This appears to be two materials bonded together who expand at different temperatures causing a coiling effect vs a single material changing properties.
The University of Texas approach was fishing line OR sewing thread. (Just a twisted single-material polymer line.)
At the start of this MIT article, I was wondering if it was just a re-hash of the fishing-line 'muscle' discovery as well.
But it's clear that the MIT muscle coils up because of the difference in thermal expansion between two bonded materials, while the Texas one seems to be more about a coiling pattern that multiplies the effect of a thermal expansion or contraction of a single material.