In fishing terms, "catch and release" generally means first impaling with a hook and then nearly suffocating a fish. For engineers at MIT, it means delicately trapping a fish with water itself—albeit in gel form—and then releasing the "quite happy" victim, in the words of Xuanhe Zhao, the mechanical engineering professor that leads a group at MIT studying hydrogel "soft" robots. For the past five years, Zhao and colleagues have been perfecting hydrogel recipes ideal for tough and forceful but also soft and biocompatible robot manipulators.
The results of Zhao's latest recipe, which are described in the current issue of Nature Communications, can be seen in the video below.
A hydrogel is more than just a soft robot. The soft robots we typically see—and there are a lot of them lately—are fashioned from materials like silicon. But because a hydrogel consists mostly of regular old water, it has the advantage of biocompatibility. A hydrogel might then be safer to use in biomedical settings.
Since hydrogel is basically water jelly, engineers have to prove that it can be useful for tasks like, well, gripping fish, which we might imagine to instead be an internal organ being gripped during surgery. A swimmy fish liver, say. Previous attempts at hydrogel actuators have been limited in this respect, according to Zhao, while the hydrogel gripper seen above is able to achieve up to 1 Newton in force with a response time of less than a second. Add to it the acoustic and optical camouflage that comes with being made out of mostly water—one of the material's big selling points—and that goldfish didn't have a chance.
Zhao and co. took their inspiration from tiny eel larvae known as glass eels. In the natural world, glass eels start their lives with a perilous trek from their ocean hatching grounds to future river homes. These tiny defenseless blobs manage the journey thanks to both speed and complete transparency, which is thought to be an evolutionary adaptation. This is what they were hoping to mimic, only swapping out eel muscles for hydraulic actuators.
"The hydrogel actuators and robots can maintain their robustness and functionality over multiple cycles (that is, over 1,000) of actuations, due to the anti-fatigue property of the hydrogel under moderate stresses," Zhao writes. "It is also expected that pneumatic actuation can be used to drive the hydrogel actuators and robots to operate in air."
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