Decoupling Ionic and Electronic Currents in Melanin

Melanin, the human skin pigment, has emerged as a model material for bioelectronic interfaces due to its biocompatibility, ability to be processed into electronic-device-grade thin films, and transducing charge transport properties. These charge transport properties have been suggested to be of a mixed protonic/electronic nature, regulated by a redox reaction that can be manipulated by changing the material's hydration state. However, to date, there are no detailed reports which clarify, quantify, or disentangle the protonic and electronic contributions to long-range current conduction in melanin. Described herein, is a systematic hydration controlled electrical study on synthetic melanin thin films utilizing impedance/dielectric spectroscopy, which rationally investigates the protonic and electronic contributions. Through modeling and inspecting the frequency dependent behavior, it is shown that the hydration dependent charge transport is due to proton currents. Results show a real dielectric constant for hydrated melanin of order approximate to 1 x 10(3). Surprisingly, this very high value is maintained over a wide frequency range of approximate to 20-10(4) Hz. The electronic component appears to have little influence on melanin's hydration dependent conductivity: thus the material should be considered a protonic conductor, and not as previously suggested, a mixed protonic/electronic hybrid.