Yes there are. They are commonly called aerogels or artificial muscles. They are currently being used mostly in solar cells because of the voltage needed to make them contract or expand. However, i was thinking, since robots have such a hard time walking, running, etc. with pneumatics and the current mechanics we use for their mobility, how come we cant make an accurate muscle group and whole robot with a similar muscle mapping to us with this material. And to compensate for the energy needed, the fibers could also act as solar cells. Unless this is already being tested or it's not capable of doing so.
http://www.nbcnews.com/id/29776085/