Bio-inspired underwater robots that can change shape push through the water much more efficiently than their conventional rigid counterparts. But they are quite difficult to build.

Now a research team at MIT has come up with an innovative approach to making deformable underwater robots. The modular system can be used to build robots of various shapes and sizes with both hard and soft elements.

The deformable robots are made with lattice-like pieces called voxels that are low in density and have high rigidity. The individual voxels are usually hollow structures of molded plastic pieces with narrow struts in complex shapes. The box-like shapes are load-bearing in one direction but soft in others, an unusual combination achieved by combining rigid and flexible components in different proportions.

The deformable robots are made with lattice-like pieces called voxels.
The deformable robots are made with lattice-like pieces called voxels. Credit: MIT

“Smooth flexibility of the body surface allows us to implement flow control that can reduce drag and improve propulsion efficiency, resulting in significant fuel savings,” says Triantafyllou, who was part of MIT’s original RoboTuna team.

The team demonstrated the new system in two different sample configurations, one as an eel and the other as a wing-like hydrofoil.

In one demonstration, they built a meter-long, snake-like structure made up of 20 voxels attached end to end in a long row. An actuator in the center can pull a wire attached to each of the two voxels on either side, causing them to contract and bend the structure. The whole structure is covered with a rib-like support structure and then a close-fitting neoprene waterproof skin.

An MIT towing tank test demonstrated its efficiency in the water, showing that it was indeed capable of generating enough forward thrust to propel itself with undulating motion.

The team also created a wing-like hydrofoil.
The team also created a wing-like hydrofoil. Credit: MIT

Researchers state that a snake-like robot built by NASA was made of thousands of unique pieces, while this group’s snake consists of only about 60 individual components. Also, compared to MIT RoboTuna, which took two years to design and build, this device was put together in about two days.

The other they demonstrated is a wing-like shape, or hydrofoil, which consists of a series of the same voxels, but which can change the profile shape and thereby control the wing’s lift-to-drag ratio and other properties.

Ultimately, the concept could be applied to a cetacean underwater craft, using the changeable body shape to create propulsion. It can also be applied to parts of other craft, such as racing yachts, where a keel or rudder that can bend slightly during a turn instead of staying straight can give it an extra edge.

Magazine reference:

  1. Alfonso Parra Rubio, Dixia Fan, Benjamin Jenett, José del Águila Ferrandis, Filippos Tourlomousis, Amira Abdel-Rahman, David Preiss, Michael Triantafyllou and Neil Gershenfeld. Modular Morphing Grids for Large-Scale Underwater Continuum Robotic Structures. Soft Robotics, 2023; DOI: 10.1089/soro.2022.0117