A handful of people already put RFID chips into their skin a good 20 years ago. The problem with a tattoo is that it cannot easily change (unless henna), and it

A handful of people already put RFID chips into their skin a good 20 years ago. The problem with a tattoo is that it cannot easily change (unless henna), and its publicly available so it can be copied/mimicked (with e.g. henna but possibly other solutions). That makes it difficult to apply it for security reasons (CIA triangle). The same was true for RFID; can be easily copied. NFC is supposedly more secure, but nothing is 100% secure, so eventually you want a chip to be replaced. We have to ensure the hardware is replaceable.