18 December 2012—Researchers have long been interested in capturing energy created by the mechanical motion of the human body and using it to power portable electronics, but the parts required for such a harvesting device tend to be bulky and stiff—not at all comfortable for humans.
Researchers in the lab of University of Auckland associate professor Iain Anderson are developing soft generators that dispense with much of the external circuitry that makes the devices cumbersome. Thomas McKay, a research fellow at the university’s Biomimetics Laboratory, demonstrated their invention at the Materials Research Society’s fall meeting in Boston last month. As McKay strode around in front of the audience, LEDs on his right sneaker lit up when his heel struck the ground, generating electricity. “This shoe feels just like a shoe,” he said. “We’re generating energy here without affecting the experience of the user.”
The device in the athletic shoe, built with his fellow students Mahdieh Nejati and Daniel Xu, was a dielectric elastomer generator (DEG). The DEG is made with stretchable electrodes on either side of a flexible silicone material that, as it deforms through compression and relaxation, converts mechanical work to electrical energy. The mechanism holds promise for harvesting energy not just from human movement but also from the motion of waves, wind, or the vibrations of buildings and bridges, say researchers.
In a typical DEG design, the generator needs an external source of electricity to prime it. The elastomer is stretched, and then a bias voltage is applied to the electrodes. When the elastomer is allowed to relax, the action pulls opposite charges further apart in space and packs like charges more closely together, increasing the voltage and energy density in the system. Unfortunately, there’s always some charge leakage from the system, and eventually an external battery would be drained.
The Auckland researchers devised a self-priming DEG, which consists of two membranes pressed against each other. When one membrane stretches, the other is compressed, and vice versa. As they alternately expand and relax, they pass charge back and forth, building up voltage with each pass. A tiny external charge is still needed at the beginning, but after that, the system is self-sustaining. In one experiment, the DEG increased voltage from just half a volt to 2 kilovolts in a few seconds.
Having solved the priming problem, the researchers were still left with six external diodes that controlled how charge was added to or extracted from the DEG. So to eliminate that circuitry, they fashioned piezoresistive electrodes. This mix of nonconductive grease and carbon black is painted onto the membranes and changes its resistance as it stretches. From a resistance of megaohms in their relaxed state, the resistance increases to gigaohms when the electrodes are stretched to 1.4 times their original length.
The resulting soft generator had a specific energy of 10 millijoules per gram and an efficiency of 12 percent. (Lithium ion batteries store hundreds of joules per gram.) The theoretical specific energy of such a device is in the thousands of millijoules, McKay said, so there’s room for improvement. Just a square centimeter of lightweight acrylic membrane and grease in the shoe could produce 30 to 40 milliwatts of electricity.
Little bursts of power wouldn’t be very useful for running a cellphone, but McKay says the device could be set up to store energy while the wearer walks, then plugged into the phone later to recharge it. Alternatively, the DEG could be used as an actuator instead of an energy generator, changing the stiffness of the shoe based on the wearer’s stride. Other designs might act as power sources for mobile robots.
In another presentation at the Materials Research Society meeting, Xiaofan Niu, a doctoral student in materials science and engineering at the University of California, Los Angeles, reported that his group is working on toughening up the easily broken elastomers for such generators. By adjusting the chemical composition of the materials, which are generally urethane, acrylic, and silicone acrylates, and curing them under ultraviolet light, they are able to produce thin films of material that can undergo many cycles of stretching and relaxing without failing. “We think it could be a very good candidate for many energy-harvesting applications, as well as actuator applications,” Niu said.