Surface Micro-discharge Plasma Research Articles

The dynamics and transport of ozone in the gas and liquid phase generated by air surface microdischarge plasma at atmospheric pressure

AbstractThis contribution focuses on the spatial‐temporal behavior and reactive pathways of O3 produced by a surface air microdischarge in the gas and liquid phase using ultraviolet absorption spectroscopy. The findings demonstrate that mode transition from ozone to nitrogen oxide over time is observed at a constant input power higher than ~0.60 W/cm2. Due to the long‐lived characteristic and ionic wind, the perpendicular distribution of O3 is almost uniform. The maximum penetration depth is around 5 mm, and the gas–liquid mass transfer efficiency is approximately 0.4‱ at a depth of 1 mm, when the treatment time is 10 min. The mass transfer of O3 between gas and liquid phases is dominated by the liquid convention induced by ionic wind.

Comparative analysis of helium and air surface micro-discharge plasma treatment on the microbial reduction and quality attributes of beef slices

This work aimed to compare the effects of helium and air surface micro-discharge (SMD) plasma on the microbial safety and quality of beef tissues. For the beef tissue model, the concentration and diffusion depth of hydroxyl radical and ozone have different change patterns over plasma treatment time and distance in helium and air SMD plasma. The inactivation efficiency of helium plasma depended on the plasma treatment time and distance, while the inactivation efficiency of air plasma only depended on the treatment time. For the fresh beef slices, air SMD plasma treatment exhibited a higher antimicrobial activity against S. aureus and E. coli than helium SMD plasma treatment (1.5 versus 0.9; 0.9 versus 0.28 log CFU/g at 10 min). However, air SMD plasma treatment caused more adverse effects on beef quality, leading to a smooth surface, extensive lipid oxidation, protein structure damage, low pH and discoloration compared to helium SMD plasma treatment. This work provides valuable guidelines for the working gas choice in the practical application of plasma to meat decontamination.

On the mechanisms of surface microdischarge plasma treatment of onychomycosis: Penetration, uptake, and chemical reactions

AbstractDespite becoming increasingly established as a treatment technique, the mechanism underlying the plasma treatment of onychomycosis has yet to be elucidated. Here, we focus on the interactions between the nail plate and a surface microdischarge plasma, including penetration, uptake, and chemical reactions. Results show that long‐lived gaseous species from plasma can penetrate the nail plate effectively and are primarily responsible for the instantaneous antionychomycosis effect. The amount of uptake far exceeded that of penetration. Attenuated total reflection‐Fourier‐transform infrared spectroscopy and X‐ray photoelectron spectroscopy results show that antimicrobial compounds form on the nail surface. Combined with the uptake of active species in the nail plate, these antimicrobial compounds may inhibit microorganism growth, thereby promoting long‐term protection against onychomycosis.

Mode transition of air surface micro-discharge and its effect on the water activation and antibacterial activity

The chemical characteristics of atmospheric pressure dielectric barrier discharge plasma generated in air are very sensitive to some factors, such as environment temperature and discharge power. There are two typical modes, namely the ozone mode and nitrogen oxides mode. In this paper, the air surface micro-discharge plasma in the ozone mode and nitrogen oxides mode are respectively generated at environment temperatures of 5 °C and 50 °C when discharge power is almost kept at 6 W, and the plasma-activated water (PAW) is prepared by means of passing the plasma exhaust gases into deionized water. By comparing the reactive species in PAW under these two modes and their sterilization effects, it is found that the concentrations of H+, H2O2, NO2 −, NO3 − and ONOO− in PAW for the nitrogen oxides mode are about 2–3 fold over those for the ozone mode, while the concentration of O3 in PAW for the ozone mode is more than 28 times that in the nitrogen oxides mode. The sterilization effect under the nitrogen oxides mode is two orders of magnitude higher than that in the ozone mode. For the biological effects, it is found that treatment of a methicillin-resistant Staphylococcus aureus suspension by PAW leads to approximately a four-log reduction in the nitrogen oxides mode, which is higher than that in the ozone mode, leading to speculations that the reactive nitrogen species represented by ONOO− in PAW may be the critical species in sterilization. Furthermore, the transition between ozone mode and nitrogen oxides mode can be achieved by adjusting the discharge power. Interestingly, both of the compositions and concentrations of the reactive species in gas and liquid phases under 5 °C & 9.5 W and 50 °C & 6 W conditions are roughly identical, indicating that the effect of environment temperature on the mode transition is consistent with that of discharge power.

