Fig. 2a shows the morphology of the AgNWs-coated fabric observed from SEM images. It can be seen that the average diameter of AgNWs is about 55 nm. The density of the AgNWs on the surface of the fabrics was adjusted by the times of dip-coating cycles. The SEM images showed that the AgNWs were distributed on the surface of the individual fibers (Fig. 2b). With increasing dip-coating cycles, the density of AgNWs coated on the fibers is higher. Meanwhile, the nanowire mesh spacings was decreased from 2–4 µm to below 1 µm accordingly. For the fibers with 5 dip-coating cycles, the AgNWs conductive networks were completely coated on the fibers.
The composition of the AgNWs-coated fabric was also investigated by the EDS, as illustrated in the inset of Fig. 2b. In addition to the C and O contents which mainly attributed from cotton, Ag element was also observed, indicating the distribution of AgNWs on the cotton.
The sensing principle of the pressure sensor is shown in Fig. 3a. In the unloading state, the initial resistance is large, which is caused by the non-contact AgNWs on the fabrics. Once pressure was applied, the increasing fiber-scale contacts of nanowires on the adjacent fabrics contributed to a decrease in the resistance. Furthermore, when the nanowires on the fabrics were contacted completely, the continued pressure loaded on the fibers then increased the yarn-scale contacts. With loading pressure increasing, fabric-scale contacts further reduced the resistance of the sensor due to the increased contact area between the facing fabrics. Therefore, the pressure sensing of the sensors was determined by the synergistic effect of the fiber/yarn/fabric multi-scale contact.
Figure 3b shows the current-voltage curves of the pressure sensor under different pressures. When the applied pressure increased from 0 kPa to 100 kPa, the resistance of the sensor was decreased. Furthermore, the response of sensor was steady and fell in line under Ohm’s law.27 The current of pressure sensor is shown in Fig. 3c, which is relatively constant under different applied pressure, revealing that the response of the sensor is stable for different pressures. Therefore, the results provide the excellent electrical stability for the potential application of the pressure sensor.
To investigate the performance of pressure sensors, the relative current changes (ΔI/I0) versus pressure with the different AgNWs dip-coating cycles and mesh hole diameters were shown in Fig. 3d and e. Here, the sensitivity of the pressure sensor was defined as S = (ΔI/I0)/P, where P denotes the applied pressure. At a mesh hole diameter of 0.25 mm, the sensitivity of the pressure sensor was strongly dependent on the AgNWs dip-coating cycles. The sensitivity of the sensors was improved from 2.12 × 103 kPa− 1 to 1.98 × 105 kPa− 1 within the range of 0–10 kPa when the dip-coating cycles increased from 1 to 5. In addition, the sensitivity improved from 764 to 1.12 × 103 kPa‒1 at 10–100 kPa. The improvement of sensitivity with high dip-coating cycles is mainly attributed to the increase of the AgNWs densities.
Furthermore, the dependence of hole diameter was subsequently characterized. The pressure sensors with 5 dip-coating cycles exhibited enhancing sensitivities with increasing diameters, which were increased from 1.12 × 103, 9.88 × 103, to 2.16 × 104 kPa‒1 within the pressure range of 10–100 kPa, respectively. The enhancement in the sensitivity was mainly attributed to the increased contact area through the larger holes. Notably, the pressure sensor clearly exhibited two linear current segments, the sensitivity increases sharply in the low-pressure range and increases gradually in the high-pressure range. According to the sensing principle as we mentioned above, in the low-pressure range, the adjacent AgNWs contact plays an important role in increased current. Furthermore, when the pressure is increased to 10–15 kPa, the AgNWs on the interface were contact completely. The current changes were mainly determined by the contact resistance between the yarns and fabrics, which was relatively stable. Contact between the yarn- and fabric-scales played a more significant role in the sensing mechanism at a dip-coating cycles of 5 times and diameter of 0.75 mm by enhancing the sensitivity and linear range. Therefore, the diameter of 0.75 mm (Fig. 3e) exhibited a higher sensitivity and larger linear range due to increased contact.28,29
The dynamic response of the device was studied under loading/unloading pressure cycles. The sensor exhibited an immediate response to the cyclic pressures. The time-resolved response was analyzed to quantify the response and relaxation times (Fig. 4a). The measured response and relaxation times were 32 and 24 ms, respectively. The ΔI/I0 with an applied pressure of 10 kPa with 1,000 loading cycles was used to verify the repeatability of the device (Fig. 4b). The results show the excellent stability of the pressure sensors.
Due to the natural flexibility of fabrics and high sensitivity of sensors, the pressure sensor was wearable and able to detect mechanical signals such as physiological pulse and respiratory rate. First, the device was attached at the wrist using an adhesive bandage to monitor the pulse pressure. Figure 4c presents the real-time recorded data, in which the pulse rates were measured to be ≈ 72 beats min‒1. In addition, the sensor was also attached on a mask to detect respiration states. Figure 4d indicates that the normal respiratory rate of 10 breaths per minute of an adult and a square-like wave for normal breathing. Furthermore, the width of the waveband indicated the maintained time of breathing. These results suggest that the pressure sensor with high sensitivity and superiority has great potential in wearable healthcare device applications.