There may be fractions of a degree difference in temperature on opposite sides of a flex implant, primarily caused by the break the implant creates in the capillaries that delivery blood to that area of the skin. That would only equate to millivolts and microamps from even the most efficient thermopile or peltier. Your body is very good at thermoregulation, so from an energy harvesting perspective I think subdermals are just not viable. Only transdermals like Zwack said.
If you want to have capacitors storing energy in an implant then check out this post for relevant calculations