## 3 Results  
In this section, the hardware and firmware of the developed piezoresistive respiratory sensor were tested by six different patients having different genders, physical constitutions, and ages, as detailed in Section 4.1. For each user, the band was worn on the upper abdomen, and the elastic band was adjusted to ensure the correct fitting of the piezoresistive and inertial sensors on the body, depending on the physical constitution of the user (Figure 8). As soon as the band is worn, the firmware carries out an initial test to check the correct positioning of the chest band on the user's body while the patient is walking or sitting. Figure 7: Screenshots of the developed Android application used to visualize the data transmitted by the developed piezoresistive band: _Connection_ screen (**a**) and _Monitoring_ screen (**b**). Specifically, the algorithm verifies that the signal amplitude from the piezoresistive sensor has an adequate value, and the IMU signal does not show an excessive variance. This control mechanism enables the chest band to be worn by users of different constitutions and genders, while maintaining the device's performance. Specifically, referring to this preliminary step before actual use, the chest band was tested on the same six users of different genders (three males and three females), ages (25.5 \(\pm\) 2.9 years), and weights (73.0 \(\pm\) 7.3 kg) (detailed features are reported below in Section 4.1); a correct calibration was obtained in all cases. Different waveforms were acquired to test the correct firmware operation, directly verifying the corresponding feedback on the user's body. Figure 9 shows the waveforms acquired along the processing chain implemented by the developed chest band. The blue trace represents the raw voltage signal taken by the piezoresistive strain sensor, whereas the red trace is the signal filtered by the second-order Butterworth filter used to eliminate all the high-frequency components caused by external factors, such as electromagnetic noise or body movements as precordial motion. Notably, a periodic signal is obtained at the filter output, characterized by a frequency range from 0.2 to 0.6 Hz, due to the abdomen movements induced by the inflation and deflation of the lungs during the breathing cycle (Figure 9a) [54]. During the abdomen expansion due to air inspiration, the piezoresistive sensor undergoes tensile stress, leading to a reduction in its electric resistance and thus of the detected signal. Conversely, the signal increases during the expiration; due to the abdomen's shrinkage, the stress on the piezoresistive sensor is released, increasing the sensor resistance and thus the output signal (Figure 9a). In addition, the band was tested over light clothing (T-shirts), and no significant variation of the signal amplitude from the piezoresistive sensor was observed. In addition, since the peak detection algorithm is based on statistical data analysis, and evaluates the deviation in the points from the moving average, the processing chain is relatively insensitive to slight reductions in the signal amplitude, thereby ensuring the correct operation of the device. Finally, the green trace is the output of the peak detector algorithm operating according to the dispersion principle; specifically, it indicates the exhalation and inhalation instants used by the firmware to derive the respiratory parameters. The peak detection algorithm records each data point based on a set number of standard deviations from the moving average (called the z-score). Since this algorithm works on the moving average of the recorded data points, and detects the deviation of the signal from the moving average, it automatically adapts to changes in the amplitude of the acquired signals. This mechanism ensures the correct operation of the chest band for different levels of band tightness on the user's chest. The graph reveals the voltage fluctuations produced by the piezoresistive signal that are attributable to the expansion and contraction of the chest (Figure 9a).