These cells are named as GBFC (Glucose biofuel cells). The first functional GBFC that researchers made recently can generate peak specific power of 24.4 µW/mL which is sufficient for a standard pacemaker. The cell can be further enhanced to produce 50 times more power to satisfy needs of all kinds of implants in humans.
At present, a number of functional artificial organs are under development. Artificial hearts, kidneys, hands, fingers, and even eyes can be implanted now. But they all share a common problem. The need of a battery to run and battery have a limited lifespan. Many times operations are done just to replace the battery.
Working Principle of GBFC
GBFC works similar to Voltaic cell or Daniell cell. Here glucose and dioxygen serve the purpose of electrolyte.
In tradition cells, since we have limited volume of electrolyte, when it decompose fully, the cell goes dead. But for a GBFC, glucose and oxygen of our body are both available in plenty. So theoretically, a biofuel cell could keep working indefinitely.