Bioelectronics made of melanin?
Italian scientists have achieved a billion-fold increase in the electrical conductivity of melanin, which could unleash its potential in safe, sustainable bioelectronics. Their work has been published in the journal Frontiers in Chemistry.
The dark brown melanin pigment known as eumelanin colours hair and eyes, and protects our skin from sun damage. It has also long been known to conduct electricity, but too little for any useful application — until researchers led by Dr Alessandro Pezzella and Dr Paolo Tassini subtly modified its structure by heating it in a vacuum.
Dr Pezzella had not even begun school when scientists first discovered that a type of melanin can conduct electricity. Excitement quickly rose around the discovery, because eumelanin is fully biocompatible.
“Melanins occur naturally in virtually all forms of life,” said Dr Pezzella, from the University of Naples Federico II. “They are non-toxic and do not elicit an immune reaction. Out in the environment, they are also completely biodegradable.”
But decades later, nobody had managed to harness melanin’s potential in implantable electronics. “To date,” Dr Pezzella said, “conductivity of synthetic as well as natural eumelanin has been far too low for valuable applications.”
Some researchers tried to increase the conductivity of eumelanin by combining it with metals, or super-heating it into a graphene-like material, but what they were left with was not truly the biocompatible conducting material promised. Determined to find the real deal, the Neapolitan group considered 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,” Dr Pezzella said. “The answer seemed obvious: neaten the stacks and align the sheets, so they can all share electrons — then the electricity will flow.”
This process, called annealing, is used already to increase electrical conductivity and other properties in materials such as metals. So the researchers put films of synthetic eumelanin through an annealing process under high vacuum to neaten them up — a little like hair straightening, but with only the pigment.
“We heated these eumelanin films — no thicker than a bacterium — under vacuum conditions, from 30 min up to 6 hours,” said Dr Tassini, from the Italian National Agency for New Technologies, Energy and Sustainable Economic Development. “We call the resulting material High Vacuum Annealed Eumelanin (HAVE).”
The annealing worked wonders for eumelanin: the films slimmed down by more than half, and picked up quite a tan — Dr Tassini reports that they were “now dark brown and about as thick as a virus”. Importantly, the films had not simply been burnt to a crisp.
“All our various analyses agree that these changes reflect reorganisation of eumelanin molecules from a random orientation to a uniform, electron-sharing stack,” Dr Tassini said. “The annealing temperatures were too low to break up the eumelanin, and we detected no combustion to elemental carbon.”
After annealing at 600°C for 2 hours, and achieving the intended structural changes to eumelanin, Dr Pezzella confirmed that the conductivity of the films had increased a billion-fold to “an unprecedented value of over 300 S/cm”.
Although this is well short of most metal conductors — copper has a conductivity of around 6 x 107 S/cm — this finding launches eumelanin well into a useful range for bioelectronics. What’s more, the conductivity of HVAE was tuneable according to the annealing conditions.
“The conductivity of the films increased with increasing temperature, from 1000-fold at 200°C,” Dr Pezzella said. “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 reorganised the films, rather than burning them.”
There is one potential dampener: immersion of the films in water results in a marked 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,” Dr Pezzella said. “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.”
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