Sticky pads and metal

Conventional highly conductive electrode (i.e. metal) concentrate all current at the edges.

image source: Bioelectrodes, Eric McAdams
Simulations available at Estimation of current density distribution under electrodes for external defibrillation

This results in current hotspots, which are rumored to result in painful sensations.

Sticky pads are engineered to avoid this problem, the design used by some cheap manufacturers uses a carbon backing film with medium resistance, the wire is attached to the center. Below are measurements of DC resistance I performed on a sticky pad bundled with the coyote. For ease of measurement I removed the sticky gel.

Despite the manufacturers failure to attach the wire precisely in the center, it is clear the electrode has been designed with a larger resistance on the edge, which should encourage current to enter the skin more uniformly.

Many sticky pad manufacturers claim their electrodes have patented techniques to promote a more uniform current density, although I am not convinced this is more than an elaborate marketing hoax.

What about conductive rubber?

Joanne sells flat sheets of conductive rubber, omega rubber. This rubber has a fairly low resistance, about 460 ohm per meter with 1khz signal and 525 ohm per meter DC.

With typical cut length of 3cm, corner-to-corner resistance is at least one order of magnitude below that of sticky pads. This suggests conductive rubber pads also concentrate most current along the edges of the electrode.