Talk about going green with a vengeance. People desperate to reduce their carbon footprint are even going to re-heat coffee the green way. This concept design not just uses a fuel cell to heat the beverage inside the mug but it is smart enough to understand that heating the container / mug is a waste of energy besides being annoying.

Since the heater stays in direct contact with the drink it is able to do both. Once the drink is heated, a thermographic ink changes color to let you know that its ready.

Source