Surface Micro-discharge Research Articles

Influence of electrode geometry on the transport of OH radicals in an atmospheric pressure surface micro-discharge

Atmospheric pressure surface micro-discharge (SMD) has recently received considerable interest as an efficient source for its potential applications in biomedical, agricultural and environmental fields. The inherent physical separation between the discharge layer and the downstream target causes a reduction in the flux of reactive chemical species, resulting in the limitation of application efficacy. To obtain the development of healthcare devices, this contribution focuses on the spatial-temporal transport behavior of OH radicals generated by SMD devices in helium using laser induced fluorescence and optical emission spectroscopy with hexagon mesh electrode of lattice distance varying between 3 mm and 11 mm. It was observed that the lattice distance has a significant impact on the distribution of OH radicals and the uniformity of the SMD. The increased lattice distance of the hexagon mesh both increased the density, delivery distance and propagation velocity of OH radicals. When the lattice distance varied from 3 to 11 mm, the generated OH was spread over twice the distance. The maximum delivery distance was found to be approximately 10.4 mm under lattice distance of 11 mm condition, and the peak propagation velocity was estimated to be about 3.8 mm ms−1. For the main distribution region of OH, the optimum lattice distance is 9 mm. In addition, the distribution region rapidly expands between 7 and 9 mm. However, the findings indicated that researchers should make a compromise between the delivery of reactive chemical species and uniformity. The decreased lattice distance facilitated a high degree of uniformity for reactive species on a downstream target, which is a key requirement in healthcare related areas.

The distribution of OH in surface micro-discharge with different electrode widths by laser-induced fluorescence

Cold atmospheric plasma has been successfully applied in the field of biomedicine. One of the most pressing challenge is the development of plasma devices, for example, large-area application in wound healing. Surface micro-discharge device is one new type of dielectric barrier discharge design with desirable plasma features, such as homogeneity over a large surface area. In this study, two devices with electrode width varying from 0.25 to 1.00 mm were fabricated. The contribution focuses on the impact of the hexagonal mesh electrode width on the degree of uniformity of hydroxyl (OH) radicals in the downstream region and the OH transport efficiency. Accordingly, OH radicals were measured as an indicator by laser-induced fluorescence. Steady state measurements of OH perpendicular to the dielectric surface demonstrated that narrow electrode width could not offer a high degree of uniformity, and will reduce the transport efficiency of reactive species.

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.

Low-energy short-term cold atmospheric plasma: Controlling the inactivation efficacy of bacterial spores in powders.

The present research work aims to elucidate kinetics and mechanisms of the inactivation of Bacillus subtilis spores by a surface micro-discharge (SMD) - cold atmospheric pressure plasma (CAPP). Regarding industrial applications, the inactivation of spores was also studied for a static layer of a biopolymer powder or film, with an air plasma and at ambient pressure. Close to 4 log10 cycles of inactivation of Bacillus subtilis spores were achieved when exposing spores on flat glass to the SMD-CAPP. This effect can be reached at a very low plasma power density of 5mW/cm2 in 7min exposure time. The maximum inactivation level of spores drops when treating corn-starch powder to 2.6 log10 cycles at 7mW/cm2 plasma power density for 5min and with a polymer load of 5mg/cm2. Similar is true for films produced with hydroxymethyl cellulose (HMC). The inactivation efficacy can be tuned and is a function of applied surface energy (product of the plasma power density and the exposure time) and the polymer load. Plasma diagnostics reveal the fundamental importance of reactive nitrogen species (RNS) in the inactivation. Etching of spore hull is supposed to be triggered by the plasma density, while UV-C and UV-B radiation do not contribute directly and significantly to the inactivation effect at least in a biopolymer matrix. Fluidization of a fixed powder layer is supposed to overcome limitations of the inactivation efficacy by reducing the diffusion distance of active plasma species between the source and the sample. The combination of low plasma power density with short treatment time is supposed to reduce the risk of the formation of side-products from the matrix.

