A small robotic frog developed at Aarhus University can swim using six artificial muscles activated by light. Behind the achievement is a systematic approach that could help researchers develop muscles for the next generation of soft robots.
It has no motor, batteries or wires. Yet a small frog created using a 3D printer by PhD student Cecilie Ryberg from the Department of Chemistry at Aarhus University can swim across the water's surface.
It is not particularly fast. In laboratory experiments, it travelled eight centimetres in almost two minutes. But what makes the frog interesting is how it moves.
It uses six artificial muscles that work together to move its legs, pushing water backwards.
The muscles are made from materials called liquid crystal elastomers, that contracts when heated and return to their original shape as they cool. The researchers added a molecule that absorbs near-infrared light and converts it into heat, allowing them to activate the muscles with a lamp.
"Getting the six muscles to work together was a major challenge. If they don't contract in a coordinated way, the frog won't swim forwards. It may simply spin around," explains Brigitte Städler, Professor at the Department of Chemistry at Aarhus University, who led the research project.
Artificial muscles from a piping bag
Other researchers have already developed soft robots capable of moving without wires. The scientific advance in the Aarhus University study lies in systematically examining how changing the chemistry of the liquid crystal elastomers influences their suitability for 3D printing and their response to heating, and how these properties translate into robotic movement.
"In a conventional 3D printer, you typically melt a solid plastic filament. Our material is more like very thick syrup or icing that you squeeze through a piping bag when decorating a cake. We simply use a printer to place the material precisely where we want it," explains Cecilie Ryberg.
Whale and frog trials
Developing artificial muscles suitable for 3D printing requires more than finding materials that can contract. They must also have the right consistency.
Cecilie Ryberg therefore systematically investigated how different molecular ingredients and chain lengths affect both the printing properties of the inks and the movements of the finished muscles.
The results provide other researchers with clearer guidance when choosing materials for artificial muscles, potentially reducing the amount of trial and error involved.
To demonstrate their approach, Cecilie Ryberg created two swimming robots. One resembles a whale, propelled by a single muscle that makes its tail undulate. The other is the frog, whose six muscles work together to produce forward movement.
Both the whale and the frog have bodies made from soft silicone, which the researchers 3D-printed together with the artificial muscles. They also developed a chemical method to bond the two materials together.
This is important because even the best artificial muscle is of little use if its movement cannot be transferred to the rest of the body.
From the laboratory to the playroom?
The researchers envisage that soft robots of this kind could eventually be used for applications such as environmental monitoring and search-and-rescue operations, potentially reaching places that conventional rigid robots struggle to access.
However, their current designs still require an external light source and cannot yet navigate independently around obstacles.
And if the technology eventually finds its way out of the laboratory, its first application may be considerably less ambitious than rescue missions.
"I could easily imagine these robots being used as toys before we see them on rescue missions. I think people would have fun playing with a frog like this using a suitable light source," says Cecilie Ryberg.
