Assuming 15C delta temp(pretty high for typical users) and 140mAh battery that lasts a day I calculate it would take roughly 46 cm2 to power it. That is about one order of magnitude too large for typical wearables. It is nice work but 46 cm2 worth of solar cell will also power this in an office environment. If you get an athlete in a cold environment the numbers do start to look reasonable.
Hello, does not work in temperate climates where the temp difference is small and heat transfer away from the 'generator' is not a sure thing. And as a Norwegian I know it's not going to work in winter. I'm not going to bare my wrists! Thus the temperature inside my jacket and mittens cafrefully covering my wrists is the same as my external body temperature ie no energy can be harvested. A lot more work is needed.
However, if someone comes up wirt a safe and efficient way to move energy from the shoes to the wrist (apart from wires or using the body's blood stream) then we start talking re how to power a werable device!
Wonder what sort of regulation this little guy needs before it's useful to be plugged in. I don't particlarly imagine watchmakers jumping at the prospect of adding a regulator IC on the already small footprint.