Carbon-nanotube artificial muscles

October 13, 2011

A scanning electron micrograph image of a 3.8-micron diameter carbon nanotube yarn that functions as a torsional muscle when filled with an ionically conducting liquid and electrochemically charged. The angle indicates the deviation between nanotube orientation and yarn direction for this helical yarn. (Credit: University of Texas at Dallas)

Researchers in Texas, Australia, Canada and Korea have constructed carbon-nanotube yarns to build artificial muscles that twist like the trunk of an elephant but provide a thousand times higher rotation per length.

The breakthrough shows promise for improved microfluidic pumps and valve drives, and other applications that require tiny motors, according to the researchers.

The research comes, in part, from the lab of UT Dallas professor Ray H. Baughman, who has done previous groundbreaking work in carbon nanotube yarns for use in artificial limbs in humans and in autonomous robots.

Microbots to repair the body

“A futuristic possible application of our torsional muscle is to propel microbots that can be placed in the body to repair the human body,” Baughman told KurzweilAI. “Just like bacteria and sperm have tails that rotate to provide propulsion, the carbon nanotube torsional muscles might be very attractive in this application.”

Unlike conventional motors, whose complexity makes them difficult to miniaturize, the torsional carbon nanotube muscles are simple to inexpensively construct in either very long or millimeter lengths. The nanotube motors consist of a yarn electrode and a counter-electrode, which are immersed in an ionically conducting liquid.

Batteries included

Electrolyte-filled electrochemical cell used for characterizing torsional and tensile actuation for a carbon nanotube muscle. Torsional actuation rotates the paddle attached to the nanotube yarn. (Credit: University of Texas at Dallas)

A low voltage battery can serve as the power source, creating electrochemical charge and discharge of the yarn to rotate it in opposite directions. In the simplest case, the researchers attach a paddle to the nanotube yarn, like mixing liquids on “micro-fluidic chips” used for chemical analysis and sensing.

These muscles accelerate a paddle that is 2,000 times heavier than the yarn up to a speed of 590 revolutions per minute (rpm) in 1.2 seconds, then reverse the rotation when the applied voltage is changed. The researchers said they achieved a rotation of 250 rpm per millimeter of muscle length — more than 1,000 times that of previous artificial muscles, which are based on ferroelectrics, shape memory alloys, or conducting organic polymers.

The output power per yarn weight is comparable to that for large electric motors, and the weight-normalized performance of these conventional electric motors severely degrades when they are downsized to millimeter scale.

Nature has used torsional rotation based on helically wound muscles for hundreds of millions of years, and exploits this action for such tasks as twisting the trunks of elephants and octopus limbs. In these natural appendages, helically wound muscle fibers cause rotation by contracting against an essentially incompressible, bone-less core.

The helically wound carbon nanotubes in the nanotube yarns are undergoing little change in length, but are instead causing the volume of liquid electrolyte within the porous yarn to increase during electrochemical charging, so that torsional rotation occurs.