The scientists also created a crawling mini-machine with a rounded body and a "leg" at the back that flexes to push the device along.
And the team made a spontaneously coiling strip as well as a clawlike gripper that could grab tightly enough to move individual cells, said study co-author Adam Feinberg.
Get One's Ducks in a Row
"I think the greatest accomplishment is not that we made these walking devices," team leader Parker said.
Rather, the achievement was that the team was able to get the muscle cells—and the molecular motors inside them—to line up properly.
"In order to have rhythmic contraction, you need to have proper alignment" of the muscle cells, Parker said. "To date, that is what has vexed tissue engineers."
To solve this, Parker's team stamped their elastic films with a pattern of stripes made of a protein called fibronectin. The researchers were able to make the heart muscle cells grow along these paths.
"If you put down a pattern, you automatically get a tissue," Parker said.
The cells then arranged themselves into working muscle fibers.
"Soft" Robots and Artificial Arms?
The team thinks that muscular origami is just the tip of the iceberg for their research.
Animals other than rats are inspiring the experts to shoot for more complex devices.
"An octopus can deform itself to get through obstacles" and can also bend its arms in all directions, Parker said.
Getting the muscular films to work in a similar way could lead to the development of "soft robotics," he pointed out.
Meanwhile, "the first application of these [rat muscle films] might be in drug assays" that look at the benefits and side effects of particular drugs on heart cells, Parker said.
"We need to try to get human cells to grow" on the films, because this would open up another possibility—making replacement tissues and organs—he added.
With sheets of muscle tissue, doctors could repair holes in the heart or diseased bowels.
Gordana Vunjak-Novakovic of Columbia University was not involved in the study, which she described as "innovative."
"What's really nice is they focus on the mechanics of single cells," she said, controlling how these cells attach to one another and work together.
In the far future, such engineered muscles might make their way into artificial arms or legs.
"It is conceivable [that] prosthetic devices would be very substantially advanced if you could make these [engineered muscles] work for you," she said.
Keith Baar is a biochemist at the University of Dundee in Scotland.
"The study is spectacular," he said. "In part because of the science, but as much because they perform and describe a number of experiments that are purely joyful, the types of things that you imagine doing when you are a kid, before you are told that they can't be done.
"It is a significant improvement," he continued, "because it will change the way that people think about the interaction between the synthetic and biological materials and how they can work together to produce machines."
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