Polymers that change shape, size, or stiffness in response to light, heat, or liquids could be useful materials that power mechanical devices. Alternatively, that induced motion could be used to generate electricity for low-power devices.

Some polymers swell in water, which can make them suitable as an artificial muscle. Current materials can’t replicate motion that’s as smooth, fast, and powerful as muscle contractions, though careful design has now helped scientists improve the performance of water-induced material actuators. And, in the process, they've made a device that can slowly charge a capacitor—just by getting wet.

Mingming Ma and co-workers at the Massachusetts Institute of Technology built a material that has strands of a water-responsive polymer called polypyrrole held between a rigid skeleton of a different polymer (made from branched ethylene glycol connected with borate groups). This construction mimics the structure of our muscle fibers, which have strings of stretchy fibers surrounded by a rigid matrix of collagen.

Things start moving when the scientists place their new material on a wet surface. Water vapor seeps into the bottom of the material, severing the links between the polypyrrole and the rigid matrix. The water also breaks some of the bonds in the rigid matrix.

These chemical changes cause the material to soften, swell and pull away from the surface. As more of the film peels off the surface, the curved sheet topples onto its side. If this film was a crunchy taco shell, the fillings would be spilling all over the table. And while tacos can’t move out of this position without help, this material can.

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 harnessed that mechanical motion to generate electricity. They covered the new film in a material that generates electricity in response to mechanical stress. This piezoelectric actuator generated about 1.0V with a 10-megohm resistor in the circuit. Capturing that motion-induced voltage in a different circuit and rectifying the signal, the researchers charged a 2.2 microF capacitor in about 7 minutes to its full capacity (about 0.66V).

These materials might be used to generate electricity for low-power devices, the researchers write. But it’s the material’s performance, not the application, that’s the advance here. Adding a rigid matrix to polypyrrole increases the water-induced stress by four orders of magnitude and increases the strain six-fold over polypyrrole alone. The mechanical power density of the new material is on the order of 1 watt per kilogram.

The idea of harvesting energy from a moisture gradient, rather than typical sources like light, heat, or wind, is interesting, write Hyoki Kim and Sunghoon Kwon, of Seoul National University, in an accompanying commentary. Though the electrical energy generated from this prototype is low, stacking these devices might help increase the power output, they add. However, they caution that the polymer film might break after many cycles of actuation.

Science, 2012. DOI: 10.1126/science.1230262 (About DOIs).