Surface modifications caused by cold atmospheric plasma sterilization treatment

Inactivation of microorganisms on sensitive surfaces by cold atmospheric plasma is one major application in the field of plasma medicine because it provides a simple and effective way to sterilize heat-sensitive materials. Therefore, one has to know whether plasma treatment affects the treated surfaces, and thus causes long-term surface modifications. In this contribution, the effect of cold atmospheric Surface micro-discharge (SMD) plasma on different materials and its sporicidal behavior was investigated.Hence, different material samples (stainless steel, different polymers and glass) were plasma-treated for 16 hours, simulating multiple plasma treatments using an SMD plasma device. Afterwards, the material samples were analyzed using surface analysis methods such as laser microscopy, contact angle measurements and x-ray photo-electron spectroscopy. Furthermore, the device was used to investigate the behavior of Bacillus atrophaeus endospores inoculated on material samples at different treatment times.The interaction results for plasma-treated endospores show, that a log reduction of the spore count between 4.3 and 6.2 can be achieved within 15 min of plasma treatment. Besides, the surface analysis revealed, that there were three different types of reactions the probed materials showed to plasma treatment, ranging from no changes to shifts of the materials’ free surface energies and oxidation.As a consequence, it should be taken into account that even though cold atmospheric plasma treatment is a non-thermal method to inactivate microorganisms on heat-sensitive materials, it still affects surface properties of the treated materials. Therefore, the focus of future work must be a further classification of plasma-caused material modifications.

A comparative study of the major antimicrobial agents against the yeast cells on the tissue model by helium and air surface micro-discharge plasma

Surface micro-discharge (SMD) plasma with a large-area and homogeneous discharge has attracted much attention in the skin disinfection due to its high antimicrobial efficiency and less side effects on tissues. Although SMD plasma sterilization is undisputedly attributed to the reactive oxygen and nitrogen species (RONS), the exact RONS speciation on the tissues and their individual contribution to the plasma inactivation are still not fully understood. Herein, we investigated the generation and distribution of hydroxyl radical (·OH), hydrogen peroxide (H2O2), ozone (O3), nitrite (NO2−), and peroxynitrite/peroxynitrous acid (OONO−/ONOOH) on the agarose tissue model and their contribution to yeast inactivation by helium (He) or air SMD plasma at different irradiation distances. The results show that He and air SMD plasma exhibited different RONS speciation and antimicrobial activity. The He SMD plasma mostly generated ·OH and H2O2 on the tissue model, which were concentrated in every hexagon micro-discharge unit and decreased with the irradiation distance, while the air SMD plasma mainly produced O3, NO2−, and OONO−/ONOOH, which were uniformly distributed on the whole tissue model. More importantly, the ·OH generation on the tissue model by the He SMD plasma was derived from the plasma delivery, while UV photolysis led to the in situ ·OH generation by the air SMD plasma. Additionally, the air SMD plasma has a higher inactivation efficiency than the He SMD plasma and the major antimicrobial agent for He and the air SMD plasma is, respectively, ·OH and O3 in this plasma–tissue interaction system.

