Artificial muscles give soft robots superpowers

Origami-inspired muscles are both soft and strong, and can be made for less than $1

Researchers at Harvard and MIT have created origami-inspired artificial muscles that add strength to soft robots, allowing them to lift objects that are up to 1,000 times their own weight using only air or water pressure.

Specifically, one of the team’s 2.6-gram muscles can lift a 3-kilogram object – which is the equivalent of a duck lifting a car.  A single muscle can be constructed within ten minutes using materials that cost less than $1, making them cheap and easy to test and iterate.

Future applications include everything from minimally invasive surgery and transformable architecture to deep-sea research to space exploration.

We were very surprised by how strong the muscles were. We expected they’d have a higher maximum functional weight than ordinary soft robots, but we didn’t expect a thousand-fold increase. It’s like giving these robots superpowers,” says CSAIL director Daniela Rus.

The study will be published this week in Proceedings of the National Academy of Sciences (PNAS).

Each artificial muscle consists of an inner “skeleton” surrounded by air or fluid and sealed inside a bag of “skin,” both of which can be made from a variety of materials. A vacuum applied to the inside of the bag initiates the muscle’s movement by causing the skin to collapse onto the skeleton, creating tension that drives the motion.

The structural geometry of artificial muscle skeleton determines the muscle’s motion. Credit: Shuguang Li / Wyss Institute at Harvard University

Incredibly, no other power source or human input is required to direct the muscle’s movement; it is determined entirely by the shape and composition of the skeleton.

One of the key aspects of these muscles is that they’re programmable, in the sense that designing how the skeleton folds defines how the whole structure moves. You essentially get that motion for free, without the need for a control system,” says postdoc Shuguang Li.

This approach allows the muscles to be very compact and simple, and thus more appropriate for mobile or body-mounted systems that cannot accommodate large or heavy machinery.

When creating robots, one always has to ask, ‘Where is the intelligence – is it in the body or in the brain?’” says Rus. “Incorporating intelligence into the body has the potential to simplify the algorithms needed to control the robot to achieve its goal. All these actuators have the same simple on/off switch, which their bodies then translate into a broad range of motions.

Origami-inspired artificial muscles are capable of lifting up to 1,000 times their own weight, simply by applying air or water pressure. Credit: Shuguang Li / Wyss Institute at Harvard University

The team constructed dozens of muscles using materials ranging from metal springs to packing foam to sheets of plastic, and experimented with different skeleton shapes to create muscles that can contract down to 10 percent of their original size, lift a delicate flower off the ground, and twist into a coil, all simply by sucking the air out of them.

They are also safer than other types of artificial muscles currently being tested in soft robotics because they rely on a vacuum rather than inflation to contract.

A lot of the applications of soft robots are human-centric, so of course it’s important to think about safety,” says Daniel Vogt, M.S., co-author of the paper and Research Engineer at the Wyss Institute.  “Vacuum-based muscles have a lower risk of rupture, failure, and damage, and they don’t expand when they’re operating, so you can integrate them into closer-fitting robots on the human body.”

In addition to their muscle-like properties, these soft actuators are highly scalable. We have built them at sizes ranging from a few millimeters up to a meter, and their performance holds up across the board,” Wood says. This feature means that the muscles can be used in numerous applications at multiple scales, such as miniature surgical devices, wearable robotic exoskeletons, transformable architecture, deep-sea manipulators for research or construction, and large deployable structures for space exploration.

The team was even able to construct the muscles out of the water-soluble polymer PVA, which opens the possibility of robots that can perform tasks in natural settings with minimal environmental impact, as well as ingestible robots that move to the proper place in the body and then dissolve to release a drug.

The possibilities really are limitless. But the very next thing I would like to build with these muscles is an elephant robot with a trunk that can manipulate the world in ways that are as flexible and powerful as you see in real elephants,” Rus says.

This research was funded by the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), and the Wyss Institute for Biologically Inspired Engineering.

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