In vitro comparison of direct plasma treatment and plasma activated water on Escherichia coli using a surface micro-discharge

In this study, our aim is to compare the inactivation of bacteria by direct plasma exposure of bacterial solution and plasma activated water (PAW) using a surface micro-discharge (SMD). Four potential factors which could affect both treatments were tested, namely the effect of low dose reactive oxygen species (ROS) pretreatment, enzymatic defence mechanism against oxidative stress, extracellular molecules, and mass transfer in liquid. We applied two power modes of the SMD, i.e. low power ozone mode and high power mode, to inactivate () in distilled water either by direct treatment (DT) or PAW. ROS pretreatment was tested by 30 M of , while superoxide dismutase (SOD) and catalase were used to investigate the effect of enzymatic antioxidant defense. Furthermore, the effect of extracellular molecules was examined by centrifuging and mass transfer in liquid by vortexing. In addition, reactive species in the solution were measured by titanium sulfonate, and nitrite/nitrate colorimetric method for , and and , respectively. Results indicate that vortexing and centrifuging significantly influence the bacterial reduction, while pretreatment could not induce resistance of bacteria against plasma treatment. Enzymatic antioxidants of SOD and catalase could potentially effect the inactivation and plasma chemistry yet only if the concentrations are sufficiently high. We assume that the continuous generation of peroxynitrite is crucial in plasma inactivation for both DT and PAW under high power condition, while under low power condition, long-term bactericidal affects are limited due to the decomposition of ozone by nitrite.

Interaction of long-lived reactive species from cold atmospheric pressure plasma with polymers: Chemical modification by ozone and reactive oxygen-nitrogen species

Atmospheric pressure plasma (APP) sources are able to generate a variety of reactive species that have different effects on materials, such as functionalization, etching, and deposition. In this article, the authors study the effect of long-lived reactive neutral species on polymers using a model plasma-surface interaction system that consists of ultrathin (∼10 nm) polystyrene (PS) films and a surface microdischarge (SMD) reactor operated with various N2/O2 working gas mixtures. The authors characterized and quantified the reactive species generated by SMD using IR and UV absorption, and they found that O3, N2O5, N2O, and HNO3 are the dominant long-lived reactants near the target surface. When exposing PS films to these reactive species, the authors observed material responses including film thickness expansion, surface and bulk oxidation, and surface organic nitrate formation. The quantity of these changes varied with the N2/O2 working gas composition. By correlating material response with gas phase species, the authors find that the chemical modification of PS strongly depends on the density of O3 in the gas phase, which is indicative of an essential role of O3 in the remote APP treatment of polymers. Authors’ results show that O3 causes polymer surface oxidation, participates in the diffusion-reaction process in the polymer bulk, and results in aromatic ring cleavage and the formation of carbonyl groups. In contrast, they did not find a correlation between surface organic nitrate and individual long-lived reactive species mentioned above. This indicates that the organic nitrate formation on polymer surfaces might result from the interaction of multiple species, including O3 and nitrogen containing reactive species. A model for the interphase mass transfer of reactive species from gas to solid was also described.

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.

Biofilm inactivation by synergistic treatment of atmospheric pressure plasma and chelating agents

In this study, we investigated the bacterial biofilm reduction by combined treatment of atmospheric pressure plasma (APP) and chelating agents. Many of hospital acquired infections (HAI) are related to biofilm infections. APP and chelating agents were reported to be effective in bacterial biofilm inactivation and eradication. It is believed that chelating agents disrupt the biofilm formation through bonding metal ions, while APP inactivates bacteria mainly through reactive species [1–3]. In our study, we used a surface micro-discharge (SMD) driven by sinusoidal power input of 2 kHz and peak to peak voltage of 9 kV to treat Escherichia coli (E. coli), Enterococcus faecalis (E. faecalis), and Staphylococcus capitis (S. capitis) biofilm on 316 L stainless steel and hopkins rod lens glass plates in combination with chelating agents of trisodium citrate (TSC), ethylenediaminetetraacetic acid (EDTA), egtazic acid (EGTA), and alizarin. Bacterial biofilm reduction was measured by means of colony count assay and BacTiter-Glo cell viability assay. The results of colony count assay showed that combined treatment of EDTA and TSC with plasma has synergistic effects on all three bacterial biofilms, while EGTA only on E. coli and none for alizarin. Experiments of BacTiter-Glo cell viability assay indicate that EDTA, EGTA, and TSC has synergistic effects on all three biofilms while also none for alizarin.

