An optically bistable element is described. The second engaged section is displaced in a direction so as to be disengaged from the second engaging sections in accordance with displacement of the operation section, and the first engaged section is elastically deformed as a result of being interlocked with the second engaged section according to displacement of the operation section. For this purpose, at least one plasma torch, which produces a plasma jet and is directed at the outlet area of the casting nozzle in a direction opposite the casting direction, is provided. The hydrogel is held under a predetermined compressive force within the sensor body such that the hydrogel exerts a base pressure. There may be multiple alignment marks for the device around the corners or at the edges of an area surrounded by the guard ring.