What Are Atmospheric-Pressure Plasmas?

Atmospheric-pressure plasmas are partially ionized gases that operate at or near standard atmospheric pressure (approximately 101 kPa), distinguishing them from the low-pressure plasmas used in conventional semiconductor fabrication. They exist in two broad categories: thermal plasmas, in which the heavy gas species and electrons are in thermodynamic equilibrium at temperatures above 6,000 K, and non-thermal (cold) plasmas, in which the electron temperature far exceeds that of the background gas, often producing reactive chemical species at near-room-temperature conditions. Non-thermal atmospheric-pressure plasmas have attracted substantial research and industrial interest because they can deliver chemically reactive environments without the heat damage that would otherwise limit application on biological tissue, polymer films, food products, and other thermally sensitive materials.

The discipline draws from plasma physics, electrical engineering, and applied chemistry. The engineering challenge in atmospheric-pressure operation is that the higher collision frequency at ambient pressure drives discharges toward thermal arc formation unless active measures such as dielectric barriers, pulsed voltage excitation, or flow geometry are used to limit current and maintain the non-equilibrium electron energy distribution.

Discharge Types and Generation

Several electrode configurations are used to sustain non-thermal plasmas at atmospheric pressure. The dielectric barrier discharge (DBD) is the most widely deployed: one or both electrodes are covered by an insulating dielectric material, which limits the charge transferred per half-cycle of the applied AC voltage and prevents the transition from streamer discharge to a thermalized arc. DBDs typically operate with sinusoidal or pulsed voltages at frequencies of a few hundred hertz to several megahertz. Atmospheric pressure plasma jets (APPJs) use a flowing noble gas (usually helium or argon) to sustain a discharge within a small tube and project a reactive effluent into open air, enabling treatment of three-dimensional or biological surfaces. IEEE Xplore publications on atmospheric pressure non-thermal plasma document the early characterization of discharge morphology and electron energy distributions in these systems. Gliding arc discharges and corona discharges are additional configurations used for specific chemical processing tasks.

Non-Thermal Plasma Chemistry

The reactive character of non-thermal atmospheric-pressure plasmas arises from electron-impact dissociation and ionization of feed gas molecules. In air or air-containing mixtures, the primary reactive species produced include ozone (O3), atomic oxygen (O), hydroxyl radicals (OH), nitric oxide (NO), and nitrogen dioxide (NO2), collectively termed reactive oxygen and nitrogen species (RONS). These species have lifetimes ranging from nanoseconds for the most reactive radicals to minutes for ozone, and they are responsible for the biological, disinfection, and surface modification effects of cold plasma treatment. Gas composition, voltage waveform, discharge gap, and feed gas flow rate all influence the balance of RONS produced, allowing plasma systems to be tuned for specific applications. Research on dielectric barrier discharge microplasma published through IntechOpen details the tunability of RONS composition for applications ranging from indoor air cleaning to polymer surface treatment and transdermal drug delivery. For biomedical uses, the plasma dose delivered to a target is characterized by the fluence of reactive species, which is correlated with measurable endpoints such as bacterial inactivation or cell signaling response.

Surface Modification and Biomedical Applications

Cold atmospheric-pressure plasmas modify polymer and biomaterial surfaces by oxidizing surface functional groups, introducing hydrophilicity, and improving adhesion without bulk heating. These effects underpin industrial use in film wetting, adhesive bonding preparation, and anti-fogging coatings. In medicine, direct plasma application has been shown to inactivate bacteria, fungi, and viruses on skin and wound surfaces through RONS-mediated mechanisms. A review published in IEEE Transactions on Radiation and Plasma Medical Sciences on cold atmospheric plasma for surface sterilization catalogs the mechanistic evidence for microbial inactivation and the engineering parameters governing plasma dose delivery. Emerging research is investigating plasma treatment of cancer cells and its role in stimulating immune responses through controlled oxidative stress.

Applications

Atmospheric-pressure plasmas have applications across a wide range of industries and research areas, including:

• Surface sterilization and decontamination: inactivation of bacteria, spores, and viruses on medical instruments, food packaging, and wound surfaces
• Material surface treatment: improving adhesion, wettability, and printability on polymers and composites without solvents
• Environmental remediation: decomposition of volatile organic compounds, NOx, and ozone-depleting species in industrial exhaust streams
• Ozone generation: large-scale DBD-based ozone production for water purification and food sanitation
• Plasma-assisted combustion: enhancing flame stability and ignition in high-speed flow environments