Natalia Yu. Babaeva

Intracellular electric fields produced by dielectric barrier discharge treatment of skin

The application of atmospheric pressure plasmas to human tissue has been shown to have therapeutic effects for wound healing and in treatment of skin diseases. These effects are attributed to both production of beneficial radicals which intersect with biological reaction chains and to the surface and intracellular generation of electric fields. In this paper, we report on computational studies of the intersection of plasma streamers in atmospheric pressure dielectric barrier discharges (DBDs) sustained in air with human skin tissue, with emphasis on the intracellular generation of electric fields. Intracellular structures and their electrical properties were incorporated into the computational mesh in order to self-consistently couple gas phase plasma transport with the charging of the surface of the skin and the intracellular production of electrical currents. The short duration of a single plasma filament in DBDs and its intersection with skin enables the intracellular penetration of electric fields. The magnitude of these electric fields can reach 100kVcm −1 which may exceed the threshold for electroporation. (Some figures in this article are in colour only in the electronic version)

Structure of positive streamers inside gaseous bubbles immersed in liquids

Electric discharges and streamers in liquids typically proceed through vapour phase channels produced by the streamer or in gaseous bubbles. The bubbles can originate by enthalpy changes produced by the discharge or can be artificially injected into the liquid. Experiments on streamers in bubbles immersed in liquids have shown that the discharge propagates either along the surface of the bubble or through the volume of the bubble as in conventional streamer propagation in air. In this paper we report on results of a computational investigation of streamer propagation through bubbles immersed in liquids. We found that the dielectric constant of the liquid in large part determines the path the streamer takes. Streamers in bubbles immersed in a liquid with a high permittivity preferentially propagate along the surface of the bubble. Liquids with low permittivity can result in the streamer propagating along the axis of the bubble. The permittivity at which this transition occurs is a function of the applied voltage, size of the bubble and the conductivity of the liquid.

Reactive fluxes delivered by dielectric barrier discharge filaments to slightly wounded skin

The application of atmospheric-pressure plasmas to human tissue has been shown to have therapeutic effects for wound healing and in treatment of skin diseases. In this paper, we report on a computational study of the intersection of plasma filaments in a dielectric barrier discharge (DBD) with a small wound in human skin in the context of plasma medicine. The wound is represented as a small cut in the epidermal layer of cells. Intracellular structures and their electrical properties were incorporated into the two-dimensional computational mesh in order to self-consistently couple gas phase plasma transport with the charging of the surface of the wound. We quantify the fluxes of reactive oxygen and nitrogen species, ions and photons produced in or diffusing into the wound as might occur during the first few discharge pulses of treatment. Comparison is made to fluxes predicted by global modelling. We show that the relative location of the plasma filament with respect to the wound is important on plasma time scales (ns) for ions and photons, and for radicals directly produced by electron impact processes. On the longer-term diffusion time scales (ms) the position of the plasma filament relative to the wound is not so critical. For typical DBD conditions, the magnitude of these fluxes to the cellular surfaces corresponds to fluences of radicals nearly equal to the surface site density. These results imply that the biological reactivity is limited by reaction probabilities and not the availability of radical fluxes.

Interaction of multiple atmospheric-pressure micro-plasma jets in small arrays: He/O2 into humid air

Arrays of atmospheric-pressure plasma jets are being considered as a means to increase the area being treated in surface modification and in plasma medicine in particular. A unique challenge of scaling plasma jet arrays is that individual plasma jets in an array tend to interact with each other, which can lead to quenching of some individual jets. To investigate these potential interactions, a computational study of one-, two- and three-tube arrays of micro-plasma jet arrays was performed. An atmospheric-pressure He/O2 = 99.8/0.2 mixture was flowed through the tubes into humid room air. We found that the jets interact through electrostatic, hydrodynamic and photolytic means. The hydrodynamic interactions result from the merging of individual He channels emerging from individual tubes as air diffuses into the extended gas jets. Ionization waves (IWs) or plasma bullets, which form the jets on the boundaries of an array, encounter higher mole fractions of air earlier compared with the center jet and so are slower or are quenched earlier. The close proximity of the jets produces electrostatic repulsion, which affects the trajectories of the IWs. If the jets are close enough, photoionizing radiation from their neighbors is an additional form of interaction. These interactions are sensitive to the spacing of the jets.

