## 3 Results
### Working Mechanism and Output Performance of the Triboelectric Respiratory Sensor  
Figure 2a shows the sensing mechanism of our triboelectric respiratory sensor [23, 24, 25]. The working process of the triboelectric respiratory sensor can be described as follows. First, in the original state, the electrification process occurs when the PTFE film contacts the nylon layer, and the same numbers of opposite-polarity charges are generated between the contact interfaces. As the most triboelectrically negative material, PTFE tends to gain electrons compared with nylon. Then, in the inhaling process, the PTFE film gradually separates from the nylon, and the potential difference increases, resulting in the instantaneous flow of electrons from the ground to the Ag electrode. In the exhalation process, the PTFE film approaches the nylon layer, and the potential difference decreases, leading to the electrons' instantaneous flow back to the ground until the two materials come into full contact. During periodic respiration, the PTFE film performs a periodic contact-separation process to generate an instantaneous AC current through an external circuit. To understand the potential distribution of the respiratory sensor more quantitatively, finite-element analysis was carried out using COMSOL software, as shown in Figure 2b. The potential balance is formed when the PTFE film is in full contact with the printed nylon layer. When the PTFE film is far away from the nylon layer, a potential difference is formed between the PTFE film with positive potential and the nylon layer with negative potential. The results of the finite-element analysis confirmed the mechanistic analysis.  
To accurately evaluate the respiratory-sensing performance, a linear motor was used to drive the PTFE film to move reciprocally and simulate the respiratory process. As shown in Figure 2c, with an area of 4 \(\times\) 4 cm\({}^{2}\), a frequency of 0.5 Hz, and a constant force of 1 N, our triboelectric respiratory sensor can output an open-circuit voltage (\(V_{\alpha\alpha}\)) of 8.3 V, a short-circuit current (\(I_{\infty}\)) of 17.6 nA, and a short-circuit charge (\(Q_{\infty}\)) of 2.5 nC. For the 0.5 Hz \(V_{\alpha\alpha}\) signal, the response and recovery times of the respiratory sensor to the airflow are 0.25 s and 0.97 s, respectively, in the exhalation and inhalation process (Figure S6).  
The electrical output of this triboelectric respiratory sensor is determined by the respiratory status, including two main factors, namely, respiratory intensity and respiratory frequency. Respiratory pressure and frequency in humans are generally lower than 1000 Pa and 3 Hz, respectively [13]. Firstly, Figure 2d shows that the output voltage of the triboelectric respiratory sensor increases with increasing pressure from 60 Pa to 3000 Pa and that the output voltage presents an approximately linear increase in the range of 60-1200 Pa, with a sensitivity of 0.0079 V/Pa. Secondly, as shown in Figures 2e and S7, with an operating frequency from 0.5 Hz to 3 Hz, the triboelectric respiratory sensor exhibits well-defined frequency characteristics, while the output voltage shows an indistinct difference and remains stable at 8.3 V.  
Moreover, the electrical output can be easily regulated by the device area (Figure 2f). In particular, for patients with dyspnea or respiratory failure, the sensing area can be appropriately increased to enhance the signal-to-noise ratio to obtain clearer respiratory signals by means of the customization of screen-printing methods. Change in temperature has no effect on the respiratory sensor, which ensures the ability of the sensor to continue to be used in a changing environment (Figure S8). Durability is another essential factor for real-time respiratory monitoring; as shown in Figure 2g, our triboelectric respiratory sensor works stably after more than 10,000 continuous contact-separation cycles. All these excellent characteristics make this triboelectric respiratory sensor ideal for continuously monitoring daily respiratory status.