Evaluating the roles of OH radicals, H2O2, ORP and pH in the inactivation of yeast cells on a tissue model by surface micro-discharge plasma

Surface micro-discharge (SMD) plasma holds great potential as an effective, economical and safe method for the treatment of diverse skin infections. Although the antimicrobial effect of SMD plasma is undisputed, the factors most responsible for the inactivation effect and their corresponding killing mechanisms are still not fully understood. Herein, we evaluate the roles of hydroxyl radicals (·OH), hydrogen peroxide (H2O2), oxidation–reduction potential (ORP) and pH in the inactivation of yeast cells on an agarose tissue model by helium SMD plasma over different irradiation distances and time periods. The ·OH distribution pattern shows that ·OH is mostly produced in the center of every hexagon mesh electrode, whose concentration is closely related to humidity and is markedly increased at a shorter irradiation distance and longer treatment time. Meanwhile, the killing pattern of yeast cells corresponds to ·OH distribution and the inactivation efficiency has a similar trend to that of ·OH concentration. The results of Pearson correlation analysis reveal that the inactivation efficiency is more dependent on ·OH and pH compared with H2O2 and ORP. Furthermore, we investigated the effects of ·OH and pH on yeast cell viability, membrane integrity and intracellular ROS and pH homeostasis by using a specific ·OH scavenger D-mannitol (D-man) and phosphate buffer solution (PBS). The results showed that D-man could significantly reduce the inactivation efficiency by maintaining cell membrane integrity and intracellular ROS and pH homeostasis, while PBS only slightly mitigates the plasma-caused damage on yeast cells. Based on the results, it is concluded that ·OH contributes most to the inactivation of yeast cells on a tissue model by helium SMD plasma studied here.

Inactivation of multidrug-resistant pathogens and Yersinia enterocolitica with cold atmospheric-pressure plasma on stainless-steel surfaces

The objective of this study was to investigate the impact of cold atmospheric-pressure plasma (CAP) produced by a surface micro-discharge plasma source as a new strategy to combat the transmission of five multidrug-resistant (MDR) pathogens and Yersinia enterocolitica on typical hospital- and food-producing surfaces, e.g. stainless-steel. Approximately 106 CFU/cm2 of vancomycin-resistant Enterococcus faecium, methicillin-resistant Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Y. enterocolitica were inoculated on a 3.14-cm2 stainless-steel surface. Bovine serum albumin (BSA) (3%) was used as a disruptive factor simulating natural organic material. The inoculated surfaces were subsequently exposed to CAP, generated by a peak-to-peak voltage of 10 kV with sinusoidal waveform and a frequency of 2 kHz, for 5, 10 and 20 min, respectively. Fluorescent staining with propidium iodide and SYTOTM 9 was used to demonstrate the manner of bacterial cell damage. Significant (P < 0.05) inactivation of 1.68 ± 0.17 up to 2.80 ± 0.17 log steps was achieved after 5 min of CAP treatment. However, bacterial reduction could be increased to 3.35 ± 0.1 up to 5.17 ± 0.67 log steps after 20 min of CAP treatment. Bacterial cells covered with BSA were statistically significantly less inactivated by CAP. Fluorescent staining showed a predominant level of orange-stained, sublethally damaged bacterial cells after 10 min of CAP treatment. In conclusion, CAP has the ability to inactivate MDR bacterial pathogens on stainless-steel surfaces. Further research is required to investigate the clinical features of CAP.

Plasma Treatment of Onychomycosis

We report on the use of three “cold” atmospheric plasma (CAP) devices in treating model nails that have been coated on the backside of the nail with either Escherichia coli (E. coli) bacteria or Trichophyton rubrum (T. rubrum) fungus. We tested a helium plasma jet; a surface microdischarge plasma device; and a floating electrode dielectric barrier discharge. All of the devices, acting through the model nail, showed significant log reductions of bacterial and fungal targets in several tens of minutes. CAP technology appears to offer promise as a safe, effective, and inexpensive therapy for fungal nail infection treatment.

