Tiny ribbons that generate electricity when flexed and flex when stimulated with electricity have been developed to flap the wings of mechanical dragonfly spy drones, but the technology can also find uses ranging from powering iPods and cell phones to charging batteries by converting to electricity the vibration of devices deployed in data centers.
Researchers at Princeton University have embedded these brittle ribbons in silicone rubber, allowing them to flex and also protect them from environments where they might be deployed, such as in shoes - to capture mechanical energy as people walk in them - or implanted within humans - to capture the motion of lungs to power pacemakers.
But the research, funded by the United States Intelligence Community, a cooperative of federal intelligence and national security agencies, was meant to develop technology to flap the wings of dragonfly spy drones, which would use the property that the material responds to electricity by flexing, says Michael McAlpine, a professor of mechanical and aerospace engineering who heads up the team.
"It's hard to get the wings to flap and make it lightweight," he says. Voltage applied to the new technology could change the shape of the wings on a type of intelligence-gathering drone made to look like a dragonfly. "It exists; I've seen it," McAlpine says.
The researchers have applied well-known materials and fabrication techniques to create piezo-rubber chips, solving the problem of how to make the ribbons - made of a brittle material called lead zirconate titanate (PZT) – durable. PZT is a piezoelectric material, meaning it generates a voltage when it is stressed.
McAlpine just published a paper in Nano Letters outlining how his team crafted and encased PZT ribbons in silicone and is working now to measure just how much electricity the technology can generate. He says he is shooting for 10 milliwatts (mW). "That's getting close to what it takes to power a cell phone or iPod," he says. "We're hoping for more." He thinks he could have results sometime this summer.
Power produced by the PZT-silicone wafers scales by area; the larger the area of the wafers, the more power they produce. McAlpine says his team has made piezo-rubber chips ranging from 1 square centimeter to 25 square centimeters.
He thinks the timeframe for producing practical devices is five years, and the possible applications are many. Placed on the housing of data center devices, for example, piezo-rubber chips could capture vibrations to generate electricity that is then stored in batteries, he says. Similarly the vibrations of vehicles could be captured as electricity. "Waste power is a huge issue," he says.
Artificial skin with PZT embedded could generate electrical impulses in the area where they come into contact with objects, acting as sensory receptors, he says.
Power generation using the PZT ribbons is similar to solar cells except that solar cells require sunshine. "Here you need to continuously generate motion to power things," he says. PZT is also more efficient than solar cells, which capture 25% of the energy they absorb. PZT captures 80%, McAlpine says.
The material is made of lead, zirconium, titanium and oxygen and has been known for decades. The Princeton researchers came up with a way to fabricate the ribbons, which are just nanometers thick and centimeters long using production techniques of liquid spin coating and etching that are used in manufacturing semiconductors, he says.
The materials are relatively inexpensive. For less than $200, he can buy what he needs to make multiple square meters of PZT.