## 4 Graphene Nanoplatelets for Strain Sensors and Stretchable Electronics  
The field of wearable and stretchable electronics pushes towards unusual material arrangements to design circuitries on curvilinear and deformable structures/organisms, and fabricate soft electrical devices and sensors [71; 94; 95; 96; 97]. The junction of a pliable and elastomeric material with a conductive nanomaterial is a favorable approach to build compliant nanocomposites for electronics [98; 99; 100]. Sensor technologies exploit conductivity and capacitance fluctuations recorded when such nanocomposites are deformed (stretched, wrapped, or compressed). These variations are used as a feedback mechanism for devices like strain, pressure, and tactile sensors [101; 102; 103]. Typically, with an external stimuli (often mechanical), conductive nanofillers inside the matrix are separated apart or connected further, changing the material's electrical features.  
Following such approaches, flexible and stretchable GnP-based sensors were investigated and designed in the past years (see Table 2) [48].  
Figure 9: Electrical characteristics of the thermoplastic polyurethane GnPs nanocomposites spray coated on cotton. (**a**) Electrical conductivity and sheet resistance measurements as a function of GnPs concentration with respect to thermoplastic polyurethane. In the inset, a conductive path is spray coated on a t-shirt and used as conductor to light-up LEDs. (**b**) Black line: modifications in the sheet resistance due to weight-pressed 0 mm bending radius folding-unfolding events. The inset displays crack formation after the 10 th cycle. Red line: performance after healing the crack by hot-pressing. Reprinted with permission from ACS Appl. Mater. Interfaces 9, 16, 13825-13830. Copyright 2017 American Chemical Society.  
Strain sensing is crucial for the advances in smart robots, human/structure health monitoring, and human-machine interactions [108; 117; 121; 122]. Indeed, the strain gauge market surpassed 4.5 billion \(\$\) in 2013, and is growing constantly [104]. The most important parameter for strain sensing is the gauge factor. This parameter is defined as \((\Delta\)R/R)/(\(\Delta\)L/L), where \((\Delta\)R/R) is the relative change in electrical resistance (R) obtained under material elongation, and \((\Delta\)L/L) is the applied strain. Graphene-based materials have shown gauge factors among the highest ever reported [103]. Two-dimensional nanoflakes usually show a piezoresisitivity one order of magnitude higher than that of nanowires, since their electrical percolation network is largely susceptible to geometrical changes and discontinuities [123]. Furthermore, carbon-based fillers can expand the utilization of strain sensors, enhancing their elongation range from a few to several hundred percent stretches [122; 124].  
Different approaches were employed to realize GnPs-based strain sensors. The most commonly-used feedback mechanism is based on stretch-induced electrical resistance changes (piezoresistivity). [PERSON] et al. [104] designed strain gauges by simply spraying pristine GnPs thin film on plastic PET substrates. Changing the spray parameters, they were able to control the coating morphology and decrease the film resistance by varying the amount of deposited dispersion. The gauge factor was stable over 4000 strain cycles, and exceeded 150. The electrical percolation behavior of the system under stretch was in good agreement with a model simulating a link between randomly-oriented conductive disks.  
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