Researchers have identified a novel approach to creating and building artificial muscles that could bring real-life technology into the realm of science fiction.
On the battlefields of the future, ground vehicles will resemble the All-Terrain Walkers in Star Wars. Shark-like submersibles propelled by swishing tails, and nearly indistinguishable from sea life, will stealthily navigate hostile waters. And infantry clad in body armor that mimics the movement of human muscles will be impervious to small-arms fire and resistant to explosives, greatly decreasing ballistic injuries and casualties.
Or at least that's the vision of the authors of a paper published March 20 in Sensors and Actuators A: Physical. In it, the researchers detail a new approach to creating artificial muscles that combines electrostatic actuation, microfluidics and 3D printing that is theoretically feasible, according to computer simulations.
Co-author Emil Kartalov, a professor of physics who studies advanced warfare at the Naval Postgraduate School, in Monterey, California, said his team is testing physical prototypes of the technology and pursuing a wide range of potential military and civilian applications. He also revealed to The Academic Times that he is a tabletop gamer who reads a lot of science fiction.
"If you read Starship Troopers, they talk about powered armor," Kartalov said. "If you play Warhammer 40,000, the space marines have powered armor. I'm also reasonably proficient in military history and understand enough to see it would be an enormous improvement for our forces if you could have armor that carries itself."
Artificial muscles are not a new concept, but earlier attempts at developing them have had various shortcomings, according to Kartalov and his colleagues.
Actuators are parts of a machine system that make movement possible. Thermal actuation — heating an object up to change its shape — doesn't work because it "takes forever to heat up and cool down," he said. "That would not produce an active muscle that would move on the same basis at which you and I move. It's not practical, in my opinion."
Piezoelectric actuators, which rely on pressure and latent heat, offer too narrow a range of motion due to the inflexibility of the material. "The reason we're able to move the way we move is because our muscles are squishy," Kartalov said, "so they can deform quite a bit."
Electromagnetic motors, on the other hand, don't work without a strong magnetic field generated by a permanent magnet, a heavy and inefficient means of locomotion that creates excessive heat. And pneumatics used in heavy machinery such as backhoes and cranes don't exert enough force when scaled down for use in compact systems.
"That's why we can't have a pneumatic steampunk kind of power," Kartalov said. "The physics don't allow you to do that."
This process of elimination led Kartalov to electrostatic actuation, the use of opposing forces of two oppositely charged electrons to move a membrane. This avoids the need to generate an electromagnetic field while still producing strong electrical forces. Back-of-the-envelope calculations suggested that thousands of tiny electrostatic capacitors could exert several hundred pounds of pressure per square inch — enough to bring an exoskeleton, a ground vehicle, or an unmanned underwater vehicle to life.
Going beyond rough calculations, Kartalov and his colleagues ran simulations with the multiphysics software COMSOL, using parameter sweeps to determine that the concept is feasible. They also found a "sweet spot" in terms of the thickness of artificial tendons relative to muscle fiber to maximize output force density.
"It showed us a general pathway toward optimizations and improvement," he said.
Manufacturing was the final piece of the puzzle. "Even if it works most of the time but it takes three Ph.D.s to run it and it's very finicky, not manufactured easily, then it isn't useful," Karalov said. "So, I tapped into my knowledge of microfluidics and 3D-printed devices for medical diagnostics and said, 'Why don't we just 3D-print the muscles?'"
The resulting technology is artificial muscle fiber with microchannels — wiring filled with conductive fluid or gel — in a comb-like arrangement. Applying voltage allows for expansion and contraction of the surrounding material. If the technology proves as scalable as Kartalov hopes, it could lead to the creation of military vehicles that move like wildlife.
"If you have a ground vehicle just like the multilegged walkers in Star Wars, you can move as fast as an ostrich — 45 mph cross-country, no problem," he said. "It would be gyro-stabilized with very large ground clearance, which makes you much less susceptible to IEDs. You can crouch; you can maybe lose a few limbs from the vehicle and still be close to fully operational."
He also imagines the possibility of soft submersibles propelled by slow-moving tails, mimicking the movement of a shark, dolphin or large tuna.
"In undersea warfare, if you're detected, you have a very good chance of getting destroyed," Kartalov said. "If you can cloak yourself to look and sound like a biological, then you have a huge advantage. … They can differentiate you from nothing, but they cannot differentiate you from ambient biologicals of similar size."
A potential advantage of artificial muscles in submersible vehicles is evading sonar, Kartalov said. Materials such as steel and copper, often used in propulsion systems, are detectable by sonar due to their density. A propulsion system based on artificial muscle technology would be made from resin and plastic and therefore less distinguishable.
Kartalov's team is currently testing prototypes and exploring multiple applications for the technology. From here, the project will involve further miniaturization and assembling of muscle bundles that mimic those of both humans and animals.
The new approach to building artificial muscles could have civilian applications, as well. In principle, it could improve prosthetics for people with missing limbs or boost mobility for people who are elderly or disabled. But the project's main focus is improving the safety of U.S. military personnel.
"That's what I'm really excited about," Kartalov said. "I want our guys to be protected."
The study, "Simulations of 3D-Printable Biomimetic Artificial Muscles Based on Microfluidic Microcapacitors for Exoskeletal Actuation and Stealthy Underwater Propulsion," published March 20 in Sensors and Actuators A: Physical, was authored by Michelangelo A. Coltelli, Jefferey Catterlin, Emil P. Katalov, Physics Department, Naval Postgraduate School, Monterey, California; and Axel Scherer, Electrical Engineering Department, California Institute of Technology in Pasadena.