A ladder made of composite material comprises a plurality of rungs having an elongated body provided with at least one connecting portion adapted to engage an upright of a ladder. The device comprises a plurality MTJ cells each comprising a free layer and a pinned layer separated by a dielectric layer. The electronic controller activates only the starter generator when only low starting torque is required. The head is fabricated by forming a first resist pattern on an inorganic insulating layer. A, B, Ar, R1, R2, R3, R3a have meanings given in the description. These VES-based fluids have faster and more complete clean-up than polymer-based fracturing fluids.