Over at the the University of Michigan they have demoed a prototype self powering computer just over one cubic millimeter in size.

This makes it the first complete millimeter-scale computing system. Designed as an implantable eye pressure monitor for glaucoma patients, the computer consists of a low-power microprocessor, a pressure sensor, memory, a thin-film battery, a solar cell and a wireless radio to transmit data to an external reader device.

The computer achieves ultra-low power consumption with a unique power gating architecture and an extreme sleep mode:

The newest system wakes every 15 minutes to take measurements and consumes an average of 5.3 nanowatts. To keep the battery charged, it requires exposure to 10 hours of indoor light each day or 1.5 hours of sunlight. It can store up to a week