Muscle Mechanics
A plastic skeleton sits astride a stationary bicycle in the basement laboratory, a microprocessor board in its abdominal cavity and a couple of plastic reservoirs where its lungs should be.
“This is the famous Mr. Bony,” says the skeleton’s owner, Mohsen Shahinpoor. If all goes well, one day soon Mr. Bony’s skeletal legs will pedal away on the bike as a demonstration of a startling new technology.
The model’s limbs will be powered by artificial muscles made from bundles of synthetic polymers that move rapidly in response to electrical or chemical stimulation.
“This is very exciting,” says Shahinpoor, 53, a professor of mechanical engineering at the University of New Mexico who has been working on the project for five years. “It’s a revolution, if it happens. I’m not quite there, but I’m close.”
Shahinpoor sees intriguing potential for the invention, ranging from robotics, artificial prostheses and ultraflexible spacesuits to noiseless submarine propellers. He even envisions using artificial muscles to boost weakened hearts.
Some government agencies share his enthusiasm. The Defense Department, NASA’s Jet Propulsion Laboratory and Sandia National Laboratory have all contributed funding. Meanwhile, the university’s regents recently established an Artificial Muscles Research Institute, of which Shahinpoor is the director.
With the university working on securing domestic and international patents, the hope is that Shahinpoor’s inventions will yield substantial royalties. “They’ve given me four years to make [the institute] self-sufficient,” he says.
With a background in chemical, mechanical and electrical engineering and a longtime interest in robotics, Shahinpoor is uniquely suited to his work. His quest to develop artificial muscles began five years ago with an idle notion.
“One day it struck me. We can move our arms and legs quietly--no noise,” he says. “Why can’t we make our mechanical [equivalents] move quietly? That was my challenge.”
Intrigued, he began by studying literature on how human muscles work. He soon discovered the similarity between muscles and polymers, materials made of long chains of molecules. “I knew the answer had to be in polymers. So I began looking at the literature.”
He found that as long ago as the early 1950s, scientists knew some synthetic polymers expanded or contracted like muscle fibers if they were treated with acidic or alkaline solutions. In the 1960s, other researchers found a similar effect from electrical stimulation. But in both cases, the polymers moved too slowly.
Joining forces with three Sandia National Laboratory scientists, Shahinpoor tested some common polymers and quickly discovered materials that responded much more rapidly--within milliseconds. “All of a sudden,” he says, “there was a promise for engineering applications.”
An especially useful polymer is Orlon, an artificial silk. Shahinpoor bakes the fiber, then boils it in an alkaline solution, a process that leaves it soft and flexible. Then he bundles together thousands of the dark brown strands to simulate real muscle tissue.
Using a tabletop model skeleton--Mr. Bony Jr.--Shahinpoor has rigged a bundle of the fibers connecting the bones of the forearm to those of the upper arm. When a weak alkaline solution is sprayed on the fibers, they relax and the arm slowly drops. A spritz of acid causes the arm to rise.
To make a usable muscle, Shahinpoor and his students must perfect an outer membrane, similar to natural muscle sheath. “At first we used condoms, because condoms are quite tough,” he says. But the latex sometimes breaks, so they are experimenting with a length of pleated, accordion-like tubing that expands and contracts.
When the kinks are ironed out, Shahinpoor hopes to have Mr. Bony Sr. bulked up with muscles strong enough to pedal the bike. The microprocessor in the abdomen will control the rapid pumping of acid and alkaline fluids from the plastic reservoirs.
Elsewhere in his lab, Shahinpoor is experimenting with rubbery sheets of ionic exchange membrane, a porous material used in water purification and desalinization. What these polymers have in common is that they are ionically active, Shahinpoor says. Applying a current causes the ions--electrically charged atoms--to attract or repel one another. That changes the ionic distribution within the material, which causes it to move.
The membrane, sandwiched between conductive layers of platinum, is Shahinpoor’s latest discovery. He places a small piece between two electrodes and gradually increases the current. The membrane flaps rapidly up and down like a bird’s wing.
Another piece of polymer, shaped like an octopus, waves oddly when the current is applied. A goldfish-shaped sliver wiggles its tail. Shahinpoor laughs delightedly. “Every day, something new,” he says. “Isn’t that spooky ? It comes alive.”
In a glass tray is a clear gel that exhibits similar properties and could be used as an adaptive lens for cataract patients.
Artificial muscles could prove useful in such things as prosthetic limbs, replacement vocal chords, robots and manufacturing processes, Shahinpoor says. Artificial muscles also could augment an astronaut’s mobility in a bulky spacesuit, he says. The fluttering motion of ionic exchange membranes has suggested the possibility of noiseless propellers, or even aircraft with flexible wings.
Earlier this year, Shahinpoor was contacted by Dr. John Alexander, a surgeon at Northwestern University Medical School, who had seen a scientific article about artificial muscles. Alexander thought Shahinpoor’s artificial muscles might help patients suffering from cardiomyopathy, a progressive weakening of the heart.
Alexander says it might be possible to wrap a damaged heart with artificial muscle, bolstering its contractions.
“I’m encouraged that there may be something here, but I’m not sure what it is yet,” Alexander says.
For Shahinpoor, the idea that his research might enable doctors to save lives is especially rewarding.
“I’d really like to pursue this,” Shahinpoor says, “because it has great potential.”
