## 3 Nanomaterials
### Carbon Nanomaterials and Others  
Carbon nanomaterials are the most used conductive materials in flexible sensor technology. Carbon nanomaterials have excellent conductive and mechanical properties and cost effectiveness but have poor transparency, which is one the of the main challenges in this research field. Another challenge in using carbon-based nanomaterials such as carbon nanotubes is that the particles tend to agglomerate due to the hydrophobic nature of carbon nanotubes and strong van der Waals forces between them. Agglomeration causes print head/cartridge nozzle clogging during inkjet printing of carbon inks. This can be counteracted by the use of organic solvents such as dimethylformamide or N-methyl2-pyrrolidone or the use of dispersants for nanotubes in water solutions. Carbon particles in water can also exhibit high surface tension leading to printing issues. This can be dealt with by the use of a surfactant [42]. Another carbon-based nanomaterial currently used in flexible sensor technology is graphene. Graphene is a semi-metal with zero bandwidth and is widely used as a sensing layer in flexible sensors. The resistance of graphene is inversely proportional to temperature changes, making it a good candidate for temperature sensors. Graphene oxide and its reduced version has been used as a sensing layer, with reduced graphene oxide having a higher conductivity and therefore a wider usage [49]. In one report, an electrochemical sensor was fabricated using single-walled carbon nanotubes (SWCNT) with sodium n-dodecyl sulfate (surfactant) [42]. The sensor was fabricated by inkjet printing the SWCNT ink using a Hewlett-Packard (HP) inkjet printer onto a thin film. The resulting sensor had a competitive sheet resistance of 132 \(\Omega\). The electrical properties of the sensor were characterised via cyclic voltammetry (CV) which involves the cycling of the electrodes voltage and measuring the output current. Another interesting research area is the use of flexible sensors on human skin. The main research challenge is that human skin does not perfectly differentiate between pressure and temperature stimuli under mixed stimulation. A research study by [PERSON] et al. overcame this limitation by developing a sensor that can not only sense both temperature and pressure at the same time but can also differentiate between them [50]. The reduced graphene oxide-based sensor exhibited linear sensitivity to both pressure and temperature variations. The sensor has a pressure sensitivity of 0.7 kPa\({}^{-1}\) up to 25 kPa and a temperature coefficient of resistance of 0.83% K\({}^{-1}\). The pressure/temperature sensor can sense temperatures in the range of 22-70\({}^{\circ}\) and has a competitive response time of 100 ms. Both pressure and temperature sensing correlated to resistance changes of the GO sheet. A highly sensitive and wearable temperature sensor was fabricated using graphene nanowalls combined with polydimethylsiloxane (PDMS) (see Figure 5a) [48]. Some researchers have incorporated nanoparticles of multiple types in polymer matrixes or on their surface to fabricate advanced materials for flexible sensors. For example, [PERSON] and his team [58] developed a strain sensor based on PDMS with silver nanoparticles and carbon nanotubes on its surface. The silver/carbon nanocolloid was drop cast onto the PDMS substrate and the resulting composite was dried to evaporate the liquid, resulting in a stretchable sensor that can attain various shapes, as shown in Figure 6b. The drop casting fabrication method is shown in Figure 6a. Recently, a sensor that is responsive to an external magnetic field was fabricated [60]. The sensor is composed of magnetic nanoparticles, Iron Oxide (Fe\({}_{3}\)O\({}_{4}\)), embedded into alginate and methylcellulose hydrogel. The hydrogel brings unique properties into the composite, including high stretchability, softness and biocompatibility, which enables the  
Figure 5: Flexible sensors based on nanomaterial and a polymer.