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It is common to see electricity being generated today from light or heat. Now, researchers at the Massachusetts Institute of Technology (MIT) have found a way to use polymers to generate electricity for low-power devices when the polymers expand after getting wet.
Although the motion of the polymers is not as smooth and powerful as typical muscle contraction, the swelling polymers developed by Mingming Ma at co-workers at MIT do create an artificial muscle, of sorts. The pumping action can slowly charge a capacitor, reports ARS Technica.
Ma and his team used a polymer called polypyrrole held by a rigid skeleton made from a different polymer, branched ethylene glycol connected with borate groups. The matrix of polymers resembles muscle tissue, with stretchy fibers bordered by collagen.
When placed on a wet surface, the water vapor seeps into the bottom layer of the material, which breaks the links between the polypyrrole and its skeleton. The rupture of the links causes the material to soften and swell. The film peels away from the surface, making it curl and fall onto its other side.
The researchers liken the curling motion to the shape of a crunchy taco shell. ARS Technica further explains how the movement continues in the wet environment:
Water evaporates from the moist side of the film (now on top) and that portion of the material stiffens. Meanwhile, a part of dry side of the sheet (now on bottom) absorbs water from the surface and curls up. This asymmetric swelling and stiffening causes the material to stretch and leap across the surface. Eventually, what was originally the dry side of the film settles on the wet surface and the cycle begins again.
The researchers used that mechanical motion to generate electricity when they covered the polymer matrix with a material that generates electricity in response to movement. A piezoelectric actuator, using the material, generated about 1.0V with a 10-megaton resister in the circuit.
The materials certainly can be used to generate power for certain devices, the researchers say. But the polymer material’s performance, not the application, is innovation that’s important, the researchers say.
Specifically, the rigid matrix of the polypyrrole is what increases the water-induced movement by four orders of magnitude over polypyrrole alone. The researchers claim that the mechanical power density of the new material is about 1 watt per kilogram.
Although the electricity generated with this method is relatively low, the power output can be increased if the devices were stacked, says Hyoki Kim and Sunghoon Kwon of Seoul National University, who commented on the development. They cautioned, however, that the films might break after several rounds of actuation.
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