S Sven Hofmann

Mechanisms of bacterial inactivation in the liquid phase induced by a remote RF cold atmospheric pressure plasma jet

A radio-frequency atmospheric pressure argon plasma jet is used for the inactivation of bacteria (Pseudomonas aeruginosa) in solutions. The source is characterized by measurements of power dissipation, gas temperature, absolute UV irradiance as well as mass spectrometry measurements of emitted ions. The plasma-induced liquid chemistry is studied by performing liquid ion chromatography and hydrogen peroxide concentration measurements on treated distilled water samples. Additionally, a quantitative estimation of an extensive liquid chemistry induced by the plasma is made by solution kinetics calculations. The role of the different active components of the plasma is evaluated based on either measurements, as mentioned above, or estimations based on published data of measurements of those components. For the experimental conditions being considered in this work, it is shown that the bactericidal effect can be solely ascribed to plasma-induced liquid chemistry, leading to the production of stable and transient chemical species. It is shown that HNO2, ONOO − and H2O2 are present in the liquid phase in similar quantities to concentrations which are reported in the literature to cause bacterial inactivation. The importance of plasma-induced chemistry at the gas‐liquid interface is illustrated and discussed in detail. (Some figures may appear in colour only in the online journal)

Power dissipation, gas temperatures and electron densities of cold atmospheric pressure helium and argon RF plasma jets

A set of diagnostic methods to obtain the plasma parameters including power dissipation, gas temperature and electron density is evaluated for an atmospheric pressure helium or argon radio frequency (RF) plasma needle for biomedical applications operated in open air. The power density of the plasma is more or less constant and equal to 1.3 ? 109?W?m?3. Different methods are investigated and evaluated to obtain the gas temperature. In this paper the gas temperatures obtained by rotational spectra of OH(A?X) and (B?X) are compared with Rayleigh scattering measurements and measurements of the line broadening of hydrogen and helium emission lines. The obtained gas temperature ranges from 300 to 650?K, depending on the gas. The electron densities are estimated from the Stark broadening of the hydrogen ? and ? lines which yield values between 1019 and 1020?m?3. In the case of helium, this is an overestimate as is shown by a power balance from the measured power density in the plasma jet. The obtained plasma parameters enable us to explain the radial contraction of the argon plasma compared with the more diffuse helium plasma. The accuracy of all considered diagnostics is discussed in detail.

Spatially resolved ozone densities and gas temperatures in a time modulated RF driven atmospheric pressure plasma jet : an analysis of the production and destruction mechanisms

In this work, a time modulated RF driven DBD-like atmospheric pressure plasma jet in Ar?+?2%O2, operating at a time averaged power of 6.5?W is investigated. Spatially resolved ozone densities and gas temperatures are obtained by UV absorption and Rayleigh scattering, respectively. Significant gas heating in the core of the plasma up to 700?K is found and at the position of this increased gas temperature a depletion of the ozone density is found. The production and destruction reactions of O3 in the jet effluent as a function of the distance from the nozzle are obtained from a zero-dimensional chemical kinetics model in plug flow mode which considers relevant air chemistry due to air entrainment in the jet fluent. A comparison of the measurements and the models show that the depletion of O3 in the core of the plasma is mainly caused by an enhanced destruction of O3 due to a large atomic oxygen density.

Nitric oxide density distributions in the effluent of an RF argon APPJ: effect of gas flow rate and substrate

The effluent of an RF argon atmospheric pressure plasma jet, the so-called kinpen, is investigated with focus on the nitric-oxide (NO) distribution for laminar and turbulent flow regimes. An additional dry air gas curtain is applied around the plasma effluent to prevent interaction with the ambient humid air. By means of laser-induced fluorescence (LIF) the absolute spatially resolved NO density is measured as well as the rotational temperature and the air concentration. While in the laminar case, the transport of NO is attributed to thermal diffusion; in the turbulent case, turbulent mixing is responsible for air diffusion. Additionally, measurements with a molecular beam mass-spectrometer (MBMS) absolutely calibrated for NO are performed and compared with the LIF measurements. Discrepancies are explained by the contribution of the NO2 and N O 2 to the MBMS NO signal. Finally, the effect of a conductive substrate in front of the plasma jet on the spatial distribution of NO and air diffusion is also investigated.

Transitions Between and Control of Guided and Branching Streamers in DC Nanosecond Pulsed Excited Plasma Jets

Plasma bullets are ionization fronts created in atmospheric-pressure plasma jets. The propagation behavior of those bullets is, in the literature, explained by the formation of an interface between the inert gas and the ambient air created by the gas flow of the plasma jet, which guides these discharges in the formed gas channel. In this paper, we examine this ionization phenomenon in uniform gases at atmospheric pressure where this interface between two gases is not present. By changing electrical parameters and adding admixtures such as oxygen, nitrogen, and air to the gas flow, the conditions for which plasma bullets are present are investigated. Nanosecond time-resolved images have been taken with an ICCD camera to observe the propagation behavior of these discharges. It is argued that the inhomogeneous spatial concentration of metastable atoms and ions, due to the laminar gas flow and the operation frequency of the discharge in the range of a few kilohertz, is responsible for the guidance of the ionization fronts. Furthermore, conditions have been observed at where the branching of the discharge is stable and reproducible over time in the case of a helium plasma by adding admixtures of oxygen. Possible mechanisms for this phenomenon are discussed.

