Everyone's heart rate is apparently unique, as much or more than fingerprints or a face if we are to believe this patent recently granted to Apple.
A heart sensor coming to the iPhone soon?££££
The document describes a technology that identifies the user's particular heart rate to give them access to their device, in the same way that Face ID does with the face, or Touch ID does with fingerprints. A system that could be implemented simply on an Apple Watch, already capable of capturing heart rate. But the patent goes further by mentioning the addition of such a sensor to the iPhone. All you would have to do then is hold your smartphone for it to recognize you thanks to your heartbeat passing through your fingers.
Note that the document promises increased security and even faster user recognition than the biometric scans (and passwords) currently in use.