## 4 Graphene Nanoplatelets for Strain Sensors and Stretchable Electronics
### PDMS and Graphene Nanoplatelets for Strain Sensing  
A widespread material for strain sensors is polydimethylsiloxane (PDMS), due to its flexibility, stretchability and ease of manufacturing [125; 126; 127; 128; 129; 130]. PDMS-based strain sensors were also investigated coupled with GnPs. For example, [PERSON] and collaborators [109] developed GnPs-PDMS composites by simple sonication and molding processes. This device reached gauge factors of approximately 230 at a GnPs concentration of around 8 vol %, and within a strain of 2%. [PERSON] and collaborators [117] produced an electrically conductive and stretchable film by mechanically pressing GnPs onto a medical tape and embedding the structure inside PDMS. The manufacturing was low-cost and fast (in the order of one minute), and therefore, large-scalable. The obtained sensor was reliable over 1000 stretch-release cycles, exhibited a time of response of less than 50 ms, and a gauge factor of approximately 110. Moreover their sensor showed sensitivity to low mechanical strains. Indeed, it could detect minuscule movements from a cricket, and air vibration caused by mobile phone speakers, as shown in Figure 10.  
Figure 10: Performance of the deformation sensor obtained pressing GnPs on a medical tape and embedding such structure inside PDMS. The sensor was employed as tiny movements and sound signal detector. (**a**) Signal generated by the footstep of a field cricket moving on the sensor and (**b**) recognition of music signal from an iPhone speaker, with inset being audio signal. Reprinted with permission from Advanced Functional Materials. Copyright 2016 Wiley.  
Another PDMS and GnPs composite for strain sensing was investigated by [PERSON] and collaborators [131]. They used layer by layer assembly to create controlled geometries of GnPs on the elastomeric substrates. Such devices can monitor subtle human movements.  
The synergic properties of GnPs and carbon nanotubes are convenient to design PDMS-based strain sensors. For example, [PERSON] et al. [112] screen printed a biocompatible gauge device employing multilayer graphene nanoflakes and multiwall carbon nanotubes (MWCNT). The gauge factor changed from 22 (for 12.5 wt % GnPs inclusion) to 100 for concentrations near the percolation threshold (i.e., 1.5 wt % GnPs and 3.5 wt % MWCNT blend). The authors attribute such behavior to the large contribution of electron tunneling at nanofiller loads close to the percolation threshold; with stretch, the distance between nanofiller units augment, however, still permitting tunneling conductivity. For higher filler loads, the mechanism of conductivity is mostly determined by direct contacts that are slightly affected by strain compared to tunneling electrons. More recently, [PERSON] and collaborators [118] used catalyst chemical vapor deposition approaches to grow nanotube forests on both sides of GnPs nanoplatelets. The nanoflakes-nanowires filler was mechanical mixed with PDMS, achieving percolation threshold at 0.64 vol %. At 0.75 vol % concentration, they reached a gauge factor in the order of 1000. Such sensors are able to sense and even distinguish between tiny finger motions, as shown in Figure 11. Furthermore, the nanocomposites were also used as compression sensors, reaching pressure sensitivity of 0.6 kPa\({}^{-1}\).  
Figure 11: Nanoflakes-nanotubes PDMS composite film glued on rubber gloves and serving as finger motion detector. (**a**) Photograph of the relaxed state of the finger; (**b–d**) are photographs of second and third joint and clenching motions respectively; (**e**) Resistance changes for each independent movement. Reprinted with permission from ACS Appl. Mater. Interfaces 7, 9652–9659. Copyright 2015 American Chemical Society.