Decontamination of raw produce by surface microdischarge and the evaluation of its damage to cellular components

AbstractThe use of surface microdischarge (SMD) for the bacterial decontamination of raw produce was evaluated. With 1 min of SMD treatment, >2 logarithmic reduction in Escherichia coli O157:H7 was consistently observed. The scanning electron microscopy of E. coli O157:H7 show that SMD damages the cell membrane, leads to cell expansion, and eventually lysis. The attenuated total reflectance‐Fourier‐transform infrared spectroscopy characterization of E. coli O157:H7 and lipopolysaccharides (LPS) shows that SMD causes (a) the oxidation of cellular components by forming COOH and COO − groups inside and on the cell wall, and (b) the modification of polysaccharides and phosphorus‐containing groups found in phospholipids and DNA. Further characterization with X‐ray photoelectron spectroscopy suggests SMD mainly modifies the O‐chain and core‐polysaccharide part of LPS.

Uptake and diffusion of plasma-generated reactive nitrogen species through keratinized membrane

We propose a mathematical model for uptake and diffusion of air plasma-generated reactive nitrogen species (RNS) in a model keratinized membrane, such as a thin slice of bovine hoof or human nail material. An experimental system consisting of a surface microdischarge (SMD) in air was designed for the purpose of developing and validating a reaction-diffusion model to describe this system. Key variables such as membrane effective diffusivity, surface reaction rate coefficients, and other parameters are determined through comparison between the model predictions and experimental measurements. The model results yield spatial and temporal concentration profiles of RNS inside the keratinized membrane, leading to an improved understanding of transport and reaction of plasma generated RNS through the membrane. This work offers insights into possible mechanisms underlying plasma treatment of toenail fungus.

Effects of plasma activated species produced by a surface micro-discharge device on growth inhibition of cyanobacteria

A kind of surface micro-discharge (SMD) device is developed to produce the reactive oxygen/nitrogen species (RONS). According to the yield of ozone and nitrides which is heavily dependent on the applied discharging voltages, the discharges of this SMD can be categorized into three modes, i.e. ozone dominating, nitrogen oxides dominating and transition modes. RONS airflow (or air plasma flow) is led into cyanobacteria solution. Not only ozone but also the pH decrease caused by nitrogen oxides can effectively inhibit the growth and reproduction of cyanobacteria. The pH threshold below which the acidity takes effect is 6.8. In the transition mode, when the solution is alkaline ozone plays a major role, and when the solution is converted to acidic the effect of pH decrease catches up even more than ozone. For this SMD device, direct RONS airflow with an applied voltage range of 20 to 30 kV and a time period of 8 to 11-min treatment is optimal to successfully inhibit the growth and reproduction of cyanobacteria, without obvious changes of environmental acidity.

The transport behaviour of OH radicals in atmospheric pressure surface micro-discharge

This work presents the transport behaviour of OH radicals in an atmospheric pressure pulse-modulated surface micro-discharge in helium. Laser-induced fluorescence is employed to measure the time development of OH radicals density distribution during the pulse duration and the inter-pulse period. It is shown that convection caused by electrohydrodynamic force enhances and dominates the transport of OH radicals from the surface plasma layer to the afterglow region with a distance up to 8 mm away from the dielectric surface. Interestingly, after the plasma switches off, OH density decreases quickly near the electrode but the downstream distribution region keeps constant, showing that the impact of convection is still present during the post-discharge. Additionally, the propagation velocity of OH declines monotonously with time. The maximum value is estimated as 1.86 m s−1 during the first 1 ms when the plasma is on. The influence of power delivered to plasma is investigated, and the results indicate that the OH density everywhere in the detection zone is directly proportional to the applied power. However, the input power has no significant influence on the propagation velocity and delivery distance, suggesting that it is unrealistic to increase the delivery distance of reactive species by increasing the power delivered to plasma.

Plasma afterglow circulation apparatus for decontamination of spacecraft equipment

A newly developed apparatus using cold atmospheric plasma (CAP) is presented, providing a useful alternative decontamination method for spacecraft equipment. The designed setup uses the plasma afterglow generated by a surface micro-discharge (SMD) technology and works with a circulating gas flow of ambient air at room temperature. Additionally, the apparatus allows the control of gas flow, plasma power and humidity, and offers O3 monitoring and a variable treatment volume. Within this study we examined the apparatus’ performance by evaluation of the inactivation efficacy of bacterial endospores Bacillus atrophaeus in different treatment volumes of 0.54 l, 1.8 l and 2.6 l. The experiments with Bacillus atrophaeus showed at least a 4.4 log reduction after the treatment times of 10, 20 and 30 min in the respective treatment chambers with a volume of 0.54 l, 1.8 l and 2.6 l. These results demonstrate that high sporicidal effects can be achieved with the newly developed apparatus, and that longer treatment times are needed for larger treatment volumes due to different filling rates of reactive components in different treatment volumes. Conclusively, these investigations illustrate the scalability of the designed apparatus up to 2.6 l for the afterglow treatment of samples with flat surfaces. The composition of the plasma afterglow was analysed by Fourier Transformation Infrared (FTIR) and UV absorption spectroscopy. The spectroscopic analyses identify O3, N2O, and HNO3 as predominant products of the CAP apparatus.

