Engineers from Princeton University have developed power-generating rubber films that could be used to harness natural body movements such as breathing or walking in order to power electronic devices such as pacemakers or mobile phones. The material, which is composed of ceramic nanoribbons embedded onto silicone rubber sheets, generates electricity when flexed and is highly efficient at converting mechanical energy into electrical energy.

Its developers say shoes made of the material could harvest the pounding of walking or running to power mobile electrical devices and, when placed against the lungs, sheets of the material could use the raising and falling breathing motions of the chest to power pacemakers. This would negate the current need for surgical replacement of the batteries which power the devices.

Plus, because the silicone is biocompatible and is already used for cosmetic implants and medical devices, “the new electricity-harvesting devices could be implanted in the body to perpetually power medical devices, and the body wouldn't reject them," said Michael McAlpine, a professor of mechanical and aerospace engineering, at Princeton, who led the project to develop the material.

To produce the material the researchers first fabricated lead zirconate titanate (PZT) nanoribbons in strips so narrow that 100 fit side by side in a space of a millimeter. PZT is a ceramic material that is piezoelectric, meaning it generates an electrical voltage when pressure is applied to it. Of all piezoelectric materials, PZT is the most efficient, able to convert 80% of the mechanical energy applied to it into electrical energy.

"PZT is 100 times more efficient than quartz, another piezoelectric material," said McAlpine. "You don't generate that much power from walking or breathing, so you want to harness it as efficiently as possible."

In a separate process, the team then embedded these ribbons into clear sheets of silicone rubber, creating what they call "piezo-rubber chips.” The Princeton team is the first to successfully combine silicone and nanoribbons of PZT.

In addition to generating electricity when it is flexed, the opposite is true: the material flexes when electrical current is applied to it. This opens the door to other kinds of applications, such as use for microsurgical devices, McAlpine said.

"The beauty of this is that it's scalable," said Yi Qi, a postdoctoral researcher who works with McAlpine. "As we get better at making these chips, we'll be able to make larger and larger sheets of them that will harvest more energy."

A paper on the new material, titled "Piezoelectric Ribbons Printed onto Rubber for Flexible Energy Conversion," was published online Jan. 26, in Nano Letters.