Decades ago, scientists were thrilled to find that eumelanin — the natural pigment responsible for darkening the skin and the hair — can conduct electricity. This immediately raised the possibility for its use in implantable electronics. And because melanin is already naturally found in the body, biocompatibility would be guaranteed. However, the electrical conductivity proved too weak for any meaningful real-life applications despite considerable effort to manipulate the pigment. In a breakthrough study, Italian researchers have found a way to rearrange messy melanin clumps into more orderly and thin sheets, thereby increasing the pigment’s conductivity a billion fold.
“Our process produced a billion-fold increase in the electrical conductivity of eumelanin,” the authors of the new study published in the journal Frontiers in Chemistry said in a statement. “This makes possible the long-anticipated design of melanin-based electronics, which can be used for implanted devices due to the pigment’s biocompatibility.”
Melanin is the pigment that gives human skin, hair, and eyes their color. Dark-skinned people have more melanin in their skin than light-skinned people. Melanin comes in two main forms called eumelanin and pheomelanin. Eumelanin is responsible primarily for brown and black hues, while pheomelanin appears as red and yellow hues. Both are produced by a specialized group of cells called melanocytes.
These compounds are of great interest to scientists since they naturally occur in virtually all life forms, are non-toxic, do not elicit an immune response, and are completely biodegradable. However, previous efforts to tame the electrical conductivity of melanin, such as combining it with other metals or superheating it with graphene, have come out empty handed.
Dr. Alessandro Pezzella of University of Naples Federico II and Dr. Paolo Tassini of Italian National Agency for New Technologies, Energy and Sustainable Economic Development took a different route from other researchers before them. Firstly, they started from the structure of eumelanin which is naturally made of millions of disordered layers on top of each other.
“All of the chemical and physical analyses of eumelanin paint the same picture—of electron-sharing molecular sheets, stacked messily together. The answer seemed obvious: neaten the stacks and align the sheets, so they can all share electrons—then the electricity will flow,” the researchers said.
In order to produce neatly arranged layers of eumelanin, the researchers turned to a method called annealing. Typically employed in metallurgy and materials science, annealing is a heat treatment process used mostly to increase the ductility and reduce the hardness of a material.
Films of synthetic eumelanin were introduced in a vacuum chamber and heated to 600°C for up to 6 hours. The annealing process resembles hair straightening, only this time it was just the pigment the researchers worked with. When the experiment was complete, the eumelanin films became dark brown and slimmed down from the thickness of a bacteria to that of a virus. Instead of disorderly layers, the melanin sheets self-arranged into a parallel configuration that enabled electron transfer. And unlike previous attempts, the films had not been burnt to a crisp.
“All our various analyses agree that these changes reflect reorganization of eumelanin molecules from a random orientation to a uniform, electron-sharing stack. The annealing temperatures were too low to break up the eumelanin, and we detected no combustion to elemental carbon.”
The researchers found that the conductivity of the melanin films increased billion-fold to 300 S/cm. This opens the possibility of using melanin in next-generation devices and biocompatible implants.
There is still much work to go, however. Despite the huge boost, the treated melanin is still half a billion times less conductive than copper. What’s more, it’s conductivity dropped considerably in the presence of water, which makes up most of our bodies.
“This contrasts with untreated eumelanin which, albeit in a much lower range, becomes more conductive with hydration (humidity) because it conducts electricity via ions as well as electrons. Further research is needed to fully understand the ionic vs. electronic contributions in eumelanin conductivity, which could be key to how eumelanin is used practically in implantable electronics.” concludes Pezzella.