## 2 Printing Techniques  
Printing is the most used technique to produce flexible electronics and flexible sensors due to its great number of advantages. It is a bottom-up process, where layers are added one by one: both electrodes and also different functional materials can be deposited with the same technique even on the same substrate. The intrinsic decoupling between the ink preparation (starting from the synthesis of functional material) and the film deposition allow the optimization of the whole procedure in terms of simplification, wastage and costs reduction. Printing permits the large-scale fabrication with low cost production. The use of this technique on a flexible organic substrate makes possible the deposition of a pattern/layer also on non-planar surfaces and on areas larger than that the conventional rigid substrates and at temperatures suitable for organic materials. The number of fabrication steps in printing techniques is lower than that for standard microfabrication technology. It could be performed in ambient condition by using ecofriendly and not hazardous starting materials [65, 66]. Printing techniques can be grouped in two main classes depending on the presence of a physical contact or non-contact between the ink and the substrate during deposition. In the first class can be included gravure printing, nano-imprinting, flexographic printing, and transfer printing, while screen printing, inkjet printing, etc. are belong to non-contact method. Detailed description of all these processes is given in a comprehensive review [65], while a summary of the more advantageous single techniques to prepare chemiresistive gas sensors is reported below [67]. _Nano-imprinting_--Involves different steps and allows to have printed layers with thickness among 1 and 20 \(\upmu\)m. A rigid substrate lodges a continuous layer that is subsequently patterned pressing with a mold. The patterned layer is demolded, thermally treated, and then transferred on the flexible substrate [65]. Until now, only one chemiresistive flexible gas sensor was prepared by nanoimprinting. It is a palladium film which was transferred into a polycarbonate film to reveal hydrogen [63]. _Gravre printing_--A simplest gravure printer is composed by a rotating printing cylinder with the printing pattern incised on its surface. The ink is released using a nozzle on the top of the cylinder while a doctor blade removes the ink excess before the deposition on the moving substrate. The printed film quality depends on the pattern and on the ink viscosity. This method allows printing speeds in the range of 8-100 m/min, greater than that of the here considered techniques [65]. As shown in the Table 1 few chemiresistive flexible gas sensors are fabricated by gravure printing [59, 60, 61, 62]. _Screen printing_--Is the most used technique to fabricate thick film chemiresistive gas sensors on ceramic (such as alumina) substrate [65, 66, 67, 68]. In the case of flexible chemoresistive gas sensors, inkjet printing is preferred in terms of R2R implementation, reliability and no waste production. However, screen printing is still widely used for electrodes fabrication. The fundamental tool is the screen that is made of a mesh (e.g., polymer or aluminum threads) mounted on a frame under tension. Finer and smaller openings of the mesh are needed to print a pattern with higher degree of detail. The screen is placed above a substrate. Ink, located on top of the screen, is pressed by a squeegee through the holes of the mesh. The ink is deposited on the substrate in a controlled amount, proportional to the thickness of the mesh. The thickness of the printed layer ranges from 5 to 30 \(\upmu\)m. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the substrate (named snap-off), leaving the ink upon the substrate surface. An example of TiO\({}_{2}\) based flexible gas sensor made by using screen printing on PET substrate is reported in the reference [57].