a mechanical drawing depicting an exemplary embodiment of percutaneous electrode array 300 the array comprises 3600 electrodes arranged in a regular grid pattern of 60 by 60 the width w 1 of each electrode is approximately 200 um a distance s 1 of about 860 um separates the electrodes these dimensions result in a 5 cm by 5 cm array of electrodes detail a shows the electrodes within the void area before they are bent upwards suitable materials for use in layer 330 are a hydrogel or sol gel construct containing an electrolyte the minimum height of the hydrogel layer h 1 is limited by the estimated evaporation time and the mechanical modulus of the gel in a preferred embodiment the array comprises a 635 um thick conductive gel e g uni patch type rg63b as the hydrogel is exposed to the air the water in the gel will evaporate drying out the array and reducing the adhesive and conductive properties of the gel the use of such an array would require a higher applied voltage if the array is flexed or the skin array mechanical interface is otherwise altered an instantaneous drop in interfacial impedance can occur giving rise to an unpleasant feeling in the patient and concentrating the current at points of good contact raising the possibility of a thermal burn adhesion layer 330 is preferably adapted to provide an indication that the array is no longer suitable for use in a preferred embodiment the hydrogel contains.