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Made using a reportedly inexpensive new fabrication method, the thermally driven polymer muscles were developed at the University of Texas at Dallas.
Dr Ray Baughman, the senior and co-corresponding author of the new research, has previously made artificial muscles from materials such as polymer fibres and carbon nanotubes, wrapping the fibres around a spindle, or mandrel, to form the coil.
The new method is mandrel-free, however, and creates polymer muscles that can stretch 97% of their original length and have a high spring index (the ratio of a spring’s mean diameter to the diameter of its component fibre or wire) of over 50, meaning they are loosely wound and flexible.
“The problem has been that there are no process reports for making mandrel-free, large-spring-index yarns other than to dissolve the mandrel after muscle coiling, which wastes the large-diameter polymer fibre that is typically used as a mandrel and creates a waste stream,” said Dr Baughman.
The new mandrel-free fabrication process involves inserting twists into individual fibres, then plying the twisted fibres to create spring-like coils. During plying, each fibre serves as a mandrel for other fibres.
The expansion and contraction of the muscles is driven by heat, which is provided by applying electricity or by the use of solvents or electrochemical means.
Along with robotics, the researchers said one potential application for the technology is comfort-adjusting jackets that automatically open large thermally insulating pores as the ambient temperature decreases. Dr Baughman and his team previously licensed their polymer artificial muscles made using a mandrel-wrapped process to a clothing manufacturer that incorporated the technology into jackets worn by members of the US team at the 2022 Winter Olympics. Because the mandrel-wrapped process was so expensive, however, the jackets are no longer commercially produced.
Development of the mandrel-free fabrication technique was led by Dr Mengmeng Zhang, lead author of the new study. “High-spring-index yarns can be made much less expensively by this mandrel-free method,” Dr Zhang said. “When heated and cooled, these muscles can significantly contract and elongate due to their large spring index.”
Dr Zhang said the fabrication method also enables the spring index to be varied along a muscle’s length. The researchers used the mandrel-free method to make high-spring-index carbon nanotube yarns, which could be used to harvest mechanical energy or as self-powered strain sensors.
The team has applied for a patent on the technology.
The research was funded by the US Office of Naval Research, the US Air Force Office of Scientific Research and The Welch Foundation. It was published in Science.
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