Here's a courtesy heads-up.
I just now stumbled across a circuit:
that allegedly can operate on as little as 10na while collecting energy, which it then uses to power a small pager motor once a threshold voltage is reached.
I have this circuit working in simulation now. Using just a plain voltage source behind a high value resistor as a crude solar cell proxy for simulating the given circuit, the circuit does seem to consume quite a lot more than the advertised 10na.
That said, someone on the EEVBlog built the circuit and reported that it consumes around 20na, whereas my first pass simulation using TINA shows it to be consuming around 8 microamps, i.e. roughly 1000x worse than the advertised 10na. Perhaps there is some way to improve the circuit to get the advertised 10na current?