Efficacy of antimicrobial 405 nm blue-light for inactivation of airborne bacteria

Airborne transmission of infectious organisms is a considerable concern within the healthcare environment. A number of novel methods for ‘whole room’ decontamination, including antimicrobial 405 nm blue light, are being developed. To date, research has focused on its effects against surface-deposited contamination; however, it is important to also establish its efficacy against airborne bacteria. This study demonstrates evidence of the dose-response kinetics of airborne bacterial contamination when exposed to 405 nm light and compares bacterial susceptibility when exposed in three different media: air, liquid and surfaces. Bacterial aerosols of Staphylococcus epidermidis, generated using a 6-Jet Collison nebulizer, were introduced into an aerosol suspension chamber. Aerosolized bacteria were exposed to increasing doses of 405 nm light, and air samples were extracted from the chamber using a BioSampler liquid impinger, with viability analysed using pour-plate culture. Results have demonstrated successful aerosol inactivation, with a 99.1% reduction achieved with a 30 minute exposure to high irradiance (22 mWcm-2) 405 nm light (P=0.001). Comparison to liquid and surface exposures proved bacteria to be 3-4 times more susceptible to 405 nm light inactivation when in aerosol form. Overall, results have provided fundamental evidence of the susceptibility of bacterial aerosols to antimicrobial 405 nm light treatment, which offers benefits in terms of increased safety for human exposure, and eradication of microbes regardless of antibiotic resistance. Such benefits provide advantages for a number of applications including ‘whole room’ environmental decontamination, in which reducing levels of airborne bacteria should reduce the number of infections arising from airborne contamination

Hydroxyl radical production in DC streamer discharge

Plasma-induced advanced oxidation processes do not suffer from the drawbacks, such as carcinogenic by-products, associated with conventional water treatment, and enable the removal of micro-pollutants. The high oxidation strength of hydroxyl radicals enables degradation of resistant contaminants. Many reactions are known to occur at the plasma-water interface; however, the mechanisms of hydroxyl radical production are still not clear. To understand the physical and chemical processes occurring at the plasma-water interface, this research involved investigation of the hydroxyl radicals produced during d.c. streamer discharges. A needle-plate electrode configuration in atmospheric air was used, with the treated solution used as the ground electrode. To understand the effects of polarity and gas type on hydroxyl radical production, both positive- and negative-polarity energization in air, nitrogen and helium were investigated. Plasma filaments were developed from the needle electrode, which was in contact with the solution. Terephthalic acid (TA) was used as a scavenger of hydroxyl (OH) radicals, with OH density subsequently being quantified by fluorescence emission from 2-hydroxyterephthalic acid (HTA), which is formed through specific reaction between TA and OH. The power inputs in positive pulsed streamer discharges were 0.125 W, 0.18 W and 0.26 W in air, nitrogen and helium, respectively; the corresponding hydroxyl radical production efficiencies were 0.56 mmol/kWh, 1.1 mmol/kWh and 5.94 mmol/kWh, respectively. For negative pulsed streamer discharges in air, the power input was 0.063 W and the efficiency was 1 mmol/kWh. The hydroxyl radical production rates were 2.6× 10-7 Ms-1 in negative air discharges, and 2.7× 10-7 Ms-1, 1.8× 10-6 Ms-1, and 2.2× 10-6 Ms-1 in positive air, nitrogen and helium discharges, respectively

Pulsed ultraviolet light decontamination of artificially-generated microbiological aerosols

Airborne transmission of infectious organisms is a major public health concern, particularly within healthcare and communal public environments. Methods of environmental decontamination utilising pulsed ultraviolet (UV) light are currently available, however it is important that germicidal efficacy against airborne contamination is established. This study demonstrates evidence of the dose-response kinetics of airborne bacterial contamination when exposed to pulsed UV-rich (PUV) light. Bacterial aerosols (Staphylococcus epidermidis) were generated using a 6-Jet Collison nebuliser, and introduced into a custom-designed aerosol chamber which enabled prolonged airborne suspension and circulation. Bacterial aerosols were exposed to short duration pulses (~20 µs) of UV-rich light emitted from a xenon-filled flashlamp. The lamp was operated using a 1 kV solid–state pulsed power source, with a pulse frequency of 1 Hz, and output energy of 20 J/pulse. Post-treatment, air samples were extracted from the chamber using a BioSampler liquid impinger, and the surviving fraction was enumerated using standard microbiological culture methods. Results demonstrate successful aerosol inactivation, with a 66.4% reduction achieved with only 10 pulses of UV-rich light (P=<0.0002). Inactivation using continuous UV light was also investigated in order to quantify the comparative efficacy of these antimicrobial light regions. In addition to determining the inactivation kinetics, the spectral outputs of the pulsed and continuous UV sources were captured and compared in order to assess their comparative UV-C content, and subsequently assess how this UV content relates to their germicidal efficiency. Overall, results provide evidence of the dose-response kinetics of bacterial aerosols to PUV-rich light. As with continuous UV light, safety restrictions limit its application to unoccupied environments, or within sealed enclosures such as air handling units, however the reduced treatment times with PUV provides operational advantages over continuous light treatment

