There is ambiguity in the question in my opinion, so I will list some information about materials in electrocardiograms. On the whole electrocardiography is a large, old and complex field that implies adding many up-to-date improvements, materials and machinery.
The first ECGs were recorded onto photographic plates, then they were replaced by ink tracers, and now thermal paper is widely used for tracing ECG.
Electrodes were connected to a patient's body through gauze napkins moistened with saline solution rarely, now those napkins are replaced by conductive ink.
A Lippmann (capillary) electrometer consisted of mercury and sulfuric acid, and was replaced by an Einthoven (string) galvanometer in 1901 that among other things contained a long taut silver-coated quartz filament.
Hope this works for you!