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Biomaterials

Biomaterials

Heat treatment turns melanin into an electrical conductor

29 Mar 2019 Isabelle Dumé
Polyindole packing evolution
Pictorial model of polyindole packing evolution during high vacuum annealing. Water molecules and carboxylic groups are shown, to show that they diminish in the material as the process temperature increases. Copyright © 2019 Migliaccio, Manini, Altamura, Giannini, Tassini, Maglione, Minarini and Pezzella. Front. Chem., 26 March 2019 https://doi.org/10.3389/fchem.2019.00162

Researchers have succeeded in increasing the conductivity of eumelanin – the dark brown pigment that colours skin, hair and eyes – to a record value of up to 318 S/cm by simply annealing it at high temperatures in vacuum. The material could thus now be employed in a variety of melanin-based bioelectronics.

Eumelanin is a form of melanin and conducts electricity – albeit weakly – in its natural state. Researchers first discovered that the polyindolic pigment was a semiconductor in the 1970s and suggested that this behaviour comes from energy bands associated with a non-localized empty molecular orbital within the eumelanin polymer chain. They also suggested that the material might be used in biocompatible electronics, but unfortunately, and not for lack of trying, they have been unable to significantly improve the conductivity of either natural or synthetic eumelanin.

Useful range for bioelectronics applications

A team led by Alessandro Pezzella of the University of Naples Federico II and Paolo Tassini of the Italian National Agency for New Technologies, Energy and Sustainable Economic Development has now increased the conductivity of synthetic eumelanin to 318 S/cm from an initial value that lies between 10-13 and 10-5 S/cm. Although still much lower than most metal conductors (copper has a conductivity of around 10S/cm, for example), this value is well in the useful range for bioelectronics applications.

Conductivity of vacuum annealed eumelanin

The researchers obtained their result by annealing a thin film of the material at different temperatures (of 230, 300, 450, and 600°C) in a high vacuum of 10−6 mbar. The samples were annealed for 30 minutes to six hours.

Well-aligned layers that are conducting

According to the researchers, the increased conductivity of their High Vacuum Annealed Eumelanin (HVAE), as they have dubbed it, comes from the fact that the heat treatment rearranges the molecular sheets in the eumelanin films into well-aligned layers that are conducting. In its natural state, the sheets are stacked in a disordered, random fashion, making electron flow between them difficult. The researchers observed this change thanks to Grazing Incidence Wide Angle X-ray Scattering measurements, among others.

Since the annealing was performed in vacuum, the temperatures employed do not degrade the eumelanin or carbonize (burn) it, but the films do lose mass and become thinner (as confirmed by thermogravimetric analysis and thickness measurements). The higher temperatures also remove both weakly and strongly bound water from the material, as well as carboxylic groups, say the researchers, but don’t damage the molecular backbones of the eumelanin.

Unfortunately, when the films are rehydrated, they do lose some of their high conductivity.

This is in marked contrast to unannealed eumelanin, whose conductivity increases in water, because it conducts electricity via ions and well as electrons, explains Pezzella. “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.”

Biocompatible devices and sensors

Nevertheless, it is now possible to start thinking about new types of biocompatible devices and sensors based on the pigment for medicine and research, says Tassini. And they are many. “Some examples include: devices for treating Parkinson’s disease through deep stimulation of the brain; human-computer interfaces for controlling artificial limbs; generating neurons and synapses from undifferentiated stem cells grown on the eumelanin; sensors to study cells and tissue behaviour in vitro in response to drugs or other stimuli; or electrodes made of eumelanin integrated in intelligent fabric to monitor the health of patients.”

The researchers, reporting their work in Frontiers in Chemistry, say they are now looking to improve the stability of the material in water and designing real-world devices. “We are just at the beginning of our study,” Tassini tells Physics World. “We would now like to better understand the HVAE chemical and physical properties and succeed in fully exploiting them for applications. We also hope that our results will be useful for other groups around the world studying eumelanin for bioelectronics.”

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