Hydroxyl radicals and hydrogen peroxide formation at nonthermal plasma-water interface

This paper investigated hydroxyl radicals and hydrogen peroxide formation under a needle-plate electrode con- figuration using positive-polarity dc discharges generated in air, nitrogen, and helium. The discharge mode in air and nitrogen was found to change above ultrapure water; initially, a nanosecond pulse discharge was observed, transitioning to a diffuse discharge due to the increasing conductivity of the water. The discharge in helium was a nanosecond pulse discharge and the repetition rate increased with increasing water conductivity. It was found that hydroxyl radicals contribute to 7%, 78%, and 70% of hydrogen peroxide formation when using the ultrapure water in air, nitrogen, and helium, respectively. It is suggested that hydroxyl radicals are formed by water reactions with energetic positive ions and the neutral particles, such as N + 2 ,He + ,O,H, and HO 2 . Part of hydrogen peroxide is directly formed from atoms and radical reactions with water in nitrogen and helium, while oxygen reactions are heavily involved for hydrogen peroxide formation in air. A fluorophotometry method, using terephthalic acid, was used to directly quantify the formation of hydroxyl radicals and compared with the tert-butanol method

Statistical analysis of pulsed micro-discharges and ozone generation in dielectric barrier discharges

Pulsed micro-discharges produced by dielectric barrier discharges (DBDs) in a sub-millimeter gap were investigated under 200 cycles of sinusoidal ac voltage at 5 kHz in this work. The impulsive current in the external circuit was accurately measured by an oscilloscope with a bandwidth of 2.5 GHz and maximum sampling rate of 40 GS/s to calculate the filamentary current in the discharge gap. The amplitude, pulse duration and transferred charge of a single filamentary current and the micro-discharge energy acquired over the 200 voltage cycles were statistically analyzed for different discharge gaps and gas pressures. The micro-discharge parameters and ozone generation efficiencies for different conditions were compared. The ozone production efficiency was found to increase with increasing pressure from 1 bar absolute to 2 bar absolute, and the gap length from 0.2 mm to 0.5 mm. The maximum ozone production efficiency achieved in the work was 217 g/kWh, with a gap length of 0.5 mm, 2.0 bar absolute pressure, and an applied voltage of 5.5 kV at 5 kHz

Impulsive discharges in water : acoustic and hydrodynamic parameters

Underwater spark discharges are used in multiple practical applications including plasma closing switches, water treatment, plasma channel drilling and mineral processing, waste recycling, treatment of metals and medical lithotripsy. Spark discharges in water have been studied for several decades, however, despite significant research efforts and progress in this area, further investigation into the efficiency of plasma-acoustic sources and their optimisation is required in order to expand their practical applications. This paper is aimed at investigation of the electrical and hydrodynamic parameters of underwater plasma-generated cavities, including plasma resistance, energy delivered into the plasma cavity, period of cavity oscillations and characteristics of pressure impulses. Different energy levels, breakdown voltages and gap distances were used in the present study to allow systematic analysis of these electrical and hydrodynamic parameters. Empirical scaling laws which link the maximum acoustic pressure and the period of cavity oscillations with the energisation parameters and the resistance of plasma have been obtained. These empirical functions can be used for optimisation of the plasma-acoustic sources and for tailoring their parameters for specific practical application