Pattern Transition in a Surface Microdischarge in Atmospheric-Pressure Helium

The surface microdischarge in atmospheric helium is studied for the dynamic evolution of plasma patterns in the glow mode. It is found that there are two distinct patterns in the positive half cycle. This indicates that the influence of space-charge electric field caused by surface deposited charges not only exists in the negative half cycle for the evolution process, but also exists between the half cycles. It is in agreement with the phenomenon which the originating regions of discharge in positive half cycle depend on the number of the current pulse in negative half cycle. The emission pattern of the positive half cycle is dominated by the last strong current pulse in the negative half cycle.

Cold atmospheric plasma treatment inhibits growth in colorectal cancer cells.

Plasma oncology is a relatively new field of research. Recent developments have indicated that cold atmospheric plasma (CAP) technology is an interesting new therapeutic approach to cancer treatment. In this study, p53 wildtype (LoVo) and human p53 mutated (HT29 and SW480) colorectal cancer cells were treated with the miniFlatPlaSter - a device particularly developed for the treatment of tumor cells - that uses the Surface Micro Discharge (SMD) technology for plasma production in air. The present study analyzed the effects of plasma on colorectal cancer cells in vitro and on normal colon tissue ex vivo. Plasma treatment had strong effects on colon cancer cells, such as inhibition of cell proliferation, induction of cell death and modulation of p21 expression. In contrast, CAP treatment of murine colon tissue ex vivo for up to 2 min did not show any toxic effect on normal colon cells compared to H2O2 positive control. In summary, these results suggest that the miniFlatPlaSter plasma device is able to kill colorectal cancer cells independent of their p53 mutation status. Thus, this device presents a promising new approach in colon cancer therapy.

Diagnosis of atmospheric pressure helium surface micro-discharge by two-dimensional temporal and spatial resolved emission spectroscopy

The time dependence of the spectral emission of atmospheric pressure surface micro-discharge (SMD) in helium has been investigated with a two-dimensional spatial resolution. The time-resolved emission spectra from OH (309 nm), N2+ (391.4 nm), He (706.5 nm), and O (777.4 nm) behave differently. The results indicate that direct electron impact excitation dominates the generation of the plasma emission in the middle part of the rim electrode where the discharge event originates during the initial period. However, the fan-shaped diffusion of the discharge patterns head is principally due to Penning ionization and charge transfer reaction in later phases. Furthermore, the dynamic evolution of SMD emission pattern has a bullet-like structure in the negative half cycle while it looks like a glow in the positive half cycle. It implies that the polarity of applied voltage plays an important role in the dynamics of the SMD emission patterns.

Stages of polymer transformation during remote plasma oxidation (RPO) at atmospheric pressure

The interaction of cold temperature plasma sources with materials can be separated into two types: ‘direct’ and ‘remote’ treatments. Compared to the ‘direct’ treatment which involves energetic charged species along with short-lived, strongly oxidative neutral species, ‘remote’ treatment by the long-lived weakly oxidative species is less invasive and better for producing uniformly treated surfaces. In this paper, we examine the prototypical case of remote plasma oxidation (RPO) of polymer materials by employing a surface micro-discharge (in a N2/O2 mixture environment) treatment on polystyrene. Using material characterization techniques including real-time ellipsometry, x-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, the time evolution of polymer film thickness, refractive index, surface, and bulk chemical composition were evaluated. These measurements revealed three consecutive stages of polymer transformation, i.e. surface adsorption and oxidation, bulk film permeation and thickness expansion followed by the material removal as a result of RPO. By correlating the observed film thickness changes with simultaneously obtained chemical information, we found that the three stages were due to the three effects of weakly oxidative species on polymers: (1) surface oxidation and nitrate (R-ONO2) chemisorption, (2) bulk oxidation, and (3) etching. Our results demonstrate that surface adsorption and oxidation, bulk oxidation, and etching can all happen during one continuous plasma treatment. We show that surface nitrate is only adsorbed on the top few nanometers of the polymer surface. The polymer film expansion also provided evidence for the diffusion and reaction of long-lived plasma species in the polymer bulk. Besides, we found that the remote plasma etched surface was relatively rich in O–C=O (ester or carboxylic acid). These findings clarify the roles of long-lived weakly oxidative plasma species on polymers and advance the understanding of plasma–polymer interactions on a molecular scale.

