## 3 Results
### Morphology and Structure of the Fiber Membrane  
Figure 2 shows the surface morphology of TPU fiber membranes, PDA/TPU fiber membranes, CB/CNT/TPU fiber membranes, and CB/CNT/PDA/TPU fiber membrane samples. As shown in Figure 2a-a\", the TPU fibers have smooth surfaces and are randomly distributed. The fibers' internal structure contains numerous pores and nodes of varying sizes, forming a typical three-dimensional network framework, which provides abundant attachment sites for CB and CNT. In Figure 2b-b\", a dense PDA layer is formed on the fiber surfaces through in-situ polymerization. After modification with PDA, the TPU fiber surfaces become rougher. The adhesive properties of the PDA layer significantly enhance the interfacial bonding strength between CB, CNT, and TPU fibers. As shown in Figure 2c-c\", CB and CNT are successfully deposited on the TPU fiber surfaces after ultrasonic treatment. During the ultrasonic process, CB and CNT are propelled toward the  
Figure 1: Schematic diagram of the fabrication process for the CB/CNT/PDA/TPU strain sensor. nanofiber surfaces, where they undergo interfacial collisions. At the same time, portions of the nanofibers soften or melt, allowing CB and CNT to anchor onto the fiber surfaces and form a three-dimensional conductive network using the nanofibers as a scaffold. As shown in Figure 2d-d\", the PDA-modified TPU fibers show greater CB and CNT deposition compared to the pure TPU fibers in Figure 2c-c\" after ultrasonic treatment. As seen in Figure 2d\", the high aspect ratio of CNT enables it to interconnect with the zero-dimensional CB aggregates, forming a synergistic CB/CNT conductive network, which significantly enhances the conductivity performance of the sensor. The thermal stability of CB/CNT/TPU and CB/CNT/PDA/TPU samples was tested using thermogravimetric analysis (TGA). Figure 3a shows the TG curves of CB/CNT/TPU and CB/CNT/PDA/TPU nanofiber membranes. From Figure 3a, it can be observed that at a heating temperature of 800 \({}^{\circ}\)C, the mass loss of CB/CNT/TPU reaches 67.87%, while that of CB/CNT/PDA/TPU is 59.89%. For TPU and PDA/TPU, the mass loss at 800 \({}^{\circ}\)C is 90.41%. Therefore, the CB/CNT contents on CB/CNT/TPU and CB/CNT/PDA/TPU nanofibers  
Figure 2: SEM images of fiber membranes: (**a–a”**) TPU, (**b–b”**) PDA/TPU, (**c–c”**) CB/CNT/TPU, and (**d–d”**) CB/CNT/PDA/TPU. are calculated to be 22.54% and 30.52%, respectively. This indicates that PDA modification enhances the adhesion of conductive materials on the TPU fiber surface, resulting in more CB and CNT coating. The results are consistent with [PERSON] et al. 's findings, where increasing the CNT suspension concentration from 0.2 g/L to 1.0 g/L demonstrated that PDA/TPU exhibited a stronger capacity to bind conductive materials compared to pure TPU [27]. The thermal decomposition temperature of CB/CNT/PDA/TPU is 449.2 \({}^{\circ}\)C, slightly higher than the 427.7 \({}^{\circ}\)C observed for CB/CNT/TPU. Additionally, as shown in Figure 3b, the maximum weight loss rate of CB/CNT/PDA/TPU occurs at 571.3 \({}^{\circ}\)C, compared to 538.6 \({}^{\circ}\)C for CB/CNT/TPU. These results demonstrate that the thermal stability of the CB/CNT/PDA/TPU nanofiber membrane is significantly improved. Fourier Transform Infrared Spectroscopy (FTIR) was used to characterize the functional groups and intermolecular interactions between the phases in TPU nanofibers and their composites. Figure 3c shows the infrared spectra of TPU, PDA/TPU, and CB/CNT/PDA/TPU samples. Due to the polymer composition and structure of TPU nanofibers, the stretching and bending vibrations of polyurethane's N-H and -CH groups produce peaks at 3323 cm\({}^{-1}\), 2956 cm\({}^{-1}\), and 2852 cm\({}^{-1}\), respectively. The peak at 1076 cm\({}^{-1}\) corresponds to the C-O-C bond, while the peaks at 1728 cm\({}^{-1}\), 1700 cm\({}^{-1}\), and 1529 cm\({}^{-1}\) are associated with the -H-N-COO- group.