Replace old or worn-out cartridges of your curling irons and other styling products with the Conair ThermaCELL refill cartridges. They're intended to be used with ThermaCELL curling irons and styling products. Each cartridge last up to 2-3 months. Remove the ThermaCELL curling iron refills protective cap and insert the cartridge. Twist the cap clockwise, so that the cartridge fits in snugly. Conair ThermaCELL refills are made of plastic.