Melanins are pigmentary macromolecules found throughout the biosphere that, in the 1970s, were discovered to conduct electricity and display bistable switching. Since then, it has been widely believed that melanins are naturally occurring amorphous organic semiconductors. Here, we report electrical conductivity, muon spin relaxation, and electron paramagnetic resonance measurements of melanin as the environmental humidity is varied. We show that hydration of melanin shifts the comproportionation equilibrium so as to dope electrons and protons into the system. This equilibrium defines the relative proportions of hydroxyquinone, semiquinone, and quinone species in the macromolecule. As such, the mechanism explains why melanin at neutral pH only conducts when "wet" and suggests that both carriers play a role in the conductivity. Understanding that melanin is an electronic-ionic hybrid conductor rather than an amorphous organic semiconductor opens exciting possibilities for bioelectronic applications such as ion-toelectron transduction given its biocompatibility.bioelectronics | electrical properties | biomacromolecules | ionic conduction T he melanins are responsible for multiple critical functions in humans such as photoprotection and free radical scavenging (1). These molecules are also found in the substantia nigra of the human brain stem where their exact biological role is unknown; however, it has been speculated that neuromelanin may be involved in neural transmission (2). Melanin phototoxicity is also implicated in deadly melanoma skin cancer (3, 4). Despite decades of intense studies across biology, chemistry, and physics, the full details of the structure and functions of the melanins are still not clearly understood. Eumelanin, the major component in human skin pigment is viewed as the archetypal "true" melanin (here we adopt the standard nomenclature in which the terms "eumelanin" and "melanin" are used interchangeably). Eumelanin is composed of aggregated oligomeric and polymeric species based upon the indolic monomers 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and their various redox forms (1). These monomers are randomly cross-linked to form planar sheets, stacked via aromatic π-interactions and with varying conjugation length and character (5, 6, 7). For many years, melanins were considered as extended linear homo-or heteropolymers with a conjugated backbone (8). However, this view no longer has any credence and the disordered 2D protomolecular sheet model is now widely accepted. This sheet model has quite profound implications for the treatment of melanins within a conventional polymer framework, and many of the standard methods and theories for probing structure property relationships in macromolecules are inadequate or simply do not apply.Two properties of melanin have particularly intrigued physicists and chemists for decades: (i) Melanins are electrical conductors showing photoconductivity in the solid state; and (ii) melanins are black with broad featureless opt...

Melanin, the human skin pigment, is found everywhere in nature. Recently it has gained significant attention for its potential bioelectronic properties. However, there remain significant obstacles in realizing its electronic potential, in particular, the identity of the solid-state free radical in eumelanin, which has been implicated in charge transport. We have therefore undertaken a hydration-controlled continuous-wave electron paramagnetic resonance study on solid-state eumelanin. Herein we show that the EPR signal from solid-state eumelanin arises predominantly from a carbon-centered radical but with an additional semiquinone free radical component. Furthermore, the spin densities of both of these radicals can be manipulated using water and pH. In the case of the semiquinone radical, the comproportionation reaction governs the pH- and hydration-dependent behavior. In contrast, the mechanism underlying the carbon-centered radical's pH- and hydration-dependent behavior is not clear; consequently, we have proposed a new destacking model in which the intermolecular structure of melanin is disordered due to π-π destacking, brought about by the addition of water or increased pH, which increases the proportion of semiquinone radicals via the comproportionation reaction.

We demonstrate experimentally for the first time the presence of two free radicals in the pigment eumelanin.

Inorganic semiconductors permeate virtually every sphere of modern human existence. Micro-fabricated memory elements, processors, sensors, circuit elements, lasers, displays, detectors, etc are ubiquitous. However, the dawn of the 21st century has brought with it immense new challenges, and indeed opportunities-some of which require a paradigm shift in the way we think about resource use and disposal, which in turn directly impacts our ongoing relationship with inorganic semiconductors such as silicon and gallium arsenide. Furthermore, advances in fields such as nano-medicine and bioelectronics, and the impending revolution of the 'ubiquitous sensor network', all require new functional materials which are bio-compatible, cheap, have minimal embedded manufacturing energy plus extremely low power consumption, and are mechanically robust and flexible for integration with tissues, building structures, fabrics and all manner of hosts. In this short review article we summarize current progress in creating materials with such properties. We focus primarily on organic and bio-organic electronic and optoelectronic systems derived from or inspired by nature, and outline the complex charge transport and photo-physics which control their behaviour. We also introduce the concept of electrical devices based upon ion or proton flow ('ionics and protonics') and focus particularly on their role as a signal interface with biological systems. Finally, we highlight recent advances in creating working devices, some of which have bio-inspired architectures, and summarize the current issues, challenges and potential solutions. This is a rich new playground for the modern materials physicist.

The melanins are an important class of multifunctional biomacromolecules that possess a number of intriguing physical and chemical properties including electrical and photoconductivity. Unusually for a conducting organic material, eumelanin is hydrophilic and its electrical properties are strongly dependent on its hydration state. We have therefore measured adsorption isotherms for two polar adsorbates, water and ethanol, in the pressed powder pellets of synthetic eumelanin typically used in electrical studies. We show that a simple kinetic monolayer Langmuir model describes the adsorption and find that there are strong adsorbate-eumelanin interactions in both cases. These isotherms allow the proper scaling of electrical conductivity data and in doing so make progress toward a better understanding of eumelanin electrical properties, which is a critical prerequisite to the design of new eumelanin-like bioelectronic materials.