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Sea cucumbers inspire liquid robots

The scientists put their robots through an obstacle course of mobility and shape-changing tests.

(CN) — To create what an international team of engineers calls "liquid" robots, they had to overcome the limitations of both traditional and so-called "soft" robots."

In their study published Wednesday in Matter, the team describes traditional robots as hard-bodied and stiff while “soft” robots are flexible but weak and their movements difficult to control. To find a compromise between the two models, Carnegie Mellon University Professor Carmel Majidi — a mechanical engineer and the senior author of the study — said the team looked to the sea cucumber — an animal that can alter the stiffness of its tissue to improve load capacity and prevent physical damage from the environment.

“The ability to change stiffness is especially important in making soft robots where in one state you want it to be highly deformable, compliant, and adaptive to its surroundings, while in another state you want it to lock in place so that it can withstand load and exert force or perform mechanical work on its environment,” Majidi said via email.

According to the study, the team embedded magnetic particles in gallium, a metal with a very low melting point of 85.6 degrees Fahrenheit, to create a new phase-shifting material. Chengfeng Pan, an engineer from the Chinese University of Hong Kong and leader of the study, said that allows robots to switch between liquid and solid states and “endows them with more functionality.”

This new phase-shifting material has an extremely fluid liquid phase that has "very low viscosity when it’s in its liquid state, meaning that it has virtually no mechanical resistance to deformation," Majidi said.

"This is in contrast to other approaches in stiffness tuning, where the material has a high viscosity when in its soft state and behaves more like a paste that does not flow as easily," he added.

With the aid of a magnetic field, the team tested the material’s mobility and strength by having the robots jump over moats and climb walls, and the robots split themselves in half to cooperatively move other objects around before rejoining. One video, the study said, shows a person-shaped robot liquify to ooze through a grid before remolding back into its original shape.

Majidi said the team designed and tested the liquid robots for biomedical applications.

The study describes his team using the robots to remove a foreign object from a human stomach model filled with water and to deliver drugs.

The phase-changing material, the scientists said, can also ooze into hard-to-reach circuits to act as both solder and conductor for smart soldering robots meant for wireless circuit assembly and repair. As a universal mechanical “screw,” the team says that the material can assemble parts in hard-to-reach spaces by melting into the threaded screw socket and then solidifying, removing the need for actual screwing.

“This is still ongoing research and to use this in medicine, we next need to show that these materials are biocompatible and safe for use in the human body," wrote Majidi. "My research group is currently exploring the biocompatibility of liquid metals and metals like gallium that have a very low melting point. Such studies require examination of the cytotoxicity of these materials when in contact with cells and biological tissue.”

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