3D Printed Swimming Soft Robot Uses Shape Memory Polymers to Paddle Forwards and Backwards

In November, researchers from the Swiss Federal Institute of Technology Zurich (ETH Zurich) published a study that explained how shape memory polymers can be the catalyst for making adaptable materials that will return to their original shape once heated, which we’ve seen before. The team used 3D printing, programmable design, and thermoviscoelastic meta-materials in the study, and Professor Kristina Shea, along with her doctoral student Tian (Tim) Chen, developed 3D printable shape memory polymer strips for the experiment.

Now, Professor Shea and her team have partnered with researchers from the California Institute of Technology (Caltech) to use these strips in a new proof of concept study about 3D printing and soft robotics. The research team created a 3D printed mini-submarine that doesn’t need propellant, a power supply, or an engine to move.

Professor Shea explained, “The main takeaway from our work is that we have developed a new and promising means of propulsion that is fully 3D printed, tuneable and works without an external power source.”

The team has developed a new propulsion concept for swimming robots, where they use temperature fluctuations in the water to paddle along. A multimaterial 3D printer was used to create a 7.5 cm mini-sub, which is equipped with paddles that are actuated with a bistable propulsion element. This element is triggered by two of Professor Shea and Chen’s shape memory polymers, which expand in warm water to act like muscles to power the robot.

A paper on the 3D printed submarine, titled “Harnessing bistability for directional propulsion of soft, untethered robots,” was recently published in the journal PNAS (Proceedings of the National Academy of Sciences of the United States of America). Co-authors include Chen, Osama R. Bilal from Caltech, Professor Shea, and Chiara Daraio from Caltech.

The abstract reads, “In most macroscale robotic systems, propulsion and controls are enabled through a physical tether or complex onboard electronics and batteries. A tether simplifies the design process but limits the range of motion of the robot, while onboard controls and power supplies are heavy and complicate the design process. Here, we present a simple design principle for an untethered, soft swimming robot with preprogrammed, directional propulsion without a battery or onboard electronics. Locomotion is achieved by using actuators that harness the large displacements of bistable elements triggered by surrounding temperature changes. Powered by shape memory polymer (SMP) muscles, the bistable elements in turn actuate the robot’s fins. Our robots are fabricated using a commercially available 3D printer in a single print. As a proof of concept, we show the ability to program a vessel, which can autonomously deliver a cargo and navigate back to the deployment point.”

One of the main challenges in soft robotics is integrating actuation, control, propulsion, and sensing in one mechanism. The team’s soft robotics design principle is a material-based approach that can swim untethered, and complete pre-programmed tasks, like deliver cargo and follow specific routes, without any electronics, power sources, or controllers on board.

If the water the mini-sub is floating in is heated, the expanding muscles cause the bistable element to snap, which triggers a paddle stroke. Each actuating element of the mini-sub can complete one paddle stroke before a necessary manual reprogramming, though in the future the researchers believe they can 3D print complex swimming robots that have several actuators. Instead of relying on controllers or power, the robot’s material and geometry define the force, timing, and directional motion of the paddle strokes.

The team’s current 3D printed robot can paddle forward in one stroke, release a coin, and return to its starting point with a stroke in the opposite direction, and it does all of this just by sensing temperature changes in the water. By changing up the geometry of the shape memory polymer muscles, the scientists can define the stroke triggering sequence – thin polymer strips will respond more quickly, because they heat up faster in warm water.

In the future, this 3D printed swimming soft robot could be further developed to make low-power vessels for exploring the ocean; additionally, the scientists could also use shape memory polymers that react to environmental factors like the salinity or acidity of water, instead of the temperature.

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[Source: ETH Zurich]