Researchers at Princeton University have created a flexible material that harvests record amounts of energy when stressed. The researchers say the material could be incorporated into the soles of shoes to power portable electronics, or even placed on a heart patient's lungs to recharge a pacemaker as he breathes.

The energy-harvesting rubber sandwiches ribbons of a piezoelectric material called PZT between pieces of silicone. When mechanically stressed, a piezoelectric material generates a voltage that can be used to produce electrical current; a current can also be converted back into mechanical movement.

The rubber material can harness 80 percent of the energy applied when it is flexed--four times more than existing flexible piezoelectric materials.

Flexibility could prove vital if energy-harvesting technology is to take off. For example, the military tested stiff-soled piezoelectric shoes as a power source, but soldiers complained of foot pain. And previous flexible energy harvesters--based on piezoelectric polymers, nanowires, or other types of crystal--put out little electrical current.

PZT is the most efficient piezoelectric material known, but its crystalline structure means that it must be grown at high temperatures, which normally melt a flexible substrate. The Princeton researchers, led by mechanical engineering professor Michael McAlpine, got around this by making PZT at high temperatures and then transferring thin ribbons of the material onto silicone.

First, the researchers treat the PZT with a chemical etching bath that removes a thin ribbon from the surface of the crystal. They then use a polymer stamp to pick up the ribbon and place it on a silicone film before covering it with a second piece of silicone and sealing it. "All the processes we use to make flexible PZT ribbons are extremely simple and straightforward," says McAlpine. Crucially, the researchers found that the process doesn't compromise PZT's energy-conversion efficiency.

Proof-of-concept tests described this week in the journal Nano Letters show that the rubber-encased PZT ribbons maintain their high power-conversion efficiency. McAlpine says the simple printing process should readily scale up to make larger sheets; he has filed a patent on the process.

McAlpine is particularly focused on biomedical applications for the material and says it could cut down on the number of surgeries that patients with implants must undergo. For example, doctors could place a power-generating sheet against the lungs during the initial surgery; the constant movement of the organs could help recharge a battery, McAlpine says.

Jim Grotberg, professor of surgery and biomedical engineering at the University of Michigan, says wireless monitoring and drug delivery for patients with chronic medical problems are other potential applications. "If you have a sensor that monitors heart-rate, brain activity, or blood pressure, or an implantable insulin-injection system, you need a battery," he says.

PZT itself is not biocompatible--the "p" comes from the chemical symbol for lead, one of its components along with zirconium and titanium. But the crystal ribbons are completely encapsulated in silicone, a material that is approved by the U.S. Food and Drug Administration for medical implants.

Even animal testing is still a ways off. But the Princeton researchers are now making prototype devices from the sheets to test how much electricity they can generate when they are built into shoes.