Title:

Mesoscale structure-ionic transport relationship of eumelanin

Recent advances in biodegradable electronics exhibit a huge potential to transform the conventional implantable biomedical devices into transient ones by obviating many challenges associated with chronic implants. Biocompatible/biodegradable packaging materials, naturally-sourced electronic components, or biodegradable charge storages would be the ideal building block to fabricate these types of devices. Recent research has shown that eumelanin pigments can be served as promising electrode materials due to the exceptional biodegradability, the ease of processing, and the capability of binding various cations. Coordination bonding with cations can be formed mostly with semiquinone radicals that are induced by comproportionation reaction of indoles. These electrochemical properties are largely dependent upon the chemical functional groups as well as the structural characteristics. Eumelanins are mainly composed of the aggregated network of oligomers that include two subunits of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) in various ratios. These subunits are randomly crosslinked into planar protomolecules by Pmel17 proteins that are further -stacked to form spherical nanostructures. The protomolecules of natural eumelanins have been known as the unique structural feature that cannot be observed in the synthetic counterparts synthesized from the template free autoxidation. Despite numerous efforts with a wide range of techniques, little has been known about the macromolecular topography that links the configuration of two subunits. Furthermore, the structural effects on charge transport are still elusive due to the disordered nature during melanogenesis. We report herein the presence of the mesoscale protomolecule structures of natural eumelanins using x-ray scattering, electrochemical techniques, and spectroscopic analysis. Increased understanding of structure-property relationship can provide the insight into the template-assisted self-assembly in melanosome. In addition, controlling the charge transport would be advantageous to design the device quality bioinspired materials as well as the high performance energy storage devices for biodegradable transient biomedical electronics.

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