Artificial muscles powered by bursts of electricity generated from water could lead to moving machines fueled solely by evaporating sweat or the humidity rising from rivers and lakes, scientists say.
The generators developed by MIT researchers are based on strong, flexible films that perform as actuators, or artificial muscles. These films expand when they absorb water and contract when they expel it.
Making mechanical energy
Made of two organic polymers, polypyrrole and polyol-borate, these artificial muscles generate significantly more force when exposed to water than comparable actuators, which are typically made from just one kind of material. Films composed of the two polymers that are just 15-40 microns thick — about one-sixth to two-fifths the width of a human hair — can lift objects 380 times heavier than themselves and transport cargo 10 times heavier than themselves.
When these films are placed on flat moist surfaces, such as sheets of wet paper, they swell and curl up. However, they dry quickly via evaporation at room temperature — in just a few seconds when on wet paper, and in just a minute or two if they get completely soaked and then removed from water. A quick drying time helps the film absorb water again, enabling the cycle to start all over. For as long as the paper emits water vapor, the film continues to dance.
"When we made this material, we found it moved automatically on human hands," says researcher Mingming Ma, a chemist at MIT. “That was an interesting phenomenon, which we thought could lead to something.”
Converting motion to electricity
The researchers next coated these artificial muscles with nine-micron-wide layers of a piezoelectric material, polyvinylidene difluoride, which converts this constant mechanical stress to electrical energy. As such, when the actuators flexed in response to moisture, they generated roughly three bursts of electricity every 10 seconds, with a peak voltage of about one volt.
"Potential commercial applications include large-scale water-vapor power generators or small generators to power wearable electronics," Ma says. One might imagine large-scale power generators placed above lakes or rivers, he says, and small-scale generators attached to clothing, where the mere evaporation of sweat could fuel devices such as wearable blood pressure and heart rate sensors. Even tinier generators could help power microelectrical mechanical systems (MEMS) or nanoelectronics.
These generators could also replace batteries in certain devices — in wireless networks of remote sensors, for example, where it would be difficult and expensive to regularly change batteries, says electrical and material engineer Sunghoon Kwon at Seoul National University, who did not take part in this research.
"Since the device only requires a wet surface to generate power, environment monitoring with wireless sensor systems will be one of its straightforward applications," Kwon says.
The researchers originally created these artificial muscles with the aim of using them as flexible, stretchable electrodes. Their research on these films continues with the goal of developing devices that stimulate muscles and nerves, Ma adds.
In the future, the researchers hope to use other piezoelectric materials and modify aspects of these films to improve their efficiency at converting mechanical energy into electricity. Kwon says it should also be possible to increase power output by stacking multiple films together, which is necessary before the films can become useful.
"The current actuator-generator is just a prototype, and there are many challenges that must be met to make the transition from the laboratory to successful commercial technology," Ma says.
The researchers have filed a U.S. patent covering related materials and technology. They detailed their findings in the Jan. 11 issue of the journal Science.
Top Image: The continuous motion of a polymer actuator driven by water vapor. Image courtesy Dr. Ning Zhang.