If you sandwich a piece of water-absorbing plastic onto a piezoelectric sensor, you create an assembly that can bend in response to water vapor and, in so doing, generate a small amount of electrical current. That's exactly what researchers at the Massachusetts Institute of Technology say they've done with a special water-responsive polymer they've created.
Researchers Mingming Ma, Liang Gue, Daniel Anderson, and Rober Langer took a stiff plastic called polypyrrole and coated it with a water-absorbing gell-like plastic called polyol-borate. The polyol-borate swells up when it absorbs water. Pieces of thin plastic made this way will actually curl up and uncurl as portions of their surface absorb water and dry out. (The curling is in response to moisture gradients on the material surface, the reserachers say. Bathing the material in water vapor so that all parts of the plastic get exposed to the same amount of moisture results in the plastic just sitting there.)
Moreover, the curling action is rapid. A video from the MIT group shows samples of the special polymer flipping around and curling over the course of a few seconds. Besides being fun to watch, all that action can do useful work, the researchers say. They've measured 27 megapascals of contractive stress generated by the pieces of film. They can also lift objects 380 times heavier than themselves and transport cargo weighing about ten times more.
The most interesting behavior comes by combining the special plastic with piezo material. One generator the researches constucted put out about a volt and would produce alternating current at about a 0.3 Hz rate. The setup generated enough juice to be stored in capacitors that could then be used to power microelectronic devices.
Researchers sandwiched a PVDF piezoelectric film to their actuator to form an electric generator. They then sent to resulting signals through a rectifier to a resistive load to generate the ac signal (below left). They also charged a capacitor (right). The movement of the plastic could be categorized into five stages, the researchers say. As the stuff touched a wet surface, the bottom face absorbed more water than the top which forced the plastic to curl away from the moist spot. This heightened the plastic's center of gravity which made it mechanically unstable and eventually forced the piece to topple over. At this point a new spot on the plastic touched the moisture and moisture released from the raised part to generate a horizontal movement. Finally, most of the contact area curled up and a new portion made contact with the wet surface to repeat the cycle.
Researchers have also figured out that the frequency of these events depend on the saturated water vapor pressure, so locomotion of the stuff can be regulated by controlling the water evaporation rate. Water-saturating the air next to the substrate forced the plastic to fold into a roll which contained a lot of elastic energy, 21 to 76 J/km, which could partially release in a quick leap. Additionally, researchers found that bigger pieces of plastic moved at about the same rate as smaller pieces, suggesting that the whole effect would scale up if need be.
When researchers measured the forces that arose when the plastic absorbed and gave up water, they found shrinking and stiffening caused by water desorption generated about 14 N. The maxium measured stress of 27 MPa is about 80 times higher than that of mamalian skeletal muscles. The expansion/contraction cycle also could be repeated hundreds of times. The maximum work the researchers found during the plastic's contraction was about 73 J/kgm with a power density of 2.5 W/kgm.
It turns out the optimum thickness for the plastic material to act as an actuator was 15 to 40 μm. Pieces thicker than this didn't move well and thinner pieces tended to stick to the moist substrate.
To figure out how the plastic would behave when coupled with piezo material, the group sandwiched it against a PVDF film. The PFDF generated up to 3 V open circuit and up to 1 V when driving a 10 MΩ resistor. Average power output was 5.6 nW, cooresponding to a power density of 56 μW/kgm. Researchers sent the current this setup generated through a full-wave bridge rectifier for storage in a 2.2 μF capacitor.
The researchers are now working to improve the efficiency of the conversion of mechanical energy to electrical energy, which could allow smaller films to power larger devices. They published their findings in the Jan. 11, 2013 issue of Science Magazine.