## 1 Introduction  
Plasma is usually defined as an ionized gas, which is produced by the decomposition of polyatomic gas molecules or the removal of electrons from monoatomic gas shells [1]. Non-thermal atmospheric plasma (NTP) is one of them, which can produce reactive oxygen species (ROS), reactive nitrogen species (RNS), ultraviolet radiation, infrared radiation, electronic ion electric field, and so on [2]. The physiological level of ROS (mainly hydrogen peroxide) generated by plasma entering cells is a key redox signaling factor which emits signals through specific protein targets and participates in cell metabolism regulation and stress response to support cells to adapt to changing environments and pressures [3]. These special ingredients make NTP a powerful tool for many applications, such as cancer treatment [4, 5], coagulation [6], oral treatment [7], sterilization [8], skin disease treatment [9], and wound healing [10, 11] etc. As the largest organ of the human body, the skin is the first line of defense against microbial invasion, physical and chemical damage, and trauma [12]. However, when this line of defense is destroyed by mechanical or chemical damage, the skin needs wound healing rapidly. Wound healing is a complex and dynamic process generally divided into four stages, starting with hemostasis, inflammation, then proliferation, and finally tissue remodeling. In the micro environment of damaged tissue, there are many biological factors and cell types involved in this process. One cell type is keratinocytes, which proliferate and migrate to the site of the injury during proliferation under the induction of growth factors [13]. In clinical practice, there are various therapeutic strategies for wound healing, such as the most common drug therapy [14], growth factor therapy [15], epidermal stem cell transplantation [16], spray and stem cell therapy [17], etc. Although there are many methods, there are still many limitations, such as drug resistance, high price, and complex operation. Therefore, there is an urgent need for new treatments to promote wound healing, especially for complex chronic wounds. Low-dose NTP is promising to achieve this strategic goal [18; 19; 20], which can promote wound healing not only by killing bacteria in the wound area [21], but also by stimulating the proliferation of vascular endothelial cells [22], inducing the expression of IL-6, IL-8, MCP-1, TGF-\(\beta\)1, and TGF-\(\beta\)2 [23], which is related to wound healing, and promoting angiogenesis [24], etc. The proliferation stage of wound healing is characterized by extensive activation of keratinocytes, fibroblasts, macrophages, and endothelial cells to coordinate wound closure matrix deposition and angiogenesis [25]. As early as 12 h after injury, keratinocytes are activated by various alterations [26], which lead to partial epithelial-mesenchymal transition in keratinocytes at the edge of the wound [27]. Furthermore, keratinocytes with altered polarity migrate horizontally through the wound to reconstruct the epidermis [28]. The keratinocytes behind the leading edge regulate their cell adhesion through PCK\(\alpha\)-mediated desmosomal adhesion changes [29] and Eph-mediated adhesion junction changes [30], so that they can rearrange their order with the migrated epithelial sheets [26]. Keratinocytes in new epidermis release matrix metalloproteinases in the neoepit dermis to assist their migration pathways and rebuilt the basement membrane [31], and then completely remodel the basement membrane and undergo terminal differentiation to stratification and regeneration of the epidermis [32; 33]. Keratinocyte is one of the most important cells in wound healing, and the HaCaT cell is a cell line of immortalized keratinocytes from adult humans. Previous studies have found that low-dose NTP treatment promotes the proliferation of basal epidermal keratinocytes [34], but the mechanism of action is not elucidated.