In situ absolute air, O3 and NO densities in the effluent of a cold RF argon atmospheric pressure plasma jet obtained by molecular beam mass spectrometry

A molecular beam mass spectrometer has been calibrated and used to measure the air entrainment, nitric oxide and ozone concentrations in the effluent of a cold atmospheric pressure argon RF driven plasma jet. The approaches for calibrating the mass spectrometer for different species are described in detail. Gas phase densities of ozone and nitric oxide up to 7.5 ppm and 4 ppm, respectively, have been measured in the far effluent of the argon plasma jet. The difference in air entrainment when the plasma is undisturbed or is close to a well, which is the case for e.g. in vitro plasma–cell interaction studies, is shown. In addition, an exponential decay of the positive ion flux as a function of distance in the effluent is obtained. Furthermore, the effect of plasma power, duty cycle and air and O2 admixtures introduced into the argon flow on the NO and O3 production is presented, including the possibility of independent control of the NO and O3 flux from the jet.

Electron densities and energies of a guided argon streamer in argon and air environments

In this study we report the temporally and spatially resolved electron densities and mean energies of a guided argon streamer in ambient argon and air obtained by Thomson laser scattering. The plasma is driven by a positive monopolar 3.5 kV pulse, with a pulse width of 500 ns and a frequency of 5 kHz which is synchronized with the high repetition rate laser system. This configuration enables us to use the spatial and temporal stability of the guided streamer to accumulate a multitude of laser/plasma shots by a triple grating spectrometer equipped with an ICCD camera and to determine the electron parameters. We found a strong initial ne-overshoot with a maximum of 7 × 1019 m−3 and a mean electron energy of 4.5 eV. This maximum is followed by a fast decay toward the streamer channel. Moreover, a 2D distribution of the electron density is obtained which exhibits a peculiar mushroom-like shape of the streamer head with a diameter significantly larger than that of the emission profile. A correlation of the width of the streamer head with the expected pre-ionization channel is found.

Antibacterial plasma at safe levels for skin cells

Plasmas produce various reactive species, which are known to be very effective in killing bacteria. Plasma conditions, at which efficient bacterial inactivation is observed, are often not compatible with leaving human cells unharmed. The purpose of this study was to determine plasma settings for inactivation of Pseudomonas aeruginosa, without damaging skin cells in vitro under the same treatment conditions. An RF argon plasma jet excited with either continuous or time modulated (20 kHz, 20% duty cycle) voltages was used. To compare these two operation modes, only the input voltage was adjusted in order to obtain the same average power (1.7 W) for both modes. All other settings, i.e. gas flow, distance plasma tip to liquid surface, were kept constant. Bacteria or skin cells in physiological salt solution were exposed to direct non-contact plasma treatments. Short plasma treatments of up to 2 min resulted in a high reduction of bacterial numbers and did not affect dermal fibroblasts or keratinocytes. Bacterial inactivation has been previously ascribed to peroxynitrite, nitrite and H2O2 while eukaryotic cell viability is proposed to be reduced in the long term by the presence of H2O2 and is less affected by reactive nitrogen species. The remote RF plasma jet treatment was highly effective for bacterial inactivation while skin cell viability was preserved.

A new flexible DBD device for treating infected wounds: in vitro and ex vivo evaluation and comparison with a RF argon plasma jet

Cold plasma has been shown to provide a promising alternative antimicrobial treatment for wound healing. We developed and tested a flexible surface dielectric barrier discharge (DBD) and compared it to an argon gas based plasma jet operated remotely with a distance between plasma plume and sample of 8 mm. Tests were conducted using different models: on cultured cells, on ex vivo human skin and on bacteria (Pseudomonas aeruginosa) (on agar, in suspension, in collagen/elastin matrix or on ex vivo human skin), allowing us to directly compare bactericidal with safety aspects under identical conditions. Both plasma devices were highly efficient when used on bacteria in non-buffered solutions, but DBD was faster in reaching the maximum bacterial reduction. Treatment of bacteria on intact skin with DBD resulted in up to 6 log reductions in 3 min. The jet was far less efficient on intact skin. Even after 8 min treatment no more than 2 log reductions were obtained with the jet. Treatment of bacteria in burn wound models with DBD for 6 min resulted in a 4.5 log reduction. Even when using DBD for 6 min on infected burn wound models with colonizing or biofilm phase bacteria, the log reductions were 3.8 or 3.2 respectively. DBD plasma treatment for 6 min did not affect fibroblast viability, whereas a treatment for 8 min was detrimental. Similarly, treatment with DBD or plasma jet for 6 min did also not affect the metabolic activity of skin biopsies. After treatment for 8 min with DBD or plasma jet, 78% or 60% of activity in skin biopsies remained, respectively. Multiple treatments of in vitro burn wound models with surface DBD for 6 min or with plasma jet for 8 min did not affect re-epithelialization. With the flexible surface DBD plasma strip we were able to quickly inactivate large numbers of bacteria on and in skin. Under the same conditions, viability of skin cells or re-epithelialization was not affected. The DBD source has potential for treating larger wound areas.

Comparison of a He and an Ar Cold RF Atmospheric-Pressure Plasma Jet Operating in Continuous and Pulsed RF Modes

In this paper, the absorbed power, power density, and absolute UV emission of a radio-frequency jet working in continuous and pulsed modes are measured. The plasma source is a plasma needle which operates under atmospheric pressure with helium or argon in air. Photographs of the discharges were taken to present the effect of radial contraction of the argon plasma for the highest power densities investigated. The contraction is explained by the electron-ion-dissociative-recombination-dominated losses in the argon case.