Amphibious Snake Robot Adapts by Shifting Internal Water

Most snake-like robots excel in one environment. Some crawl across land, while others are designed to swim. Moving between the two is where things usually fall apart, requiring specialized appendages, entirely different gaits, or other complex mechanisms to cope with changing terrain. Researchers have now demonstrated a simpler alternative with an amphibious robot called WorMa that adapts by doing something many animals already do naturally: shifting its weight.

Developed by researchers at New York University, WorMa uses the same undulating motion whether it's traveling across flat ground, climbing a slope, swimming through water, or pulling itself back onto land. Instead of changing the way it moves, the robot changes where its weight is concentrated by pumping water between reservoirs located in its head and tail. This approach eliminates the need for additional reconfiguration mechanisms while allowing a single robot to tackle environments that normally require very different locomotion strategies.

The robot measures about 50 cm long and consists of five 3D-printed body segments connected by four motorized joints. Silicone pads mounted underneath the body provide directional grip while crawling, and foam disks attached along the sides provide enough buoyancy for aquatic operation. A pair of latex balloon reservoirs, connected by silicone tubing, hold approximately 300 grams of water — roughly 28% of the robot's total mass. A centrally mounted 12 V peristaltic pump transfers that water between the head and tail, shifting the robot's center of gravity as conditions change. An ESP32 oversees the system, while four servos generate the wavelike motion that propels the robot forward.

Adjustable weight distribution turns out to have a dramatic effect on performance. When climbing, pumping the water toward the front presses the leading segments more firmly against the ground, increasing traction where it's needed most. During testing, the head-heavy configuration was the only one capable of climbing a 19.5-degree incline, while also reducing the robot's cost of transport by at least 33% compared to the other weight distributions.

Swimming favors the opposite arrangement. Moving the water toward the tail increased swimming speed by as much as 26% while improving efficiency by 52%. Exiting the water once again required shifting the weight forward so the robot could generate enough grip to haul itself onto shore. Rather than searching for a single compromise that works everywhere, the robot continuously tailors its balance to the terrain it's about to encounter.

The researchers also showed that changing the center of mass during movement can solve problems that no fixed weight distribution can. To climb a 15-cm step, WorMa approaches with its weight forward for traction, transfers water toward the rear to lift its front section higher, then shifts the weight forward again after hooking the top edge to complete the climb.

The current prototype still needs some improvements before it's ready for real-world use. Completely transferring water between the reservoirs takes about 110 seconds, making weight changes relatively slow. Faster pumps and onboard sensing could eventually allow the robot to recognize changing terrain and automatically rebalance itself in real time. Even in its current form, though, WorMa demonstrates that adapting to new environments doesn't necessarily require more legs, more motors, or more complicated mechanics.