Suppose I want to implant a bio-chip that consumes a lot of power into my body, for example, a full-fledged CPU, and I don't want to lug around a heavy battery like the ones used in pacemakers, is it possible to be in Rome and do as the Romans do, i.e. draw and utilize the sugar inside my blood as fuel, like a huge living and breathing body cell or an organ like the liver? Let's assume I can put this device where there's a lot of blood flow, say the jugular vein.
One possible way to do this could be a microbial fuel cell.
This has, apparently, been tested ex-vivo: A pacemaker powered by an implantable biofuel cell operating under conditions mimicking the human blood circulatory system - battery not included
Another approach, which also works at least ex-vivo, is called Abiotic Biofuel Cell: Pacemaker Activated by an Abiotic Biofuel Cell Operated in Human Serum Solution
Keywords here are biofuel cell and microbial fuel cells, knowing these you can google the rest yourself.