opposite sides 10b and lOd of the substrate 20. The first pair of electrodes El and E3 are arranged opposite each other along a first direction indicated by x in Figure 1(b), and the second pair of electrodes are arranged opposite each other along a second direction indicated by y , where the second direction is different, for example orthogonal, to the first direction. Also disposed on the substrate 20 on an upper surface 20a thereof is a grid structure 22. The grid structure 22 is disposed on the substrate between the electrodes El, E2, E3 and E4. In the example shown in Figure 1(b), the grid structure is a rectilinear grid having a first plurality of regularly spaced grid lines 22h that are aligned along the first direction, and a second plurality of regularly spaced grid lines 22v that are aligned along the second direction. Substrate 20 may be flexible, so that it may, for example, be suitable for attaching to a limb of a user such that a surface of the substrate 20 conforms to the contours of the user’s limb as shown in Figure 1(a). Additionally, electrodes E1-E4 and grid structure 22 may be flexible. Electrodes E1-E4 are arranged to collect triboelectric charge that is generated at the surface 20a of the substrate 20 when an object, such as the user’s finger F, is slid over the surface 20a such that the object crosses at least part of the grid 22. To this end, electrodes E1-E4 may each be connected to an external circuit, as will be described in further detail below, for measuring the signal generated by the sliding motion of the user’s finger F. The grid structure 22 is a spacing structure that comprises a plurality of raised portions of substantially equal height above the surface 20a of the substrate 20. I n the embodiment shown in Figure 1, the grid lines 22v and 22h are the raised portions. Transport of the user’s finger F along the surface 20a and over the raised portions (grid lines) 22v, 22h of the spacing structure 22 generates, in a manner which will be described in more detail below, a separation of charges between the substrate surface 20a and the surface of the finger F when the finger F encounters one of the raised portions 22v or 22h. The charges are collected at the respective electrodes E1-E4 and the electric potentials of the