## 2 Various Types of Flexible Sensors (Temperature, Pressure, Humidity and Chemical)
### Chemical Sensors  
Chemical sensors can detect a certain chemical element or compound (the analyte) within a sample which may be in gas or liquid form. In 2014, a research team developed an electrochemical ionic sensor based on single-walled carbon nanotubes (SWCNTs) with sodium n-dodecyl sulfate (SDS), an anionic surfactant [42]. The sensor has potential use in flexible electronics and the authors demonstrated its ease of fabrication via a simple inkjet printing of colloidal ninks on flexible material using a commercial desktop printer (Hewlett-Packard (HP) inkjet printer). High-accuracy gas detection has become an essential property for preventing environmental pollution caused by Nitrogen Dioxide (NO\({}_{2}\)), Ammonia (NH\({}_{3}\)), Carbon Monoxide (CO), Carbon Dioxide (CO\({}_{2}\)), fine dust, etc. This makes the monitoring of these gases essential for preventing pollutant emissions accompanying industrial and recycling processes. Recent reports have used transition metal dichalcogenides (TMDs) in flexible chemical sensors for increasing the sensitivity. TDMs examples include molybdenum disulfide (MoS\({}_{2}\)), MXenes (2D carbides/nitrides), molybdenum diselenide (MoSe\({}_{2}\)), tungsten disulfide (WS\({}_{2}\)), tungsten diselenide (WSe\({}_{2}\)), borophene (2D boron), silicene (2D silicon), germanene (2D germanium) and hexagonal boron nitride (h-BN) [43]. TDMs have a high surface-area-to-volume ratio which provides more reaction sites for the reaction with specific molecules [44]. In another study, MoS\({}_{2}\)-SWCNT nanosheets were used to fabricate a chemical sensor that can detect NH\({}_{3}\) and NO\({}_{2}\) gases due to the excellent gas adsorption property of SWCNTs [13]. The MoS\({}_{2}\) nanosheets were fabricated via a chemical vapor deposition (CVD) technique using a porphyrin-type organic promoter. The team reported that the sensors exhibited stable chemical sensing with low loss in sensitivity even after \(10^{5}\) bending cycles.  
In another report, a hydrogen gas flexible sensor was fabricated from (Tungsten trioxide) WO\({}_{3}\) nanowires grown on Kapton via an aerosol-assisted chemical vapour deposition method [45]. The sensors could measure various concentrations of hydrogen gas. Silver and silver-platinum were used to coat the sensors. The sensors were then subjected to  
Figure 4: A textile temperature sensor, metal rods are intertwined with the fabric, reprinted with permission from [39].  
bending cycles and it was found that the silver-platinum coating reduced the flexibility of the sensor, rendering it unusable, while the silver coating was successful after cyclic bending tests.  
In a review paper on recent research in flexible sensors based on nanomaterial that was published in 2020 [3], the uses of flexible sensors were explored including biomedicine, smart devices, environmental monitoring and automobile manufacturing. Practical examples of uses of flexible sensors were mentioned, including sensors for monitoring glucose [46] and pulse [47]. Table 1 summarises examples of temperature, pressure, humidity and chemical flexible sensors from the literature.