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From pacemakers to neurostimulators, implantable medical devices rely on batteries to keep the heart beating and reduce pain. But the batteries discharge over time, and their replacement requires invasive operations. To solve these problems, researchers from China have developed an implantable battery that runs on oxygen in the body. The study, published March 27 in the journal Chem, shows that in rats, the proof-of-concept design can provide stable energy and is compatible with biological systems.
"If you think about it, oxygen is the source of our life," says corresponding author Xizhen Liu, who specializes in energy materials and devices at Tianjin University of Technology. "If we can use a continuous supply of oxygen to the body, battery life will not be limited by the finite materials in conventional batteries."
To build a safe and efficient battery, the researchers made its electrodes from an alloy based on sodium and nanoporous gold, a material with pores thousands of times smaller than the width of a human hair. Gold is known for its compatibility with living systems, and sodium is an important and ubiquitous element in the human body. Electrodes enter into a chemical reaction with oxygen in the body to produce electricity. To protect the battery, the researchers placed it in a porous polymer film that is soft and flexible.
The researchers then implanted the battery under the skin on the rats' backs and measured its electrical output. After two weeks, they found that the battery could produce a stable voltage of 1,3 V to 1,4 V with a maximum power density of 2,6 μW/cm 2 . Although the output power is not sufficient to power medical devices, the design shows that it is possible to use the oxygen in the body to generate energy.
The team also assessed inflammatory responses, metabolic changes and tissue regeneration around the battery. No visible inflammation was found in rats. Byproducts of the battery's chemical reactions, including sodium ions, hydroxide ions, and low levels of hydrogen peroxide, were easily metabolized by the body and did not affect the kidneys and liver. The rats healed well after the implantation, and after four weeks the fur on their backs had fully grown back. To the researchers' surprise, blood vessels also regenerated around the battery.
"We were puzzled by the erratic power output immediately after implantation," says Liu. "It turns out that we have to give the wound time to heal, so that the blood vessels around the battery can regenerate and supply oxygen before the battery can provide stable electricity. This is a surprising and exciting discovery because it means the battery may help control wound healing."
Next, the team plans to increase the battery's energy output by researching more efficient electrode materials and optimizing the battery's structure and design. Liu also noted that the battery is easy to scale up in manufacturing, and choosing cost-effective materials could lower the price even further. The team's battery can also find other purposes beyond powering medical devices.
"Because tumor cells are sensitive to oxygen levels, implanting this oxygen-consuming battery around them could help prevent cancer. It is also possible to convert battery energy into heat to kill cancer cells,” says Liu. "From a new energy source to potential biotherapies, the prospects for this battery are exciting."
