Evolution is a problem-solver, and one of the problems it solved in many different ways is locomotion. Birds fly. Fish swim. Animals walk.
But earthworms found another way to move around the niche they occupy. Can we copy them to explore other worlds?
Earthworms are adapted to moving through the soil, and their segmented bodies allow them to do it. An earthworm has between 100 and 150 segments, and they also have two types of muscles that allow them to move: circular and longitudinal. The muscles and segments allow them to move via crawling. Tiny bristle-like appendages called setae help the earthworms move by preventing them from slipping backward.
An earthworm segment with all the parts labelled. Image Credit: By KDS444 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=33297054
Earthworms use the flexibility of their bodies to move using peristalsis. Peristalsis is wave-like motions that move up and down the earthworm’s body segment by segment. It’s similar to how we swallow food: when we swallow, it sends a muscle wave travelling down our esophagus that pushes the food into our stomach.
This close-up video shows an earthworm moving.
A team of scientists at the Istituto Italiano di Tecnologia (Italian Institute of Technology) is developing a robot that mimics earthworms to move underground and even help explore other worlds. The robot is segmented like an earthworm but uses air to expand and contract the segments and provide locomotion. The segments are called peristaltic soft actuators (PSA.)
This figure from the paper shows the peristaltic soft actuator. The prototype robot has five of them. Image Credit: Das et al. 2023.
The Bioinspired Soft Robotics lab at the ITT created the ‘worm-bot,’ and it’s all part of the lab’s effort to “… develop robotic solutions capable of working in unstructured environments,” according to the lab’s website. “These robots will be able to interact with living beings safely and substitute humans in challenging conditions.”
The principal investigator at the Bioinspired Soft Robotics lab is Barbara Mazzolai, and she’s also a co-author of a new paper presenting the team’s ‘worm-bot.’ The paper is “An earthworm-like modular soft robot for locomotion in multi-terrain environments,” and the lead author is Riddhi Das, a post-Doctoral researcher at the ITT. The paper is published in Nature Scientific Reports.
“The potential applications for this technology are vast, including underground exploration, excavation, search and rescue operations in subterranean environments and the exploration of other planets.”
An Earthworm’s body segments are called metameres, and they contain fluid that controls their internal pressure. The pressure exerts forces on the metamere allowing the earthworm to perform “independent, localized and variable movement patterns,” according to a press release. The quantity of fluid in each segment never changes; it’s just manipulated in different ways. This is an example of a hydrostatic skeleton, where fluid pressure supports a soft, flexible skeleton. Jellyfish and sea anemones have them, too.
The ITT team worked out a way to mimic the fluid in earthworms by using air. Their robot’s PSA elongates when air is pumped into it and contracts when air is released. The worm-bot has five PSAs, is 45 cm (18 inches) long and weighs 605 grams (21 ounces.)
The team tested their robot with and without small passive friction pads that mimic an earthworm’s setae. On a simple flat surface, the setae helped the robot move much more efficiently.
They also tested it on different granular surfaces with different depths, with and without setae.
The worm-bot is just a prototype, and it allows the team to understand biological locomotion in more detail. They’re optimistic about the future of this type of bio-inspired locomotion. “The potential applications for this technology are vast, including underground exploration, excavation, search and rescue operations in subterranean environments and the exploration of other planets,” their press release says.
This isn’t the first robot inspired by earthworms and other similar organisms. The idea has been around for years. In 2012 a Japanese team developed a robotic prototype that tunnels and digs. It had two units: a propulsion unit and an excavating unit. The excavating unit creates a space for the robot to tunnel into.
These schematics from the 2012 Underground Excavator robotic prototype show some of the detail. Image Credit: Omori et al. 2012.
Researchers keep working on the worm-bot and associated efforts because they hold so much promise. We have rovers with auxiliary helicopters on Mars, and there’ll be more of them in the future. Could tunnelling robots one day make the journey to Mars and expand our exploration efforts there?
The InSight lander is an instructive example of how tunnelling robots could aid exploration.
InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander was a joint NASA/DLR mission. Its mission was to investigate the interior of Mars. The mission ended in December 2022 after meeting some of its objectives, but the lander’s main instrument, the Heat flow and Physical Properties Package (HP3), failed.
HP3 was also called ‘the mole’ because its job was to penetrate into the Martian regolith. Once it reached the designed depth, heat sensors along its length would measure the heat flow from the planet’s interior to the surface. That data would’ve told us a lot about the interior of Mars. But it couldn’t complete its mission because it couldn’t penetrate the surface. The instrument’s jackhammer approach to tunnelling below the surface was ineffective.
InSight’s Heat Probe (HP3) popped out of its hole and couldn’t complete its mission. Image Credit: NASA/JPL-Caltech
A tunnelling robot based on an earthworm design might have made the difference. The Mole relied on the friction between the regolith and itself to avoid backing out of the hole, but the consistency of the regolith prevented that. There was no way for mission designers to prepare for that.
But a tunnelling robot based on biology may have succeeded where the Mole failed. Some kind of setae on the outsides of the robot may have made all the difference. If setae could’ve gripped the soil in between percussive movements, the device might have succeeded.
The InSight lander was a well-conceived idea. We would’ve learned a lot about terrestrial planets from it. It’s unfortunate that the HP3 failed, though the mission’s other instruments were successful. Who knows? Maybe there’ll be an InSight 2 to take care of the unfinished business.
If there is, we could see an entirely different instrument penetrate Mars’ surface. One that’s based on the mighty earthworm.