Origami-Inspired Muscles Provide Both Soft and Strong Features to Robots

Soft robotics has succeeded in making leaps and bounds over the last decade as researchers from all over the world have experimented with different designs and materials to allow once rigid, jerky machines to bend and then flex in ways that imitate and interact in a more natural way with living organisms.

However, increased dexterity and flexibility has a trade-off of decreased strength, as softer materials are normally not as resilient or strong as inflexible ones, which restricts their use.

We were very surprised by how strong the actuators [aka, “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.

Daniela Rus, Ph.D., the Andrew and Erna Viterbi Professor of Electrical Engineering and Computer Science at MIT and one of the senior authors of the paper.

“Artificial muscle-like actuators are one of the most important grand challenges in all of engineering,” adds  Rob Wood, Ph.D., corresponding author of the paper and Founding Core Faculty member of the Wyss Institute, who is also the Charles River Professor of Engineering and Applied Sciences at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS). “Now that we have created actuators with properties similar to natural muscle, we can imagine building almost any robot for almost any task.”

Each artificial muscle is made up of an inner “skeleton” that can be made of different materials, such as a sheet of plastic folded into a certain pattern or a metal coil, surrounded by fluid or air and sealed within a textile or plastic bag that functions as the “skin.” A vacuum applied to the inside of the bag activates the muscle’s movement by making the skin collapse onto the skeleton, producing tension that drives the motion. Incredibly, no human input or any other power source is needed for directing the muscle’s movement; it is wholly determined by the composition and shape 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 first author Shuguang Li, Ph.D., a Postdoctoral Fellow at the Wyss Institute and MIT CSAIL. This approach permits the muscles to be extremely compact and simple, and thus more suitable for body-mounted or mobile systems that cannot accommodate heavy or large machinery.

When creating robots, one always has to ask, ‘Where is the intelligence – is it in the body, or in the brain?’ Incorporating intelligence into the body (via specific folding patterns, in the case of our actuators) has the potential to simplify the algorithms needed to direct 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.

Daniela Rus, Ph.D., the Andrew and Erna Viterbi Professor of Electrical Engineering and Computer Science at MIT and one of the senior authors of the paper.

The team developed dozens of muscles by employing materials ranging from metal springs to sheets of plastic to packing foam, and experimented with varied skeleton shapes to produce muscles capable of contracting down to 10% of their original size, lifting a fragile flower off the ground, and then twisting into a coil, all by just sucking the air out of them.

The artificial muscles can move in a number of ways, and they do so with impressive resilience. They can produce about six times more force per unit area than that produced by mammalian skeletal muscle, and are also extremely lightweight; a 2.6-gram muscle is capable of lifting a 3-kilogram object, which is the same as that of a mallard duck lifting a car. Furthermore, a single muscle can be developed within ten minutes using materials costing less than $1, making them cost-effective and easy to test and iterate.

It is possible to power these muscles by a vacuum, a feature that enables them to be safer than most of the other artificial muscles presently being tested.

A lot of the applications of soft robots are human-centric, so of course it’s important to think about safety, 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.

Daniel Vogt, M.S., co-author of the paper and Research Engineer at the Wyss Institute.

“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 explains the fact that the muscles can be used in a number of applications at multiple scales, such as wearable robotic exoskeletons, miniature surgical devices, deep-sea manipulators for research or construction, transformable architecture, and huge deployable structures for space exploration.

The team also succeeded in constructing the muscles out of the water-soluble polymer PVA, which is capable of opening the possibility of robots that can carry out tasks in natural settings with slight environmental impact, and also ingestible robots that move to the correct place in the body and then dissolve to discharge 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.

“The actuators developed through this collaboration between the Wood laboratory at Harvard and Rus group at MIT exemplify the Wyss’ approach of taking inspiration from nature without being limited by its conventions, which can result in systems that not only imitate nature, but surpass it,” says the Wyss Institute’s Founding Director Donald Ingber, M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at HMS and the Vascular Biology Program at Boston Children’s Hospital, as well as Professor of Bioengineering at SEAS.

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