Cold Atmospheric Plasma: A Promising and Safe Therapeutic Strategy for Atopic Dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease characterized by severe itching and recurrent eczema-like changes, often accompanied by other atopic diseases such as asthma and allergic rhinitis [1]. In the overall population, the prevalence of adult AD was 4.9% in the USA, 3.5% in Canada, 4.4% in the European Union, and 2.1% in Japan [2]. The clinical manifestations of AD vary with age, and they are usually divided into the following three stages: erythema, papules, erosion and exudation involving the face and limbs in infancy. Inflammatory infiltration is reduced in childhood, and it develops into lichenified lesions involving the head, neck, and flexion surfaces of the limbs during adolescence or adulthood [3]. Skin dysbacteriosis, immune system dysfunction, and skin barrier defunctionalization are currently recognized as the main pathogenesis events involved in AD [4]. Genetic barrier defects and environmental stimuli might be involved in the pathogenesis of epidermal-barrier dysfunction with AD; the latter enhances the penetration of transdermal microorganisms and allergens, reducing the inflammatory threshold of haptens and irritants. Local infection and stimulation will further activate autoimmunity and induce keratinocytes to release pro-inflammatory cytokines and chemokines [5]. The skin barrier abnormalities hypothesis is an “outside-in” that theory suggests that epidermal-barrier dysfunction is a prerequisite to immune activation; whereas, the “inside-out” immunology hypothesis indicates that AD is primarily an immune dysregulation disease with a reactive epidermal dysfunction [6]. CD4⁺ and CD8⁺ T cells are activated memory/effector T-cell subpopulations with skin lymphocyte-associated antigens. Th1/Th2 imbalance promotes excessive production of Th2-related cytokines, such as interleukin (IL)-4, IL-5, and IL-13. IL-5 induces an increase in eosinophil activity and prolongs the lifespan of eosinophils. IL-13 upregulates IgE antibodies in peripheral blood and tissues [7]. Repeated stimulation of activated T cells and cytokines induces the damage of keratinocytes. The destruction of the skin barrier in turn irritates the production of cytokines derived from keratinocytes such as IL-6, IL-18, IL-23, granulocyte macrophage colony-stimulating factor, and tumor necrosis factor (TNF)-α, which further promotes immune disorders [8]. There is crosstalk between immune system dysfunction and skin barrier defunctionalization in AD pathology, however, the fact prove that they are mutually influence and mutually stimulation to exacerbate the symptoms of AD [9]. So far, there is no complete cure for AD. Long-term topical use of glucocorticoids and traditional systemic immunosuppressive agents lead to drug side effects or low curative effect. Wide application of biologic agents is limited by the high medical cost and unclear long-term effectiveness and safety. Thus, it is imperative to identify a medicine with good balance between the price and effect.

Plasma is the fourth state of matter, which shows special properties other than solids, liquids, and gases. It can be produced when the gas is ionized by radio frequency, microwave frequencies, high-voltage alternating current, or direct current (DC) [10]. According to the electron temperature (Te) of plasma, it can be divided into high-temperature plasma (up to 10⁹ K) and low-temperature plasma (≤10⁴ K). High-temperature plasma is in a fully ionized state, such as in solar core, nuclear fusion, etc., and the ionization degree (the ratio of electron density to neutral particle density) is close to 1. This kind of plasma is also called fully ionized plasma. Low-temperature plasma is in a partially ionized state (ionization degree 10⁻⁷∼10⁻⁴), so it can be called a partially ionized plasma, and its electron temperature is relatively low. According to the thermodynamic properties of plasma, it can be further divided into thermal equilibrium and nonthermal equilibrium plasma. The temperature of all particles in the thermal balance plasma is the same, that is, the temperature of heavy particles (usually expressed by the gas temperature, Tg) is the same as the electron temperature. Accordingly, when the electron temperature is much higher than the gas temperature (Te ≥ Tg), the plasma is in a nonthermal equilibrium state [11]. High-temperature (or thermal balance) plasma can be used for metal smelting, magnetic confinement nuclear fusion, etc. Low-temperature (or nonthermal equilibrium) plasma, due to its relatively low gas temperature and high chemical properties, has a wide range of applications in many fields, such as materials, environment, and chemical engineering [12]. In recent years, plasma has received great attention in the field of biology and medicine [13, 14]. To enable the plasma to make a better and more direct contact with human tissues and produce a series of biomedical effects, the emerging cold atmospheric plasma (CAP) technology has developed rapidly, and thus a new interdisciplinary – “plasma medicine” has been derived.

