Mar 27 2019
Eumelanin, the dark brown melanin pigment, colors eyes and hair, and safeguards the skin from sun damage. For a long time, scientists have been aware that it conducts electricity, but very little for any practical application—so far.
In a breakthrough research published in Frontiers in Chemistry, Italian scientists subtly altered eumelanin’s structure by heating it in a vacuum.
"Our process produced a billion-fold increase in the electrical conductivity of eumelanin," say study senior authors 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. "This makes possible the long-anticipated design of melanin-based electronics, which can be used for implanted devices due to the pigment's biocompatibility."
Eumelanin is a biocompatible conductor
A young Pezzella had not even started school when researchers first discovered that a type of melanin can transmit electricity. Excitement rapidly grew around the discovery because eumelanin—the dark brown pigment found in skin, hair, and eyes—is totally biocompatible.
Melanins occur naturally in virtually all forms of life. They are non-toxic and do not elicit an immune reaction. Out in the environment, they are also completely biodegradable.
Dr. Alessandro Pezzella, Study Senior Author, University of Naples Federico II.
Several years later, and notwithstanding widespread research on the structure of melanin, nobody has discovered a way to harness its potential in implantable electronics.
"To date, conductivity of synthetic as well as natural eumelanin has been far too low for valuable applications," he adds.
A few scientists attempted to increase the conductivity of eumelanin by super-heating it into a graphene-like material or merging it with metals—but what remained was not actually the biocompatible conducting material hoped for.
Resolute to discover the real deal, the Neapolitan group studied the structure of eumelanin.
"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."
Heat treatment straightens out hair pigment
This process, known as annealing, is used already to boost electrical conductivity and other properties in materials like metals.
For the first time, the scientists tested films of synthetic eumelanin in an annealing process under high vacuum to sort out them out—similar to hair straightening, but with just the pigment.
"We heated these eumelanin films—no thicker than a bacterium—under vacuum conditions, from 30 min up to 6 hours," describes Tassini. "We call the resulting material High Vacuum Annealed Eumelanin, HVAE."
The annealing worked miracles for eumelanin: the films reduced down by more than half, and developed quite a tan.
"The HVAE films were now dark brown and about as thick as a virus," Tassini states.
Importantly, the films had not just 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."
A billion-fold increase in conductivity
Having accomplished the planned structural alterations to eumelanin, the scientists proved their hypothesis in a remarkable manner.
"The conductivity of the films increased billion-fold to an unprecedented value of over 300 S/cm, after annealing at 600 °C for 2 hours," Pezzella confirms.
Although quite short of most metal conductors—copper has a conductivity of about 6 x 107 S/cm—this finding propels eumelanin well into a practical range for bioelectronics.
Furthermore, the conductivity of HVAE was modifiable consistent with the annealing conditions.
"The conductivity of the films increased with increasing temperature, from 1000-fold at 200 °C. This opens the possibility of tailoring eumelanin for a wide range of applications in organic electronics and bioelectronics. It also strongly supports the conclusion from structural analysis that annealing reorganized the films, rather than burning them."
There is one potential downside: immersion of the films in water causes a noticeable decrease in conductivity.
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.
Dr. Alessandro Pezzella, Study Senior Author, University of Naples Federico II.
Source: http://www.frontiersin.org