Put a coil outside the metal body of the device and not too far from the skin. Then have another coil that you place on the skin, in the same region as the under-skin coil. Run a 1kHz sine wave through the external coil and you'll make SOME voltage on the internal coil. That allows you to charge.

For bonus points you can also run a communication protocol at say 100kHz (gotta get a couple of decades of frequency difference) and since you're charging the device, you can afford to "waste" a lot of power to get the signal out.

I don't do this stuff myself, but my dad did for quite a number of years.