Various embodiments provide an electrosurgical instrument with a disposable electrosurgical cartridge. In one embodiment, the cartridge has first and second energy-delivery surfaces that carry first and second opposing polarity conductors coupled to a voltage source, together with first and second temperature-responsive variable impedance bodies exposed partly in the respective-delivery surfaces. The cartridge further carries a slidable blade member. The temperature-responsive variable impedance bodies are coupled to the voltage source by series and parallel circuitry. In use, the variable impedance bodies are adapted to modulate current flow and ohmic heating in engaged tissue by providing controlled current paths in the tissue and through the variable impedance bodies as the temperature-responsive bodies sense the temperature of adjacent engaged tissue. The engagement surfaces are capable of highly localized modulation of Rf energy application to engaged tissue to provide high and low temperatures, voltage and current in the tissue to create high strength welds.