Field-time breakdown characteristics of air, N2, CO2, and SF6

The dielectric performance of gases in insulation systems used in high voltage power and pulsed power applications is a subject of intensive theoretical and experimental investigations. Transient breakdown processes in gases stressed with short, high-field impulses, have been studied for many decades. However, there are still significant gaps in the understanding of the main breakdown processes and mechanisms associated with fast transient breakdown processes in gases. This knowledge is important for the optimization of gaseous insulating systems and for the coordination of gaseous insulation in power and pulsed power apparatuses. This information is also required for the development of gas-filled components such as circuit breakers and plasma closing switches. This article is aimed at the analysis of the field-time breakdown characteristics of air, N2, CO2, and SF6, using kinetic and drift-diffusion approaches. The kinetic approach is based upon the avalanche-to-streamer transition criterion, while the fluid drift-diffusion model requires self-consistent numerical solution of the continuity equations for charged species, and the Poisson equation for the electric field. The time to breakdown as a function of the applied field was obtained for all investigated gases. The obtained analytical results agree well with the experimental data reported in the literature, which suggests that both approaches can be used for insulation coordination, and for the development of gas-insulated power and pulsed power systems and components

Transient spark discharges in high velocity airflow

Air-filled, self-breakdown plasma closing switches can operate at high pulse repetition frequency, when overstressed with HVDC voltage. However, their recovery characteristics, breakdown voltage, the achievable pulse repetition rate and variation in this rate are affected by the thermal effects local to the electrodes and by gas by-products generated by spark discharges. As a potential improvement to the performance of plasma closing switches, flowing gas can be forced through the inter-electrode gap. In this paper, it was shown that purging an inter-electrode gap of the plasma closing switch with air is a viable way of controlling the pulse repetition rate and variation of this rate. It was found that the difference between breakdown voltages of the first (single) breakdown events in static air and at 100 m/s air flow was less than 10%. In static air, the breakdown voltage of repetitive breakdown events decreases after the first and reaches its saturation value. It was shown that introduction of the air flow at 100 m/s leads to recovery of the repetitive breakdown voltage, which could reach its new saturation value, up to 30% higher than the first recorded breakdown voltage. Also, purging of the spark gap with air resulted in decreasing the pulse repetition rate, however it leads to higher breakdown voltage, lower variation in pulse repetition rate and increasing plasma conductivity in the spark channel, thereby increasing the efficiency of the overall high voltage system via reduced switching losses

Modelling of propagation characteristics of acoustic pulse from partial discharge in polymeric insulating materials

The partial discharge (PD) event in high-voltage insulation releases energy, exerts mechanical pressure, and generates elastic waves. Detecting and locating these PD events through short-duration acoustic pulses is well established, particularly in gas-insulated systems and oil-insulated transformers. However, its full potential remains untapped in solid insulation systems, where the propagation capability of the acoustic pulse and the acoustic reflections pose fundamental challenges to the acoustic emission (AE) detection technique. This study investigates the influence of reflections and multiple paths on the propagating acoustic pulse in polymeric insulating materials using a finite element method (FEM) in COMSOL. It was observed that the reflections from the boundary influence the propagating pulse’s shape, peak magnitude, and arrival time. An analytical MATLAB model further quantifies the impact of multiple propagation paths on the shape, magnitude, and arrival time of the pulse travelling in a cylinder. Additionally, a Perfect Matched Layer (PML) was implemented in the COMSOL model to eliminate the reflections from the boundary, and it revealed that the acoustic pulse magnitude decreases with distance following the inverse square law. In essence, the models aid in measuring how reflections contribute to the observed signals, facilitating the precise identification of the source of the PD event in the tested system

Effect of applied field and rate of voltage rise on surface breakdown of oil-immersed polymers

In sub-systems of high-voltage, pulsed-power machines, the introduction of a solid into bulk liquid insulation located between two conductors is often necessary to provide mechanical support. Breakdown events on or around the surface of the solid can result in permanent damage to the insulation system. Described in the present paper are experimental results pertaining to surface breakdown of five different solid dielectrics held between plane-parallel electrodes immersed in mineral oil. The effect of varying level of peak applied field from 200 kV/cm (dV/dt 70 kV/µs) to 1 MV/cm (dV/dt 350 kV/µs) is investigated, and the breakdown voltages and times to breakdown are compared to those for an open oil gap. The time to breakdown is shown to be reduced by the introduction of a solid spacer into the gap. Rexolite and Torlon samples suffered significant mechanical damage, and consistently showed lower breakdown voltage than the other materials - average streamer propagation velocity up to 125 km/s was implied by the short times to breakdown. Although ultra-high molecular weight polyethylene yielded the longest times to breakdown of the five types of liquid-solid gap, breakdown events could be initiated at lower levels of applied field for spacers of this material than those with permittivity closely matched to that of the surrounding mineral oil. Polypropylene and low-density polyethylene are concluded to provide the most stable performance in mineral oil. Due to the similarity of the applied voltage wave-shape (1/6.5 µs) to short-tail lightning impulses, the results may also be of interest to high-voltage system designers in the power industry