I seem to recall hearing about people freaking out about just this sot of thing…

A smart tattoo implanted under your skin? Meet the next wave in wearables

The backbone of Project Underskin is a sub-dermal tattoo that would be implanted into your hand and would interact with everything you touch. You’d be branded with the ability to unlock your door, track your health or exchange information with a handshake.

Useful? Yes, very likely. Creepy? Oh, you betcha.