Adding a solar cell below the dial has given the PowerWatch 2 enough juice to add the sort of features most folks expect from a smartwatch. That includes built-in heart-rate monitoring, a full-color display and, most surprising of all, onboard GPS. The cell is, like with other watches of this type, sat in a ring below the crystal and over the face, so better roll your sleeves up.
Thermocouples and solar cells are old-school pieces of technology that have been used to drive battery free devices for decades. Thermocouples rely upon the difference in temperature between your wrist and the case, using the disparity to generate energy. But that power is only sufficient for a dainty wristwatch, and so Matrix's efforts building a low-power smartwatch have to be applauded.