## 3 Flexible Substrates  
Thin glass, metal foils and plastics could be used to fabricate flexible electronics [70; 71]. However, only plastic foils permit a low cost, high-speed production over large areas by using various printing technologies in an R2R production line. Thin glass has an intrinsic fragile nature that limits its flexibility,  
Figure 5: Scheme of the most used printing techniques to prepare chemiresistive gas sensors on flexible substrates. while metal foils have good resistance to high temperatures, but are inadequate due to their surface roughness and high cost. Furthermore, recent advances have involved paper and textile as flexible substrates to lodge electronics and sensor devices. Until now, printed flexible chemiresistive gas sensors were prepared using plastic, paper and textile substrates. _Plastic substrates_--In general, the polymer substrates should mimic the properties of planar rigid substrates. Dimensional and thermal stability, low coefficient of thermal expansion, good solvent resistance and good barrier properties for moisture, air and gases are necessary for plastic substrates [69; 70; 71; 72]. In addition to physical, chemical, mechanical and optical performances, also the glass transition temperatures of different polymers have to be evaluated depending on the final application and the fabrication process involved. In a previous review [65] characteristic properties of most used polymers in flexible electronics were reported. To fabricate flexible gas sensors, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), polycarbonate are mainly chosen [73; 74; 75]. They are characterized by glass transition temperatures lower than 300 \({}^{\circ}\)C. Thermal treatments in fabrication steps could induce a substrate expansion damaging the device. Humidity affects the plastic resistivity, water absorption increases the substrate weight and alters the dielectric constant. To bypass this problem, a thin coating made of transparent oxides is applied on its surface acting as a barrier mainly for sensors used in food and medical packaging [65]. _Paper substrates_--Paper could be used as a flexible substrate with numerous advantages. It is easily accessible, eco-friendly, recyclable, light, thin, with low thermal expansion coefficient, roll to roll compatible and low cost. Its main drawbacks are the surface roughness and great absorption capacity. Paper substrates are usually coated with several materials such as kaolin, and polymers [76]. Different gas sensors printed on paper substrates were fabricated through inkjet printing by using both polymer and inorganic functional materials. _Textiles_--The first and most simple method to implement a flexible gas sensor on a textile is the sensor fabrication by using a polymer substrate after weaving on the textile. In [77] this method was applied using a cotton textile. Flexible printed gas sensors could be developed also by using textile as substrate. In the case reported in reference [49], textile (polypropylene (PP) spun-bonded non-woven) is the substrate on which the sensing layer was deposited. In other studies, through the functionalization of nylon or cotton-based yarns, sensitive textiles were obtained [78; 79]. However, in this configuration the yarn could be stressed during the weaving process. Regarding the influence of the substrate materials on the gas sensor performances, a useful comparative study has been carried out by [PERSON] et al. in [80]. They prepared VOCs sensors through inkjet printing depositing a carbon black paste as functional material on PET (polyethylene terephthalate), paper and cotton fabric substrates. About the ink printability, the authors observed that the sensing material impregnates the paper and the cotton fabric substrates merging ink and substrate into one. This is an ideal feature when the sensors have to be bended or mounted on planar structures.