Electrolyte-filled twist-spun carbon nanotube yarn that is thinner than a human hair is able to function as a torsional artificial muscle in a simple three-electrode electrochemical system.

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By constructing a yarn that twists, Dr. Forougi from Australia and researchers around the world (including Ray Baughman of the University of Texas at Dallas) have created a design that may be applicable to small (meaning very small–thinner than a human hair) motor designs. The yarns are twist-spun from forests of multiwalled carbon nanotubes (MWCNTs). When these yarns are half-way inserted into a fluid that conducts electricity with a counterelectrode (a device that simply conducts a charge opposite to that of the electrolyte fluid) the yarn partially untwists–a process known as torsional actuation. The untwisting is hypothesized to occur due to an increase in ions flowing though the yarn which increases pressure, similar to the increase in ions flowing through our muscles when we run. To alleviate the pressure flux, the yarn untwists to create a larger surface area.    The end that is left outside the fluid (the nonactuating length of yarn) allows for the reversibility of the twist that occurred under electrical impulse, thus preserving the torque power. For the process to work correctly, both ends of the yarn must be tethered (think of holding a string between two hands and twisting the string–this works much better than dangling a string from just one hand and attempting to twist it without the other end being fixed).

Key Differentiators

While carbon nanotube artificial muscles using linear or bending modes and powered by electricity, fuels, light, or heat are well known, there is still much to be learned about rotational artificial muscles. In fact, Dr. Foroughi and his research team are among the first to offer a design solution for an artificial muscle that provides large torsional rotation per muscle length. Their new nanotube yarn has reversibility of its torsional spin due to tethering at both ends that prevents full rotation. This particular design twists about 1,000 times as much as any previous nanotube yarn, producing a torque that is comparable to many large motors. Most other carbon nanotubes struggle with the reversibility of their movement and do not offer torsional (circular) movement, rather they utilize stretch movement through contraction and expansion. In addition, this design is the most efficient of its class; when comparing this artificial muscle design to others, “the observed torsional rotation of 250° per millimeter of actuator length is more than 1000 times the values previously reported for materials that torsionally actuate” (Science 2011: 495).

Biomimicry Story

The video below features researcher, Ray Baughman, and the inspiration he has received from nature’s design. He discusses how these nanotubes are of much interest to a variety of application fields including the Army and fields that require rotational positioning and high torque generation (such as microfluidic pumps, valve drives, and mixers).Biomimicry Artificial Muscles

Challenges Solved

Most electric motors are large and complex which makes it difficult to miniaturize their designs. As you decrease the size of motors, they lose the torque strength per unit mass and become extremely expensive. Losing torque makes motors less efficient and incapable of performing tasks that demand a lot of mechanical work. The extra twist provided by this design solves the problem of losing torque and strength; in fact, these small nanotubes are more than a 100 times the strength of steel (BBC News 2011: 6).

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