1. Current Status of CAP Application in Skincare

Beauty and skincare products based on CAP technology are gradually emerging in the global market and receiving positive feedback from users.

In 2013, a German company launched a CAP product for treating chronic, infected wounds and skin diseases caused by pathogens, marking the beginning of CAP application in clinical medicine [1][2][3]. This device has been used to treat long-term chronic and infected wounds, especially in cases where traditional treatments fail. Over 80% of patients reported that the wound healing process started or accelerated until the wounds fully closed. Additionally, clinical users emphasized that the product has a certain therapeutic effect in eradicating multi-drug resistant bacteria (such as MRSA).

【See Figure 1 for the application of CAP in wound healing】

Figure 1 Application of CAP in Wound Healing

At present, plasma medical and beauty products are gradually emerging in the global market, with corresponding brands and products available in Germany, the United Kingdom, the United States, and South Korea. These products utilize the physical and chemical properties of CAP for precise treatment of skin tissue, featuring significant efficacy, safety, and non-invasiveness.

Another German brand dedicated to the development of cold plasma technology products has applied its products in fields such as medical treatment and air purification. A British brand offers cold plasma devices that can treat various skin problems; this technology is safe, non-ablative, and painless. The brand has a certain influence in the field of beauty and skin treatment, and its products and technologies have attracted the attention of some professional beauticians and medical personnel. User feedback indicates that relevant devices have good effects in improving skin conditions and treating acne. A U.S. brand claims to provide users with an unparalleled CAP experience through its unique suction mechanism and controllable emission technology.

2. Shortcomings of Existing Applications

By observing the aforementioned foreign CAP devices, it is found that some devices have poor uniformity and diffusion of discharge during use, with frequent spark discharges or bright filamentary discharges; most products directly adopt the form of dielectric barrier discharge, making it difficult to form stable and diffused low-temperature plasma. Numerous studies have shown that uniformly diffused discharge (diffuse discharge) and spark filament discharge are essentially different types of discharges in physics [4][5][6][7][8][9][10], which not only affect the user experience (e.g., strong electric shock, stinging, and burning sensations) but also undermine the therapeutic effect.

【See Figure 2 for low-temperature plasma with inefficient diffusion】

Figure 2 Low-Temperature Plasma with Inefficient Diffusion

【See Figure 3 for the form of dielectric barrier discharge】

Figure 3 Form of Dielectric Barrier Discharge

【See Figure 4 for stable and diffused low-temperature plasma】

Figure 4 Stable and Diffused Low-Temperature Plasma

3. Advantages of CAP Diffusion

Experiments have proven that uniformly diffused CAP offers better therapeutic effects and a more comfortable user experience.

The so-called CAP diffuse discharge refers to low-temperature plasma presenting a large-area, uniform, and stable blue-purple color. During the physical process, the discharge channels are diffusely distributed throughout the discharge gap, fully connecting the two electrodes and existing stably without transitioning to spark discharge. Studies have shown that diffuse discharge has the following significant advantages [11][12][13][14][15][16][17][18]:

(1) Large-Area Uniformity

This characteristic is determined by the nature of diffuse discharge. Large-area uniformity is crucial for CAP applications, as it ensures the uniformity and stability of application and therapeutic effects.

(2) Normal or Low Temperature

Diffuse discharge maintains the plasma in a stable non-equilibrium state, limits the degree of air ionization, and prevents excessive energy from being injected into the plasma (which would cause the plasma to transition to thermal equilibrium and rapidly heat the air medium). When the plasma and air medium are significantly heated, the skin will feel a strong stinging sensation, affecting the comfort of the treatment experience. Diffused CAP ensures the air medium remains at normal temperature, significantly improving the comfort of application.

(3) Higher Generation Efficiency of Metastable Active Particles

Spark discharge leads to excessively high electron density in the discharge filaments; this high electron density causes metastable active particles to be excited to higher energy levels through rapid collisions. Spectral measurement results show that the intensity of various active particles in pulsed CAP is significantly higher.

(4) Significantly Better Sterilization Effect

Sterilization experiments also indicate that the sterilization effect of pulsed CAP is significantly superior to that of AC discharge.

4. Prospects

When CAP is generated in air, due to the high gas pressure of atmospheric pressure air discharge and the strong electronegativity of air, the electric field intensity required for air ionization is high, and the non-equilibrium plasma can easily transition to thermal equilibrium plasma in a short time—manifested externally as the conversion of diffuse discharge to bright filamentary spark discharge [4][5][9]. How to generate stable, uniform, and large-area CAP in open air has long been a research focus among many scholars. Considering applications in civil fields such as skincare, hospital clinics, and beauty salons, CAP devices also need to be compact, portable, and aesthetically pleasing, which further increases the difficulty of R&D for generation devices. Although CAP technology has achieved substantial research progress and been widely applied at home and abroad, there is still enormous room for improvement in CAP technology that meets the aforementioned multiple requirements. Technical barriers remain to be overcome, and this will also be a key focus of future research and applications.

