pairs (El, E3) and (E2, E4) of electrodes may be used to infer motion parameters of the user’s finger F. 

The flexible substrate 20 may be made of silicone rubber, which has good stretchability. The substrate 20 acts as a friction layer to contact with objects in a sliding or touching manner to generate triboelectric charges. The grid 22 may be a layer formed of resin, and may be deposited on the substrate 20 by 3D printing, for example. To this end, any suitable 3D printable resin, such as polyamide (PA) resin 11, may be used to apply the grid 22 to the substrate 20. The material of the grid 22 is not limited to resin, and other materials can be used so long as they do not exhibit a strong triboelectric effect with the object that is to be slid across the sensor 10 (e.g., a finger). The grid material may have a similar degree of flexibility to the substrate 20. For example, if the substrate 20 is flexible so that the sensor 10 can be worn as shown in Figure 1(a), then the material of grid 22 should also be flexible. 

The electrodes E1-E4 on the edges of the substrate 20 may be fabricated from starch-based hydrogel PDMS elastomer (HPE), i.e., a mixture of starch-based hydrogel and PDMS. The starch-based hydrogel may be obtained by dissolving corn-starch in an electrolyte at 60°C and the resultant starch-based hydrogel may be maintained at a relative humidity (RH) of 60%. 

Similarly to the 2D sensor 10, the I D sensor 12 shown in Figure 1(c) comprises a flexible substrate 24, and has respective electrodes El’ and E2’ at opposite ends thereof. The flexible substrate 24 may be formed from silicone rubber, as for the substrate 20. The electrodes El’ and E2’ may also be flexible. For example, electrodes El’ and E2’ may be formed from HPE, as for electrodes El- E4. 

Turning to Figure 1(d), the operating principle of the 2D sensor 10 will now be described. I n Figure 1(d), electrode El is connected to a load resistor R1 and electrode E3 is connected to a load resistor E3. The operating mode begins with a single contact between the finger F and the top of a raised portion 22v of the grid structure 22. When the finger F slides to the right, the finger F approaches 

