Melanin is a material responsible for the creation of pigments – it is able to absorb ultraviolet light and can also bind and unbind metallic ions. To use it as part of a battery, the team added it to a few other ingredients to create a mix that could be poured onto a steel mesh after adding sodium ions. The battery that resulted, the team found, was capable of discharging for up to five hours, although offering less power than traditional batteries.
"For decades, people have been envisioning that one day, we would have edible electronic devices to diagnose or treat disease," said team member Christopher Bettinger, Ph.D. "But if you want to take a device every day, you have to think about toxicity issues.
"An ingestible, degradable device is in the body for no longer than 20 hours, so even if you have marginal performance, which we currently do, that's all you need."
The team has experimented with battery designs that use melanin pigments at either the positive or negative terminals; various electrode materials such as manganese oxide and sodium titanium phosphate; and cations such as copper and iron that the body uses for normal functioning.
"We found that they work," said Hang-Ah Park, a post-doctoral researcher at CMU. "We can power a 5mW device for up to 18 hours using 600milligrams of active melanin material as a cathode."
Although the capacity of a melanin battery is low relative to a lithium-ion, it should be high enough to power an ingestible drug-delivery or sensing device for delivering bursts of a vaccine over several hours before degrading.
In parallel with the melanin batteries, the team is also making edible batteries with other biomaterials such as pectin, a natural compound derived from plants and used as a gelling agent in jams and jellies.
