Is that available as a circle as well? Because in that case, I could use the fact that it will be clearly visible (being 2mm thick) as a visual effect which would be quite fitting… but I guess probably not, because there has to be some kind of chip inside for it to work with NFC 
Okay… in the picture, it looks like there is some kind of “metal”-ish ring around the thing, is that right? If so, what material might that be…? I’m highly allergic to nickel, so I’m not all too happy to implant unknown metals into my body…
But still, veeeery interesting thing!
As for the second, single-LED-blinky, looks quite nice too - has anyone experience with installation on more unusual places, like chest, shoulders, near (not directly over!) joints, whatever? I know that silicone implants over softer tissue are idiotic, because they simply sink in and are not visible - but what about these? Any idea? 