1. What is Plasma?

Plasma is the fourth state of matter besides solid, liquid, and gas. Due to the fact that the way particles are carried in uniformly discharged plasma is similar to how blood plasma carries red blood cells, it is vividly referred to as "electrical plasma".

Plasma is the fourth state of matter apart from solid, liquid, and gas. In the universe, plasma is the main form of matter existence, accounting for more than 99% of the total matter in the universe. For example, stars (including the Sun), interstellar matter, and the ionosphere around the Earth are all composed of plasma. Common plasmas on Earth include lightning, auroras, and flames. Generally speaking, plasma refers to an ionized gas that is electrically neutral on a macroscopic scale.

Plasma can be generated under low-pressure, atmospheric-pressure, and high-pressure conditions. The generation method involves using high-energy particles produced by electric fields, irradiation, or thermal excitation to convert neutral gases into the plasma state. In plasma, a "mixture" is formed, which includes charged particles, free radicals, metastable particles, and photons. Since plasma, as a matrix, carries various types of particles, it is quite similar to the way blood plasma carries red blood cells, white blood cells, bacteria, and other substances. Therefore, in the early days, uniform discharge was vividly called "electrical plasma", which is plasma [1][2][3][4][5][6][7][8].

[See Figure 1 for the schematic diagram of the four states of matter transformation]

Figure 1 Schematic diagram of the four states of matter transformation (taking water as an example). As the temperature rises, the distance between water molecules gradually increases. Water gradually changes from solid ice to liquid water, then to water vapor, and finally, as electrons escape from the bondage of atoms, it turns into plasma.

2. Why Choose Cold Atmospheric Plasma?

CAP (Cold Atmospheric Plasma): Cold plasma with a temperature close to the human body temperature was first concerned about and applied in the medical field.

Plasma can be classified in different ways according to research perspectives or specific needs. When classified based on thermodynamic equilibrium or temperature, plasma can be divided into fully thermally equilibrium plasma (high-temperature plasma, such as the interior of the Sun and nuclear fusion), locally thermally equilibrium plasma (thermal plasma, such as electric arcs and argon arc welding), and non-thermally equilibrium plasma (cold plasma, such as corona) [9][10].

With the development of electrical engineering and materials science, cold atmospheric plasma (CAP) has been developed. Plasma used in medicine needs to be in direct contact with human tissues. To avoid burning human tissues, medical plasma usually requires a temperature close to room temperature or human body temperature (< 40℃). Therefore, except for medical thermal plasma that specifically uses the heat generated by plasma to ablate tissues, all medical plasmas are atmospheric-pressure non-equilibrium plasmas (cold atmospheric plasmas) [10].

The cold atmospheric plasma (CAP) technology produces non-thermally equilibrium plasma that can be directly contacted by the human body, with a perceived temperature of approximately 30℃. Common generation methods of CAP include dielectric barrier discharge (DBD) and non-equilibrium atmospheric pressure plasma jet (N-APPJ). According to different gas media, N-APPJ can be further divided into inert gas N-APPJ and air N-APPJ.

[See Figure 2 for the schematic diagram of cold plasma (CAP)]

Figure 2 Cold Atmospheric Plasma

3. Applications of CAP

At present, due to its unique functions and characteristics, CAP has been attracting increasing attention from various fields and industries.

The charged particles (electrons and ions), reactive oxygen/nitrogen species (RONS for short), and electric fields contained in cold atmospheric plasma (CAP) all play critical functional roles [8]. Therefore, it has been widely applied in many fields such as biomedicine, material processing, environmental treatment, agricultural food, and energy conversion [11][12][13]. Among these fields, the macroscopic temperature of cold atmospheric plasma (CAP) is close to or slightly higher than room temperature. When it is used to treat biological tissues and heat-sensitive substrates, it can ensure the efficiency of treatments like sterilization and disinfection without causing thermal damage. Thus, it has received extensive attention in the biomedical field, including applications in sterilization and disinfection, wound healing, skin care, tumor treatment, stomatology, and dermatological disease treatment [11].

