"We are particularly excited about using the nanofibre-based generators in bio-compatible situations, like embedding the devices in shoes and clothing to harvest energy from the motion of the human body to charge personal electronics such as iPod batteries and cell phones," says team leader Yong Shi, who is a mechanical engineer at Stevens.

The new high power output devices are based on lead zirconate titanate (PZT) nanofibres. PZT has a high piezoelectric voltage and dielectric constants – ideal properties for converting mechanical energy into electrical energy. Unlike bulk thin films or microfibres, PZT nanofibres prepared by electrospinning processes are also highly bendable and mechanically strong.

Embedded in a soft polymer
Shi's team made the nanogenerator by depositing electrospun PZT nanofibres on preformed arrays of electrodes on a silicon substrate. The nanofibres are around 60 nm in diameter and they were embedded in a soft polymer (polydimethylsiloxane, PDMS) matrix. The finished device can be released from the silicon substrate or prepared on flexible substrates, depending on the application desired.

When mechanical pressure is applied on the top surface of the ensemble, it is transferred to the nanofibres via the PDMS matrix. This results in electrical charge being generated thanks to the combined tensile and bending stresses in the nanofibres as they move. This results in a voltage between two adjacent electrodes.

The researchers say that, for a given volume of nanogenerator, the nanofibre device generates much higher voltages and power than devices made from semiconductor piezoelectric nanowires for the same energy input. In theory, the maximum output power from a piezoelectric nanogenerator depends on the properties of the active materials, so the higher the piezoelectric voltage constant of the material between two electrodes, the higher the output voltage and power. What is more, varying the length of the active materials between the two electrodes will also vary the voltage output and current at the same time, explains Shi.

The devices could be used to power wireless sensors, personal electronics and, in the future, biosensors and bioactuators that are directly injected into the human body.

Powered by blood flow
Arthur Ritter – who is director of biomedical engineering at Stevens and was not involved in the research – says, "One of the major limitations of current active implantable biomedical devices is that they are battery powered. This means that they either have to be recharged or replaced periodically. Shi's group has demonstrated a technology that will allow implantable devices to recover some of the mechanical energy in flowing blood or peristaltic fluid movement in the gastro-intestinal tract to power smart implantable biomedical devices."

And, because the technology is based on nanostructures, it could provide power to nanorobots in the blood stream for extended periods of time, he adds. Such robots could transmit diagnostic data, take biopsy samples and/or send wireless images directly to an external database for analysis.

The team now plans to optimize the structure of its nanodevice and simplify the fabrication process. "We are also working hard on implantable bio-applications," revealed Shi.

The work was reported in Nano Letters.