CAP is a highly reactive ionized physical state with standard conditions for temperature and pressure (p ≈ 1 atm, Tg ≈300 K) generated by inert gases, molecular gases, mixture gases, or air [15]. CAP can be driven in various frequency ranges, for example, DC and pulsed DC, alternating current (∼kHz), RF (∼MHz), as well as microwave (∼GHz) [16]. Glow discharge, corona discharge, dielectric barrier discharge (DBD), and atmospheric pressure-plasma jet (APPJ) are the commonly used plasma sources for generating CAP [17]. DBD belongs to direct plasma devices, consisting of an air gap, two electrodes, and a dielectric layer. The dielectric layer is covered on one or two electrodes, or suspended between two electrodes. When a high-voltage pulse is applied to one or two electrodes, plasma discharge can be generated [18‒20]. The APPJ usually consists of two electrodes, a dielectric tube, feed gas, and a power supply. When rare gas flows through the dielectric tube, a high-voltage pulse is applied to the electrode to generate a plasma discharge. APPJ can directly generate stable plasma in the atmospheric environment without the need for any complex vacuum equipment [21‒23]. kINPen^® MED device (INP Greifswald/neoplas tools GmbH) from APPJ source that has been CE-certified to treat chronic wounds in humans in 2013 [24]. The kINPen^® MED consists of a pin-shaped power electrode in a dielectric ceramic tube and an external ground electrode. It can be operated with inert gas or molecular gas (such as oxygen, nitrogen, or compressed air). The gas flow is usually between 3 and 5 standard liters per minute [25]. There is a spacer between the plasma jet and the skin to maintain a suitable distance, and the velocity of the plasma jet moving in the treated skin area is generally 5 mm/s [26]. PlasmaDerm^® VU-2010 (CINOGY Technologies GmbH, Duderstadt, Germany) from DBD source has also been CE-certified in Germany by MEDCERT [24]. The PlasmaDerm^® consists of a control unit with a power, a hand-held unit with cables, an integrated high-voltage generator, and finally a plasma electrode. Using an AC voltage pulse with amplitudes >10 kV and a power density of 120 mW/cm² to generate plasma discharge[27]. PlasmaDerm^® is suitable for the treatment of large skin lesions. The main indications include chronic wound healing, arteriovenous ulcer, bedsore, etc. [26]. SteriPlas and Adtec MicroPlaSter (Adtec Ltd., London, UK) has been passed the ISO13485 quality management system and CE certificate of Europe, and be defined as class IIa device [24, 26]. SteriPlas and MicroPlaSter have a six-electrode plasma torch, respectively. Generally, the microwave frequency is 2.45 GHz, the power is 110 W, argon flow rate is 2–5 L/min, and the treatment area is about 4–5 square centimeters [28]. Recently, a new approach of CAP has emerged. When CAP acts on water, medium, or hydrogels, active ingredients can be imported into the inactive media, namely, plasma-activated media (PAM), plasma-activated water, or plasma‐treated hydrogels. PAM, plasma-activated water, and plasma‐treated hydrogels have also been studied in biomedical applications [29, 30]. They offer a convenient and feasible way of combining the advantages of current treatment modalities.

