## 6 Conclusions  
Flexible electronics, nanomaterials and polymers are the basis of the future generation of sensors. Indeed, with the advent of flexible electronics, sensors have become low-cost, thin, with large sensing area, lightweight, wearable, flexible and transparent, and therefore the number of new produced devices have multiplied as well as their applications in many aspects of our daily life. The sensor demand, related to the IoT world, has stimulated the development of new sensor solutions as well as the mass production. In the coming years sensors will spread throughout the IoT world to monitor parameters related to human healthcare, to the environment, to machine operation, to food quality, to security, etc., and essential to have \"smart things\".  
Printing is the most used technique to produce flexible electronics and flexible sensors due to its great number of advantages, such as mass production, low costs and the opportunity to implement different functional materials in to the ink. Among the printing techniques, inkjet printing is the most used, while to realize mass production the new R2R approach is preferred and is rapidly growing. The integration of different fabrication techniques in a single production line comprising the control electronics, data processing and transmission will enable low-cost applications in the emerging scenarios. R2R ideally enables this integration, however additional efforts should be profuse to successfully achieve the target.  
In this review, the study of the current state of research and development of the printed chemiresistive gas sensors has confirmed the global trend regarding sensors and flexible sensors. Among the flexible substrates, polymer bases (PET, PI, etc.) are the most common, although there are some papers about gas sensors printed on paper or textiles. The choice of the substrate material is one of the main steps to fabricate a flexible sensor, because it has to be compatible with the device operational conditions and it affects directly the sensing performance. As for the rigid substrates, the functional materials range from metals, metals oxides, carbon nanotubes, graphene, graphene oxide, and conductive polymers. They are usually mixed to obtain better sensibility, selectivity, and reduced response and recovery times. A key factor to guarantee the durability and stability of the device is the coupling of the chemically selective layer to the physical part of the sensor, especially in developing flexible substrates-based gas sensors. A literature survey on the target gases related to the gas sensor applications has shown a great number of analytes correlated with many application fields. To date, printed flexible chemiresistive gas sensors covered only a few (H\({}_{2}\), H\({}_{2}\)O, H\({}_{2}\)S, NH\({}_{3}\), NO\({}_{2}\), CH\({}_{2}\)O, C\({}_{2}\)H\({}_{3}\)OH, C\({}_{6}\)H\({}_{3}\)O) of the many target gases reported above. Besides the improvement of sensitivity, selectivity, response and recovery times, the development of new printed flexible sensors to monitor further gases down to low concentrations is the future perspective. The first use of chemiresistive gas sensors was the detection of explosive gases. Today, besides its application in environmental monitoring and in the industrial area, the major attention is paid to the control of human analytes, in order to prevent medical diseases and ensure safe, security and wellbeing.  
Conceptualization, A.F.; writing--original draft preparation, A.F.; writing--review and editing, A.F. and M.C.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors acknowledge support by Consiglio Nazionale delle Ricerche, IAMOTER, Ferrara, Italy. The authors declare no conflict of interest.