1. Principles of CAP Sterilization

Since Dr. Laroussi from the United States first conducted research on the application of cold atmospheric plasma (CAP) in sterilization in 1966 [1][2], CAP has been increasingly widely used in disinfection and sterilization. In addition to the physical properties such as light, heat, and electricity generated during its formation, CAP also contains a variety of active components, including ultraviolet (UV) radiation, charged particles (electrons, positive and negative ions, etc.), and chemically active particles (reactive oxygen species/ROS, reactive nitrogen species/RNS, etc.). All these components play a role when CAP interacts with pathogenic microorganisms (viruses, bacteria, fungi) [3].

According to molecular biology studies, the membrane surfaces of bacteria and viruses are positively charged. The normal distribution of charges on the membrane is conducive to the absorption of nutrients by bacteria and viruses. However, when the charges on the membrane are disturbed by charged particles, the normal charge distribution is disrupted, which breaks the normal physiological activities of bacteria or viruses and leads to their death. Active particles in CAP collide with the microbial cell membrane; when the number of active particles reaches a certain threshold, they can penetrate the cell membrane, undergo chemical reactions with internal substances such as proteins and nucleic acids, disrupt the cell’s electrolyte balance, and even penetrate into the cell to break down the nucleus, resulting in cell death [4][5][6]. Multiple factors, including the treatment time of CAP on bacteria, voltage, gas composition, temperature, and humidity, affect the sterilization effect of plasma. Research results show that CAP has the ability to inactivate pathogens, such as Gram-negative bacteria, Gram-positive bacteria, bacterial spores, bacterial biofilms, and fungi [1].

2. Typical Applications of CAP Sterilization

The specific applications of CAP in disinfection and sterilization include: medical disinfection and sterilization, air purification, skin disinfection, oral medicine, food and agricultural disinfection, etc. [7].

(1) Medical Disinfection and Sterilization

The sterilization effect of plasma in hospital wards is twice that of ultraviolet sterilization [8]. CAP disinfection and sterilization technology is also applied to the disinfection of heat-sensitive medical items and equipment—such as physical examination tools, plastic and rubber polymer materials, and heat-sensitive equipment in the respiratory department. This technology not only extends the service life of medical equipment but also reduces the contamination of disposable medical supplies [4].

(2) Air Purification

With the successive emergence of viruses such as SARS, avian influenza, and COVID-19, the biosafety of air has become increasingly important. Studies have shown that CAP has a disinfection effect on microorganisms in the air [9], which can effectively solve the problem that microorganisms intercepted by air filter membranes remain viable and reproduce. The oxygen-containing and nitrogen-containing active particles in CAP have strong chemical activity and can react specifically with the polymer matrix of cell membranes or viral spike proteins, leading to pathogen inactivation. CAP has great potential in various application scenarios, such as the disinfection of COVID-19 virus aerosols, in-situ disinfection of curved surfaces, and disinfection of nucleic acid detection tubes [10][11][12].

(3) Skin Disinfection and Wound Treatment

A large number of studies have shown that CAP has a significant in vitro antibacterial effect and also exerts an antibacterial effect on chronic wounds without any side effects. Given the continuous development of bacterial resistance to antibiotics, this non-antibiotic-based method for treating infected wounds theoretically has greater advantages [7][9].

(4) Oral Medicine

Biofilms formed on the tooth surface can cause dental caries, gingival and periodontal diseases, and oral mucositis. These biofilms can also affect dental implants by causing peri-mucosal inflammation and peri-implantitis. Most dental treatments aim to remove or destroy oral biofilms. Studies have confirmed that CAP is a promising tool against dental biofilms, as it can destroy the biofilm matrix without causing any damage to oral tissues. The application indications of CAP in dentistry include: bonding with dentin and ceramics, curing and bleaching of composites, surface activation of dental implants, and antibacterial therapeutic intervention in cariology, endodontics, periodontology, and implantology.

(5) Food Sterilization

Although traditional thermal sterilization technology can effectively inactivate microorganisms, it has adverse effects on the nutritional and sensory quality of ready-to-eat meat products. CAP technology uses electricity and reactive carrier gases (e.g., oxygen, nitrogen, or helium) to inactivate enzymes, destroy microorganisms, and preserve food, thus avoiding the use of chemical antibacterial agents. Currently, a large amount of data shows that CAP can be used for the disinfection and sterilization of food (vegetables, fruits, meat, milk powder, water, fruit juice, etc.) [7][13]. The application of CAP technology to kill microorganisms carried by food itself, on the surface of processing equipment, generated during processing, or carried by inner packaging is safe and can retain the original taste and nutrition of food [14].

