"One single implanted GBFC device of 0.24 mL volume (2.4 mL for the whole implant) produced the power required to operate, using a specially designed electronic circuit to charge a capacitor, two types of electronic devices: a LED and a digital thermometer."

https://www.nature.com/articles/srep01516

I'm waiting for someone to invent the implantable chip that converts blood sugar into heat/light purely with the intention of wasting that energy.

Grab enough power to monitor blood sugar, to make sure you aren't putting someone into a coma, but otherwise just try to burn an extra few hundred calories a day.

The real question is can that conversion be done in an efficient enough way with materials that are safe to put inside someone and in a small enough package.