There was a paper just released in Science (Materials) about "Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles." This is a rare case where I believe the research material far exceeds the buzzword hype! The new material responds to applied voltages by expanding 220% in a few milliseconds, operating in temperatures as low as liquid-nitrogen and as high as the melting point of iron. It has the strength and stiffness of steel (by weight) in one direction and yet is as compliant as rubber in the other two. It has extremely low density due its airy (aerogel) properties, is conductive, and transparent. This materials innovation has the potential to rejuvenate research on artificial muscles, which has generally been focused on shape memory alloys (i.e. nickle-titanium or Nitinol), piezoelectrics (such as PZT), or electroactive polymers (EAPs). Read on for a discussion about these alternative technologies, their drawbacks, and why this new material may be a game-changer!
Most people know that I'm fascinated by robots and sensing. However, material science is another oft-neglected passion -- in a parallel universe, that would be my vocation. Thus, I'm fascinated by anything that combines the three, such as artificial muscles. Basically, existing efforts come in three categories:
1. Shape Memory Alloys:
The most common shape memory alloy employed as an "artificial muscle" is nickel-titanium, or Nitinol. This material has been used in small novelty robots such as Stiquito (pictured) and in medical implants. Unfortunately, this material is power-hungry and has very slow (re)activation times, making it less compelling as a serious artificial muscle candidate.
There is a fair amount of work on piezoelectrics (such as PZT) as artificial muscles. These materials have high power density, high bandwidth, and high efficiency. Unfortunately, the strain (and stroke length) is very small -- on the order of 0.1%. This means that an artificial muscle of PZT must rely on strain amplification structures (shown) that add points of failure and seriously diminish their power-to-weight (and volume) ratio. In this subject, I am particularly fond of the work by Georgia Tech professor, Jun Ueda. If you're interested, his ICRA 2008 paper entitled "Static Lumped Parameter Model for Nested PZT Cellular Actuators with Exponential Strain Amplification Mechanisms" is worth reading.
3. Electroactive polymers (EAPs):
There are numerous varieties of electroactive polymers -- one great place to read about them is in Electroactive Polymer (EAP) Actuators as Artificial Muscles: Reality, Potential, and Challenges. Generally, I find these extremely interesting and compelling as well. Despite the quest for the "ultimate" EAP, there are still numerous (technology-dependent) challenges: poor power-weight ratios, poor strain/force performance, issues with controllability, the requirement to keep them submerged (for the wet variety), etc. You may also check out the NASA webpage dedicated to the subject (they previously developed an EAP-driven hand, shown).
Researchers at the UT Dallas Alan G. MacDiarmid NanoTech Institute have demonstrated a fundamentally new type of artificial muscle, which can operate at extreme temperatures where no other artificial muscle can be used -- from below the temperature of liquid nitrogen (-196° C) to above the melting point of iron (1538° C).
The discovery is reported in the March 20 issue of Science under the title “Giant Stroke, Superelastic Carbon Nanotube Aerogel Muscles.”
Once actuated (or put into motion) in a certain direction, these new artificial muscles can elongate 10 times more than natural muscles and at rates 1,000 times higher than a natural muscle. In another direction, when densified, they can generate thirty times the force of a natural muscle having the same cross-sectional area. While natural muscles can contract at about 20 percent per second, the new artificial muscles can contract at about 30,000 percent per second.
These artificial muscles are carbon nanotube aerogel sheets made by a novel solid-state process developed at UT Dallas. Sometimes called frozen smoke, aerogel is a low-density solid-state material derived from a gel in which the liquid component of the gel has been replaced with gas. Aerogels are comprised mostly of air. The starting material is an array of vertically aligned carbon nanotubes manufactured under a chemical heat process. Because of the special arrangement of these nanotube arrays, which are called forests because they look like a bamboo forest, the carbon nanotubes can be pulled into sheets at speeds of up two meters per second. The sheets have such low density that an ounce would cover an acre.
When scientists apply a voltage to the carbon nanotube aerogel sheets, the nanotubes repulse, or push away from one another, which in effect works the muscle. These transparent sheets have strange properties that are important for muscle operation. While having about the density of air, in one direction, they have higher specific strength (strength/density) than a steel plate. When stretched in another direction, they provide rubber-like stretchability, but by a mechanism quite different than for ordinary rubber. Because of their nanoscale and microscale structure, they amplify a percent stretch in the nanotube orientation direction to a percent 15 times larger than the percent they contract laterally.
This sounds compelling for applications involving extreme temperature variations. One example is space applications, where these artificial muscles would also offer extremely valuable mass-savings.
However, only time will tell if this technology will yield the "holy grail" of artificial muscles: an "artificial muscle" comparable to their biological counterparts (power, weight, efficiency, volume, etc). For while the new nanotube-aerogel muscles appear to have a good power-to-weight ratio, the extremely low density may necessitate exorbitant volumes to produce the forces/torques of biological systems.
Oh, and here is a picture of a meter-long ribbon of carbon nanotube aerogel from an article at NewScientist.