speeds performed by the finger have been characterized according to example embodiments, i.e., slow (~38.0 mm/s), normal (~97.4 mm/s) and fast (~152.0 mm/s). The respective signals obtained with the device 200 supported on a table (Figures 5(d),(e),(f)) and on a human hand (Figures 5(g),(h),(i)) are shown. During the measurements, the finger is lifted and moved back into the centre each time. When the finger is not sliding across a specific pattern, whether it is on the surface or in air, the signal is 0 . Accordingly, 0 level is measured between two sliding motions as seen in the graphs. 

The operating principle of the device 200 may be further understood with reference to Figure 6(c). I n Figure 6(c), two grating elements 202a, 202b of electrode 202 are taken as an example. All the grating elements are connected together to form the electrode 202, and are connected to an external resistor load R. After a finger wearing a nitrile glove contacts with PTFE 206, electrons are injected from the nitrile glove into PTFE at the interface, due to the different electron affinity of the two materials. Thus the PTFE surface becomes negatively charged and the nitrile glove surface becomes positively charged. Then when the finger slides on the first grating element 202a, the positive charges on the nitrile glove surface balance the negative charges on the PTFE surface, inducing an electric potential difference. Thus electrons are driven to flow from ground to the first grating element 202a, generating a positive output peak 302 on the external circuit. Next, when the finger slides away from the first grating element 202a, the generated electric potential difference disappears and electrons flow back to ground, generating a negative output peak 304 on the external circuit. Similarly, when the finger slides on and away from the second grating element 202b, another output peak 306 (including both the positive peak and the negative peak) is generated. That is, one output signal peak is generated when the finger slides across one grating element. I n this way, different directions with different numbers of grating elements can be differentiated based on the number of generated output peaks, e.g., one output peak will be generated when sliding in the direction with one grating element, two output peaks will be generated when sliding in the direction with two grating elements, etc. 

