"What is combined with the CBD/THC in order to help it penetrate the skin?"
Patches deliver cannabinoids to the circulation at a slow and steady rate, making them an excellent choice for treating most chronic conditions.
THC, CBD and other cannabinoids do actually penetrate skin. However, negligible amounts ever make it past the skin due to a combination of two factors: the low permeability of cannabinoids through the skin; and that the cannabinoids have no incentive to penetrate the skin to enter the circulation.
Patches attack both problems: Specialized molecules are attached to the cannabinoids to increase their permeability through the skin; and specialized molecules are attached to the backing of the patch that repel the cannabinoids towards and through the skin.
I hope this helps!