Danish researchers from Aarhus University have created a soft robot in the shape of a frog that moves in water without a motor, batteries or wires. Instead, six artificial muscles activated by light do the work. The work was published in the journal Advanced Materials Technologies.
The robot was printed on a 3D printer by graduate student Cecilie Ryberg of the Department of Chemistry. In laboratory tests, the frog swam 8 centimeters in almost two minutes - a low speed, but the principle of movement itself is what matters.
The muscles are made of liquid crystal elastomers - materials that contract when heated and return to their shape when cooled. Molecules that absorb near-infrared light and convert it into heat were added to them. This way the muscles can be switched on with a lamp, without wires.
"Getting six muscles to work together was the main difficulty. If they contract out of sync, the frog will not swim forward but simply spin in place," explains project leader Professor Brigitte Stadler.
The novelty is not the idea of a soft robot itself - such devices already exist. The scientists systematically studied how changes in chemical composition and the length of molecular chains affect the material's suitability for 3D printing and its response to heating, as well as the movement of the finished muscles. This should help other researchers choose materials more consciously rather than by trial and error.
The printing material is more like a very thick syrup or icing from a pastry bag than the usual plastic filament for a 3D printer. The printer simply lays it down in the right place, Ryberg explains.
Besides the frog, the researchers printed a robot whale with a single muscle that makes its tail oscillate. The bodies of both robots are made of soft silicone and printed together with the muscles. The scientists also developed a chemical method to firmly bond these two materials - otherwise even the best muscle is useless if its movement is not transmitted to the body.
The authors suggest that such soft robots may someday be useful for environmental monitoring and rescue operations, where rigid machines have difficulty reaching. However, the current models require an external light source and cannot yet navigate around obstacles on their own.
If the technology leaves the laboratory, the first application may not be saving people at all. "I can easily imagine these robots as toys long before rescue missions. I think people would enjoy playing with such a frog by directing a suitable light at it," Ryberg says.