CAP has great potential and broad prospects in the application of dermatology treatment due to it emits a large number of reactive molecules, excited species accompanied by electric field, ultraviolet (UV) radiation, as well as heat [12]. These substances vary in composition and content depending on the working gas and plasma source used. For example, changes in the working gas can alter the wavelength of UV radiation generated by the plasma, ranging from vacuum UV radiation to 380 nm [31]. The predominant characteristic of CAP related to medicine is the unique mixture of reactive oxygen species (ROS), reactive nitrogen species (RNS), and their derivatives, including ozone (O₃), atomic oxygen (O), superoxide (O₂⁻), singlet (¹O₂), hydrogen peroxide (H₂O₂), hydroxyl radicals (•OH), nitric oxide (NO), nitrogen dioxide (NO₂), nitrate radical (NO₃), and peroxynitrite anion (ONOO–) [32‒34]. The O atom density is in the order of 10¹⁴ cm⁻³ in the plasma plume but reach 10¹⁶ cm⁻³ inside the discharge gap. The densities of OH, O₂ (1Δg), and NO are 10¹⁵ cm⁻³, respectively [35]. The main active species concentration and proportion are not fixed but are influenced by the parameters of the plasma generator, including humidity, pressure, gas used, and irradiation time. O₃ is 15.5 ppm in 0.8% O₂ with 99.2% Ar, while O₃ is 27.1 ppm in 1.2% O₂ with 98.8% Ar. The spectra of H₂O₂ and OH at 0.8% O₂ were below 190 nm and NO₂⁻ and NO₃⁻ were at 210 nm. The increase in irradiation time leads to a decrease in the concentration of O₂, while the concentrations of NO₂⁻ and NO₃⁻ decrease due to oxidation by O₃ [36]. Researchers regulate the composition and dosage of active particles by changing the composition of working gas and water vapor to achieve optimal results. For helium humid air plasma jets, adjusting the voltage amplitude increases the average electron energy, and the average density of OH, O₃, O(¹D), O⁻ correspondingly increases, but H₂O₂ decreases. In addition, as the concentration of wet air increases, the electron energy gradually decreases, and the average density shows varying degrees of increase. The main reaction for producing OH is e+H₂O→e+H+OH, and the increase in electron energy intensifies the reaction, resulting in an increase in OH density with the increase in electron energy. In practical applications, at appropriate impurity concentrations, while obtaining the expected electron energy, a rich variety of active particles can also be generated [37]. Appropriate levels of radicals and non-radicals are closely related to intracellular homeostasis, signal transduction, immunity, and metabolism; thus, they play an important role in the regulation of biological function of mammalian cells [38]. For example, NO is responsible for mediating intracellular signal transduction [39]; O₂⁻ is involved in a variety of physiological processes, including improving the immune function and maintaining homeostasis in the internal environment [40]; and H₂O₂ has an antimicrobial effect [41]. Actually, CAP produces complex and powerful biological effects due to its excellent physical, chemical, and biochemistry properties. In practical application, CAP regulates the activity of cancer cells, making them more sensitive to the toxicity of active substances [42], and increases the expression of sestrin-2 in cells, which further activates downstream signal transduction to induce apoptosis of melanoma cell lines [43]. Psoriatic lesions can also be improved in vitro or in psoriasis mouse models with CAP, possibly due to its complex effects on keratinocytes and various immune cells [44, 45]. CAP holds great potential prospect in chronic wound healing with its good performance in inactivating multiple microorganisms, stimulating cell proliferation, and angiogenesis [46]. Increasing evidence shows that CAP can promote the recovery of inflammatory skin diseases through complex physiological effects. We summarized the applications of different types of CAPs in inflammatory and immune skin diseases in Table 1.

In patients with AD, the diversity of skin microorganisms is decreased. Meanwhile, the colonization of pathogenic bacteria, especially Staphylococcus aureus (S. aureus) is increased [57, 58]. Cell membrane proteins and virulence factors secreted by S. aureus correlate with the severity of AD [59, 60]. S. aureus adheres to the components of the extracellular matrix, which is often mediated by a class of adhesins termed microbial surface components recognizing adhesive matrix molecules [61]. AD-specific S. aureus strains stimulate T cells to produce large number of cytokines and tilt the T cell immune response toward Th1/Th2 via Langerhans cells [62, 63]. The S. aureus superantigen expressed by S. aureus is also involved in mast cell degranulation, leading to anaphylaxis [59]. Therefore, treatment of patients with evident S. aureus skin infections is common clinical practice for AD.

