 Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) polymers.There are two types of eumelanin, which are brown eumelanin and black eumelanin.Those two types of eumelanin chemically differ from each other in their pattern of polymeric bonds.