Decoding the arms of an octopus

The human body’s ability to move around and pick things up generally comes courtesy of bones, muscles, and tendons. The skeleton provides a strong and rigid structure for humans to depend on, but there is much to be learned about movement from the arms of an octopus, which are biological structures with incredible strength despite an absence of bones.

Octopus arms are also strong, wielding the power to crush shells and propel the animal rapidly across both land and sea. Despite that power, its eight arms contain no bones. Where does its strength come from?

This is the question Assistant Professor Noel Naughton sought to answer, starting when he was part of the CyberOctopus project based at the University of Illinois. He has continued the investigation since to Virginia Tech, and now his team’s research is a cover feature of the Proceedings of the National Academy of Sciences (PNAS).

“We wanted to understand how the octopus is able to control its arms, which is very challenging due to how flexible they are,” Naughton said. “Our thinking was that the octopus must have a way to simplify this complexity built into its arm’s muscular organization.”

Making a muscle

The one exception to a human’s skeletal method of mobility is the tongue. The tongue has no bones but works diligently in the mouth to process food. The scientific term for such an appendage is “muscular hydrostat.” Naughton has spent some time during his career learning about these unique muscles.

“Muscular hydrostats are really fascinating structures,” Naughton said. “They have evolved separately in multiple different species so nature clearly finds them useful. They are incredibly flexible, yet also very strong. This makes them excellent multi-functional appendages which are interesting not only to biology but also robotics and engineering.”

Engineering a synthetic version of the muscular hydrostat could be significant for the field of robotics. The strength and range of motion that a muscular hydrostat offers – also shown in elephant trunks and even the human tongue—are as equally suited to crushing and lifting as they are to gentler work. Robotics that use the same principle would have the potential to set a new horizon for the tools they would make possible. Perhaps a lifesaving device capable of both pulling the door off of a car and carefully removing an injured occupant? Maybe something on the smaller scale, like a kitchen gadget modeled on the human tongue?

Wrapping arms around research

To better understand the way a muscular hydrostat works, Naughton and team used a combination of simulations, medical imaging, and live behavioral experiments to create virtual models of the interwoven muscle mechanisms of the arm of an octopus.

As the digitized arm came to life on the screen, a modeling framework emerged which reproduced many of the complex 3D motions of a real octopus. 

In doing so, the research team found that the key to recreating motion was not in managing every part of the arm at once, but the dominant motions of the different types of muscles contained beneath the skin. One type of muscle had particular strength swinging back and forth, while another was more suited to twisting. They adjusted their modeling to match that discovery, finding that they could achieve some of the complex motions commonly used by the octopus by focusing only on those specific muscular dynamics. Instead of controlling muscle contractions along the entire arm, unique motions such as wrapping around an object like a spiral could be achieved by bringing the individual muscle motions together. 

When they programmed reaching motions into the simulation, they also found that the softness of the arm allowed it to sneak into a crevice even if the reaching motion was not perfectly on target. 

“There is still a lot we don’t know about how the octopus is able to control its arms, such as exactly which muscle groups it is activating during specific motions,” Naughton said. “However, our model allows us to form and test a hypothesis that can help guide future biological experiments. It also allows us to develop and test new control strategies for soft robotic arms that we are working on building.”

The next step: material science 

As Naughton’s team fine-tunes its virtual recreations, they will need to find physical materials that can reproduce the movements that make octopus-inspired robotics a reality. An octopus is able to circulate its blood to contract and relax different kinds of muscles, but robotics doesn’t typically use that approach. The answer may lie in materials that respond to changes in temperature, contain some sort of contracting device, or something similar. 

"Soft, octopus-inspired robots have the potential to revolutionize robotic capabilities and even change what we think of as a robot,” Naughton said. “There is still a long way to go, but this work helps provide the basis for understanding how we could eventually build and control this new class of robots. It is a very exciting time to be working on this."