When the body 11 of the tube 24 is made from metal or metal alloy such as nitinol, the inner surface 13 and the outer surface 17 of the tube 24 may be electrically insulated by coating or sheathing (as described in FIGS. 11-13) with silica, silicone elastomer, insulating plastics such as Ultem??? from General Electric Co., nylon, acrylic, polypropylene, polyethylene or polytetrafluoroethylene or s