## 4 Functional Materials  
For obtaining printed gas sensors, it is necessary to prepare an ink by adding to the functional material, in form of powder, an organic vehicle consisting of a mixture of rheological agents in volatile solvents. The amount and composition of the organic media make the ink printable and give to the films some electrical properties and the macroscopic appearance. The organic vehicle is a sacrificial ingredient of the ink that completely disappears during the thermal processes carried out onto the films. The composition of the organic vehicle is determined by the requested rheological properties of the ink suitable for the specific printing technique. Generally, the proper ink viscosity for printing on flexible substrates ranges from cP to few tens of P. Both the organic vehicle composition and the film temperature process are depending on the material substrates [65]. The sensing materials employed in the development of chemiresistive gas sensors (using a rigid or flexible substrate) are inorganic, organic and organic-inorganic composites. The detection mechanism in chemiresistive gas sensors is based on the variation of the electrical conductance as a result of surface chemical reactions with environmental gases. Among the inorganic materials, there are metals, metals oxides, carbon nanotubes, graphene and graphene oxide. The conductive polymers constitute the group of organic materials and the organic-inorganic composites are obtained by mixing inorganic and organic materials with the aim to improve the gas sensing performance of the device. Hereinafter, the above listed functional materials are discussed and examples of some flexible printed chemiresistive gas sensors are reported for each type of sensing material. _Noble metals_ (_gold_, _platinum_, _palladium_, _silver_, _medium_, _etc._)--Are well known because they are incorporated in metal oxides, in carbon nanotubes, in graphene and graphene oxide to enhance their gas sensing properties in terms of sensibility, selectivity, response and recovery times and for lowering the working temperature. Among the noble metals, only palladium could be individually used to prepare gas sensors because it offers good performance towards hydrogen. At the same time, it is singly used to prepare flexible printed hydrogen gas sensors [63] and as doped agent of SnO\({}_{2}\)[51]. _Metal oxides_--Among the great variety of materials which can be used to prepare a device able to detect a gaseous compound with optimal characteristics of sensitivity, selectivity and electrical stability, certainly the metal oxides have shown the desired properties for using them in real working conditions. They belong to the class of wide-gap semiconductor oxides which have become of widespread interest in gas sensing due to their peculiarity of modifying surface properties when interacting with reducing or oxidizing gases. Most of them are semiconductors of type \(n\), such as SnO\({}_{2}\), TiO\({}_{2}\), In\({}_{2}\)O\({}_{3}\), WO\({}_{3}\), ZnO, Fe\({}_{2}\)O\({}_{3}\), CuO, etc., and solid solutions of them, while noble metals or foreign ions are added as catalysts or conductivity modifiers, the working temperature ranging between 200 and 500 \({}^{\circ}\)C. Few are of \(p\) type, like NiO or LaFeO\({}_{3}\). A case in point is tin dioxide, the most widely used material for gas sensing. Indeed, it is able to sense a great variety of gases, both reducing and oxidizing. On the other hand, it fails in selectivity, reason why a lot of efforts have been addressed to improve the sensing and selectivity properties modifying the material with the addition in particular of noble metals (Pd, Pt, Au) enhancing the sensitivity toward different gases, specifically methane, carbon monoxide and benzene. For all gas sensors based on metal oxides, it is of paramount importance the grain size reduction, which, leading to an enhancement in the surface-volume ratio, has opened the way to further improvements toward the sub-ppm gas detection.