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Artificial muscles already help human eyes blink, robotic fish swim and mechanical arms in space replace solar panels. Now a new, potentially wearable type of artificial muscle is expected to do all of those things while being lighter, smaller, softer and cheaper.
These improved artificial muscles use the same type of rubbery, electrically activated polymer membrane as their predecessors but rely on a new design that integrates circuit elements into the membrane itself.
Artificial muscles have been around for decades. They typically consist of a rubbery insulated material sandwiched between conductors—for example, a pair of electrodes. A charge applied to the rubbery material builds up on its conductive surfaces and produces a compressive stress. This stress makes the polymer move just like a real muscle does—it thins and expands in area. This shape change provides a force that can be used to move an object such as a prosthetic or robotic limb. Depending on the polymer material and conditions, the membrane can stretch up to 300 percent, achieving performance comparable with—and sometimes better than—real muscle.
Scientists at the University of Auckland's Bioengineering Institute in New Zealand are developing an advanced artificial muscle that essentially does the same thing without the need for external conductors. The researchers start by applying grease infused with carbon particles—known as carbon grease—onto a polymer. "By carefully laying down soft electrodes on the membrane's surface, we can control the charge in a clever way, and create something that is smaller, lighter and more portable than previously possible," says group leader Iain Anderson of the university's Biomimetics Laboratory.
The team has already made the advanced artificial muscle the foundation for a dielectric elastomer generator—a device for converting mechanical energy into electricity. A wearable generator, which is made by printing switches developed by the researchers directly onto the soft artificial muscle, could harvest the energy created by walking or some other physical exertion to charge a cell phone, for instance. Energy captured from the movement of one's legs while walking generated up to roughly 10 milliwatts of electrical power in lab conditions. (Put in perspective, a typical laser pointer produces five milliwatts. A milliwatt is one thousandth of a watt.)
The artificial muscle is also self-sensing, providing high-fidelity feedback from the natural muscle while it works. This feedback is similar to proprioception in the human body that enables us to sense the position of our limbs. Humans can use this capability to, for example, touch their noses with their eyes closed. This is possible because the muscles in our arms contain cells that can sense strain.
In another experiment, the Auckland researchers designed a motor with a new hub-and-spoke configuration that harnessed the collective energy of several artificial muscles. Each piece of muscle connected to the outside of a plastic ring, which encircled a shaft. As voltage was applied, the muscles deformed intermittently, converting electricity into mechanical energy. The undulating muscles pressed against the central ring in sequence causing the shaft to rotate, much like a pencil might be rolled between a thumb and forefinger. The researchers estimate that these muscles cost about $5 each to produce, depending on the materials used—cheaper if produced in large volumes.
These developments bring artificial muscles a step closer to widespread commercial use, particularly in the toy market. "Developing innovative, niche technologies could push the artificial muscles field from a novel to a fully commercialized market and help finance research," says Yoseph Bar-Cohen, a physicist specializing in electroactive materials and mechanisms at NASA's Jet Propulsion Laboratory. Bar-Cohen is particularly impressed by the stability of the new motor. "Great potential exists for these materials in space applications," he adds.
The new work is the latest in a long line of attempts to bring artificial muscles into the mainstream. Last year scientists at the University of California, Davis, Medical Center announced a way to help stroke victims blink, via an artificial muscle and sling. Researchers at The University of Texas at Dallas's NanoTech Institute in 2009 developed a carbon nanotube–based artificial muscle capable of operating in extreme temperatures, making it ideal for use in space. Several years earlier, advances incorporating artificial muscles into TV and computer screens promised more vivid screen colors. Nearly a decade ago, artificial muscles in synthetic fins were used to propel artificial fish.
Some versions of artificial muscle are already commercially available. Artificial Muscle, Inc. makes ViviTouch technology behind the Mophie Pulse case for the iPod Touch that provides self-sensing videogame feedback using vibrations, making the game more realistic and interactive.
