Exploring the chemistry and composition of black soldier fly eumelanin, a material for a circular economy

Eumelanin is a black-brown biopigment that provides photoprotection and pigmentation in mammals, insects, and invertebrates. It can be obtained by oxidative polymerisation of 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA). Due to its unique physical and chemical properties and its biocompatibility, eumelanin is a promising biomaterial for applications in energy storage, biomedicine, and sensing. However, poor solubility in water and lack of sustainable and low-cost sources of eumelanin have so far limited the full exploitation of this biomaterial. Insect farming is rapidly emerging as an alternative source of eumelanin. Unlike other types of eumelanin, BSF eumelanin, which is extracted from the exoskeleton of the black soldier fly (BSF, Hermetia illucens ), is water-dispersible; however, its fundamental chemical properties are not completely understood. Here, we report the characterisation of BSF eumelanin using various spectroscopy techniques. Contrary to what is known about other insect eumelanins, which are believed to contain exclusively DHI, our results indicate that BSF eumelanin may contain both DHI and DHICA moieties. We discuss the potential reasons for this discrepancy. Demonstrated is the first example of dihydroxyindole carboxylic acid presence within an insect eumelanin, a conductive biomaterial material for the circular economy.