Simple Device to Study Influence of Nanosecond Surface Microdischarge Plasma on Biomaterials

This paper presents the development and description of research aimed at investigation of nanosecond surface microdischarge plasma effect on biomaterials. The technique is based on application of a simple discharge device. The device is assembled on a standard microscope slide from inexpensive materials. An optical microscope equipped with a digital camera is used for visualization of the studied samples and microdischarges. Many images of the microdischarges on the biological objects are presented in this paper. Spatial resolution of the images allows studying the discharges along the cellular structure of the tissue.

Sporicidal properties from surface micro-discharge plasma under different plasma conditions at different humidities

In the current study, bacterial endospores of Geobacillus stearothermophilus are exposed to the surface micro-discharge plasma for 5 min and the humidity and power consumption are varied. At the low humidity of 5.5 ± 0.5 g m−3, almost no sporicidal effect (<0.5 log) is observed. At the high humidity of 17.9 ± 0.6 g m−3, the spore reduction increases monotonically up to 3.5 log with increasing power consumption. At a humidity of 10.4 ± 0.6 g m−3, the spores are inactivated in a limited range of power consumption with a maximum reduction of ∼2.5 log. The survival curves show a single-slope decrease of the spores. The contribution of heat and UV to the sporicidal effect as well as the inactivation of spores by the short-lived species from the plasma are ruled out. The concentration of ozone, one indicator for the long-lived species, is measured and no correlation with the sporicidal effect is found. In conclusion, water-related reactive species, e.g. hydrogen peroxide, appear to be responsible for the sporicidal effect under the investigated conditions. Furthermore, condensation of water at high humidity enables the plasma-activated water containing both long-lived and short-lived reactive species to contribute to the sporicidal effect.

Bactericidal Agents Produced by Surface Micro-Discharge (SMD) Plasma by Controlling Gas Compositions

The composition of the gas for the Surface Micro-Discharge (SMD) plasma system is changed by applying variety of binary mixtures with oxygen, nitrogen and argon. Gram-negative Escherichia coli and gram-positive Enterococcus mundtii bacteria strains are inoculated on agar and exposed to the plasma using different gas mixture ratios either with a fixed applied voltage or with a fixed dissipated power. No bacterial reduction is observed in the absence of oxygen. Higher bactericidal efficacy is observed with higher oxygen fraction in the gas composition for the plasma. A threshold of the oxygen fraction is found where the bacterial reduction does not change much. The strong correlation between the bactericidal efficacy and the ozone concentration measured by optical absorption spectroscopy suggests that ozone generated by the SMD plasma is the major agent for the bacterial inactivation under the investigated conditions.

The dynamics of ozone generation and mode transition in air surface micro-discharge plasma at atmospheric pressure

We present the transient, dynamic behavior of ozone production in surface micro-discharge (SMD) plasma in ambient air. Ultraviolet absorption spectroscopy at 254 nm was used to measure the time development of ozone density in a confined volume. We observed that ozone density increases monotonically over 1000 ppm for at least a few minutes when the input power is lower than ∼0.1 W/cm2. Interestingly, when input power is higher than ∼0.1 W/cm2, ozone density starts to decrease in a few tens of seconds at a constant power density, showing a peak ozone density. A model calculation suggests that the ozone depletion at higher power density is caused by quenching reactions with nitrogen oxides that are in turn created by vibrationally excited nitrogen molecules reacting with O atoms. The observed mode transition is significantly different from classical ozone reactors in that the transition takes place over time at a constant power. In addition, we observed a positive correlation between time-averaged ozone density and the inactivation rate of Escherichia coli on adjacent agar plates, suggesting that ozone plays a key role in inactivating bacteria under the conditions considered here.

