You are unique. This is one of the more obscure ways you're unique: An alternating current of different frequencies running through you causes a reaction that's noticeably different from anyone else's. Researchers from Dartmouth University are trying to put this difference to use by creating wearable electronics that respond to--and only to--their intended user.
The design they're discussing is called "Amulet," a device "not unlike a watch" that could take a measurement like this, confirming the identity of a person. The device would use small electrodes to measure how the body's tissue react to the alternating current, which changes from person to person. It's a lock that's keyed into your biology; when it's set up with the device, it only unlocks it for you.
After that, it gets even better: once that connection has been established, researchers say, that device can coordinate with others. Those devices would join the party through physical contact--maybe as easily as being slipped into a pocket, and staying securely rooted in your unique biology.
A system like that could be used to better monitora person's health; a single device attached directly to the body could monitor that person from anywhere, without causing wireless security concerns. But researchers are conceding that a better way of reliably interpreting the data coming from the sensor will still take time, and reliability is more than a little important for something like this.