Chinese researchers engineer implantable oxygen-run battery
Two weeks after implantation of the oxygen-run battery in rats, the researchers observed stable voltage output between 1.3-1.4 V.
Took me a little bit this is the article that's being referenced. It's late so I'll look at this tomorrow. First impression is 2.6uW/cm2 is surprisingly high given that a human body doesn't produce that much energy at rest.
https://www.sciencedirect.com/science/article/pii/S2451929424000743