## 1 Toward Flexible Gas Sensors Era
Figure 2 shows the trend of publications number focused on gas sensors obtained by using Scopus database [2]. In 1815, [PERSON] developed the first indispensable gas detector (Davy's lamp) for coalminers against methane [18]; in 1926, [PERSON] produced the first commercial catalytic combustion gas detector [19]. Afterwards, only few papers can be found until 1970. The first significant studies related to the development of gas sensors started at the beginning of the seventies, they rapidly increased since 2002 reaching today more than two-thousand documents per year. In the same way, also the gas sensor market is continuously increasing. Recent published reports stated that the gas sensor global market isn't just growing, but it is accelerating. Indeed, it is  
Figure 1: Number of publications per year with titles, abstract or keywords including the term “sensor” (Source: Scopus [2]). expected to grow at a Compound Annual Growth Rate (CAGR) of approximately 7% during the period 2017-2023 [20] until reaching 3 billion dollars in 2027 [21]. A gas sensor is a complex system consisting of different elements (generally a sensing element, a substrate, electronics and case) chosen according to the final application and each one responsible for the good operation of the device. With this in mind, the suitable sensing mechanism, the appropriate substrate and the most convenient and compatible fabrication method have to be selected. Figure 3 summarizes some sensing mechanisms, substrates and fabrication methods, typically used. The most common sensing mechanisms are based on:  
* A non-Nernstian potential is caused at each electrode/electrolyte/gas interface by differences in the redox kinetics of various gases (mixed-potential sensors among the electrochemical sensors) [22; 23];
* A variation of sensing layer conductance proportional to the concentration of the target gas (chemiresistive gas sensors) [24];
* A variation of the capacitance of the sensing element proportionally to the concentration of the target gas when an optimized signal frequency is applied (capacitive gas sensors) [25];
* A variation of the source-drain current as a function of the concentration of the target gas (field-effect transistor-based gas sensors) [26];
* A variation in terms of amplitude or frequency of a wave propagating on the surface of the sensing layer due to the presence of the target gas (mechanical gas sensors) [27]; the measurement of the optical absorption at specific wavelengths depending on each gas (optical gas sensors) [28; 29]. To produce gas sensors, many fabrication methods are available, among them self-assembly [30], chemical vapor deposition (CVD) [31], physical vapor deposition (FVD) [32], micromachining [33], printing [34], and coating [35]. In some cases, the same methods are used to develop also sensor electronics, for example by means of printing [36]. Substrates can be subdivided in two main classes: traditional rigid substrates such as silicon, silicon carbide or ceramics (Al\({}_{2}\)O\({}_{3}\), ZrO\({}_{2}\)) [37] and recent flexible substrates, generally based on organic materials (paper, polymers and textiles) [38; 39]. Flexible substrates have immediately attracted the attention of research and industry due to their suitability to be used in wearable and flexible electronic devices, in RFID smart tags, and in general to be easily implemented in smart objects meeting the IoT requirements. Flexible substrates have been also used in the area of gas sensors, because they have been found to offer more opportunities in terms of new possible applications. Simultaneously, the number of researches devoted to substrate  
Figure 2: Number of publications per year with titles, abstract or keywords including the terms “gas sensor” (Source: Scopus [2]). characterizations and its coupling with the sensing element has grown, leading to the development of new functional materials and to optimization of the fabrication techniques.