CAP has been employed in inactivation of different kinds of microorganisms and toxin degradation in the field of disinfection and sterilization. Microwave-induced argon plasma at atmospheric pressure completely sterilized Escherichia coli (E. coli) and methicillin-resistant S. aureus in 1 s [64]. The inactivation rate of S. aureus and Enterococcus faecalis (E. faecalis) in the treatment area by cold atmospheric-pressure air plasma microjet reached 100% in 1 min under the treatment distance of 1 cm and the inactivation rate of bacteria in the whole dish was 100% in 4–5 min [65]. The metabolic activity of Candida albicans SC5314 biofilms was decreased by 41% within 30 s and by 89% within 60 s under the effects of the nonthermal plasma jet [66]. The mechanisms by which plasma causes microbial inactivation may involve heat, UV radiation, free radicals, charging, shear stress, and desiccation [67] (shown in Fig. 1). UV radiation from gas plasma can directly attack the microbial structures, especially the inner membrane and DNA of the bacterial endospore core [68]. However, UV radiation produced by CAP may be reabsorbed by the volume of the plasma, thereby limiting its effect [69]. The pulse voltage of plasma may cause structure change in spores, such as cracks [68]. In addition, the effect of CAP on microorganisms is also related to ROS. ROS mainly attacks the cell envelope of E. coli causing leakage of cell contents. For S. aureus, ROS causes damage to the intracellular components but does not cause cell leakage [70]. It should be emphasized that the inactivation effect of CAP on S. aureus is also related to treatment times, input power, and treatment distances. For example, on setting the same input power as 60 W and treatment time as 30 s, when the treatment distance is 2 mm, the inactivation rate of S. aureus is close to 100%; when the treatment distance is 6 mm, the S. aureus is only reduced by a log; Similarly, on setting the same input power as 60 W and treatment distance as 4 mm, from 5 s to 45 s, the reduction of S. aureus is increased from 0.09 log CFU/mL to 4.95 log CFU/mL [71]. These results indicate that strong treatment can lead to a very high inactivation rate, thus leading to further optimization of the treatment effect in the treatment process.

AD is a chronic inflammatory disease closely related to T-cell immunity [72]. Abnormal activation of the immune system triggers an inflammatory cascade in AD. The interactions between dendritic cells and T lymphocytes cause CD4⁺ T cells to differentiate into Th1 and Th2 cells [73]. Substantial evidences showed that significant Th1/Th2 homeostasis imbalance and Th2 polarization exist in AD [74]. Th2 cytokines could promote eosinophil recruitment and mast cell proliferation. Besides, they could induce homoconversion of B lymphocytes to increase the serum IgE levels [75]. Activation of the Th17 pathway has also been reported in Asian patients [76]. Hence, AD is a disorder of immunologic function, which is jointly completed by different helper T-cell pathways and multiple cytokines [77].

