A team of researchers at MIT’s David H. Koch Institute for Integrative Cancer Research has developed a new polymer film that generates power from water vapor. Consisting of two polymer films, the material makes remarkably acrobatic somersaults in the presence of even tiny traces of evaporated water, opening the way for new types of artificial muscles for controlling robotic limbs or powering micro and nanoscopic devices.
The new material is very simple in design. It consists of a 20-micrometer thick film in two layers. The first is made of polypyrrole, which is a hard polymer that provides structural support, and the second is polyol-borate, a soft gel that swells as it absorbs water. Previous efforts at developing a similar “artificial muscle” have relied on one layer, but two turned out to be a distinct improvement.
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“By incorporating the two different kinds of polymers, you can generate a much bigger displacement, as well as a stronger force,” postdoctoral student Liang Guo said.
The film exploits the water gradient between dry and moist environments. When it lies on a surface with even a small amount of moisture present, the bottom layer curls away. This exposes the polymer to the air where the water evaporates from its surface. The bit of film does a somersault and the cycle starts over.
This isn’t just a laboratory curiosity. The polymer film exerts a surprising amount of force as it curls. A 25-milligram film can lift 380 times its own weight or carry along a load of silver wires ten times its weight. According to the researchers, that’s enough force to replace electric actuators in small robotic limbs. What’s more, it can do so without manipulating the environment. If water is available, the film will work.
This property gives it an advantage not only as a mini-motor, but as a power source. The film could use piezoelectric materials to generate electricity. Currently, the film can produce 5.6 nanowatts, which is enough to run ultra-low power microelectronic devices.
According to Mingming Ma, a postdoctoral student and lead author of a paper describing the new material, “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.”
The researchers also envision the film creating power by being placed over a body of water or incorporated into clothing where sweat could power wearable electronics. For the immediate future, the team is working to improve the film’s efficiency to allow smaller films to power larger devices.
The findings of the team were published in the January 11 issue of Science.
The video below describes the new material.