From the article: "Our system can run nearly perpetually if periodically exposed to reasonable lighting conditions, even indoors," said David Blaauw, an electrical and computer engineering professor. "Its only limiting factor is battery wear-out, but the battery would last many years."
That's cool! The Arm Cortex-M3 is a very standard microcontroller - it supports USB! Article doesn't say what the battery chemistry is... shame the device is limited to the lifespan of a battery (I don't imagine the battery is replaceable) but that's hard to avoid! This is really cool idea and achievement.
Some relevant links: