Polymer film moves when wet, creates electricity

Ars Technica : Muscles function smoothly and powerfully because they are structured as bundles of stretchy fibers surrounded by a rigid structure of collagen. Mingming Ma of MIT and his colleagues have created a similar material from strands of a water-responsive polymer called polypyrrole woven into a rigid sheet of another polymer. When exposed to a wet surface, the material absorbs the water vapor, which causes the bonds between the two polymers to break and makes the polypyrrole expand. As the material flexes and turns, the wet area dries and becomes rigid again and another area becomes wet and flexes. Ma’s team took this material and added a piezoelectric layer, which created a material that generated electrical energy from the bending and flexing. The amount of energy generated was not largeâmdash;just 1.0 V across a 1 MΩ resistorâmdash;nor was it fast, taking 7 minutes to fully charge a 2.2-μF capacitor. However, it’s the mechanical power density, on the order of 1 W/kg, that is a significant step forward in polymer-based electricity generation.