A frog that takes nearly two minutes to swim about the length of a credit card is unlikely to make a splash at the next Amphibian Olympics. Then again, most frogs have the benefit of some 200 million years of evolution. This one was just hatched out of a 3D printer.
Built by chemist Brigitte Städler and PhD student Cecilie Ryberg from Aarhus University in Denmark, together with colleagues from the University of Luxembourg, the frog-shaped robot weighs less than a gram and swims without a motor, battery, or wires.
To move, a lamp supplies it with flashes of energy, pulsing near-infrared light onto half a dozen liquid crystal muscles attached to a pair of hind legs. Working as a unit, the muscle strands propelled the tiny swimmer across the surface of a water bath a distance of about 80 millimeters (3.1 in), taking 1 minute 54 seconds on average to complete the lap.
While a speed of just 1.5 body lengths per minute is closer to my high school personal best than I’d care to admit, the robot’s true success lies more in its coordination than its velocity.
"Getting the six muscles to work together was a major challenge," says Städler. "If they don't contract in a coordinated way, the frog won't swim forwards. It may simply spin around."
The team printed the muscles and the soft silicone body in a single run. Ryberg likens the printing process to piping icing. By squeezing the rubbery, liquid-crystal elastomer "ink" through a printer nozzle, the team coaxed its molecules into alignment.
The hard part was getting a recipe that would flow through the nozzle, hold its shape, and still move predictably.
Shining a near-infrared light on the dye produced heat that disrupted the polymer’s molecules, shortening the strips and gradually forcing the whole muscle to contract. As the strips cooled, the muscle lengthened.
Getting an individual muscle to shrink is one thing. Getting six of them to agree on which way to send their froggy body is another. In an early design, the muscles pulled the legs inward when the light came on, pushing water toward the frog’s head.
By rearranging muscle attachments and changing joint shape, the researchers straightened the legs and drove water back.
The frog requires light pulses to be evenly distributed to swim straight, and suffers from water drawing heat away from its muscles. The team suggests that a steerable beam could eventually fire up one leg at a time, giving the frog a way to turn.
Earlier this year, researchers at MIT shared their design of a paper-thin, light-powered swimmer that could be steered through a maze by flashing light at living muscle cells engineered to twitch on cue. The cells pulled on a soft gel frame, making its fins flap.
If Städler’s team can find a similar solution for their frog, their methods of printing muscles directly into soft bodies could make it easier to build small swimmers that squeeze through tight spaces to inspect flooded structures or carry sensors into shallow, cluttered waterways.
For now, the researchers see a more immediate prospect in toys. A frog you can send paddling with a lamp sounds like a decent afternoon, provided you’re in no hurry to get to the other side of the bathtub.
The research was published in Advanced Materials Technologies.
Source: Eurekalert
