## 3 Results
### Low Electrical Percolation Threshold Triboelectric Sensors  
The triboelectric effect is a combination of contact electrification and electrostatic induction; the static polarised charges, resulting from the contact between the two friction surfaces with different charge affinities, are generated on the friction surfaces, causing different surface potentials, inducing charges among the two attached electrodes [26; 27; 28]. In this specific case, the PDMS nanocomposites present a high triboelectric charge density. When the mechanical testing equipment touches the surface of the PDMS, the flow of electrons generates a measurable electrical voltage. The same happens at release, but the output voltage measured has an opposite sign since the electrons are leaving the surface of the PDMS [29]. When the materials are in contact, the output voltage is zero since there is no significant exchange of electrons between the materials. Therefore, triboelectric sensors help detect instantaneous contact and release. Even for applied forces lower than 6 N, a generated output voltage may be detected, indicating that these sensors have a fast response and are relatively sensitive. Figure 14 shows that all the nanocomposites generate output voltage under contact and release. The signal is low for the random distribution nanocomposite with 0.5 wt.% due to the low electrical conductivity across the sample. The magnetic-patterned nanocomposites and the randomly-distributed nanocomposite with 1 wt.% MWCNT, which have lower electrical resistance, generate a considerable output voltage that may be used to detect instantaneous impact and release.  
When analysing the triboelectric response of the nanocomposite produced with a magnetic pattern of parallel lines, represented in Figure 15, it is observed that the contact and release are detected within different applied force ranges. When the applied force is maintained for 10 s, the output voltage is zero due to the lack of interaction between the two surfaces; however, the output voltage is visible at the moment of release.  
Figure 13: Piezoresistive effect for the sample with the parallel lines pattern with 0.5 and 1 wt.% of MWCNT/PDMS. (**a**) unloading-loading peaks, (**b**) unloading-loading delayed peaks.  
The piezoresistive sensors detect the moment the force is applied to the material, as shown in Figure 16; however, it does not detect the touch if the sensor contacts a surface without deforming the sensor. The triboelectric effect, however, can detect contact without  
Figure 14: Triboelectric effect for nanocomposites with (**a**) 0.5 and (**b**) 1 wt.% of MWCNT with random distribution and with different magnetic patterns (zig-zag, rectangle, and parallel lines).  
Figure 15: Triboelectric effect obtained for the nanocomposites prepared with the parallel line magnetic pattern with 0.5 and 1 wt.% of MWCNT; (**a**) unloading-loading peaks, (**b**) unloading-loading delayed peaks. The top images represent the applied force by the testing machine.  
bending the sensor. Therefore, the two sensor types can complement each other by having a high sensitivity to touch due to the triboelectric properties and stable monitoring of the applied force or deformation due to the piezoresistive properties.  
Figure 16 shows in detail the difference between the response of the sensors for fast (Figure 16a) and slow movements (Figure 16b). When analysing the presented results, it is possible to confirm that the triboelectric sensors work well to detect the contact and release of objects on the sensor's surface, independently of the time frame and applied force. The piezoresistive effect works better for monitoring the deformation of the sensor, being capable of monitoring both fast and slow deformations.