New robot ‘muscles’ are strong enough to lift a baseball—and nimble enough to pluck a raspberry

Robots are wielding a new kind of soft power. Researchers have outfitted them with a new class of muscles, which—like our own—offer both strength and sensitivity. By not overpowering and damaging the objects they’re manipulating, the muscles could one day help a new generation of soft robots perform delicate tasks—everything from picking fruit to assisting elderly people.

Robots, particularly those used in manufacturing, have long been good at repetitive tasks that require lots of power, such as welding pieces of a car chassis. But their motions tend to be rigid and ill-suited for tasks that require variable amounts of force. They’re also potentially dangerous to anyone who wanders too close. Robotics researchers are trying to create softer versions that can work alongside humans—or even as a part of them, like prostheses that can help paralyzed people walk.

Two soft muscle technologies have jumped to the fore: pneumatic actuators, which pump gases or liquids into soft pouches to create particular movements, and devices called dielectric elastomer actuators, which apply an electric field across an insulating flexible plastic to make it deform with a particular movement. Pneumatic actuators are both powerful and easy to make, but pumps can be bulky and moving gases and fluids around can be slow. Dielectric elastomer actuators are fast and energy efficient. But they often fail catastrophically when a bolt of electricity blasts through the plastic.

Now, researchers led by Christoph Keplinger, a physicist at the University of Colorado in Boulder, have married the best of both technologies, creating soft musclelike actuators that use electricity to drive the movement of liquids inside small pouches. The design is simple. The actuators start with small plastic pouches that contain an insulating liquid, such as regular canola oil from the supermarket. When researchers apply a voltage between electrodes placed on both sides of the pouch, they are drawn together, squeezing the liquid and causing it to flow to nearby regions. The upshot is that the actuator changes shape, and whatever is connected to it moves.

Keplinger and his colleagues report today in a pair of papers in Science and Science Robotics that they created three soft muscle designs that contract with the precision and force of mammalian skeletal muscles. In their Science paper, Keplinger’s team showed that a series of doughnut-shaped actuators had the dexterity to enable a robotic gripper to pick up and hold a raspberry. They also showed that if a bolt of electricity did arc through the insulating liquid between the electrodes, any “damage” was instantly repaired when the arcing stopped, and new liquid flowed into the region. And in Science Robotics, Keplinger’s team reports creating two other muscle designs that contract linearly, much like a human bicep, enabling them to lift far more than their own weight at a rapid repetition rate.

“This is a very big step,” says Robert Shepherd, a soft robotics expert at Cornell University. “What they’ve got is the benefit of the fluid system with direct electrical control.” That combination, Shepherd says, could spur numerous applications, including more lifelike prosthetic devices and soft robots that can work alongside people without safety concerns. And because the materials for making the soft muscles are cheap and readily available, other new designs are sure to follow.