Recently, a simple muscle fibre has been made from fishing line. It is
wound much like the rubber band in a toy plane, where the twist takes
on a second twist.
Heating the result contracts the length.
Apparently it is rather a strong effect. There are many advantages of
this approach: cost being
one. However, the efficiency is not so good,
as the fibre needs to be heated. There is no fibre, so far, that satisfies
all requirements for weight, strength, efficiency, control, cost etc.
So I was thinking about what could be tried, and was trying to find a
hard material that was piezoelectric, but in fibre form was flexible
enough to be wound like the fishing line, much like fiber optic cable is
made from glass. I didn't find anything along those lines, but I found
that silk is a soft material that is piezoelectric.
So the idea is to use silk and arrange conductive layers on the outside
that make an electric field that hopefully cause a multiplied
piezoelectric effect.
I think that piezoelectric movement is more efficient than that from
thermal expansion, but I don't have any proof of that.