Polymer etching by atmospheric‐pressure plasma jet and surface micro‐discharge sources: Activation energy analysis and etching directionality

Treatments of polymer films using either a MHz atmospheric pressure plasma jet (APPJ) or an atmospheric pressure surface micro‐discharge (SMD) plasma are investigated. While the typical approach to determine relevant reactive species is to correlate surface effects with gas phase species measurement, this does not capture potential synergistic or other complex effects that may be occurring. Activation energy and directionality of the etching process can characterize what is occurring at the surface for these processes in more detail. The APPJ source shows an apparent activation energy of ∼0.18 eV at 8 mm distance and up to ∼0.34 eV at 16 mm distance for a temperature range of 20–80 °C tested with thin polymer films. The APPJ source shows directional etching at 8 mm distance with less anisotropy the more distance is increased. The SMD source has an apparent activation energy of ∼0.8–0.9 eV at a distance of 3 mm. The SMD also only shows isotropic etching behavior. However the SMD surface chemistry changes significantly to less oxidation with increased temperature while the APPJ source induced modifications remain very similar with temperature change. The lower apparent activation energy of the APPJ‐induced etching reactions as compared with low pressure work (0.5 eV) and observation of line‐of‐sight contribution to etching suggests the involvement of a directional species at closer distances facilitating the etching which falls off with increasing distance. The high activation energy of the SMD suggests that species with less capability for etching is responsible compared to the APPJ and low pressure plasma. The high surface oxidation from low temperature SMD treatments shows that the surface is being oxidized but not sufficiently to reach the desorption step of the etching process.

Aqueous reactive species induced by a PCB surface micro-discharge air plasma device: a quantitative study

This paper presents a quantitative investigation on aqueous reactive species induced by air plasma generated from a printed circuit board surface micro-discharge (SMD) device. Under the conditions amenable for proliferation of mammalian cells, concentrations of ten types of reactive oxygen and nitrogen species (RONS) in phosphate buffering solution (PBS) are measured by chemical fluorescent assays and electron spin resonance spectroscopy (ESR). Results show that concentrations of several detected RNS (, , peroxynitrites, and ) are higher than those of ROS (H2O2, , and 1O2) in the air plasma treated solution. Concentrations of can reach 150 times of H2O2 with 60 s plasma treatment. For short-lived species, the air plasma generates more copious peroxynitrite than other RONS including , , 1O2, and in PBS. In addition, the existence of reaction between H2O2 and /HNO2 to produce peroxynitrite is verified by the chemical scavenger experiments. The reaction relations between detected RONS are also discussed.

Self-consistent plasma chemistry model for surface microdischarge in humid air including effects of ohmic heating and gas flow

A numerical model is presented for surface microdischarges (SMDs) in flowing humid air at atmospheric pressure, to investigate the effects of the direct ohmic heating of gases in the discharge layer, and the transports of heat and particles by gas flow. Using a simplified configuration of heat transfer and gas flow, the proposed model calculated not only the densities of neutral species but also the temperatures of gases as time dependent variables. The calculated dynamics for various reactive oxygen and nitrogen species showed reasonable agreement with the experimental results obtained by Fourier transformed infrared absorption spectroscopy, while the calculated dynamics without ohmic heating of gases in the discharge layer showed significant disagreement. These results imply that local ohmic heating of the thin discharge layer by the microdischarge itself considerably affected the rate constants of the temperature dependent chemical reactions. The dynamics of the neutral species were also affected by gas flow, both directly through particle transport, and indirectly through cooling. Accordingly, to properly simulate the dynamics of reactive neutral species in SMDs, plasma chemistry models should treat plasmas as sources of both particles and heat which can be deliberately transported by gas flow.