Crazy pondering of the day
I recently came across this chip:http://www.linear.com/product/LTC3107
It's able to boost from 20mV up to 5v at 3mA and could theoretically be used to charge a capacitor from nerve impulses.
Add a Kinetis KL02 (which can run on 0.036mA) and you have a complete system that can run completely on nerve impulses (or other low-current power supplies).
As for practical uses - well, human nerve impulses are generally about 80mV and not a lot of current is needed to activate a nerve cell and make it take over transmission of the impulse.
With some development it could even be possible to swap out a few neural circuits for software implementations - you just need a mathematical model of the whole circuit and the right interfaces.