## 3 Results and Discussion
### SEM Characterization of Hair Surface Morphology Before and After Plasma-Activated Water Treatments
Figure 3d illustrates coily/kinky hair (Sample 4, Type 4), which, compared to its pristine state in Figure 2d--marked by a densely textured surface and naturally lifted cuticles--exhibits moderate roughness and localized cuticle lifting following PAW-N exposure. Despite the harsh chemical environment (low pH, high nitrate concentration), the overall structural integrity remains relatively preserved. This response may be attributed to the denser cuticle structure and lower uniformity of surface exposure inherent to Type 4 hair, which potentially mitigates oxidative penetration in certain regions. These findings suggest that while coily/kinky hair is generally more porous and thus susceptible to oxidative agents, its tightly coiled morphology may, in specific cases, afford partial protection against uniform damage. This contrasts with the more severe fragmentation observed in straight hair (e.g., Figure 3b), reinforcing the idea that damage patterns are not solely dependent on porosity but also on geometry and exposure dynamics. Figure 3: Scanning Electron Microscope (SEM) images of hair samples after treatment with nitrate-rich plasma-activated water (PAW-N) at 2000\(\times\) magnification: (**a**) Sample 1—wavy hair (Type 2); (**b**) Sample 2—straight hair (Type 1); (**c**) Sample 3—straight hair (Type 1); (**d**) Sample 4—coily/kinky hair (Type 4). Compared to the untreated controls (Figure 2), the images reveal marked structural alterations, including cuticle lifting, surface roughness, and fragmentation. These effects are attributed to the acidic pH, elevated nitrate concentration, and high ionic strength of PAW-N. The SEM images in Figure 4 illustrate the morphological effects of peroxide-rich plasma-activated water (PAW-P) on hair samples, revealing milder structural alterations compared to the more aggressive PAW-N treatment. The moderate acidity (pH 2.54), reduced conductivity (1926 \(\upmu\)S/cm), and lower total dissolved solids (768.7 mg/L) in PAW-P, combined with a predominant concentration of hydrogen peroxide (>25 mg/L), result in less pronounced cuticular degradation. Figure 4a shows the wavy hair sample (Sample 1, Type 2), which, compared to its pristine counterpart in Figure 2a, displays only slight cuticle lifting and subtle surface roughening. The lipid layer appears partially disrupted, but the general cuticle architecture remains intact, indicating limited oxidative penetration. In Figure 4b, straight hair (Sample 2, Type 1) shows mild cuticle elevation without visible fragmentation, contrasting with the severe damage seen under PAW-N (Figure 3b). The original smooth and uniform cuticle  
Figure 4: Scanning Electron Microscope (SEM) images of hair samples after treatment with peroxide-rich plasma-activated water (PAW-P) at 2000\(\times\) magnification: (**a**) Sample 1—wavy hair (Type 2); (**b**) Sample 2—straight hair (Type 1); (**c**) Sample 3—straight hair (Type 1); (**d**) Sample 4—coily/kinky hair (Type 4). Relative to both untreated controls (Figure 2) and PAW-N-treated samples (Figure 3), these images show moderate cuticle lifting and surface roughness, with less fragmentation. The milder effects are associated with the lower ionic strength and oxidative activity of PAW-P, primarily due to its elevated hydrogen peroxide content (>25 mg/L). structure from Figure 2b is preserved, suggesting that PAW-P induces only superficial oxidative effects in compact hair types. Figure 4c, corresponding to another straight hair sample (Sample 3, Type 1), demonstrates minimal roughening and well-aligned cuticles, closely resembling its pristine form (Figure 2c). This preservation supports the hypothesis that PAW-P's low ionic strength and oxidative character do not significantly compromise keratin integrity. Finally, Figure 4d presents the coily/kinky hair sample (Sample 4, Type 4), where moderate roughness and localized lifting are evident when compared to its untreated state (Figure 2d).