## 4 Graphene Nanoplatelets for Strain Sensors and Stretchable Electronics
### Other Approaches for Graphene Nanoplatelets Based Strain Sensing  
Apart from PDMS, other rubbery materials are also candidates for possible implementation for strain sensing. For example, [PERSON] and coauthors [110] used a highly viscoelastic, lightly cross-linked silicone polymer (trademark name Silly-Putty(r)) mixed with GnPs to design a material that can monitor impact, pressure, and deformation. Its electrical resistance augmented linearly when stretched below 1%, and surprisingly decreased, even below its initial value for elongations between 1% and 10%. Such a trend is unique considering the monotonic increases of resistance usually observed with stretch. Silly-Putty-GnPs nanocomposites exhibit gauge factors of around 500 at 6.8 vol % of nanofillers. Such device can measure blood pressure, pulse rate, and even the footstep impact associated with spiders. The GnPs (Lateral size \(\approx\) 500 nm, thickness \(\approx\) 20 layers) employed for this work were prepared by ultrasonic tip-sonication of graphite (Branwell, Graphite Grade RFL 99.5) in NMP. These GnPs were then dispersed in chloroform after NMP drying. Recently, our group [111] showed that electrical conductivity of TPU-GnPs nanocomposites can be tuned and improved by repeated stretching cycles, without exceeding 20% of the maximum strain. A decrease of up to 60% in electrical resistance was measured after 1000 stretch-release cycles. We discovered that the described changes were caused by stretch-induced redistribution of the GnPs within the polymer matrix. Such TPU-GnPs nanocomposites can be used for strain sensing applications after stretch-induced electrical feature optimization. The rearrangement of the disposition of nanoflakes inside TPU was noted also by [PERSON] and collaborators with single layer graphene [132]. Wearable and stretchable textiles are ubiquitous. Another common strategy for strain sensing is the functionalization of such fabrics with conductive materials. For example, [PERSON] and collaborators [106] obtained a conductive (\(\approx\)200 S/m) textile functionalizing cotton through a simple spray of PEDOT:PSS-GnPs dispersions. The material exhibited strain sensing capabilities, with a gauge factor of approximately 5 at strain of 5% and 10%. It could resist repeated washing and bending events, thereby ensuring possible commercialization. The combination of high sensitivity to tiny deformation and broad sensing range is often unusual in the field of strain sensors [133]. In such a context, promising results were obtained by [PERSON] et al. [116], who reported on the fabrication of stretchable yarns realized through simple layer-by-layer assembly. In particular, different yarns (rubbery, nylon-rubbery and wool) were immersed first in a poly(vinyl alcohol) solution, and, when dry, in a GnPs dispersion. All these fabrics maintained a remarkable stretchability (see Figure 12). When needed, a PDMS coating was used to prevent GnP detachment during stretch. Depending on the fiber used, different performances were achieved. Indeed, the highly sensitive rubbery sensors exhibited a gauge factor of \(\approx\)1800 and a maximum stretchability of around 100%, the nylon-rubbery device led to a gauge factor of 1.4 and a maximum elongation of 150%, and the wool-based sensor displayed an atypical negative gauge factor of \(-0.1\) with a maximum stretch of the 50% of the initial length. These performances are suitable for monitoring a wide range of body movements depending on the fiber employed, from tiny breathing to finger-bending. [PERSON] et al. [114] investigated the use of fabric-like material coupled with GnPs for strain sensing applications. They dip coated glass fiber with conductive nanoflakes, observing that this coating was more effective when GnPs were NH\({}_{2}\) functionalized. Also, electrical conductivity and strain sensing were strongly enhanced when GnPs were doped with nitrogen. Such GnP textiles exhibited an exponential increase in resistance with stretch, reaching gauge factors in the order of 16,000.