A chief objective of the new institute is to create a combat uniform that has built-in strength-the strength to help a soldier lift heavy objects, to pump cooling fluids through embedded channels or to stiffen around a bleeding wound. Hunter’s twitching black ribbon is an early indication that nano materials might be able to deliver that sort of strength.The ribbon is made of an electroactive polymer that can move or change shape in response to an electrical signal. Researchers have long envisioned using these polymers-which can be 100 times stronger than human muscle-as artificial muscles. But so far, they’ve proved impractical as musclelike machines, largely because their movements are relatively sluggish and also because they’ve been able to contract or expand by only a few percent of their length. Human muscle can contract and expand by 20 percent.
In Hunter’s and Swager’s labs, however, researchers have recently worked together to make great strides toward a material with enough range of motion to be useful. The key is a series of molecules that operate like rods and hinges. Pivoting on the hinges, the rods repel or attract one another when a charge is applied or removed. By attaching millions of these rods and hinges end to end like segments of a folding ruler, the researchers were able to create polymers that lengthen and shorten in response to electrical stimuli (see “Molecular Muscle,” below). A film made of these polymers produces musclelike movements. “Within the last few months,” says Hunter, “we’ve doubled the range of motion,” approaching that of human muscle cells.
|A polymer that contracts and expands as much as human muscle uses molecular “hinges” and “rods.” The rods repel and attract one another when a charge is applied (top) and removed (bottom). (Illustration by John MacNeill)|
That increase in how much the polymer can expand and contract, combined with its impressive strength-which the researchers haven’t measured yet but predict to be ten times that of human muscle-could conceivably allow a combat uniform embedded with 1.4 kilograms of the material to lift 80 kilograms one meter high. In other words, a soldier could effortlessly hoist a heavy piece of equipment or even a fallen comrade. The problem: this would take at least a minute, says John Madden, an electrical engineer at the University of British Columbia in Vancouver, who until recently headed electroactive-polymer research in Hunter’s lab.
Giving these electroactive polymers useful speed is the next hurdle. It will require cutting back on the materials’ electrical resistance, so an applied charge can do its work more rapidly. The researchers plan to reduce resistance by incorporating carbon nanotubes -long, pipelike molecules-into future generations of the materials. Certain versions of carbon nanotubes are excellent electrical conductors that could deliver charge throughout the material much more rapidly. The Hunter and Swager groups hope to make artificial muscles that are as fast as human muscle in five years.
Integrating the muscle material with the rest of the soldier’s suit is the larger challenge. The electroactive polymers need, for instance, to be wired into a power distribution and signaling system; conventional wiring is simply too rigid for the job of hooking up a twitching, flexing material. So in the past year, Hunter and his coworkers have developed ribbonlike wires made of flexible electrically conducting polymers. “Instead of stiff copper wires going into polymer tissue,’ we will have tissuelike wires going into tissue,” Hunter says.