## 3 Nanomaterials  
Nanomaterials have gained much attention in the flexible sensor industry. There exists two main ways of fabricating flexible sensors with nanomaterials incorporated. One of the ways involves the inclusion of nanoparticles/nanorods/nanowires within a polymer matrix. The polymer provides flexibility while the nanomaterial provides conductivity or a sensing property. The advantage of this strategy is that multiple types of nanomaterials can be included within the polymer matrix which enables advanced sensing properties, innovations and high sensitivity [53; 54]. The second way involves the incorporation of a low Young's modulus laminar conductive material on a flexible substrate. The nano-sized laminar has better conductive properties than nanoparticles/nanorods/nanowires due to the lack of insulating material within the laminar structure (increased surface area without polymer matrix in contact).  
Nanomaterials have exceptional electrical properties compared to their bulk counterparts [55; 56]. This makes them a prime candidate in flexible sensor technology whereby the device's weight and volume must be minimised while its electrical properties and bendability needs to be maximised. Nanomaterials are often incorporated within the  
\begin{table}
\begin{tabular}{c c c} \hline
**Sensor Type** & **Materials** & **Potential Applications** \\ \hline \multirow{8}{*}{Temperature} & 1. PDMS and graphene nanowalls (GNWs) & Monitoring body temperature. [48] \\  & 2. Cellulose and graphene oxide & Electronics. [49] \\  & 3. PDMS and graphene oxide & Electronic skin. [50] \\  & 4. Parylene and silver nanoparticles & Environmental sensing. [14] \\  & 5. Kapton and silver nanoparticles & Monitoring body temperature. [41] \\  & 6. PDMS, chromel and alumel & Microactuators. [33] \\  & 7. PEDOT:PSS and carbon nanoparticles & Skin temperature sensing. [36] \\  & 8. PEDOT:PSS, graphene oxide and silver & Robotics. [38] \\  & 9. Polypropylene and graphene & Clothing. [40] \\ \hline \multirow{8}{*}{Pressure} & 1. PDMS and graphene oxide & Electronic skin. [50] \\  & 2. Cellulose and MXene & Wearables. [20] \\  & 3. Silicon and AlGaN/GaN & Wearables. [27] \\  & 4. Silicon nitride and graphene oxide & Wearables. [28] \\  & 5. Tissue paper, PDMS and Au nanorods & Wearables. [12] \\  & 6. Silicon and PDMS & Electronic skin. [29] \\  & 7. Arialid Paper (AP) and Carbon Black & Healthcare/wearables. [47] \\  & 8. PEDOT:PSS and PDMS & Wearables. [51] \\  & 9. Silk and graphene & Clothing/skin sensing. [37] \\ \hline \multirow{8}{*}{Humidity} & 1. PDMS, ZnO and graphene oxide & Flexible electronics. [8] \\  & 2. Parylene and silver nanoparticles & Environmental sensing. [14] \\  & 3. Fabric and graphene oxide & Respiration Monitoring. [15] \\  & 4. PET, Au nanoparticles and graphene oxide & Environmental sensing. [52] \\ \hline \multirow{4}{*}{Chemical} & 1. Sodium n-dodecyl sulfate and SWCNTs & Electrochemical sensing. [42] \\  & 2. MoS\({}_{2}\) and SWCNTs & NH\({}_{3}\) and NO\({}_{2}\) gas sensing. [13] \\ \cline{1-1}  & 3. Kapton and Ag/Pt and WO\({}_{3}\) nanowires & H\({}_{2}\) gas sensing. [45] \\ \hline \end{tabular}
\end{table}
Table 1: Examples of types of flexible sensors (temperature, pressure, humidity and chemical), the materials used in their fabrication and their potential uses.  
flexible substrate matrix to induce electrical properties into the device. Nanomaterials of carbon, copper, iron, gold, magnesium and others have been used in the fabrication of flexible sensors. The market share of conductive inks based on nanomaterials has risen in the last decade and is predicted to continue rising. The report here [57] shows the market share of conductive inks in China alone; carbon-based inks are the most used followed by silver-based nanoinks, mainly due to the lower costs, attractive electrical properties and biocompatibility.