Capacitive touch on off power switch
You could use a pSOC device of which there are quite a few variants. These are a Programmable System On a Chip made by cypress semiconductors and they comprise of a microcontroller with a set of configurable analog and digital subsystems. Tseveralmof the devices have inbuilt capacitive touch inputs and they can operate at very low power.

Alternatively if you already have a chosen microcontroller, then a CMOS gate could be used as a capacitive touch input d this could be used to bring your controller out of standby/sleep or even switch a transistor to allow full powerup.