Understanding the way eumelanin works: a unique example of properties and skills driven by molecular heterogeneity

Here we revisit recent scientific discoveries and technological advancements related to the eumelanin (EU) biopolymer. Starting from EU extracted from living species, the settling of laboratory-scale synthesis strategies and of characterization methodologies are now allowing to clarify and reproduce the unique combination of chemical/compositional/structural features of this biomaterial. The 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole 2-carboxylic acid (DHICA) substructures constituting EU present indeed a remarkable variability in electron delocalization, redox states, types of free radicals and localization of paramagnetic centers. Moreover, the EU building blocks can be packed following several supramolecular organizations. The high heterogeneity accounts for the abandonment of the notion "one-material/one-set-of-properties". The interplay of mixed and sometimes conflicting functionalities - electronic/protonic conduction, radiative/non-radiative decays of optical active states, antioxidant/pro-oxidant behavior, chemical/physical mechanisms for withstand radioactivity - leads to countless opportunities to employ EU in several cutting-edge technologies. The critical analysis of literature highlights advantages and drawbacks related to the realization of biomedical supports, electronic/ optoelectronic devices, energy storage/delivery systems, bioremediation and radioprotection means. The focus is on the state-of-art, on the translation of major concepts into novel applications, and on the main remaining challenges for the use of a biomaterial characterized by unusual precious characteristics.