An apparatus comprising: first and second circuit boards with respective electrodes thereon, the first and second circuit boards in a bonded configuration; one or more first layers positioned to be proximal to the one or more of the electrodes; electrolyte proximal to the respective electrodes; one or more second layers configured to provide for the bonded configuration in which the first and second circuit boards are bonded together, under curing, such that the respective one or more first layers are positioned between the one or more second layers and the electrodes, the bonding defining a chamber therebetween with the electrodes therein and facing one another, the chamber comprising the electrolyte; and wherein the one or more first layers are configured to inhibit interaction of the electrolyte with the one or more second layers during curing.