Natural Eumelanin and Its Derivatives as Multifunctional Materials for Bioinspired Applications: A Review

Abstract

Natural melanin is a ubiquitous material that plays critical biofunctional roles in different living organisms. Scientists have dedicated significant efforts to elucidate the biofunctional roles of melanin since its discovery. It has been confirmed that natural melanin possesses a number of intriguing properties such as broadband light absorption, free-radical scavenging ability, redox activity, metal ion chelating, and electronic−ionic conductivity, enabling it to be a versatile functional material in various applications. Natural eumelanin has been the most investigated type of melanin in the past few decades and is discussed in this Review. Here we have comprehensively discussed the latest advances and associated mechanisms in emerging applications of natural eumelanin in different fields such as functional polymers, energy storage, energy conversion, photocatalysis, photothermal therapy, and wastewater treatment considering its bioinspired properties. Important applications developed based on polydopamine, which is a "eumelanin-like" material, will also be presented to give guidelines to further develop natural-melanin-based applications. Authors' perspectives on the challenges and potentials of developing natural-eumelanin-based applications will also be included. This Review provides a clear picture of the natural-eumelanin-based applications and, in turn, accelerates the expansion of the relevant emerging fields.

Bio-Applications of Multifunctional Melanin Nanoparticles: From Nanomedicine to Nanocosmetics

“…In nature, brown-black eumelanin and red-yellow pheomelanin are derived from tyrosine precursor. More in detail, natural eumelanin is biosynthesized from the tyrosinase-catalyzed oxidation of tyrosine or 3,4-Dihydroxy-L-phenylalanine (L-DOPA) to DOPAquinone [49]. Following, DOPAquinone is converted to L-Dopachrome through auto-oxidation, which tautomerizes to form 5,6-dihydroxyindole-2-carboxylic acid (DHICA) or reaction proceeds spontaneously with decarboxylation to give mainly 5,6-dihydroxyindole (DHI).…”

“…In recent years, sepiomelanin, mainly composed by eumelanin, has been utilized as the standard melanin material. The extraction method includes centrifugation with water or washing with salts [49]. In two different studies, melanin has also been successfully extracted from black sesame seeds and human hair [59][60][61].…”

Melanin and Melanin-Like Hybrid Materials in Regenerative Medicine

“…The abundance of metal ions coordination sites in melanins’ structure results in a high affinity for metal cations. This attitude has been extensively exploited to bind different metal ions to improve melanin’s intrinsic features, such as antioxidant and light absorption performance [ 52 , 53 ], but also to confer further non-native contrast properties [ 48 , 49 , 52 ]. Loading strategies include postdoping, predoping, and metal ion-exchange approaches ( Figure 3 ).…”

A Melanin-Related Phenolic Polymer with Potent Photoprotective and Antioxidant Activities for Dermo-Cosmetic Applications

“…Melanins are the primary determinants of skin, hair and exoskeletal pigmentation in mammals, birds and insects [1][2][3][4][5][6], and their importance has been growing over the past few years not only from a biological point of view, related to their role in the human body, but also for the exploitation of their unique properties in biomedicine or in the cosmetic and health sectors [7][8][9][10][11][12][13][14][15][16]. These pigments are biosynthesized in melanocytes, starting with the oxidation of tyrosine to dopaquinone catalyzed by the enzyme tyrosinase ( Figure 1) [1,17,18].…”