Air plasma is an excellent source of ROS and RNS [78]. For nonthermal DBD, a plasma dose of 3.9 J cm⁻² can generate 7.32 × 10¹⁶∼1.22 × 10¹⁷ ROS in the gas phase [79]. During plasma discharge, collisions between electrons stimulate the production of a variety of reactive materials; the latter, in turn, regulate the immune response by inducing the body to produce active substances. Low to medium levels of oxygen radicals have multiple regulatory effects on inflammation and play a key role in regulating immune response [80, 81]. Experiments have proved that CAP can not only directly increase the synthesis of NO₂⁻ and NO₃⁻ in endothelial cells, but it can also improve the expression of endothelial NO synthase (eNOS) in vivo, thus indirectly regulating the generation of NO [82]. NO, a ligand of the soluble guanylyl cyclase, mediates the production of cyclic guanosine monophosphate, and regulates the function of the vasomotor system [83]. Regulatory T cells, whose activity appears to be closely related to ROS, are deemed to be critical for maintaining immune homeostasis and tolerance. The function of regulatory T cells is increased when 2,3-dimethoxy-1,4-naphthoquinone is added to raise the intracellular levels of ROS [30]. It has been found that CAP treatment enhanced the expression of nuclear factor erythroid 2-related factor 2 and decreased the activity of inducible nitric oxide synthase, and thus it enhanced the resistance of cells to oxidative stress and excessive immune response [50, 84]. IL-12 is known to be a pro-inflammatory cytokine in the development of chronic AD [73]. Plasma can downregulate the level of IL-12 with increasing CAP doses [44]. Besides, it has been reported that PAM selectively inhibited the activity of NF-κB in triple-negative breast cancers [85]. NF-κB, a transcription factor that regulates the expression of pro-inflammatory cytokines, carries considerable weight in the inflammatory response to AD [73, 86]. Nonthermal plasma showed a strong inhibitory effect on the expression of CCL17 in keratinocytes, which is related to the severity of AD [87]. The possible immunomodulatory effect of CAP on AD is shown in Figure 2. However, the underlying physical and physiological mechanisms by which CAP regulates the immune function still remain to be further elucidated.

Skin barrier is an important line of defense to protect the organisms from chemical and physical attacks and prevent the invasion of pathogenic microorganisms [88]. AD is a common disorder of barrier dysfunction [1]. Lipid alterations, filaggrin deficiency, and tight junction defects in the stratum corneum (SC) may contribute to the disorder of the skin barrier in AD [89]. Ceramides (CER), free fatty acids, and cholesterol are the key components of the lipid composition in SC. The free fatty acid chain length of nonlesional skin in AD patients is reduced, which is accompanied by long‐chain CERs reduction and increase in short-chain CERs [90, 91]. The reduction in long-chain CERs results in an abnormal or missing skin permeability barrier [92]. Filaggrin plays an important and indispensable part in maintaining the homeostasis of the epidermis. Mutations in the human filaggrin gene (FLG) and downregulation of Th2 cytokines on filaggrin expression are the main mechanisms of filaggrin deficiency, which lead to increased infection of skin microorganisms [93].

The effects of CAP on the lipid composition of human skin have been analyzed using the latest X-ray photoelectron spectroscopy technology. The results showed that CAP could significantly change the lipid stoichiometry of the skin barrier, reducing the total carbon content to 76.7% and increasing the oxygen content to 16.5%. The nitrogen content was also slightly increased to 6.8%. The changes may be beneficial in the treatment of impaired skin barrier diseases, such as ichthyosis or AD [94]. Other studies have investigated the human skin’s tolerance to cold plasma and have evaluated the effect of plasma on skin barrier function. All three different plasma sources (pulsed, non-pulsed APPJ, and DBD) were found to be well tolerated by the skin and they did not damage the skin barrier [95]. The skin physiological parameters of 7 healthy volunteers were evaluated after the application of tissue-tolerable electrical plasma in vivo. The results showed that the reversibility of TEWL was increased after plasma treatment; however, tissue-tolerable electrical plasma had almost no effect on the water content in the deep layer of SC [96]. In addition, after CAP pretreatment, the skin permeability to drugs increased [97, 98]. Although the number of CAP studies on the skin barrier is limited, the current conclusion is that plasma causes no definite damage to the skin barrier and may be beneficial for the recovery of diseases with impaired skin barrier function. The possible mechanism by which CAP improves the skin barrier function in AD is shown in Figure 3.

