## 4 Discussion
When a constant voltage of 0.6 V was applied for 5 min, the surface temperature stabilized within a range of 79-80 \({}^{\circ}\)C (Figure 4c), further confirming stable thermal output under steady-state conditions. In repeated ON/OFF cycling, the ENMH displayed a heating time of 4.7 s and a cooling time of 3.4 s (Figure 4d), reflecting rapid thermal switching and good reproducibility. The rapid temperature drop upon voltage removal supports the system's efficient cooling response. The ENMH demonstrated robust performance under various mechanical deformation conditions. Even under a tensile strain of up to 100%, the device maintained its heating function without structural damage and recovered approximately 94% of its initial surface temperature (Figure 5a). During repeated bending tests, the thermal output remained stable for more than eight cycles (Figure 5b). When attached to a finger joint, the device exhibited only a ~12% fluctuation in surface temperature during flexion, while consistently maintaining overall heating performance (Figure 5c). Notably, when the ENMH was mounted onto a nonwoven textile and applied to a curved body surface such as the wrist, it stably maintained a surface temperature between 47 \({}^{\circ}\)C and 49 \({}^{\circ}\)C over a 5 min period. Localized heating was clearly visualized using infrared thermal imaging (Figure 5d). These results collectively confirm that the ENMH can maintain structural integrity, stable thermal behavior, and consistent surface temperature under a broad range of mechanical deformation scenarios, including stretching, repeated bending, joint flexion, and conformal attachment to curved surfaces. Performance degradation was observed under both bending and tensile deformation, indicating the susceptibility of the ENMH system to dynamic mechanical stress (Figure 5c). In the case of repeated finger flexion, a surface temperature drop of approximately 5 \({}^{\circ}\)C (12.1%) was recorded, primarily attributed to an increase in electrical resistance induced by mechanical strain. A comparable decline in heating performance was also noted during tensile loading, likely due to strain-induced damage that disrupted the continuity of the conductive layer [52]. This degradation can be explained by three predominant mechanisms. First, localized mechanical stress may cause the formation of microcracks in the copper layer, interrupting continuous current pathways and thereby reducing Joule heating efficiency [52]. Second, interfacial delamination or sliding can occur in regions with insufficient Cu-polymer adhesion, further compromising electrical continuity [53, 54]. Third, deformation-induced geometric rearrangement of the nanofiber network, or structural rupture under tensile strain, may increase contact resistance and reduce the effective conductive area, ultimately lowering the overall thermal output [55]. These combined effects offer a plausible explanation for the observed thermal instability under repeated mechanical loading. An effective strategy to mitigate such degradation is the optimization of electrode pattern design. For instance, a horseshoe-shaped pattern can be engineered to accommodate mechanical deformation more flexibly, thereby relieving localized stress and minimizing structural damage. This approach has been reported to enhance both the mechanical durability and thermal stability of flexible heaters under repeated bending or tensile strain [1; 56]. Another effective strategy to mitigate such performance degradation involves the incorporation of carbon nanostructures, such as reduced graphene oxide (rGO), into the conductive network. rGO offers high electrical conductivity, excellent mechanical flexibility, and good dispersibility, making it a suitable mechanical backbone in metal-polymer composite systems. When embedded in the conductive layer, such nanostructured networks can help improve stress distribution and suppress crack formation, thereby stabilizing current pathways and reducing thermally induced performance loss during repeated deformation.