Naturally occurring melanin possesses electronic properties that allow it to serve as a battery electrode, scientists report for the first time in the Proceedings of the National Academy of Sciences. The electronics that power iPhones and laptops are optimized for performance, but for the circuitry that powers implantable or ingestible medical devices the focus is biocompatibility. The challenge has been converting traditionally exotic and potentially toxic circuitry components into equivalent biomaterials that are less toxic but that can also provide a reasonable amount of power.
Ingestible electronics range in scale from edible cameras used during endoscopies of the upper GI tract to simple, cheap edible devices on pills that send alerts to cell phones or other receiving devices when a pill has been taken.
“We’re positioning ourselves in the middle,” says Christopher Bettinger, assistant professor of materials science and biomedical engineering at Carnegie Mellon University and a co-author on the study. Bettinger and his colleagues are interested in ingestible electronics that might, for example, measure and track the core body temperature of an athlete over a few hours. But the first step in creating such a device is breaking down the power source and rebuilding it in a biocompatible way that would make a user comfortable eating it.
An ideal electrode for a sodium ion battery, the type of energy storage device pioneered by Jay Whitacre, coauthor of the study, would have an anode with a surface area that can attract and support as many sodium ions as possible, and it would be redox active, capable of shuttling electrons from anode to cathode in a circuit. The spongy microstructure of natural melanin offers both.
The authors compared the efficiency of natural melanin taken from the ink sack of a cuttlefish with commercially available synthetic melanins. Their findings show that natural melanin, as an electrode material, has a specific capacity fifty percent larger than synthetic melanins.
Differences in their microstructures noted as variations in surface area could be to blame.
“Melanin is basically a disordered molecule, it’s very enigmatic as a structural pigment, and not all melanins are made alike,” Bettinger says. “If I get a protein from a bacteria, I can make the same protein synthetically, and it will look the same. Melanins are different. We can’t replicate the structure of natural melanin.”
The electronic properties of melanin are the discovery here, Bettinger adds. “We’re still unraveling how it works.”