Polydopamine and Eumelanin: From Structure–PropertyRelationships to a Unified Tailoring Strategy.

Polydopamine (PDA), a black insoluble biopolymerproduced by autoxidationof the catecholamine neurotransmitter dopamine (DA), and syntheticeumelanin polymers modeled to the black functional pigments of humanskin, hair, and eyes have burst into the scene of materials scienceas versatile bioinspired functional systems for a very broad rangeof applications. PDA is characterized by extraordinary adhesion propertiesproviding efficient and universal surface coating for diverse settingsthat include drug delivery, microfluidic systems, and water-treatmentdevices. Synthetic eumelanins from dopa or 5,6-dihydroxyindolesare the focus of increasing interest as UV-absorbing agents, antioxidants,free radical scavengers, and water-dependent hybrid electronic–ionicsemiconductors. Because of their peculiar physicochemical properties,eumelanins and PDA hold considerable promise in nanomedicine and bioelectronics,as they are biocompatible, biodegradable, and exhibit suitable mechanicalproperties for integration with biological tissues. Despite considerablesimilarities, very few attempts have so far been made to provide anintegrated unifying perspective of these two fields of technology-orientedchemical research, and progress toward application has been basedmore on empirical approaches than on a solid conceptual frameworkof structure–property relationships. The present Account isan attempt to fill this gap. Following a vis-à-vis of PDA andeumelanin chemistries, it provides an overall view of the variouslevels of chemical disorder in both systems and draws simple correlationswith physicochemical properties based on experimental and computationalapproaches. The potential of large-scale simulations to capture themacroproperties of eumelanin-like materials and their hierarchicalstructures, to predict the physicochemical properties of new melanin-inspiredmaterials, to understand the structure–property–functionrelationships of these materials from the bottom up, and to designand optimize materials to achieve desired properties is illustrated.The impact of synthetic conditions on melanin structure and physicochemicalproperties is systematically discussed for the first time. Rationaltailoring strategies directed to critical control points of the syntheticpathways, such as dopaquinone, DAquinone, and dopachrome, are thenproposed, with a view to translating basic chemical knowledge intopractical guidelines for material manipulation and tailoring. Thiskey concept is exemplified by the recent demonstration that varyingDA concentration, or using Tris instead of phosphate as the buffer,results in PDA materials with quite different structural properties.Realizing that PDA and synthetic eumelanins belong to the same familyof functional materials may foster unprecedented synergisms betweenresearch fields that have so far been apart in the pursuit of tailorableand marketable materials for energy, biomedical, and environmentalapplications. [ABSTRACT FROM AUTHOR]