(6) Agricultural Applications

The agricultural applications of CAP include the disinfection of agricultural products and seeds using plasma to extend the shelf life of agricultural products. The combined action of various particles in CAP can shorten the seed germination time, increase the germination rate, enhance the drought resistance and disease resistance of crops, promote vigorous crop growth, and improve crop yield. It is worth noting that plasma nitrogen fixation converts nitrogen in the air into useful substances such as ammonia and nitrogen oxides at room temperature and atmospheric pressure, which can provide green fertilizers for crop production.

【See Figure 1 for the device for inactivating methicillin-resistant Staphylococcus aureus (MRSA) biofilms via surface discharge and discharge images under different gas conditions [15]】

Figure 1 Device for inactivating MRSA biofilms via surface discharge and discharge images under different gas conditions

References

[1] Xiong Z L, Lu X P, Cao Y G. Plasma Medicine [J]. Science China Technological Sciences, 2011, 41(10): 1279-1292. DOI:10.1360/ze2011-41-10-1279.

[2] Laroussi M. Sterilization of Contaminated Matter with an Atmospheric Pressure Plasma [J]. IEEE Transactions on Plasma Science, 1996, 24(4): 1188–1191.

[3] Cao J X. Methods and Characteristics of Low-Temperature Plasma Sterilization [J]. Modern Physics, 1999, (1): 11-12.

[4] Ni Y, Chen Y C, Wang M L, et al. Experimental Study on Low-Temperature Plasma Sterilization [J]. Chinese Journal of Environmental Engineering, 2009, 3(11): 1951-1955.

[5] Zhang Z, Yue Y J. Study on the Bactericidal Effect and Mechanism of Atmospheric Pressure Dielectric Barrier Discharge Air Plasma on Staphylococcus aureus [J]. Chinese Journal of Disinfection, 2015, 32(9): 853-856.

[6] Gaunt L F, Beggs C B, Georghiou G E. Bactericidal Action of the Reactive Species Produced by Gas-Discharge Nonthermal Plasma at Atmospheric Pressure: A Review [J]. IEEE Transactions on Plasma Science, 2006, 34(4): 1257-1269.

[7] Wang Y L, Xu L J, et al. Research Progress in the Field of Disinfection and Sterilization by Low Temperature Plasma Technology [J]. Theoretical Research, 2022, 4.

[8] Gentile R D. Renuvion/J-Plasma for Subdermal Skin Tightening, Facial Contouring and Skin Rejuvenation of the Face and Neck [J]. Facial Plastic Surgery Clinics of North America, 2019, 27(3): 273-290.

[9] Zheng C, Xu Y Z, et al. State-of-the-Art Non-Thermal Plasma Disinfection and Medicine [J]. Chemical Industry and Engineering Progress, 2013, 32(9).

[10] Wen T, Xiang N W, et al. Research Status and Development Trend of High Voltage Discharge Plasma [J]. High Voltage Engineering, 2023, Vol.49, No.8: 3226-3239.

[11] Guo Y T, Zhang D H Y, Zhang L Y, et al. Air Disinfection for SARS-CoV-2 and Other Pathogens: A Review [J]. Journal of Tsinghua University (Science and Technology), 2021, 61(12): 1438-1451.

[12] Zhang D H Y, Guo Y T, Zhang L Y, et al. Key Technologies of Integrated System for the Detection and Disinfection of COVID-19 Samples [J]. Proceedings of the CSEE, 2022, 42(12): 4623-4632.

[13] Chen Y, Chen G, Wei R, et al. Quality Characteristics of Fresh Wet Noodles Treated with Nonthermal Plasma Sterilization [J]. Food Chemistry, 2019, 297: 124900.

[14] Yadav B, Spinelli A C, Misra N N, et al. Effect of In-Package Atmospheric Cold Plasma Discharge on Microbial Safety and Quality of Ready-to-Eat Ham in Modified Atmospheric Packaging During Storage [J]. Journal of Food Science, 2020, 85(4): 1203-1212.

[15] Guo L, Xu R B, Liu D X, et al. Eradication of Methicillin-Resistant Staphylococcus aureus Biofilms by Surface Discharge Plasmas with Various Working Gases [J]. Journal of Physics D: Applied Physics, 2019, 52(42): 425202.