Carbon nanotube yarn: a new spin on artificial muscles

It has been a very good couple of years for nanotechnology. That much can be easily said in between last year's Nobel prize for graphene (which is one atom thin and promises to keep Moore's Law alive with teeny-tiny semiconductors) and the recent announcement of nanotube cable wire that finally reached the milestone conductivity of copper. 


And now a new twist: carbon nanotube yarns that spin when exposed to electric current.  The remarkable ability to take an electrical signal and convert it efficiently into torque is particularly useful for mimicking our very own biological actuators -muscles!


Artificial muscles traditionally use electroactive polymers, shape-memory alloys, or ceramics that have the ability to change shape. However, very few of these materials are strong enough, yet have the torsion required to apply enough twisting force to be useful in robotics or prosthetic limbs. The nanotube yarn, developed by the research team of Ray Baughman at the University of Texas, Dallas, in conjunction with University of Wollongong, has both of those qualities. 


The material and it's potential application for artificial muscles was published in the journal Science this week.  Especially fascinating is the ability of the nanotube yarn, which is about 15 micrometers in thickness, to produce as much torque (per mass of the yarn) as that produced by electric motors.  The yarn was created by spinning strands from nanotubes grown as tall as 400 microns in height, and subsequently suspending them in an ion-conducting fluid. As the electric current is applied, the ions enter the yarn in a process called electrochemical charging. As a result, the fibers rapidly twist length-wise while simultaneously contracting -just like muscular hydrostats such as tongues!


In addition to potential applications in prosthetics, these twisty fibers may become an indispensable base for microscopic motors, pumps, or actuators in microfluidics.  Microfluidics, a field that aims to execute small-scale chemical or biochemical processes in micro-scale engineered environments, would find much use for such devices, since moving parts have been notoriously difficult to miniaturize. Hence, yarn-based artificial muscles could become mixers for microfluidic lab-on-a chip sensors, or provide the mechanical work for pumps, turbines, or motors, thus eliminating the need to connect human-sized devices to the microsystem. 


The thinner-than-hair yarn's properties were demonstrated in the report by using it to spin a paddle at 590 rotations per minute while turning a weight 2,000 times heavier than the weight of the fiber. The paddle was used to mix two different-colored solutions. 


A more far-reaching goal is to utilize the nanotube yarn as tiny propellers. The yarn could be used to propel drug capsules, tiny cameras, or microscopic devices through the bloodstream. "Just like bacteria and sperm have tails that rotate to provide propulsion, the carbon nanotube torsional muscles might be very attractive in this application,” said Baughman, who filed several patents for the yarn production method as well as several potential applications.