Plasma disinfection and sterilization
Plasmas reduce concentrations of pathogens upon contact with contaminated surfaces. Plasma destroys bacteria, viruses, fungi and prions; it also removes toxins and further organic contaminants. Even complete sterilization is possible with prolonged exposure to the plasma. Most importantly, strong disinfection can be achieved with atmospheric pressure air plasmas, produced by a Piezoelectric direct discharge (PDD). Thus, plasma disinfection and sterilization does not require expensive vacuum equipment or toxic chemicals, making the process cheap and environmentally friendly. Atmospheric pressure plasmas of PDD discharge is cold, making the plasma disinfection very gentle comparing to the standard disinfection procedures using pressurized hot air, pressurized hot water vapor or toxic substances. It affects only a very thin surface layer. Because of this, plasmas can disinfect a variety of sensitive surfaces, including living tissues and even open wounds.
Cold atmospheric plasmas
Plasma is a partially ionized gas. Electric arcs, dielectric barrier, corona and piezoelectric direct discharges ionize gases at atmospheric pressures creating plasmas. The charged particles – the electrons and the ions – accelerate within the discharge electric fields to high energies. Only a small fraction of the gas molecules is turned into the energetic electrons and ions; the rest of the gas remains neutral and cold. In the case of the piezoelectric direct discharge, its temperature reaches only 30-50 C. These temperatures do not damage even sensitive surfaces. While the plasma remains cold, the very energetic electrons and ions collide with the gas molecules producing large quantities of short-lived chemical species, such as atomic H, N and O species, OH and ON radicals, ozone, nitrous and nitric acids, as well as various other molecules in metastable excited states. They make this plasma chemically very active.
Plasma disinfection mechanisms
Here, the chemically active, cold atmospheric plasma triggers a multitude of physical and chemical processes. The decomposition of organic molecules of living microorganisms is achieved on the one hand by bombarding them with electrons, ions and short-lived neutral chemical species and on the other hand by the oxidizing effect of the highly reactive chemical species in the plasma. In addition, the ion bombardment breaks the hydrogen bonds of organic molecules. Thus, these processes together produce lighter and volatile organic molecules and fragments of decomposed microorganisms that evaporate from the surface. This leads to a reduction of ultrafine organic impurities and germs up to the complete sterilization of the treated surfaces. The following reaction agents and processes contribute to the disinfection:
- Ions and electrons bombard the surface in direct contact with the ionized plasma volume. Having high energies, they are by far the strongest agents of disinfection, as they break hydrogen bonds of organic molecules, such as those forming cellular membranes. While the density of the charged particles is by many orders of magnitude lower than the density of the reactive neutral species, the charged particles contribute significantly to disinfection, or even dominate it. Among the charged particles, the electrons have the highest fluxes and the highest penetration depth. This makes them active under the layer of water or other biological liquids covering the surfaces of living tissues. There they react with water and dissolved oxygen molecules to produce reactive oxygen species. The latter kill the microorganisms, which otherwise would be protected by a layer of bio-film. Ions also dissolve in water initiating chemical reactions that make water acidic.
- Reactive oxygen species are oxygen atoms and chemically active molecules containing oxygen in ground or excited states, as well as their ions. Hydroxyl radicals (OH) react very quickly with organic molecules removing hydrogen atoms and producing alkyl radicals, which rapidly oxidize in air. With this mechanism, they indiscriminately attack all organic molecules of living organisms. Most importantly, they destroy bacteria capsules and cell walls. When the rate of the cell wall damage is higher than the rate of its own reparation, the cell dies. Hydrogen peroxide (H2O2) is also very reactive, although less than OH radicals. Therefore, it has a higher chance of penetrating into the bacterial cell nucleus, where it can damage the DNA molecules killing the cell. This makes it a strong disinfectant. Ozone (O3) is a stable long-lived molecule known as a powerful oxidant. Similar to the hydroxyl radicals, it destroys the cell walls.
- Reactive nitrogen species are a family of antimicrobial molecules including nitric oxide (NO), nitrogen dioxide (NO2), peroxynitrite anion (ONOO−), nitrous acid (HNO2). They are less reactive comparing to the oxygen species, but their lifetimes are sufficient to penetrate into deeper layers of the tissues. Although the reactive nitrogen species have shown their disinfection effectiveness, only electric arcs produce them in sufficient quantities, and in corona, DBD and PDD discharges, their concentrations are significantly lower than those of the reactive oxygen species.
- Additional mechanisms
Acidity level in the surface water layer increases due to dissolution of the ions and the reactive oxygen species. Acids corrode the bacterial cell walls causing their mortality.
Ultraviolet radiation is a well-known strong disinfectant damaging DNA molecules and inhibiting bacterial replication. High-energy part of UV radiation, produced by atmospheric pressure plasmas, is most potent. However, its intensity is usually insufficient for effective treatment.
Heat produced by atmospheric plasmas, such as the ones of piezoelectric direct discharge, is generally low, reaching temperatures of only 30-50 C. However, the microscopic points, where the discharge filament contacts the surface, are very locally overheated and sterilized. Still, they cover insignificant surface area producing insignificant disinfection.
Advantages of disinfection with atmospheric pressure cold air plasmas
Exceptional disinfection properties of plasmas are well known and documented. Atmospheric pressure plasmas, and especially the plasma of the piezoelectric direct discharge, have demonstrated a strong disinfection capacity. Comparing to the standard disinfection and sterilization methods using pressurized hot air of 170 C, pressurized hot water vapor of 120 C, wet chemistry, or low-pressure plasmas, the cold atmospheric pressure air plasmas offer the following advantages:
- Treatment at the atmospheric pressure enables disinfection of objects that can be damaged by vacuum or overpressure
- No expensive vacuum or pressurized chambers and pumps
- No wet chemistry
- Ultra-fine cleaning, no residues
- Low operating temperature
- Gentle treatment of sensitive surfaces, including living tissues and open wounds
- Air or cheap non-toxic working gases
- Environmental friendliness
Plasma disinfection by relyon plasma GmbH
To cover a wide spectrum of medical and laboratory applications, Relyon Plasma GmbH developed a Piezoelectric Direct Discharge (PDD) Technology, which enables construction of exceptionally compact and cheap plasma generators, well suited for plasma disinfection and sterilization. This technology is still very new, but it already found many applications. We constantly search for new application areas and develop specialized plasma instruments. Currently we offer the following universal plasma generator:
- piezobrush® PZ2 is a low power handheld plasma generator. It enables manual plasma processing work in laboratories. It can create corona and dielectric barrier discharges and apply them for disinfection as well as for precision ultrafine plasma cleaning and chemical functionalization of small components.