Soft, strong artificial muscle could enable more lifelike robots

People often imagine robots as stiff, inhuman metal or plastic creations. However, the development of soft robots could see the emergence of more flexible, biologically-inspired machines, with soft parts better suited to handling delicate objects and humans - in the care of dementia patients, for example.

A major obstacle in the field of soft robotics, however, has been the difficulty of developing self-contained, moving robots. Tethered soft robots – while still useful – have a limited ability to integrate into real world settings.

Now, a team based in Columbia University’s School of Engineering and Applied Science has created a new actuator which could enable the development of untethered soft robots. This mechanical muscle is 3D-printable and is capable of expanding without being connected to an electricity supply or external compressor.

The team used a silicone rubber matrix with ethanol distributed in micro-bubbles; this produced a material with high elasticity, but also useful volume change properties. According to the researchers, this material is cheap, environmentally friendly and easy to recreate.

The material was 3D printed into the desired shape and the resulting artificial muscle was actuated using a thin wire and small voltage. It was capable of expanding 900 per cent in a variety of robotic applications when heated.

“Our soft functional material may serve as robust soft muscle, possibly revolutionising the way that soft robotic solutions are engineered today,” said Dr Aslan Miriyev, who led the Nature Communications study.

“It can push, pull, bend, twist and lift weight. It’s the closest artificial material equivalent we have to a natural muscle.”

The muscle has a strain density (expansion per gram) 15 times greater than natural biological muscle and it can lift 1,000 times its own weight. By comparison, ants - one of nature’s most famous benchmark lifting creatures - are only capable of lifting approximately 100 times their own body weight.

“We’ve been making great strides [towards] making robots’ minds, but robot bodies are still primitive,” said Professor Hod Lipson, also of Columbia University. “This is a big piece of the puzzle and, like biology, the new actuator can be shaped and reshaped a thousand ways. We’ve overcome one of the final barriers to making lifelike robots.”

The development of such an actuator is particularly surprising due to all previous technologies – such as those based on hydraulic inflation of elastomer skins – not having sufficient stress and strain to function as a soft muscle.

In the future, the team will continue to improve the artificial muscle, replacing the embedded wire with conductive materials and improving its response time and shelf life. Eventually, they hope to use artificial intelligence to learn to control the muscle, replicating natural motion.

Artificial muscles created by materials engineers and roboticists often demonstrate properties unparalleled by human muscles. For instance, a ‘fishing line’ muscle has been shown to lift weight 100 times more effectively than human muscle.