I did this three years ago. I have the Dangerous Things chip with 886 bytes of user read/write memory. I implanted it myself, using sensible precautions and had no infection or migration problems. I could, I guess, have a private key on it, but that would involve putting my key on my phone - maybe you could air gap an arduino with NFC...A recent earlobe piercing caused way more problems, and I think hurt a lot more.

I did it as soon as I heard it was a thing and bought a kit on the internet - at the time the dangerous things one was about more (shipping was crazy outside the USA) and I paid $99. A surgeon friend has looked at it since, and said its very much like the under skin contraceptive implant - and could easily be removed. I've had a CT and an xray on that hand as a result of a completely unrelated accident and no problems. It sits on just under the skin on the fat pad between the thumb and index finger. I've used it to unlock my phone, direct people to my website, and now its got my bitcoin address on it. Its more of a novelty than anything but its pretty cool imho. I was at a class where having a chip in your hand in the future came up and I was able to show off, lol.
I'd love to be able to pay for small items just by scanning my hand.

This is the same thing https://store.dangerousthings.com/product/xnt/

So for now if you scan my hand with an NFC phone you can send me bitcoin...