The mouldable polymer 27 may comprise any polymer material which is fluid enough to embed the electrodes 25 and layer of two dimensional material 23. Once the mouldable polymer 27 is provided around the electrodes 25 and the second release layer 37 the mouldable polymer 27 may be cured or otherwise hardened. Once the mouldable polymer 27 has hardened it may form a substrate for the at least two electrodes 25. Once the mouldable polymer 27 has hardened the second release layer 37 may be removed.

The mouldable polymer 27 has different thicknesses across the cross section of the apparatus 21. In the example of FIG. 2 the mouldable polymer 27 comprises a first portion 24, a second portion 26 and a third portion 28. The first portion 24 supports a first electrode 25 and the second portion supports a second electrode 25. The first and second portions 24, 26 of the mouldable polymer 27 may contact the electrodes 25. The first and second portions 24, 26 of the mouldable polymer 27 may have the same thickness.