Abstract
The huge development of bioengineering during the last years has boosted the search for new bioinspired materials, with tunable chemical, mechanical, and optoelectronic properties for the design of semiconductors, batteries, biosensors, imaging and therapy probes, adhesive hydrogels, tissue restoration, photoprotectors, etc. These new materials should complement or replace metallic or organic polymers that cause cytotoxicity and some adverse health effects. One of the most interesting biomaterials is melanin and synthetic melanin-related molecules. Melanin has a controversial molecular structure, dependent on the conditions of polymerization, and therefore tunable. It is found in animal hair and skin, although one of the common sources is cuttlefish (Sepia officinalis) ink. On the other hand, mussels synthesize adhesive proteins to anchor these marine animals to wet surfaces. Both melanin and mussel foot proteins contain a high number of catecholic residues, and their properties are related to these groups. Dopamine (DA) can easily polymerize to get polydopamine melanin (PDAM), that somehow shares properties with melanin and mussel proteins. Furthermore, PDAM can easily be conjugated with other components. This review accounts for the main aspects of melanin, as well as DA-based melanin-like materials, related to their biomedical and biotechnological applications.
Highlights
Melanin as a Relevant BiomaterialNatural melanin is a structurally ill-defined polyphenolic material widely found in all types of living organisms, from bacteria to mammals
Sometimes endogenous melanin has been proposed as a putative target for chemotherapy of heavily pigmented melanotic melanoma, due to the pro-oxidant, rather than antioxidant melanin effect, once this endogenous pigment is highly exposed to oxygen and metal ions
Gadolinium-based nanoparticles formed by absorption of this metal in a polysiloxane network have been tested for theranostic treatment of melanoma, due to the affinity of the melanin contained in malignant melanocytes for these nanoparticles [66]
Summary
Natural melanin is a structurally ill-defined polyphenolic material widely found in all types of living organisms, from bacteria to mammals. The details on DA polymerization and final structure of the PDAM depend on the conditions of the synthesis, it is likely to involve the oxidation of catechol to o-quinone and cross-linkages among units. Hekstra et al [47] synthesized some tyrosine-containing tripeptides, in combination with phenylalanine and aspartic acid as the other two components After tyrosinase oxidation, these tripeptides form melanin-like pigments, but the spatial arrangements in the generated polymers remarkably depend on the position of the catecholic residue, especially on the positions of aspartic acid. These tripeptides form melanin-like pigments, but the spatial arrangements in the generated polymers remarkably depend on the position of the catecholic residue, especially on the positions of aspartic acid This seems to be due to covalent and non-covalent disorder factors that give place to several structural arrangements over a considerable range. Synthetic tyrosine-containing oligopeptides open up a new way of tuning melanin-bioinspired molecules after activation with tyrosinase, for the addition of catechol groups
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