A laser shined from outside the body heats up one side of the power generation device allowing a temperature difference that creates a small voltage via the Seebeck effect. That small temperature change is then turned into energy to keep the device powered. The carbon nanotubes absorb heat very well and the implant for power would not need to be any larger than a half centimeter cubed. This could be one of the most life altering inventions for people depending on implanted battery powered devices to live in years.