References

[1] Metelmann H R, Vu T T, Do H T, et al. Scar formation of laser skin lesions after cold atmospheric pressure plasma (CAP) treatment: A clinical long term observation[J]. Clinical Plasma Medicine, 2013, 1(1): 30-35.

[2] Fluhr J W, Sassning S, Lademann O, et al. In vivo skin treatment with tissue‐tolerable plasma influences skin physiology and antioxidant profile in human stratum corneum[J]. Experimental Dermatology, 2012, 21(2): 130-134.

[3] Reuter S, Von Woedtke T, Weltmann K D. The kINPen—a review on physics and chemistry of the atmospheric pressure plasma jet and its applications[J]. Journal of Physics D: Applied Physics, 2018, 51(23): 233001.

[4] Pai D Z, Lacoste D A, Laux C O. Transitions between corona, glow, and spark regimes of nanosecond repetitively pulsed discharges in air at atmospheric pressure[J]. Journal of Applied Physics, 2010, 107(9).

[5] Machala Z, Jedlovsky I, Martisovits V. DC discharges in atmospheric air and their transitions[J]. IEEE Transactions on Plasma Science, 2008.

[6] Tao Shao, Cheng Zhang, Zheng Niu, et al. Diffuse discharge, runaway electron, and x-ray in atmospheric pressure air in an inhomogeneous electrical field in repetitive pulsed modes[J]. Applied Physics Letters, 2011, 98(2): 021503. doi: 10.1063/1.3540504.

[7] Cheng Zhang, Tao Shao, Hao Ma, et al. Experimental Study on Conduction Current of Positive Nanosecond-pulse Diffuse Discharge at Atmospheric Pressure[J]. IEEE Transactions on Dielectrics and Electrical Insulation, 2013, 20(4): 1304-1314.

[8] C. Zhang, T. Shao, Z. Niu, H. Jiang, Y. Yang, P. Yan, and Y. Zhou. Diffuse and filamentary discharges in open air driven by repetitive high-voltage nanosecond pulses[J]. IEEE Transactions on Plasma Science, 2011, 39: 2208-2209.

[9] C. Zhang, T. Shao, Z. Niu, C. Li, G. Wang, J. Tian, and P. Yan. Pulse repetition frequency effect on nanosecond pulse diffuse discharge in atmospheric air with a point-to-plane gap[J]. IEEE Transactions on Plasma Science, 2011, 39: 2070-2071.

[10] T. Shao, V. F. Tarasenko, C. Zhang, Yu. V. Shut’ko, and P. Yan. X-ray and runaway electron generation in repetitive pulsed discharges in atmospheric pressure air with a point-to-plane gap[J]. Physics of Plasmas, 2011, 18(5): 053502.

[11] Pin YAN, XinPei LU, Tao S, et al. Review on atmospheric pressure pulsed DC discharge[J]. Scientia Sinica Physica, Mechanica & Astronomica, 2011, 41(7): 801-815.

[12] Q. Xiong, X. P. Lu, Ken Ostrikov, et al. Pulsed dc-and sine-wave-excited cold atmospheric plasma plumes: A comparative analysis[J]. Physics of Plasmas, 2010, 17(4).

[13] Walsh J L, Shi J J, Kong M G. Contrasting characteristics of pulsed and sinusoidal cold atmospheric plasma jets[J]. Applied Physics Letters, 2006, 88(17).

[14] Martens T, Bogaerts A, Van Dijk J. Pulse shape influence on the atmospheric barrier discharge[J]. Applied Physics Letters, 2010, 96(13).

[15] Williamson J M, Trump D D, Bletzinger P, et al. Comparison of high-voltage ac and pulsed operation of a surface dielectric barrier discharge[J]. Journal of Physics D: Applied Physics, 2006, 39(20): 4400.

[16] Ayan H, Fridman G, Gutsol A F, et al. Nanosecond-pulsed uniform dielectric-barrier discharge[J]. IEEE Transactions on Plasma Science, 2008, 36(2): 504-508.

[17] Zhang C, Shao T, Long K, et al. Surface treatment of polyethylene terephthalate films using DBD excited by repetitive unipolar nanosecond pulses in air at atmospheric pressure[J]. IEEE Transactions on Plasma Science, 2010, 38(6): 1517-1526.

[18] Walsh J L, Kong M G. 10ns pulsed atmospheric air plasma for uniform treatment of polymeric surfaces[J]. Applied Physics Letters, 2007, 91(25).