I understand that they used two different materials and combined them through fiber drawing process but I don’t truly understand which materials were used. Here is where it is mentioned in the article. “Using two different polymers bonded together, a very stretchable cyclic copolymer elastomer and a much stiffer thermoplastic polyethylene“. From this I was able to understand that it could have been any number of materials from silicone to rubber to normal plastic. Please anyone able to even get a close guess please do so but also please have a material science/engineering background or something similar. I’m trying to replicate their results so before I call up any fiber drawing companies(who seem to all be fiber optic manufacturers) and see if they would be willing to do a custom job and make this material for me I would need to be able to tell them materials to use. Here is the articula please read it all to have a better understanding. https://news.mit.edu/2019/artificial-fiber-muscles-0711.

Thank you and also please keep in mind I have a planned use for them so I would need them to be about 50 feet long and plan on bundle them together so I don’t know if that would effect the type of materials used but thought I’d mention it. And will be planning on making them as thick as possible for added strength.

EDITED: Please use laymen’s terms as much as possible!

  • 1
    $\begingroup$ Half the answer is there: a kind of polyethylene. The copolymer might be their secret ingredient. What's got more fatigue capability than PE? $\endgroup$
    – Abel
    Sep 16, 2023 at 2:16
  • $\begingroup$ HDPE is already pretty high. To beat that in fatigue... makes me think of rubber for the other- like silicone or polyurethane $\endgroup$
    – Abel
    Sep 16, 2023 at 2:30
  • $\begingroup$ Do you have any idea what would be the best kind of polyethylene to use? $\endgroup$
    – Ink
    Sep 16, 2023 at 2:43
  • $\begingroup$ As high a density as manageable. They did say it had to be stiff. The drawing may not be as simple as you think. Something has to put a bias in the material for it to curl in such a neat and organized manner. $\endgroup$
    – Abel
    Sep 16, 2023 at 2:52
  • $\begingroup$ I’m sorry material Science is not my speciality what do you by mean “bias”? $\endgroup$
    – Ink
    Sep 16, 2023 at 3:09

1 Answer 1


The mechanism is pretty simple in terms of materials with different thermal expansion coefficients. Normally when two such materials are bonded together at a surface, heating causes the result to bend away from the one with greater expansion. In this case, the high expansion entity is inside a spiral with the lesser expansion material on the outside.

When heated, the material on the inside grows, increasing the diameter of the spiral. The material on the outside fights an overall length expansion, but (un)bends much easier than it lengthens, allowing the diameter to grow.

To make something similar, take a silicone rubber piece and stretch it really far. Melt a strip of HDPE onto one side of it, letting it bond (it will need both heat and pressure). When the HDPE has cooled (hopefully bonded), release tension in the silicone. The silicone (hdpe side won't want to shrink since it was not stretched) will pull the shape into curling up with the silicone on the inside. Annoying part is getting the thicknesses and stretch right to get a tight enough curl.

Theoretically might also work with other types of materials too. In fact, it'd be similar to how bimetal thermometers or the bimetal open/close mechanism on foundation vents work. Key difference in the thermometers, the spiral is usually used to rotate a dial, whereas here that rotation is prevented in order to generate tension.

  • $\begingroup$ Wait so you’re saying it would curls the moment it was done being made? It is supposed to curl on command when ever it is heated at anytime not immediately after it is taken out of the fiber drawing machine. $\endgroup$
    – Ink
    Sep 16, 2023 at 19:57
  • $\begingroup$ Not according to the videos. Obvious bit is that the material is a spiral in both elongated and compressed forms when doing lifting. This is also one of the reasons when used to free lift the weight, they use multiple and with different spiraling directions. In the arm, the spiral can't turn due to the joint of the arm. In the free hanging lift, the spin is cancelled by pairing spirals. $\endgroup$
    – Abel
    Sep 17, 2023 at 2:29
  • $\begingroup$ Huh I see I missed that thank you very much. $\endgroup$
    – Ink
    Sep 17, 2023 at 5:32

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