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Sheath-run artificial muscles

July 2019: New evidence in the field of artificial muscles for use in robots, amongst others, proposes that twisting materials such as nylon into coils, coating it in polymer, and heating or cooling it to the effect of contracting or actuating, creates a new and cheaper form of “smart material” that may have the potential to replace pneumatic cylinders and electric motors.


Over the last 30 years, there has been rising interdisciplinary interest in exploring a class of materials and devices that are able to create artificial muscles. These materials can reversibly contract, expand, or rotate within one component in response to external stimuli such as voltage, current, pressure, temperature, glucose, etc. They mimic types of motion and dexterity, such as bending or squeezing, that can be created by real muscles. New evidence proposes that Sheath-run Artificial Muscles (SRAM) might offer cheaper and more flexible solutions. Current methods focus on the use of expensive materials, such as high-tech carbon nanotubes. In contrast, SRAMs use inexpensive materials ranging from fishing lines, to silk or bamboo yarns, which are twisted into coils and coated in temperature-sensitive polymer Using this technique, researchers have created muscles that contract or expand along their length, providing mechanisms with higher work-per-cycle and power densities than previous muscles. In adding a sheath in form of polymer coating, the researcher developed a way of using materials that can be taken from waste production and recycling processes, significantly reducing production costs and promoting a wider application of artificial muscles.

Implications & Opportunities

Producing cheaper artificial muscles is attractive for application in intelligent structures such as robotics, artificial intelligence, prosthetic limbs, exoskeletons or comfort-adjusting clothing. For example, the team created a SRAM made from polymer-coated nylon thread that linearly contracts when exposed to glucose. This ‘muscle’ could be used to squeeze a pouch and release insulin to balance high blood sugar. This emerging disruptive technology may have the potential to replace conventional pumps, compressors, fans or escalators, thereby providing a cheaper and slim-lined alternative to bulkier electric motors or pneumatic cylinders.


Producing SRAMs remains pre-industrial stage and the study serves as proof of concept. More research is needed to fully develop the potential of SRAMs and to pave the way for commercialisation.


Mu,J., (2019). Sheath-run artificial muscles. Science, 365, 150-155. DOI: 10.1126/science.aaw2403

New Scientist. (2019). Super strong artificial muscle can lift 1000 times its own weight. Retrieved from

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Adele Wiliams

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The views expressed in these external research papers are those of the authors and do not represent an official position of CISL, the University of Cambridge, or any of its individual business partners or clients.