## 3 Results and Discussion  
Figure 1b shows the screen-printing process for the fabrication of the CB/AgNP-based strain sensor on the PET substrate. After the CB/AgNP paste penetrated the substrate through the opening mesh owing to the pressure of the squeeze, the screen mesh was snapped off the substrate by the tension, resulting in the specific pattern of the paste. The rheological property of the CB/AgNP paste and the related pattern fidelity should be considered to obtain the desired sensor. Figure 2 shows the rheological behavior of the CB/AgNP paste using a cone-plate rheometer to determine the effect of the solid content of CB on the pattern quality. The ratios of CB/AgNP were 9:1, 5:5, and 1:9 for CB/AgNP-L, CB/AgNP-M, and CB/AgNP-H, respectively. The measured viscosities at different shear rates from the steady-state flow test shown in Figure 2a reveal that all the three pastes exhibit shear-thinning behavior. The viscosity of the paste ink decreased as the shear rate increased from 1.4 to 0.4 Pa.s, 130.4 to 1.0 Pa.s, and 486.0 to 0.4 Pa.s for CB/AgNP-L, CB/AgNP-M, and CB/AgNP-H, respectively. This indicates the suitability of all the formulated inks for the screen-printing process. In addition, the CB/AgNP paste ink with a higher solid content of CB exhibited a higher viscosity at zero shear viscosity and above the shear rate, which indicates that the concentration of CB affects processability during the screen-printing process. Subsequently, a peak hold step (PHS) test involving holding the sample at different shear rates at three intervals, namely, the reference interval (0-30 s), high-shear interval (30-60 s), and regeneration interval (60-150 s), was conducted as shown in Figure 2b. This process mimics the actual screen-printing process, and the applied stress value varies according to each time interval, which elucidates the paste flow with a viscoelastic feature. The results show the general flow behavior of thixotropy, where a structural transition occurs due to mechanical pressure (through squeezing in this process). In addition, the recovery value of the three pastes to determine the pattern  
Figure 1: (**a**) The schematic diagram of printing process and (**b**) overall working principle with CB/AgNPs for wearable strain sensor. fidelity was proportional to the CB concentration; the recovery values were in the order of CB/AgNP-H (3.5%), CB/AgNP-M (17.5%), and CB/AgNP-L (31.5%). These values are listed in Table 1. Figure 2c indicates that the result of the PHS test was significantly related to the pattern fidelity according to the recovery behavior. As the recovery value increased in the order of CB/AgNP-L, CB/AgNP-M, and CB/AgNP-H at the same time interval, the dimensions of the pattern were closer to the desired dimensions, and the line edge sharpness increased. However, the recovery time (leveling time) is also an important factor in printability, as the abrupt recovery of CB/AgNP-H did not yield the best quality, as shown in Figure 2c. In this case, the excessively short recovery time after printing was not suitable for the full leveling of the paste, resulting in an empty spot during evaporation. Moreover, trapping of the paste into the mesh becomes likelier due to the excessively high viscosity. Thus, CB/AgNP-M was chosen as the optimized paste. Six strain sensors with various line widths (100 and 300 \(\upmu\)m) and lengths (25, 52, and 78 mm) at a fixed thickness of 110 \(\upmu\)m were fabricated, as shown in Figure 3a, to investigate the effect of pattern dimensions on the sensitivity. As the resistive-type sensor consists of electrodes, the electrical performance is important to attain a high sensitivity. Figure 3b reveals the _I-V_ curves of the samples at a gradient voltage from 1 to 5 V, where the current output changes in accordance with the increase in the applied voltage. Based on the linear slope of each line, the six different strain sensors showed a uniform conductivity.