## 1 Introduction  
Plasma-activated water (PAW) is an emerging technology that has garnered significant attention across diverse scientific fields for its unique physicochemical properties. Generated by exposing water to non-thermal plasma, PAW becomes a metastable aqueous solution enriched with a complex mixture of reactive oxygen and nitrogen species (RONS), including hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), nitrites (NO\({}_{2}\)\({}^{-}\)), nitrates (NO\({}_{3}\)\({}^{-}\)), and various transient radicals [1,2]. These reactive species impart potent oxidative and antimicrobial characteristics to PAW, establishing it as a promising tool in fields ranging from agriculture and medicine to environmental decontamination. Established applications of PAW highlight its versatility and efficacy. In agriculture, it serves as an eco-friendly alternative to conventional agrochemicals, enhancing seed germination, promoting plant growth, and neutralizing microbial pathogens on crops without leaving harmful residues [3, 4, 5]. Its oxidative capacity also aids in the degradation of pollutants in soil and water [6]. In medicine, PAW is recognized for its therapeutic potential in wound healing, infection control, and sterilization [1,7]. It effectively combats pathogenic bacteria, including antibiotic-resistant strains like _Staphylococcus aureus_ and _Pseudomonas aeruginosa_, thereby promoting tissue regeneration and accelerating healing [8,9]. Its utility is also being explored in dentistry and oncology, positioning PAW as a cost-effective and non-invasive therapeutic agent [1,7,10]. Recently, plasma technology has been extended to the field of trichology, with investigations into both direct and indirect application methods for hair and scalp care [11,12]. It is critical to distinguish PAW from unrelated plasma-based treatments such as platelet-rich plasma (PRP), a blood-derived concentrate used to stimulate hair regrowth [13]. Unlike PRP, PAW is a purely aqueous solution whose activity derives from plasma-generated RONS, not biological growth factors. Promising clinical results have demonstrated the feasibility of plasma-based hair treatments. For instance, a six-month study by [PERSON] et al. (2020) found that the topical application of plasma-activated liquids (PALs) was well-tolerated and improved subjective hair density in individuals with androgenetic alopecia [11]. Furthermore, [PERSON] et al. (2025) reported that direct cold atmospheric plasma application led to a measurable increase in hair density with no significant side effects, reinforcing its therapeutic potential [12]. The indirect application of PAW or PALs presents a compelling alternative to conventional hair care products, which often rely on aggressive chemical agents. Formulations such as shampoos with sulfates, dyes with ammonia, or bleaches with high concentrations of hydrogen peroxide (3-12%) can compromise the structural integrity of hair, leading to increased porosity, dryness, and breakage [14]. In contrast, PALs contain RONS at much lower micromolar to millimolar concentrations, suggesting a gentler mechanism of action. The mild oxidative power of PAW may facilitate the removal of organic residues and reduce microbial load on the scalp. However, its acidic nature (typically pH < 3) causes the hair cuticle to contract and remain closed, which contradicts the assumption that it could enhance dye uptake by lifting the cuticle scales [15]. Instead, its benefits are more likely attributable to enhanced surface cleaning and subtle chemical modifications, though the precise mechanisms remain poorly understood. To evaluate the impact of PAW on hair, it is essential to consider the hair fiber's complex structure. The hair shaft is primarily composed of the protein keratin and consists of three concentric layers: the outer cuticle, the cortex, and the inner medulla. The cuticle's overlapping scales protect the cortex, which contains the keratin filaments and melanin pigments that determine the hair's strength, elasticity, and color [16].