Not only does melanin color our skin, hair, and protect us from sun damage, it could also be used to build electronic, bionic implants, according to a recent study published in Frontiers in Chemistry.
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Paolo Tassini and colleagues at the Laboratory for Nanomaterials and Devices in Italy have produced a study that reports an “abrupt increase” in the electrical conductivity of eumelanin, one of the three basic types of melanin.
Although the conductivity potential of melanin has been known since 1974, valuable applications for both synthetic and natural melanin have been hard to develop. This is due to the conductivity of melanin being “far too low”, as explained by Dr. Alessandro Pezzella of the University of Naples Frederico II.
Researchers have attempted to increase the conductivity potential of eumelanin by pairing it with metals, or super-heating it into a material similar to graphene. However, the material that resulted from these experiments was not fully biocompatible, unlike eumelanin.
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, Senior Author
Focusing on the structure of eumelanin, senior authors of the study Pezzella and Dr. Paolo Tassini of Italian National Agency for New Technologies, Energy and Sustainable Economic Development used a process called annealing to attempt to increase the conductivity of eumelanin.
Annealing is a heat process typically used in metallurgy and materials science that alters physical properties of materials to remove internal stresses and reduce hardness, making materials easier to work with and more ductile.
The researchers applied the annealing process to synthetic eumelanin films under high vacuum conditions from 30 minutes to six hours. The consequent material produced was dubbed “High Vacuum Annealed Eumelanin, HVAE” by the research team.
“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.”
Pezzella and Tassini described the success of using this novel process:
Our process produced a billion-fold increase in the electrical conductivity of eumelanin. This makes possible the long-anticipated design of melanin-based electronics, which can be used for implanted devices due to the pigment’s biocompatibility.”
Dr. Alessandro Pezzella, Senior Author
Although the conductivity of the eumelanin films increased to an “unprecedented value of over 300S/cm”, copper boasts higher conductivity as 6 x 107 S/cm, which limits the applications eumelanin-based bioelectronics could be used for.
The study itself acknowledges that “for valuable applications, higher conductivity values are needed yet, thus several studies explored the integration of the eumelanin with other more conductive materials.”
Additionally, conductivity decreases significantly when the eumelanin films come into contact with water, a significant disadvantage when considering the water content of the human body reaches 60 percent.
Pezzella noted: “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.”
Possible applications for eumelanin-based bioelectronics include replacing the brain implants used to treat epilepsy and Parkinson’s disease. The main challenge faced by developers of bioelectronics is finding materials that conduct both electrons and ions, as a large number of processes in the human body use ionic signals.
Other studies considering melanin-based bioelectronics have started to use a material that is very similar to biological melanin in electrical contacts, pH sensors, and photovoltaic cells.
The electronics industry is also being driven to look for new materials that are cheaper and environmentally friendly. With melanins being completely biodegradable and biocompatible, their advantages for both clinical use and the environment, and their potential for use in medical devices and sensors is clear.
Evidence of Unprecedented High Electronic Conductivity in Mammalian Pigment Based Eumelanin Thin Films After Thermal Annealing in Vacuum. Front. Chem. 26 March 2019. https://doi.org/10.3389/fchem.2019.00162
Will cyborgs be made from melanin? Pigment breakthrough enables biocompatible electronics. Phys.Org. 26 March 2019.