Melanins are a broad range of macromolecules derived from the oxidation of phenolic compunds. Their unique properties have proven useful in both medical and industral applications. Natural melanin formation is driven by the activity of oxidoreductases on phenolic substrates. For example, melanins in human skin are formed from tyrosine and catecholamine subtrates via tyrosinase activity. Similar reaction schemes result in the brown blemishs observed on the surfaces of bruised fruits and vegetables, and some shellfish. Despite their ubiquity in nature melanins remain poorly understood. Enzymatic melanogenesis is poorly controlled and the resulting melanins are difficult to characterize. They are amorphous insoluble polymer networks with a high density of zero length crosslinks. We theorized that by introducing hydrophilic spacers between crosslinks a novel hydrogel could be obtained with melanic properties.
We designed synthetic poly(ethylene glycol derived macromers with terminal gallate groups. Melanogenesis was achieved by oxidizing these macromers to obtain hydrogels. Polyphenols are oxidized to highly reactive quinones during melanin formation. These intermediates can form conjugate adducts with nucleophilic functional groups on proteins, such as thiols and amines. The reaction scheme allows the introduction of additional heteroatoms into a melanin network. We exploited this quinoid reactivity as a simple way to introduce cell adhesive ligands into the hydrogel networks. These substrates provide a versatile tool for controlling cell adhesion, adding biofunctionality to surfaces, and potentially for regenerating tissue.