## 2 Flexible Electronics Based on GnPs
### GnPs Functionalized Substrate  
Another promising approach for flexible electronics is the functionalization of bendable substrates with GnPs-based conductive ink. [PERSON] et al. [54] fabricated temperature-dependent resistors by simple drop-casting of conductive GnPs suspensions on polyimide. Such temperature sensors were stable at high relative humidity conditions, and performed more efficiently compared to carbon nanotubes  
Figure 3: Self-standing GnPs paper and its flexibility. (**a**,**b**) are photographs of the paper before and after folding. (**c**,**d**) are SEM images of the morphology of the surface of the GnPs paper (plane and at the folding edge, respectively). Reprinted with permission from Carbon 50, 3, 1135–1145. Copyright 2012 Elsevier. devices. Printing and spraying of conducting inks are convenient techniques to functionalize substrates, since the necessary tools are already largely diffused in the manufacturing industry [50; 66]. Indeed, researchers took advantage of both methods to functionalize different flexible materials employing GnPs as conductive nanomaterials. For example, [PERSON] and [PERSON] [55] screen-printed flexible paste made of PMMA-GnPs, realizing electrodes on diverse substrates (glass, Al\({}_{2}\)O\({}_{3}\), PET). This conductive paste, made with 1.5 wt % GnPs, had a sheet resistance in the order of 20 k\(\Omega\)/sq, and transparency near 17%, enough to utilize this coating as an electrode for electroluminescent displays. [PERSON] and coauthors [56] ink-jet printed conductive GnPs-PEDOT:PSS dispersion on top of a transparent substrate, manufacturing a sensor for ammonia detection. The fabrication steps and the obtained device are presented in Figure 4. The addition of only 2.33 wt % of GnPs enhanced the electrical conduction of the PEDOT:PSS conductive ink from \(\alpha\approx 0.8\times 10^{2}\) S/m to \(\approx 1.8\times 10^{2}\) S/m. Moreover, the sensing capability of the device was improved after GnPs addition. Indeed, GnPs enhanced the active surface area of the sample (increasing the surface roughness), and augmented the electron interaction between the sample and ammonia gas. More recently, [PERSON] et al. [57] used a combination of screen printing technology and rolling compression to develop GnPs-based radio frequency flexible antenna. They functionalized paper with the GnPs, obtaining electrical conductivity of 4.3 \(\times\) 10\({}^{4}\) S/m. To verify the antenna's flexibility, they measured the reflection coefficient of bended devices, recording almost the same performance as with the un-bent antenna. To perform the described experiments, [PERSON] et al. employed commercially available GnPs-based conductive ink (grade Grat-ink 102E from BGT Materials Ltd., Manchester, UK) which contains graphene nanoflakes, dispersants, and solvents. The described approaches result in flexible and conductive materials with remarkable applications. However, often mechanical durability and electrical features are not balanced [67]. Indeed, the lack of resistance to bend cycles and mechanical stresses limits the range of uses of such electronics materials in applications such  
Figure 4: Schematic diagram of the manufacturing process of the ammonia sensor. (**a,b**) a silver interdigitated electrode was screen printed on transparent paper. The GnPs-PEDOT:PSS sensing film was deposited trough ink-jet printing; (**c**) schematic of the ammonia gas sensor. (**d**) photo of the obtained device. Reprinted with permission from Organic Electronics 15, 11, 2971–2981. Copyright 2014 Elsevier. as wearable and motile sensors technologies. Certainly, in the case of GnPs inclusion inside plastics, increasing the filler loading inside the polymer matrices can transform the latter into brittle materials and lead to complications in manufacturing [67]. [PERSON] and collaborators [58] took one step towards the creation of GnPs-based durable materials for flexible electronics. Indeed, they realized a conducting composite coating dispersing GnPs of different sizes inside acrylic paint emulsions.