SkullConduct Uses Your Skull as a Password

The SkullConduct system replaces conventional passwords with the sound of your skull keeping smart glasses secure. The system will activate automatically when placed on the head, playing an imperceptible ultrasonic hum that resonates through the user’s skull to create a unique sound in each individual.

https://www.newscientist.com/article/2085430-the-buzz-of-your-skull-can-be-used-to-tell-exactly-who-you-are/