Conductive rubber is an often overlooked material for making cheap and highly effective e-stim electrodes
It is its simplicity that makes it such a good choice for homemade DIY electrodes.
Conductive rubber is actually conductive silicone, and graphite powder is mixed in with the silicone. As it cures the graphite particles touch and allows a current to flow through the previously insulating silicone.
Initially created for the pharmaceutical industry as a way of reducing static in hoses it has found a kinkier use within the e-stim community.
Rough use and stretching reduce the life of conductive rubber as it breaks down the bonds between the graphite particles.