## 2 Various Types of Flexible Sensors (Temperature, Pressure, Humidity and Chemical)  
### Overview of Current Materials and Fabrication Methods  
Up until the last decade, single-crystal silicon has been the most used substrate in sensor development owing to its distinct advantages including high sensitivity, low power  
Figure 1: A picture of a collection of types of flexible sensors made from a flexible polymer and nanomaterials as the sensing layer. (**a**) Flexible pressure sensor with Au nanorods intertwined with tissue paper and deposited on PDMS polymer, reprinted with permission from [12]. (**b**) Hydrogen gas sensor with carbon/MoS\({}_{2}\) nanomaterials on PET substrate, reprinted with permission from [13]. (**c**) Silver nanoparticles inkjet printed on paper to fabricate a flexible temperature sensor, reprinted with permission from [14]. (**d**) Graphene Oxide deposited on fabric for humidity sensing, reprinted with permission from [15]. Figure 2: Demonstration of everyday uses of flexible sensors. Flexible sensors coupled with the internet allows for the monitoring of things such as heart rate at any location and time. Reprinted with permission from [10]. losses and low cost. Although these rigid silicon sensors have been extensively used, they have certain drawbacks including high stiffness (brittleness), high fabrication cost, high operation power and susceptibility to breakage. Due to the aforementioned drawbacks, flexible sensors have gained much attention in literature and industry [16]. The materials for flexible sensors are processed via various methods depending on the required resolution, size of the sensor and cost. For instance, aerosol jet printing can be used for developing nano/micro sensors at a very high resolution, but the method is highly expensive in comparison to inkjet printing which can also print the same sensors with a lower resolution. Aerosol jet printing is a 3D printing method that involves using metal/non-metal nanoparticle-based inks to print conductive patterns on various substrates, including flexible substrates. The nanoinks are atomised in a chamber and accelerated in a gas (usually nitrogen gas). The ink particles are focused onto the substrate by a nozzle at high speeds to enable a strong attachment of ink particles and substrate. Inkjet printing involves two types of drop-on-demand methods, piezoelectric and thermal. This method has advantages over other printing methods including the lack of warmup time, improved picture quality, good resolution, low cost of printers and ease of implementation. When selecting a fabrication method for flexible sensors, it boils down to a balance among cost, resolution and how easy it is to implement the method. Other methods for the development of micro-sensors include photolithography [17], screen printing [18] and laser cutting [19]; these are discussed in detail in Section 6. In a report by [PERSON] et al., flexible Hall sensors based on graphene nanoparticles and Kapton were developed via photolithography [17]. These sensors were calibrated with a commercial Hall sensor (Allegro MicroSystems A1324) to give normalised sensitivity values. The graphene sensors were found to have sensitivity values of orders of magnitudes higher than the commercial sensor. The sensors have a thickness of 50 \(\upmu\)m, a minimum bending radius of 4 mm and a voltage and current normalised sensitivity of up to 0.093 VVT\({}^{-1}\) and 75 VAT\({}^{-1}\), respectively. These values of sensitivity are comparable to a silicon-based rigid sensor which have voltage and current sensitivities of 0.1 VVT\({}^{-1}\) and 100 VAT\({}^{-1}\) respectively. No degradation of the flexible sensors was found after 1000 bending cycles with bending radius of 5 mm, making the sensors compatible in uses such as biomedicine and wearable electronics, where the precise monitoring of current and position are imperative. When fabricating wearable devices, strain, pressure and temperature sensors are among the most important components.