Nature has been developing her technologies for many hundreds of millions of years, said Ray Baughman. “By looking at the way in which nature has solved problems like muscles, we can advance our own technologies.” Baughman is director of the NanoTech Institute at the University of Texas at Dallas. His lab creates very tiny artificial muscles by spinning filaments of invisibly small carbon nanotubes into an extraordinary yarn. Pound for pound, this nano-yarn is stronger than steel – yet is so light it almost floats in air. This interview is part of a special EarthSky series, Biomimicry: Nature of Innovation, produced in partnership with Fast Company and sponsored by Dow. Baughman spoke with EarthSky’s Jorge Salazar.
What are your thoughts on biomimicry? How can we learn to use nature’s methods to solve human problems?
We can do this in several ways. We can try to mimic exactly what nature is doing, or as close to mimicking her as possible. This is called a biomimicry approach. We can also use what is called bioinspiration. We can look at what nature does, look at what we can do with our technologies, and try to merge them together to produce a result that is sometimes even better than nature can do.
Tell us about the artificial muscles that you’re developing. How do the body’s natural muscles inspire that result?
The muscles in our body contract in order to do work. And the muscles, for example, in the limbs of an octopus contract. But as a result of this contraction they provide a rotation. Likewise the muscles in the trunk of an elephant. They’re helically wound, so that when these muscles contract, the trunk of the elephant rotates about a turn. Using nanotechnology, we’ve developed artificial muscles that can rotate 1,000 times greater degree per length than the muscles found in an octopus or an elephant’s trunk. These muscles are based on yarns of carbon nanotubes.
A carbon nanotube is a little cylinder of carbon that can be one ten-thousandth the diameter of a human hair. These yarns perhaps can be smaller than one tenth the diameter of the human hair. But these yarns are spun by twisting them, twisting the individual carbon nanotubes together.
They have a sense of handedness, like I have a left and a right hand. These yarns have a left and a right hand sense. Depending upon whether we’ve inserted a left-handed twist or a right-handed twist, these carbon nanotube yarns will rotate in different directions when we apply a voltage to them. So these carbon nanotube torsional muscles provide a very unusual type of motor.
And even here nature has beat us to a very small size motor. For example, certain bacteria have corkscrew-like motors on their rear ends. And sperm, of course, is propelled by minute rotational motors. Our motors are extremely simple. We take a carbon nanotube yarn which is twist spun. It’s twist spun just like in ancient times humans spun yarns by twisting together fibers.
The motor consists of this twist spun carbon nanotube yarn, a counter electrode and electrolyte. Electrolyte is just a liquid that conducts ions. It allows ions to move. We apply a battery between a carbon nanotube yarn and a counter electrode yarn. And the carbon nanotube artificial muscle, the torsional muscle starts to move. In fact, if we have a paddle attached to these carbon nanotube torsional muscles, this paddle will rotate and accelerate up to 600 revolutions per minute in about 1.2 seconds. It will rotate about 10,000 times larger extent per unit length than has been previously possible by any torsional muscles developed by humankind. These torsional artificial muscles can develop similar torque as giant electric motors. Torque is basically a measure of the force that can be developed by twisting. When I say they developed comparable torque, I mean torque normalized to the weight of the motor or the weight of the artificial muscle.
The power output for these minute muscles can approach that of these large electric motors when measured on a per weight basis. But when you downsize conventional electric motors, their performance dramatically degrades. And on this very small scale our torsional artificial muscles provide just a remarkable performance.
How do these carbon nanotube torsional muscles operate?
They operate in ways that are somewhat like the way an octopus limb rotates and somewhat the same as the way certain plants can follow the sun. Remember these torsional artificial muscles provide motors that are extremely simple. You have a carbon nanotube yarn and you have a counter electrode, and you apply voltage between them. When you apply a voltage between the carbon nanotube yarn and this other electrode, you inject electronic charge into the carbon nanotube. To balance this electronic charge, ions from the electrolytes – remember this is just a salt solution – migrate into the yarn. As these ions migrate into the yarn, they cause the yarn to expand.
Tell us about the design of the artificial muscles. How do you make an artificial muscle?
We start from a forest of carbon nanotubes. A carbon nanotube is a nano-size cylinder of carbon. To give you an idea about what the nano scale is: a nanometer compared to the length of a meter is the ratio of the diameter of a marble to the diameter of this world. In carbon nanotube forests these extremely small diameter carbon nanotubes are arranged like bamboo trees in a bamboo forest. If you scaled a bamboo tree with a two-inch diameter and it had the same height to diameter ratio of the carbon nanotubes that we’re using, the bamboo tree would be a mile and a-half tall.
We draw these carbon nanotubes from the carbon nanotube forest in very simple ways. For example, we can take Post-It Notes like the type made by 3M and which has an adhesive backing. We attach this adhesive layer to the sidewall of this carbon nanotube forest and draw. And we obtain a sheet of carbon nanotubes.
This sheet of carbon nanotubes is really a remarkable state of matter. It has a density that’s about that of of air. We can make it in fact have a density that’s ten times lower than that of air, and ten times lower than the density of any material that’s self-supporting that has been previously made by humankind. Despite this very low density – in other words, weight per unit of volume – these carbon nanotube sheets are, on a pound per pound basis, stronger than the strongest steel and stronger than the polymers that are used for ultralight air vehicles. The thickness of these sheets when they’re densified is so small that four ounces of these carbon nanotube sheets could cover an acre of land.
To make our carbon nanotube yarns that we use for our artificial muscles, we insert twists in these carbon nanotube sheets as we draw them from a carbon nanotube forest. By inserting twists we’re basically downsizing a technology that humans have been practicing for at least 10,000 years. By twisting natural fibers together, early humans were able to make clothing to keep them warm. We’re practicing the same technology using nano-size fibers. We use these twist spun carbon nanotube fibers to make our artificial muscles.
How are these artificial muscles you’re developing in the lab going to be used in the real world?
Presently we’ve made prototype devices in which we used these very small diameter carbon nanotube yarns to rotate paddles in what are called microfluidic chips. Technologists want to downsize the synthesis of chemicals and the analysis of chemicals in the same way as technologists have been able to downsize the dimensions of electronic circuits. But one major problem has been that these microfluidic circuits require pumps. The size of the pumps that people had available are much larger than the size of the chips that they could make. They had an incompatibility. You have a small chip, a large pump, so why is there is a benefit of having the chip be so small. Using our carbon nanotube torsional artificial muscles we can make pumps that are similarly dimensioned to the chips – much smaller, of course, than the dimension of the overall chip. We can make valves, we can make mixers that have very small dimensions.
Our carbon nanotube torsional artificial muscles can rotate paddles that are several thousand times heavier than the mass of the artificial muscle yarn. They can provide a very large work output. They can generate very large forces and this is important for a variety of different applications. Now we can talk about what we can do today, and that is to use our torsional artificial muscles for microfluidic chips. But what is possible in the future might be even more exciting.
In nature we see sperm and bacteria being propelled by a corkscrew shaped devices on their rear ends. In the future, scientists imagine having nanoscale robots that could be injected in the human body and can move through the human body doing repairs. Perhaps our torsional artificial muscles might help enable this future.