FYI it generates electricity via thermo-voltaic (Seebeck effect) ceramics and beta decay.
Why not use an alpha emitter? Far easier to shield.
Is the reason for using beta decay that it produces more heat than helium nuclei?
By the way, when I was biking out in the snow one night I came up with an idea to preserve the heat that you lose from breathing. My scarf was wrapped around my head in such a way that exhaling would bring warm air to my ears, and I thought, I want that for my whole body! To do that, you'd simply need a sort of tracksuit with a branching series of plastic tubes, perforated with small holes all over, sewn in it. The largest tube would be about the thickness of your windpipe and branches into ever smaller tubes that spread across your chest, back, arms and legs. It functions as a reverse scuba, in that you inhale fresh air and exhale into the tubes. In this way, the heat that would be lost to the air normally, is used to warm up your skin so you don't get cold.
I imagine it wouldn't find much use as a consumer product but it could be very useful for arctic explorers and the like.