Based on the abovementioned theoretical research efforts, several approaches and animal experiments were adopted to determine the CAP effects in AD from different aspects. An animal model of AD induced by Dermatophagoides farinae extracts received CAP treatment for 6 times within 17 days, while the control group did not receive any treatment. Then, the change of dermatitis severity score (DSS), TEWL value, serum IgE level, and epidermal thickness were evaluated between the two groups. CAP-exposed group showed a more significant decline in DSS on day 17, with a lower TEWL value and the mean epidermal thickness than the AD group. In addition, the level of serum IgE has declined since the seventh day and until the end of the experiment in CAP-exposed group [54]. Meanwhile, CAP increases the expression of HIF-1α in cells, which combines with the promoter region of the MANF gene and enhances the transcription and expression of MANF, thus inhibits inflammation by interacting with the key subunits of NF-κB/p65 [55]. A prospective pilot study included 22 patients with mild to moderate AD. CAP treatment was performed 3 times at 0, 1, and 2 weeks and clinical severity indices were evaluated at 0, 1, 2, and 4 weeks after treatment. Modified atopic dermatitis antecubital severity (ADAS) score, eczema area and severity index (EASI) score, Scoring of AD (SCORAD) were significantly decreased in CAP-treated group, the proportion of S. aureus in the microbial population of skin lesions also decreased [56]. All results reflect that CAP can improve AD in vivo experiments.

In the process of CAP formation, a variety of different active ingredients may be produced, mainly including free charges (electrons and positive and negative ions), excited atoms and molecules, energetic photons (e.g., UV radiation), heat, electrical fields, and chemically active particles (e.g., ROS and RNS) [67, 99]. These physical or chemical ingredients play a role in the interaction between plasma and organisms; however, they may also cause different degrees of chemical and physical threats to the cells or tissues.

The UV radiation band accompanied with plasma seems inevitable, and different working gas and parameter settings have different contributions to the formation of UV radiation [100]. Studies have measured the UV spectrum produced by atmospheric-pressure air plasma and found no significant UV emission below 285 nm. A calibrated UV detector was used to measure the power within the wavelength range of 200–300 nm. The power density of UV radiation was less than 50 μW/cm², and it was basically independent of the air velocity [101]. A risk assessment of the application of plasma in dermatology showed that the intensity of UV radiation during plasma treatment is much lower than the intensity of UV radiation received during sunbathing in the summer. The UV radiation dose of the plasma is one order of magnitude lower than the minimal erythema dose, basically ruling out the risk of potential damage to the skin caused by UV radiation during plasma treatment [102]. In fact, a specific wavelength of UV radiation has therapeutic significance for AD. UV radiation inhibits inflammatory cells in the skin, increases skin collagen synthesis, induces normalization of epidermal differentiation to limit eczema-like reactions and prevents foreign antigen invasion [103].

CAP is a kind of ionized gas, in which gas atoms and molecules collide with high-energy electrons. In contact with the skin tissue, plasma may cause pain and protein degeneration due to electrical and thermal stimulation [104]. Therefore, the temperature of CAP is generally below 40°C, thus maintaining a cold state to avoid damage [105]. Laser scanning microscopy and histological analysis were used to study the thermal effect of argon plasma on the skin. The depth of damage varied with the moving speed of the plasma beam. However, the results showed that thermal damage was always limited to the upper cells of the SC. Also, these structural changes are so superficial that it is difficult to detect by histological section analysis. When the plasma beam moved on the skin surface at a velocity of 5–8 mm/s, the laser scanning microscopy did not detect thermal damage and the keratinocytes were intact. When the velocity was reduced to 2 mm/s, lipid fusion occurred in two or three layers of cells at the top of keratinocytes, but thermal damage was not detected in the deep layer of the skin [106].

In addition to electricity and heat, different plasma doses have different effects on cell tissues. Low plasma doses (<1 J cm⁻²): inactivation of bacteria, normal cell survival. Intermediate doses (2–6 J cm⁻²): repair DNA damage, release cell growth factors, increase in proliferation and migration, and control the development of apoptosis in cancers. Higher doses (>7 J cm⁻²): normal cell death. Ultra-high doses (>10 J cm⁻²): cell necrosis [107]. Hence in the application of CAP, the appropriate plasma dose should be selected according to different application purposes so as to achieve the purpose of treatment and avoid unnecessary damage.

