Electrodes are the surfaces used to interface the OpenStim's power and ground outputs to your head. tDCS electrodes are typically large(20-35 square centimeters) and use salt water to provide a conductive interface with your head.
Saline-soaked electrodes require an absorbent material to soak up salt water, as well as a metal current distributor. The type of materials used influences both the total resistance through the electrode, and how evenly current is distributed over its surface.
The resistance of the absorbent layer of the electrode(the only part that should touch the skin) is determined by its thickness and material. Generally, most types of absorbent material(cotton, rubber sponges, gauze, etc) are not good conductors, but they absorb salt water(which is). This means that you should minimize the thickness of the lining(while still keeping enough material to absorb sufficient water) and choose a material that is extremely porous and has low density, such as gauze or cotton pads. Cleaning sponges typically make poor electrodes because they do not absorb enough water to provide good paths between the metal current distributor and the skin. However, if the considerable pressure is applied to the entire surface of the electrode, cleaning sponges will usually suffice).
The metal current distributor is needed to ensure constant current flow across all parts of the electrode. This is necessarily because the water-soaked absorbent layer has a relatively high resistance compared to electric wire. Therefore, if a wire was connected to one part of the absorbent layer, a large amount of current would reach the skin directly below the wire, but very little would reach the skin under the electrode but farther from the wire, because the electricity would have to travel a longer distance through the high-resistance layer. A metal current distributor avoids this problem, because it allows current to spread out first over the entire current distributor, and then proceed down through the absorbent layer to the skin. Because the absorbent layer is the same thickness under all parts of the current distributor, the current emitted by the electrode is kept homogeneous over its surface.