## 3 Results and Discussion
### SEM Characterization of Hair Surface Morphology Before and After Plasma-Activated Water Treatments
While Type 4 hair is inherently more porous and thus prone to oxidative stress, the cuticular disruption here remains moderate, highlighting PAW-P's less aggressive impact compared to nitrate-dominated PAW-N exposure (Figure 3d). The SEM images in Figures 2-4 highlight the structural and morphological differences observed among untreated and PAW-treated hair samples of various types. These observations are further corroborated by quantitative elemental analysis via energy-dispersive X-ray spectroscopy (EDS), summarized in Table 3 and detailed in Supplementary Figures S2-S13. Table 3 reveals that for Sample 1 (Type 2--wavy hair), PAW-N treatment induced a significant increase in carbon content (from 74.0% to 87.0%) and a marked decrease in oxygen (from 21.7% to 9.4%), suggesting degradation of surface lipids and keratin oxidation. These changes align with the SEM observations of surface erosion and cuticle lifting (Figure 3a; Supplementary Figure S3). In contrast, treatment with PAW-P resulted in milder modifications, with only a moderate increase in oxygen (17.1%) and sulfur (5.0%) levels (Figure 4a; Supplementary Figure S4), indicative of disulfide bond alteration and superficial oxidative action without major structural compromise. For Sample 2 (Type 1--Straight hair), SEM images showed pronounced cuticle disruption under PAW-N (Figures 3b and S6), with EDS confirming elevated sulfur content (5.5%) and relatively preserved carbon levels (74.7%), suggesting deep oxidative penetration. Conversely, PAW-P exposure preserved cuticle integrity (Figures 4b and S7), with only minor fluctuations in elemental composition, corroborating its milder chemical profile. In Sample 3 (Type 1--Straight hair with smooth strands), PAW-N caused moderate surface alterations (Figures 3c and S9) and a slight decrease in sulfur and oxygen, while PAW-P maintained structural features close to the control, as seen in the SEM image (Figures 4c and S10) and supported by an increase in oxygen (21.2%) and sulfur (4.0%), possibly reflecting superficial keratin oxidation with minimal damage. Interestingly, aluminum was detected only in the control condition (0.1%), suggesting possible environmental con \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Control** & **PAW-N** & **PAW-P** \\ \hline \multirow{3}{*}{1} & C: 74.0, O: 21.7, S: 3.5, & C: 87.0, O: 9.4, S: 2.8, Ca: & C: 77.4, O: 17.1, S: 5.0, \\  & Ca: 0.7, Mg: 0.1 & 0.7, Mg: 0.1 & Ca: 0.4, Mg: 0.1 \\  & C: 75.0, O: 20.6, S: 4.2, & C: 74.7, O: 19.7, S: 5.5, & C: 75.1, O: 20.2, S: 4.6, \\  & Ca: O: 1, Mg: 0.0 & Ca: 0.1, Mg: 0.0 & Ca: 0.0, Mg: 0.0 \\  & C: 79.0, O: 17.6, S: 3.2, & C: 82.4, O: 14.4, S: 3.1, & C: 74.8, O: 21.2, S: 4.0, \\  & Al: 0.1, Ca: 0.1, Mg: 0.0 & Ca: 0.1, Mg: 0.0 & Ca: 0.0, Mg: 0.0 \\  & C: 72.6, O: 22.0, S: 5.0, & C: 72.8, O: 21.4, S: 5.2, & C: 77.4, O: 18.7, S: 3.3, \\  & Ca: 0.4, Mg: 0.1 & Ca: 0.4, Mg: 0.1 & Ca: 0.1, Mg: 0.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Elemental composition (in weight%) of hair surface for each sample type (1–4), under untreated conditions (Control) and after treatment with plasma-activated water enriched with nitrate species (PAW-N) and peroxide species (PAW-P), as determined by energy-dispersive X-ray spectroscopy (EDS). Major elements include carbon (C), oxygen (O), sulfur (S), calcium (Ca), magnesium (Mg), and, in one case, aluminum (Al), reflecting the characteristic keratin structure and the effects of plasma-induced oxidative modification. tamination or residue from prior cosmetic products, as aluminum compounds are common in haircare formulations. The absence of aluminum after PAW treatment may indicate its removal through plasma-assisted surface cleaning. Sample 4 (Type 4--coily/kinky hair) showed unique behavior due to its inherently porous and dense structure. The EDS results indicate modest elemental shifts after PAW-N (e.g., sulfur increase from 5.0% to 5.2%) and PAW-P (sulfur decrease to 3.3%), with SEM images showing only moderate surface disruption (Figures 3d and 4d; Figures S12 and S13).