[See Figure 3 for the schematic diagram of the interaction between plasma and human tissues [14]]

Figure 3 Interaction between Plasma and Human Tissues

References

[1] Gan Lu, Duan Jiangwei, Zhang Song, Liu Xin, Poorun Devesh, Liu Xinxin, Lu Xinpei, Duan Xiaoru, Liu Dawei, Chen Hongxiang. Cold atmospheric plasma ameliorates imiquimod-induced psoriasiform dermatitis in mice by mediating antiproliferative effects.[J]. Free Radical Research, 2019.

[2] Brett A. Shook, Renee R. Wasko, Guillermo C. Rivera-Gonzalez, Emilio Salazar-Gatzimas, Francesc López-Giráldez, Biraja C. Dash, Andrés R. Muñoz-Rojas, Krystal D. Aultman, Rachel K. Zwick, Vivian Lei, Jack L. Arbiser, Kathryn Miller-Jensen, Damon A. Clark, Henry C. Hsia, Valerie Horsley. Myofibroblast proliferation and heterogeneity are supported by macrophages during skin repair[J]. Science, 2018.

[3] Lu XinPei, Ostrikov Kostya (Ken). Guided ionization waves: The physics of repeatability[J]. Applied Physics Reviews, 2018.

[4] Vanraes Patrick, Bogaerts Annemie. Plasma physics of liquids - A focused review[J]. Applied Physics Reviews, 2018.

[5] Endre J. Szili, Sung-Ha Hong, Jun-Seok Oh, Nishtha Gaur, Robert D. Short. Tracking the Penetration of Plasma Reactive Species in Tissue Models[J]. Trends in Biotechnology, 2018.

[6] Gan Lu, Zhang Song, Poorun Devesh, Liu Dawei, Lu Xinpei, He Mengwen, Duan Xiaoru, Chen Hongxiang. Medical applications of nonthermal atmospheric pressure plasma in dermatology.[J]. Journal der Deutschen Dermatologischen Gesellschaft = Journal of the German Society of Dermatology: JDDG, 2018.

[7] Ryugo Tero, Ryuma Yamashita, Hiroshi Hashizume, Yoshiyuki Suda, Hirofumi Takikawa, Masaru Hori, Masafumi Ito. Nanopore formation process in artificial cell membrane induced by plasma-generated reactive oxygen species[J]. Archives of Biochemistry and Biophysics, 2016.

[8] Liu D, Zhang Y, Xu M, et al. Cold atmospheric pressure plasmas in dermatology: Sources, reactive agents, and therapeutic effects[J]. Plasma Processes and Polymers, 2020, 17(4). DOI:10.1002/ppap.201900218.

[9] Duan J, Lu X, He G. On the penetration depth of reactive oxygen and nitrogen species generated by a plasma jet through real biological tissue[J]. Clinical Plasma Medicine, 2017, 24(7):291-84. DOI:10.1063/1.4990554.

[10] Duan J. On the penetration of reactive oxygen and nitrogen species (RONS) generated by the non-equilibrium atmospheric pressure plasma Jet Across Biological Tissues[D]. 2020.

[11] Lu X P. Plasma jets and their biomedical application[J]. High Voltage Engineering, 2011, 37(6):1416-1425. DOI:10.1080/17415993.2010.547197.

[12] MEI Danhua, FANG Zhi 1, SHAO Tao. Recent Progress on Characteristics and Applications of Atmospheric Pressure Low Temperature Plasmas[J]. Proceedings of the CSEE, 2020, 2.

[13] WEN Tao, XIANG Nianwen, ZHANG Cheng, et al. Research Status and Development Trend of High Voltage Discharge Plasma[J]. High Voltage Engineering, 2023, 8.

[14] LIU Dingxin, HE Tongtong, ZHANG Hao. Penetration Effect of Gas Plasmas on Human Tissues: State-of-the-art and Current Issues[J]. High Voltage Engineering, 2019, Vol.45, No.7: 2329-2342.