I’m a little behind the times on this one, but I just happened to stumble across it on the LiveScience website.  Michael Goldfarb, of the Vanderbilt University in Nashville, and colleagues are designing rocket powered bionic arms and legs.  They have a four pound prototype arm capable of lifting 20 - 25 pounds and operating for 18 hours of normal use on a single fill-up.  They say it is so quiet you wouldn’t hear it in a room with people talking at very low levels.