## 3 Results  
### Engineering Mechanism of the Intelligent Respiratory Monitoring System  
As shown in Figure 1a, the typical characteristic of physiological respiration is the cycle of inhalation and exhalation of air, which directly reflects respiratory status. Our main idea was to prepare an intelligent respiratory monitoring system which included a portable respiratory sensor to monitor the cycle of air inhalation and exhalation; a circuit board for real-time data acquisition, preprocessing, and wireless transmission; and a machine learning algorithm to enhance the recognition accuracy of respiratory behaviors. Finally, all test data were to be processed and presented in the user interfaces of various terminals to display real-time respiratory results and possible health-status alarms. Based on the above considerations, a respiratory sensor combined with a daily mask is an effective tool for daily respiratory monitoring. However, disposable masks are limited in their use time, and the average person cannot wear a mask for more than 8 h. In the COVID-19 pandemic environment, doctors suggested that the cumulative use time of a mask should not exceed 4 h and insisted that it should not be reused. Therefore, the wearing experience and production cost of the respiratory sensor are important factors that must be considered. As shown in Figure 1b,c, a triboelectric respiratory sensor with a single electrode mode was utilized and fabricated by directly screen-printing biocompatible nylon and Ag inks on a daily mask. In the commercial production process of daily masks, screen printing is a mature customized processing technology, and this method is commonly used to print logos, models, and patterns on clothes. The nylon/Ag/nylon inks were layer-by-layer screen-printed onto the mask (Figure S1), then the PTFE film was tailored and fixed above the nylon layer with a small curvature. The thickness of the upper PTFE layer was 20 \(\upmu\)m, and the maximum distance between the PTFE and nylon was 1 mm. A surface SEM image of the nylon and Ag ink layers is shown in Figure S2. During the exhalation and inhalation process, the airflow pushes the inner PTFE film so that it contacts or separates from the printed nylon layer. Based on the coupled effect of triboelectrification and electrostatic induction, this triboelectric respiratory sensor can directly sense the driving force of the respiratory airflow, which can provide intuitive respiratory information, with high-fidelity data, non-invasive acquisition, and a non-irritating interface. Figures S3 and S4 illustrate the thickness and the weight of our printed triboelectric respiratory sensor, which are only 50 \(\upmu\)m and 0.05 g, respectively. The thicknesses of the top nylon layer, the Ag electrode, the bottom nylon layer, and the cotton textile are 23, 12, 16, and 500 \(\upmu\)m, respectively. In practical use, with the merits of light weight, superior flexibility, and high integration with the mask, the triboelectric respiratory sensor can fit comfortably on the user's face (Figure 1d) and is fixed near the respiratory valve of the mask, so that it will not affect the filtering ability of the mask. The costs of each of the materials used in the 4 \(\times\) 4 cmrespiratory sensor are listed in Table S1, and the total cost of replacing the respiratory monitoring mask is only 0.377 dollars, meaning that it does not carry an economic burden. The output signal of the triboelectric respiratory sensor is recorded by a self-built portable data-acquisition circuit board, as shown in Figures 1e and S5. This circuit board can be reused, and only the low-cost respiratory sensing mask would need to be replaced in long-term use, which significantly reduces the cost of the whole system. A logical flow chart for this wireless respiratory system is shown in Figure 1f.