Florina Truica-Marasescu, PhD
Cold plasma can be described as a partially ionized gas containing molecular fragments (aka radicals), charged particles (positive and negative ions), free electrons, and photons. When in contact with surfaces, these reactive and energetic species kill or inactivate microbial cells through interactions that:
· disrupt, damage, or oxidize their membranes / shells,
· damage the microbes’ DNA.
Cold plasmas are characterized by an energy non-equilibrium between the electrons and the heavy species, making the temperature of plasma fall in the range of 30–60 °C, which is very low compared with hot plasmas, such as solar corona.
Atmospheric pressure plasma jets (APPJs) and low-pressure cold plasma (LPCP) are two distinct forms of plasma used for sterilization of many different products, including foods and medical devices. While both methods harness the power of cold plasma to eliminate harmful microorganisms, they operate under different conditions, and therefore exhibit unique characteristics that influence their effectiveness and practicality.
APPJs operate at or near atmospheric pressure, making them suitable for surface treatments and continuous treatments in open environments without the need for vacuum chambers. A typical APPJ system is depicted in Figure 1 below.
Figure 1. The experimental arrangement scheme of the plasma jet source and the
photography of a human finger under an atmospheric pressure plasma jet. Reproduced
from [2].
However:
The application of a plasma jet in open air may involve (entrain) other species from the surroundings (air), making the environment of the treatment less controllable and more prone to post treatment cross-contamination.
APPJs treatments are considered “line-of-sight”, as the concentration of reactive (germicidal) plasma species decreases significantly with increasing the frontal and lateral distance from centre of the nozzle (see Figure 2 below). This aspect is particularly important for the treatment of 3D objects, as the sterilization efficiency may be different across the surface of the object.
Figure 2. Spatial density distribution of O atoms in the effluent of the plasma jet. Reproduced from [3].
3. As seen in Figure 1, APPJs devices are small and narrow plasma sources, with widths of the plasma “plume” or “jet” typically in the range of a few mm up to a few cm. This makes the scale-up for commercial application complex, as many such sources must be coupled together to ensure coverage of large surfaces.
4. APPJs often use expensive gases, such as argon or helium as the working or carrier gas, which contributes to an increase in the operational costs, and adds complexity and safety requirements in the processing plants due to the need for pressurized gas bottles.
5. The application of APPJs takes place in air at ambient conditions; as such, the mean free path (MFP), or the average distance a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy is very small. For air at atmospheric pressure, MFP is less than 100 nm (10^-4 mm), which means that every particle in the plasma can travel a very short distance beyond the point of contact with the surface before being neutralised. When treating porous materials, such as cannabis flowers, this aspect limits the penetration of the plasma state inside the pores of the material, leaving pores with depths greater than 0.0001 mm largely untreated, thereby providing a safe heaven for microbes.
Low-pressure cold plasma (LPCPs) systems operate at reduced pressures, typically within vacuum chambers (see Figure 3).
Figure 3. Typical images of low-pressure plasmas.
These systems create a more controlled environment, allowing for precise tuning of plasma parameters such as gas composition, temperature, and density. The process can be controlled to minimize the post-treatment exposure to atmosphere, and thus limit the risk of cross-contamination.
Sterilizing LPPs use cheaper gases, such as air, humid air, O2, N2, water vapour, or a combination of these gases which makes it cheaper and safer to operate these plasma machines in commercial applications.
Low-pressure cold plasmas generate a uniform discharge across the treated surface, ensuring consistent microbial inactivation. Additionally, this method can be tailored to produce specific reactive species, optimizing its effectiveness against specific microorganisms while minimizing damage to sensitive materials.
LPPs can be scaled up to create a uniform plasma “cloud” across large surfaces, and to deliver a uniform and omnidirectional treatment of large quantities of material suspended in the plasma “cloud” (see Figure 3).
Activated species in plasmas operating at low pressures have much larger mean free paths (MFP) of roughly between 0.1 and 100 mm. This means that the plasma species and their effects can propagate inside the pores of a porous material over distances that are roughly 1 000 000 larger than the APPJ plasma species.
To summarize, plasmas are effective at sterilization; however, their unique characteristics should be considered carefully when selecting a candidate for a particular application. For example, APPJs are effective for wound healing or sterilization of flat surfaces. On the other hand, LPPs are best suited for simple, rapid, and uniform treatments that can effectively penetrate complex structures, making LPPs ideal for sterilizing cannabis plants, equipment, and packaging materials.
References:
V. Scholtz, J. Pazlarova , H. Souskova , J. Khun, J. Julak, Biotechnol. Adv. 33 (2015) 1108.
A. V. Nastuta, I, Topala, C. Grigoras, V. Pohoata, G. Popa, J. Phys. D: Appl. Phys. 44 (2011) 105204.
X. Lu, G. V. Naidis, M. Laroussi, S. Reuter, D. B. Graves, K. Ostrikov, Phys. Rep. 630 (2016) 1.
Dr. Florina Truica-Marasescu is a plasma scientist and engineer with 20 years of commercial R&D experience and a passion for developing plasma technology as safe, cost-effective, and eco-friendly methods for microbial remediation. She has started Cold Plasma Group (CPG) to commercialize this innovative platform technology for the Cannabis and the Agriculture sectors. CPG now offers technology-integrated plasma machines and services as turnkey, customized solutions.
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