## 2 Materials and Methods
### Architecture of Piezoresistive Chest Band Based on Strain-Sensitive Textile
#### 2.1.1 Development of the Piezoresistive Sensor Band  
The breathing sensor consists of a piezoresistive sensing band that changes its electrical resistance when stretched or released, connected to an electronic conditioning and acquisition section. The piezoresistive sensing band is realized with an elastic band, including a custom piezoresistive strain sensor based on the EeonTex fabric (model LTT-SLPA-20K, Figure 1).  
The sensor comprises a layer of EeonTex conductive stretchable fabric having dimensions 7 \(\times\) 2 cm. Two flexible electrodes are realized on the layer's edges by covering two squares of 2 \(\times\) 0.5 cm with silver-loaded conductive glue (model 8331, manufactured by MG Chemicals), which fixes the piezoresistive layer and improves the overall conductivity [40]. Underneath the electrode areas, the piezoresistive layer is impregnated with LiquiWire(r) conductive paint to enhance its conductivity, thus obtaining two thin and flexible electrodes (Figure 1) [41]. This band is supposed to be worn by the user, exploiting the piezoresistive behavior of the EeonTex material (model LTT-SLPA-20K) to detect the abdominal and chest movements related to the respiratory activity. Highly flexible, thin stainless steel threads connect the sensor electrodes to the conditioning section, which are sown directly onto the elastic band. In particular, the thread's material is SUS316L steel, which is characterized by low resistance, good electrical conductivity, and extreme resistance to heat (up to 1300 \({}^{\circ}\)C) and corrosion, in addition to providing an electromagnetic shield.  
The EeonTex fabric is a bidirectionally stretchy nylon/spandex elastic fabric having a long-lasting conductive composition. This material is used for pressure and strain sensor applications. EeonTex fabric has a tunable surface resistivity from 10\({}^{4}\) to 10\({}^{7}\)\(\Omega\)/square (Ohm/square), a mass per unit area of 4.8 oz/sqyd, and a warp in the range 198-248 yams/inch with an 85% warp recovery, making it highly flexible. In particular, the selected model is the LTT-SLPA-20K, featuring 20 K\(\Omega\)/square surface resistivity, which decreases by 1/2 when stretched [42]. The material is washable, demonstrating no significant increase in resistivity after 30 wash cycles. The main specifications of the EeonTex LTT-SLPA fabric are:  
* Composition: nylon 72%/spandex 28%;
* Mass per unit area: 4.8 ounces/square yard;  
Figure 1: Structure of the developed piezoresistive band for RR detection.  
* Surface resistivity: 20 K\(\Omega\)/ square;
* Thickness: \(\sim\)15 mils (0.38 mm);
* Elongation: warp 198-248 yarns/inch, warp recovery 85%;
* Dynamic range: 5 g-150 kg.  
However, it must be mentioned that the EeonTex stretchable fabric has a reduced non-linearity of the resistance vs. strain characteristic (0.0479 RMSE/\(\overline{\text{R}}\)) and wide linearity strain range (20-70%), as demonstrated in [43]. In addition, the material presents a low hysteresis (at most 7%) and fast responsivity, making it suitable for dynamic movement tracking. Moreover, the EeonTex fabric decreases its resistance when subjected to several stretching and relaxation cycles [43]. Considering the performance above, the EeonTex fabric was deemed suitable for the presented application.  
The resulting sensing band is compact (2 cm width) and light (5 g weight), and requires no moving parts. In addition, its positioning only on the lower part of the chest makes it discreet and non-obstructive.