The physiological concentration of ROS can regulate cell growth, differentiation, and apoptosis through complex signal transduction. For example, ROS stimulates the proliferation and migration of tumor cells, inhibits apoptosis and autophagy by activating signaling pathways, such as MAPK/ERK, PI3K/AKT/mTOR. However, long-term excessive production of ROS is harmful to the human body, and it can lead to the occurrence of various inflammatory diseases or cutaneous tumors. It induces genome mutations, which leads to epigenetic instability [108, 109]. CAP affects histone methylation by inducing JARID1A demethyltransferase through ROS-mediated cell conduction [110]. Park et al. [111] showed the MDA-MB-231 breast cancer cells underwent a higher rate of apoptosis and a decreased proliferation rate upon plasma treatment. After CAP treatment of MDA-c-231 cells, there were 76 CpGs with high methylation and 63 CpGs with hypomethylation. Meanwhile, significant methylation changes were observed in 17 miRNAs and 25 lncRNAs. These results suggest that CAP may have a significant effect on DNA methylation, especially in cancer cells. The CAP jet was used to treat normal cells and cancer cells, respectively, in order to evaluate the safety of ROS induced by CAP. In the experiment, the CAP jet produced a high level of free radicals and minimized the impact of charged particles and UV radiation on the tissue. The results show that cancer cells are more sensitive to CAP-induced free radicals than normal cells. Under the effect of ROS and RNS, the activity of cancer cells was decreased and apoptosis was more likely to occur, but the effect on normal cells around cancer cells was not significant [112]. Phosphorylated histone 2AX (γH2AX) is a marker of DNA double-strand breaks in ionizing radiation. γH2AX induced by cold plasma is a secondary event of redox or apoptosis signal, rather than the result of DNA damage directly mediated by ROS [113]. Therefore, damage to the organism caused by ROS produced by plasma may be limited.

Through a series of in vitro and in vivo experiments, the potential value of powerful anti-infection, immune regulation, and skin barrier improvement functions of CAP has been gradually confirmed and it has become a research hotspot in the medical-biological community. The effects of inhibiting inflammatory factors, intercellular communication, immune response, and signal transduction mediated by CAP have been widely studied and utilized. In addition, inactivation of drug-resistant bacteria and fungi by CAP plays a unique role in wound healing. Being a chronic inflammatory skin disease, patients with AD suffer from skin microbial infection, immune disorder, skin barrier loss, and other problems. Therefore, we reasonably speculate that CAP can be used in the clinical treatment of AD. However, being a newly discovered medical treatment, the biological function and molecular mechanism of plasma have not been fully elucidated. There are still many deficiencies in its research. For example, the penetration of charged particles and active particles produced by plasma in human skin tissues are important obstacles restricting the process of plasma research, and how to improve the particle penetration depth is a tricky problem that remains to be solved. Second, the research scope of plasma is relatively limited and most stay at the cellular stage. Related research on signal pathway conduction is still scarce and how to achieve precise treatment of different diseases requires further in-depth research. These scientific problems are related to the application scope and development direction of plasma in dermatology and are the key scientific and technical problems that need to be solved urgently. However, it is undeniable that plasma presents great innovation opportunities in the field of dermatology. The technology of plasma in skin beauty has been certified by the US Food and Drug Administration. It is believed that with further improvement of the plasma device, plasma medical technology can provide more refined, safe, and convenient medical treatment.

The authors have no conflict of interest to declare.

This study was supported by the National Natural Science Foundation of China (No. 82203898) and the World-Class Universities (Disciplines) and the Characteristic Development Guidance Funds for the Central Universities (No. PY3A038).

Fan Bai wrote the manuscript and created the figures; Yutong Ran edited the study; Siyue Zhai revised the manuscript; and Yumin Xia conceived and finalized the manuscript. All authors have read and approved the manuscript.

Edited by: H.-U. Simon, Bern.