Humanity is increasingly turning to robots to perform a wide range of tasks such as manufacturing and even exploration. One of the most common and traditional forms of locomotion for exploratory robots are wheels, but wheeled robots face difficulties in tackling different terrains, something that researchers are attempting to solve with different approaches to locomotion. 

Taking their inspiration from nature, engineers have developed earthworm-like robots that travel by crawling that can more easily traverse tricky terrain. But these crawling machines still suffer from another issue incumbent with other robots.

Thus far these robots have been built using inflexible hard parts that can make navigating tight spots tricky or can even pose a risk to life forms in the area under exploration, and in extreme cases, human co-workers.

Earthworm robots made from soft polymers

In a new paper published in the journal Advanced Materials, researchers take the next natural step in the evolution of worm robots, replacing hard segments with more flexible sections just like their counterparts in the animal kingdom to help these machines out in a tight spot. 

“Worms and other limbless locomotors  —  snakes, caterpillars, snails, and the like  —  have a distinct advantage in constrained environments where wheels pose an issue, such as pipes and tunnels. These would be difficult for a wheeled robot to navigate as rigid structures may become stuck at sharp turns and narrowing passageways while the wheels are prone to catching on irregular surfaces,” said Shane Riddle a Ph.D. student at Case Western Reserve University. “The peristaltic locomotion method is capable of overcoming surface inconsistencies and narrowing that would otherwise impede a wheeled robot.”

Riddle and the team created their worm-like robot using soft polymers, which adds to the locomotive abilities of these machines. 

“Most existing worm-like robots contain rigid components which restrict how they can adapt or deform to various environments or obstacles,” said Austin Mills, on of the study’s authors and a researcher at Case Western Reserve University. “Conversely, our soft polymer body design allows our robot to squeeze itself through constrictions and undergo deformations that would not be possible with traditional rigid-bodied robots.”

Mills added that soft-bodied robots like the one designed by the team are able to more closely mimic the complex movement of their biological counterparts, while also being inherently safer for human-machine interaction. 

Mimicking modular worms

The worm robot designed by the team has a modular design with multiple segments that lead to the precise, well-timed, and repeatable actuation of the individual segment that facilitates its worm-like locomotion. This means its discrete segments allow it to move more like a biological earthworm as each can be moved in a specific way with a measured amount of force, rather than the same applied force and direction across the whole of the robot.

“These segments share a relationship between diameter and length; as the segment diameter contracts, the segment length increases and vice versa,” Mills continued. “A biological earthworm typically has over 100 of these segments, and if examined during movement, you would notice a wave of segment muscle contractions and relaxations traveling down the body of the worm, called peristalsis.”

The robotic segments are made of polymer bilayers that function much like inflexible metal counterparts known as “bimetallic strips”, which Mills points out have been in use for many years. 

“Both metal and polymer bilayers work on a principle of having two different materials sandwiched together that expand at different rates during heating, causing the bilayer to curl,” he added.

Mills’ fellow co-author Livius Muff, who developed the materials used in the robot as a Ph.D. student at the Adolphe Merkle Institute in Fribourg, Switzerland explained: “Combined into a bilayer actuator, we were able to exploit the polymer’s material properties to generate a bending motion, using an integrated electrical heating system.”

Curling bilayers

The team achieved increased flexibility or a “high bending deflection” by combining this new class of high-thermal-expansion polymers with a commercial low-thermal-expansion polyimide film and flattened at high temperatures. When they are allowed to cool at room temperature the layers begin to curl. 

“Using eight bilayers per segment, we joined six segments together in series to form the worm-like robot,” Mills added. “Using a microcontroller, we control when certain segments are being heated to achieve peristaltic locomotion. By only heating up the left or right four bilayers of a segment we can cause the robot to turn for steering purposes.”

The flexible bilayer polymer actuators make up the structure of the robot’s body, resulting in a nearly fully soft-bodied worm robot whose only rigid components are those that the team could not replace with current technology; mainly the battery and microcontroller. This means that the new robot is still burdened with one problem that literally weighs down its hard-bodied counterparts: it still has to be tethered to a controller.

“While this doesn’t solve our issue with tethering, it is a step in the right direction for making these robots completely soft,” Riddle said. “This design, though quite soft, could still be improved in terms of speed and energy efficiency.”

He pointed out that these improvements could come when advancements to high-thermal expansion material are made that allow for faster or more energy-efficient heating. Mills added improvements could come via the use of actuators not powered by heating, such as electrically powered elastomer bending actuators.

“The robot’s existing structure could also be used for attaching various sensors, which would allow it to gather important environmental data and give it localization capabilities,” Mills said. 

From exploration to medicine

Muff explained that thanks to the robust and flexible design of the robot, the team envisions it being used to navigate complex, unpredictable, and potentially unstable terrain. Additionally, the polyimide skeleton of the worm robot means it could be used as a flexible circuit board, thus opening up the possibility for the on-board integration of cameras, lighting, and other sensors. 

“The robot can overcome narrow constrictions, much smaller than its resting diameter, and explore hollow spaces while suspended in mid-air,” said Muff. “These skills make our worm a fantastic device for underground exploration of cave systems, subterranean infrastructure inspection, or surveillance operations.

“Furthermore, the worm robot’s hollow tubular structure could be used to deliver cargo such as medication or emergency supplies to trapped individuals in collapsed buildings or rubble,” he concluded.

Reference: L.F Livius., A.S. Mills., S. Riddle, V. Buclin., et al., Modular design of a polymer-bilayer-based mechanically compliant worm-like robot, Advanced Materials, (2023). DOI: 10.1002/adma.202210409