By using the skin pigment melanin as an electrode material, researchers have made batteries that people could swallow. Such edible batteries could potentially power medical implants that disintegrate in the body instead of having to be surgically removed.
The idea behind biodegradable electronics is to make devices that go into the body, for example, to measure temperature, monitor wounds, or deliver drugs. Once their job is done, the devices could crumble into smaller pieces that are easily eliminated by the gastrointestinal tract, said Christopher Bettinger, a materials science and engineering professor at Carnegie Mellon University (CMU). “Unlike a pacemaker that has to last for five years, such edible electronics only need to last about 20 hours,” he said. “So we want simple devices that are cheap and biocompatible.”
Bettinger and others have already made biodegradable transistors and capacitors using various natural and synthetic materials. But the essential power sources for edible electronics were missing. Lithium-ion batteries could be used in the body if they are packaged safely, said Bettinger, but the toxic lithium and electrolytes in them could be a problem if the battery leaks or gets stuck in the body.
So he teamed up with materials science and engineering professor Jay Whitacre to use melanin electrodes in the sodium-ion battery chemistry that Whitacre has developed. Melanin shares some key properties with conventional electrode materials: it binds reversibly with ions, it generates electrons, and it is composed of uniform nanoscale granules that give it a very high surface area.
The sodium-ion battery relies on a water-based electrolyte to move sodium ions between its electrodes. The researchers replaced the activated carbon anode with those made of melanin. To make the anodes, they extracted melanin from cuttlefish, loaded it with sodium ions, and encased it in a steel mesh. The result is a battery that uses benign, abundant materials such as water, sodium, and melanin. “The cathode is manganese, which people need in their diets,” Bettinger said. “We’re using materials that are found in the body or that we eat anyway.”
The battery could power a body temperature sensor for five hours, although its power output was less than that of a traditional battery. The melanin anodes, meanwhile, could provide as much current per mass as some well-known anode materials. Bettinger, Whitacre, W.J. Kim, and W. Wu of CMU and S.-E. Chun of the University of Oregon reported their results in the December 24 issue of the Proceedings of the National Academy of Sciences (DOI:10.1073/pnas.1314345110; p. 20912).
“This introduces a route to batteries built with biodegradable organic materials,” said John Rogers, a materials science and engineering professor at the University of Illinois at Urbana-Champaign commenting on this study. Other possibilities to power edible electronics have been demonstrated recently, he said. These include mechanical energy harvesters; degradable solar cells; and RF antennas and rectifiers that could be used to power devices from outside the body. But, he said, batteries are an important power supply option.
“This new system appears to offer a scalable route to high power output,” Rogers said. “Successful integration of these batteries with electronics and sensors and wireless communication components, all of which now appear possible due to rapid advances in chemistries and materials, can enable devices that go into the body and then naturally disappear.”
