In one embodiment, a medical system includes a medical instrument having a grasper head including first and second complementary grasping jaws, and first and second conducting surfaces disposed on respective distal portions of the first and second grasping jaws, the conducting surfaces being electrically isolated from each other in the grasper head, an actuator configured to close the grasping jaws so as to bring the conducting surfaces into contact with a tissue of a body part of a living subject, and a proximity sensor configured to output at least one proximity signal responsive to a displacement between the grasping jaws, and processing circuitry coupled to sense the displacement between the first and second grasping jaws responsively to the at least one proximity signal, and apply an electrical current between the first and second conducting surfaces of the grasping jaws when the sensed displacement is less than a given threshold displacement.