## 3 Results
### Electrical Properties  
To investigate the effect of the mass ratio of CB and CNT in the conductive filler on the sensing performance, nanofiber membranes with different CB/CNT ratios were fabricated. As shown in Figure 5a, under the same strain, the slope of the curve becomes steeper with an increasing CB content, while it becomes gentler as the CNT content increases. This is because zero-dimensional CB particles tend to aggregate, while CNTs with a high aspect ratio are more likely to entangle with each other. As a result, under the same strain, nanofiber membranes with higher CB content exhibit greater relative resistance changes compared to those with higher CNT content, leading to higher sensitivity. When the CB/CNT ratio is 9:1, the tensile properties of the nanofiber membrane decrease, and the working range of the flexible sensor becomes smaller. At a strain of 250%, the gauge factors (GF) for CB/CNT ratios of 7:3, 5:5, 3:7, and 1:9 were 1063, 334.3, 167, and 98, respectively, indicating that the sensor sensitivity can be controlled by adjusting the mass ratio of CB/CNT in the conductive filler. Among these, the nanofiber membrane with a CB/CNT ratio of 7:3 better meets the comprehensive performance requirements of high sensitivity and a wide strain range. As shown in Figure 5b, the strain sensitivity of the CB/CNT 7:3 nanofiber membrane increases with increasing strain. The strain sensitivity curve for the flexible strain sensor was divided into three stages (0-100%, 100-220%, 220-300%) through linear fitting. In the strain range of 0-100%, the GF is 141; in the range of 100-220%, the GF is 312.9; and in the range of 220-300%, the GF reaches 1063. As strain is applied to the nanofiber membrane, the distance between the mixed conductive fillers increases. Larger strains cause more significant damage to the conductive network, leading to a larger GF value. Figure 5e shows that the CB/CNT/PDA/TPU strain sensor exhibits a fast strain response, with response and recovery times around 137 ms. In Figure 5d, it is evident that most of the flexible strain sensors reported in the literature do not simultaneously achieve  
Figure 4: (**a**–**c**) Thickness and images of CB/CNT/PDA/TPU under twisting and bending. The thickness of the sensor is 0.22 mm; (**d**,**e**) Stress–strain curves, and bar charts of tensile strength and elongation at break for TPU, PDA/TPU, CB/CNT/TPU, and CB/CNT/PDA/TPU. a wide detection range and high sensitivity. However, the CB/CNT/PDA/TPU flexible strain sensor fabricated in this study offers both a wide detection range (0-300%) and high sensitivity (GF = 1063). To verify the stability and long-term durability of the flexible strain sensor, the sensing performance of the CB/CNT (7:3) flexible strain sensor under different strain conditions was tested. As shown in Figure 5c, the sensor's performance was evaluated under stepwise cyclic stretching at strains of 50%, 150%, and 250%. The experimental results indicate that during stepwise cyclic stretching, the change in \(\Delta\)R/R follows the cyclical strain variations, showing excellent repeatability and stability. Figure 6a,b demonstrate that the CB/CNT/PDA/TPU flexible strain sensor exhibits reproducible and stable cyclic strain sensing behavior at different strains. It can accurately detect stable response signals even at a small strain of 1%, indicating that the CB/CNT/PDA/TPU flexible strain sensor has an ultra-low detection limit. From 1% small strain to 150% large strain, the CB/CNT/PDA/TPU flexible strain sensor can detect periodic signals, making it suitable  
Figure 5: **(a)** Relative resistance change in mixed conductive fillers with different mass ratios during tensile process; **(b)** Relative resistance change in CB/CNT 7:3; **(c)** Response and recovery curves at 1% strain; **(d)** Stepwise cyclic response of CB/CNT 7:3; **(e)** Comparison of GF and working range of the sensor prepared in this work with those in the literature [18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. for detecting human body movements in various scenarios.