions is correlated only with the negative part of discharge current, with one main peak in the detected ionic flux seen per cycle. The rise time of the negative ions is about 10 µs, independent of mass; however, we observed longer decay times, from 100 to 150 µs, increasing with mass. With increased driving frequency, the time modulation in the ionic fluxes is reduced, particularly for the negative species that show almost constant fluxes at 25 kHz throughout the cycle. The observations can be understood through a simple picture of the interaction of the He jet and the moist ambient air. The results indicate that the discrete plasma ‘bullets’ and their afterglow tail, that forms the jet, carry an associated positive or negative current depending on the time of their creation in the voltage cycle.

Cancer Therapy: Single‐Cell‐Level Microplasma Cancer Therapy (Small 16/2011)

The image shows the apoptotic response in B16F0 tumor cells induced by a 15 μm hollow-core optical fiber microplasma jet assay using the TUNEL assay, which detects DNA nicks–an important cell apoptotic indicator. Apoptotic tumor cells treated by the plasma device show DNA nicks, and their nuclei are dyed to a bright color by the imaging assay kit, whereas no response was found in untreated cells. A well-defined boundary between plasma-treated and nontreated areas can be observed. This work enables new directed cancer therapies based on a highly flexible and precise hollow, optical-fiber-based microplasma cancer endoscope, and it offers unique advantages to understanding plasmainduced tumor cell apoptosis.

Differential responses of human liver cancer and normal cells to atmospheric pressure plasma

When treated by atmospheric pressure plasma, human liver cancer cells (SK-HEP-1) and normal cells (THLE-2) exhibited distinctive cellular responses, especially in relation to their adhesion behavior. We discovered the critical threshold voltage of 950 V, biased at the electrode of the micro-plasma jet source, above which SK-HEP-1 started to detach from the substrate while THLE-2 remained intact. Our mechanical and biochemical analyses confirmed the presence of intrinsic differences in the adhesion properties between the cancer and the normal liver cells, which provide a clue to the differential detachment characteristics of cancer and normal cells to the atmospheric pressure plasma.

Impurity intrusion in radio-frequency micro-plasma jets operated in ambient air

Space and time resolved concentrations of helium He (3S1) metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Spatial profiles and lifetime measurements show significant influences of air entering the discharge from the front nozzle and of impurities originating from the gas supply system. Quenching of metastables was used to deduce quantitative concentrations of intruding impurities. The impurity profile along the jet axis was determined from optical emission spectroscopy as well as their dependence on the feed gas flow through the jet.

Apoptosis of lung carcinoma cells induced by a flexible optical fiber-based cold microplasma

Atmospheric pressure plasmas have been used as a therapy for cancer. However, the fairly large size and rigidity of present plasma-delivery systems obstructs the precise treatment of tumors in harder-to-reach internal organs such as the lungs, pancreas, and duodenum. In order to improve the targeted delivery of plasmas a highly flexible microplasma jet device is fabricated using a hollow-core optical fiber with an inner diameter of either 15μm, 55μm, or 200μm. Described herein, based on this device, are results on lung carcinoma therapy using a microplasma cancer endoscope. Despite the small inner diameter and the low gas flow rate, the generated plasma jets are shown to be sufficiently effective to induce apoptosis, but not necrosis, in both cultured mouse lung carcinoma and fibroblast cells. Further, the lung carcinoma cells were found to be more sensitive to plasma treatment than the fibroblast cells based on the overall plasma dose conditions. This work enables directed cancer therapies using on highly flexible and precise hollow optical fiber-based plasma device and offers enhancements to microplasma cancer endoscopy using an improved method of plasma targeting and delivery.

Single‐Cell‐Level Microplasma Cancer Therapy

A flexible microplasma endoscope based on a 15 μm hollow-core glass optical fiber is fabricated, and tumor cell apoptotic analysis supports its potential use in targeted cancer therapies. The optical-fiber microplasma jet reveals antitumor activity at a certain plasma dose in animal studies.