Contact-free cold atmospheric plasma treatment of Deinococcus radiodurans

In this study we investigated the sensitivity of Deinococcus radiodurans to contact-free cold atmospheric plasma treatment as part of a project to establish new efficient procedures for disinfection of inanimate surfaces. The Gram-positive D. radiodurans is one of the most resistant microorganisms worldwide. Stationary phases of D. radiodurans were exposed to cold atmospheric plasma for different time intervals or to ultraviolet C (UVC) radiation at dose rates of 0.001-0.0656 J cm⁻², respectively. A methicillin-resistant Staphylococcus aureus strain (MRSA) served as control for Gram-positive bacteria. The surface microdischarge plasma technology was used for generation of cold atmospheric plasma. A plasma discharge was ignited using ambient air. Surprisingly, D. radiodurans was sensitive to the cold atmospheric plasma treatment in the same range as the MRSA strain. Survival of both bacteria decreased with increasing plasma exposure times up to 6 log₁₀ cycles (>99.999 %) within 20 s of plasma treatment. In contrast, UVC radiation of both bacteria demonstrated that D. radiodurans was more resistant to UVC treatment than MRSA. Cold atmospheric plasma seems to be a promising tool for industrial and clinical purposes where time-saving is a critical point to achieve efficient disinfection of inanimate surfaces and where protection from corrosive materials is needed.

Cold Atmospheric Air Plasma Sterilization against Spores and Other Microorganisms of Clinical Interest

Physical cold atmospheric surface microdischarge (SMD) plasma operating in ambient air has promising properties for the sterilization of sensitive medical devices where conventional methods are not applicable. Furthermore, SMD plasma could revolutionize the field of disinfection at health care facilities. The antimicrobial effects on Gram-negative and Gram-positive bacteria of clinical relevance, as well as the fungus Candida albicans, were tested. Thirty seconds of plasma treatment led to a 4 to 6 log(10) CFU reduction on agar plates. C. albicans was the hardest to inactivate. The sterilizing effect on standard bioindicators (bacterial endospores) was evaluated on dry test specimens that were wrapped in Tyvek coupons. The experimental D(23)(°)(C) values for Bacillus subtilis, Bacillus pumilus, Bacillus atrophaeus, and Geobacillus stearothermophilus were determined as 0.3 min, 0.5 min, 0.6 min, and 0.9 min, respectively. These decimal reduction times (D values) are distinctly lower than D values obtained with other reference methods. Importantly, the high inactivation rate was independent of the material of the test specimen. Possible inactivation mechanisms for relevant microorganisms are briefly discussed, emphasizing the important role of neutral reactive plasma species and pointing to recent diagnostic methods that will contribute to a better understanding of the strong biocidal effect of SMD air plasma.

Optimizing the distance for bacterial treatment using surface micro-discharge plasma

Reactive plasma species generated by a surface micro-discharge (SMD) electrode are delivered to the target by diffusion and/or convection. In humid air conditions, the diffusion process is coupled with complicated plasma chemical reactions, which affect the density profiles of bactericidal agents. One may expect that the production of reactive plasma species can be optimized at a certain distance. Our experimental results found the optimum distance for achieving the highest bactericidal efficiency with plasma treatment using an SMD electrode. The optimum distance is about 2–4 mm from the SMD electrode to the target and depends on the geometry of the experiment. The bactericidal efficiency in the plasma-treated area can be improved by a factor of 30 if the bacterial samples are placed at the optimum distance. The results show the predominant role of the long-lived reactive plasma species. It is seen that the diffusion model of multi-plasma species with coupled plasma chemical reactions would be highly important for understanding the bactericidal property of cold atmospheric plasmas and therefore for optimizing cold atmospheric plasma sources for medical and biological applications.

The bactericidal effect of surface micro-discharge plasma under different ambient conditions

A series of experiments on the bactericidal properties of plasmas using a surface micro-discharge (SMD) device in an atmosphere under different ambient temperatures and humidities was carried out. This plasma dispenser was developed for use as a disinfection system in private and public places (hospitals, medical practices, etc). The bactericidal effect is due to the interplay of the plasma and the chemical products produced via interactions with O2/N2 and H2O vapour in air. To use this device in different countries and therefore under various ambient conditions, it is important to understand its behaviour and efficiency, especially with respect to air temperature and humidity. The experimental results obtained in this study show that the bactericidal properties of the SMD plasma dispenser are not sensitive to the different temperatures and humidities.