A medical device comprising a tube, a first electrode and a second electrode, and a structure at a distal portion of the tube, the structure defining a first section and a second section, wherein the first and second sections are configured to be filled with a conductive medium, wherein the first electrode is contained within the first section, and the second electrode is contained within the second section, and wherein the structure includes a central barrier separating the first section from the second section, and the central barrier is insulative.