Scientists have significantly increased the conductivity of a type of melanin skin pigment, opening up the possibility of biocompatible electronics for implantable devices or, more speculatively – human cyborgs.

Melanin has been known to conduct electricity for decades, but researchers had been unable to increase electrical conductivity enough to power devices.

But scientists at the University of Naples Federico II subtly say they have created a “billion-fold increase” in eumelanin’s conductivity.

They did so by modifying the structure of eumelanin – a dark brown type of melanin – by heating it in a vacuum.

The process, known as annealing, neatened the structure of the synthetic eumelanin, reorganising the molecules from a random structure to a “uniform, electron-sharing stack”.

This structure let electricity flow more freely, dramatically increasing conductivity for biocompatible electronics.

“Our process produced a billion-fold increase in the electrical conductivity of eumelanin,” said 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.”

Pigment power

Although the annealing process is already used to increase conductivity in metals, the researchers say it is the first time it has been used on synthetic eumelanin under a high vacuum.

“We heated these eumelanin films – no thicker than a bacterium – under vacuum conditions, from 30 min up to 6 hours,” said Tassini. “We call the resulting material High Vacuum Annealed Eumelanin, HVAE.”

The films did not burn up, but turned “dark brown” and became “about as thick as a virus”, added Tassini.

“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,” said Pezzella.

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To caveat, though, it is far less conductive than most metals. Copper, for example, has a conductivity of around 6 x 107 S/cm.

And the researchers add that immersing the eumelanin structure in water notably reduced conductivity – the opposite to untreated eumalanin, which slightly increases conductivity when hydrated.

The scientists also found that treating the eumelanin films at higher temperatures increased their conductivity, which they say “opens the possibility of tailoring eumelanin for a wide range of applications in organics and bioelectronics”.

The study was published today in the scientific journal Frontiers in Chemistry.


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