## 4 Discussion
The differences in Cu residue and electrical degradation between the two groups are further evidenced by the increasing trend in error bar magnitude observed across peel cycles. This reflects not only differences in average interfacial strength but also the variability in delamination behavior within each group. In the ENMH(-BSA step) group, large error bars were already evident after the first peel cycle, indicating poor initial adhesion and highly inconsistent interfacial failure across samples (Figure 3g). The continued increase in variability over subsequent cycles suggests the inherent instability of the metal-polymer interface in the absence of BSA pretreatment. In contrast, the ENMH group maintained low Cu residue and minimal variability for the first two cycles, demonstrating superior mechanical durability and interfacial stability. However, a noticeable increase in error bars was observed from the third cycle onward in both groups, indicating that even with BSA, repeated mechanical stress eventually initiates partial delamination. Nonetheless, the overall level of Cu residue in the ENMH group remained substantially lower than that of the control, reinforcing the conclusion that BSA significantly enhances metal adhesion and resistance to fatigue-induced interfacial failure under cyclic stress. This interpretation is further supported by the changes in electrical properties, which exhibit a consistent trend (Figures 4h and S4). The gradual increase in sheet resistance over successive peel cycles reflects the progressive loss of conductive pathways due to interfacial degradation. Notably, the ENMH samples showed a much smaller increase in resistance and narrower error margins compared to the ENMH(-BSA step) group, indicating that improved interfacial adhesion contributes not only to mechanical robustness but also to enhanced electrical stability. To ensure stable heating performance and safe operation of the flexible and conductive planar heater (ENMH), two key design strategies were employed. First, a BSA-assisted electroless plating process was used to form a continuous and uniform copper layer over the entire electrospun PVDF-HFP nanofiber network. This uniform metallization promotes even current distribution, enabling consistent and spatially homogeneous Joule heating (Figures 2 and 4a). Second, the 3D fibrous architecture of the PVDF-HFP scaffold enhances in-plane thermal dissipation and prevents localized heat buildup, thereby reducing the risk of hotspot formation. This design was further evaluated in terms of electrical and thermal safety. The ENMH operates at low input voltages, minimizing the risk of electric shock and thermal injury, while maintaining surface temperatures within the safety thresholds established for skin-contact devices under typical operating conditions. In addition, infrared thermal imaging conducted under repeated mechanical deformation and skin attachment (Figure 5c,d) confirmed that the heating behavior remained stable and reproducible, with no evidence of thermal runaway or localized overheating. The thermal performance of the ENMH represents a key strength of this study, supported by its demonstrated safety and operational stability. The device achieved a surface temperature of 48.8 \({}^{\circ}\)C under an applied voltage of only 0.4 V, aligning well with the typical operational range (40-50 \({}^{\circ}\)C) required for wearable heating applications (Figures 4a and 5d) [49, 50, 51]. Given that most commercial wearable heaters are designed to operate at or below this range for safety and comfort, the demonstrated performance at such a low voltage highlights the practical applicability of the ENMH. At a higher input of 1.0 V, the surface temperature increased to 124.1 \({}^{\circ}\)C, indicating high thermal efficiency (Figure 4a). The device also exhibited precise temperature responsiveness to stepwise voltage increments of 0.1 V applied every 30 s (Figure 4b), demonstrating controllable and reproducible heating behavior.