Harnessing this continuous motion could drive robotic limbs or generate enough electricity to power micro- and nanoelectronic devices, such as environmental sensors. "With a sensor powered by a battery, you have to replace it periodically. If you have this device, you can harvest energy from the environment so you don't have to replace it very often," says Mingming Ma, a postdoc at MIT's David H. Koch Institute for Integrative Cancer Research and lead author of a paper describing the new material.
"We are very excited about this new material, and we expect as we achieve higher efficiency in converting mechanical energy into electricity, this material will find even broader applications," says Robert Langer, the David H. Koch Institute Professor at MIT and senior author of the paper. Those potential applications include large-scale, water-vapor-powered generators, or smaller generators to power wearable electronics.
The new film is made from an interlocking network of two different polymers. One of the polymers, polypyrrole, forms a hard but flexible matrix that provides structural support. The other polymer, polyol-borate, is a soft gel that swells when it absorbs water. Previous efforts to make water-responsive films have used only polypyrrole, which shows a much weaker response on its own. "By incorporating the two different kinds of polymers, you can generate a much bigger displacement, as well as a stronger force," Guo says.
The film harvests energy found in the water gradient between dry and water-rich environments. When the 20-micrometer-thick film lies on a surface that contains even a small amount of moisture, the bottom layer absorbs evaporated water, forcing the film to curl away from the surface. Once the bottom of the film is exposed to air, it quickly releases the moisture, somersaults forward, and starts to curl up again. As this cycle is repeated, the continuous motion converts the chemical energy of the water gradient into mechanical energy.
Such films could act as either actuators (a