Here is a nice little blurb in popular science on a newer compact nuclear battery.  This is the same type of power generator used in some currently operating space probes, although those have a much larger size.  This is a great power generating technology for cybernetic uses since the power generation lasts much longer than current batteries (decades) and is much more efficent.  Of course there is a risk of contamination from the radioactive materials that are used.

http://www.popsci.com/science/article/2009-10/scientists-develop-penny-sized-nuclear-battery