Ion activation energy delivered to wounds by atmospheric pressure dielectric-barrier discharges: sputtering of lipid-like surfaces

The application of atmospheric pressure plasmas to human tissue has been shown to have therapeutic effects for wound healing and in treatment of skin diseases. These effects are attributed to production of UV photon fluxes, electric fields and beneficial radicals which intersect with biological reaction chains, and to energetic ions bombarding the surface. In this paper we report on results from a computational investigation of the ion energy and angular distributions (IEADs) in a dielectric-barrier discharge sustained in air incident directly on cell membranes for small dry and wet wounds in human skin. We found that ion energies in excess of 20‐30eV can be delivered onto cell membranes of dry wounds, and up to 60eV onto the liquid interface of the wet wound. The details of the IEADs depend on the orientation of the cell membrane and on the relative location of the plasma streamer to the wound. Using results from a molecular dynamics simulation of ion sputter probabilities of typical lipid-like material, we show that prolonged exposure of the cell membrane to such IEADs can produce significant carbon removal. (Some figures may appear in colour only in the online journal)

Effect of inhomogeneities on streamer propagation: I. Intersection with isolated bubbles and particles

The branching of streamers in high pressure gas discharges and discharges in liquids is an almost universal occurrence having many causes. In this paper, we discuss results of an investigation of one possible cause—inhomogeneities in the media through which the streamer propagates. These inhomogeneities produce corresponding enhancements or decreases in ionization and excitation as the avalanche front encounters them, some of which may produce branching. Three types of inhomogeneities were investigated—negative bubbles (regions having a lower density than ambient), positive bubbles (having a higher density) and solid bubbles (particles). Depending on the size and density of the bubble, the streamer can be focused into the bubble (negative small bubble), deflected and split (positive bubbles and particles) or refracted (large negative bubble). In the case of gaseous bubbles, this behavior is partly explained by the larger E/N (electric field/gas number density) in the negative bubble, producing more ionization by electron avalanche, and smaller E/N in the positive bubble, producing less ionization. A streamer may diverge into a negative bubble located off axis due to seeding of electrons in the bubble by photoionization and subsequent avalanching in the large E/N. (Some figures in this article are in colour only in the electronic version)

Streamer dynamics in gases containing dust particles

Atmospheric pressure plasmas, and streamers in particular, sustained in air often encounter dust or aerosol particles having sizes of a few to tens of micrometres. The dynamics of streamers intersecting such particles are of interest due to their possible use for functionalizing the surfaces of the particles. Using a 2-dimensional plasma hydrodynamics model having an unstructured mesh, the consequences of dust particles on streamer dynamics were investigated while varying the particle size, shape and material properties. We found that while small dielectric particles (< tens of micrometres) are enveloped by the streamer, larger particles can intercept and reinitiate streamers. By increasing the permittivity and capacitance of the particles, streamer interception and re-initiation may also occur on smaller particles. The presence of multiple particles in the path of a streamer can increase the speed of the avalanche front due to synergistic polarization of the particles.

The interaction between plasma filaments in dielectric barrier discharges and liquid covered wounds: electric fields delivered to model platelets and cells

The treatment of wounds by atmospheric pressure plasmas in the context of plasma medicine typically proceeds through a liquid layer covering exposed cells. The wounds and their liquid covering often have irregular shapes with electrical properties (i.e. conductivity and permittivities) that may differ not only from wound-to-wound but also for a single wound as healing proceeds. The differing shapes and electrical properties extend into the liquid within the wound that typically contains cellular materials such as blood platelets. The plasma, wound, liquid and intra-liquid cellular components represent an interacting system of mutual dependence. In this paper, we discuss the results from a computational investigation of the treatment of small, liquid-covered wounds by filamentary dielectric barrier discharges. The sizes of the wounds are of the order of the plasma filaments and the liquid within the wound, an approximation of blood serum, contains idealized blood platelets. We find that the electrical properties of a wound can have significant effects on the spreading of the plasma on its surface by virtue of the deformation of the vacuum electric fields due to the shape, the effective capacitance of the wound and the discontinuities in electrical permittivity. This in turn effects the penetration of the electric field to cells under the liquid. The orientation and permittivity of the platelets relative to the liquid determines the electric fields that may stimulate the platelets.

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.

Ion energy and angular distributions onto polymer surfaces delivered by dielectric barrier discharge filaments in air: II. Particles

Atmospheric pressure streamers intersecting particles are of interest in the context of plasma aided combustion, where the particle may be a fuel aerosol droplet, or in sterilization of air, where the particle may be a bacterium. The ion energy and angular distributions (IEADs) incident on the particles, small curved dielectric surfaces, then in part determine the propensity for activating chemical reactions or, in the case of bacteria, the plasma’s sterilization capability. In this paper, we discuss results from a computational investigation of IEADs on small particles (45 µm radius) produced by atmospheric pressure discharge. Streamers intersecting a particle momentarily generate a large sheath potential as the streamer passes by as the particle charges towards the plasma floating potential. During that time, ions of energies up to 3‐10eV can strike the particle. The permittivity of the particle and the streamer polarity in part determine the character of the IEAD. (Some figures in this article are in colour only in the electronic version)