A tiny, nearly invisible nanowire can convert the energy of pulsing, flexing muscles inside a rat's body into electric current, researchers at Georgia Institute of Technology have shown. Their nano generator could someday lead to medical implants and sensors powered by heartbeats or breathing.

Zinc oxide nanowires show the piezoelectric effect, producing electricity when they are under mechanical stress. Georgia Tech professor of materials science and engineering Zhong Lin Wang and his group first demonstrated these nanowire generators in 2005. Since then they have made devices that can harness the energy of a running hamster and tapping fingers, and have also combined their piezoelectric nanowires with solar cells.

In their latest work, published in the journal Advanced Materials, Wang's team shows that the nanogenerator works inside a live animal. The researchers deposited a zinc oxide nanowire on a flexible polymer substrate and encapsulated the device in a polymer casing to shield it from body fluids. It was then attached to a rat's diaphragm. The rodent's breathing stretched the nanowire, and the device generated four picoamperes of current at two millivolts. When attached to a rat's heart, the device gave 30 picoamperes at three millivolts.

Zinc oxide nanogenerators would be an ideal power source for nano-scale sensors that monitor blood pressure or glucose levels and detect cancer biomarkers. These can run on low power levels of about one microwatt, but they need a long-lasting nano-sized power source instead of a battery to be truly nano scale. "Our ultimate goal is to make self-powered nano devices for medical applications," says Wang.

The femtowatt scale of power generated by the devices is far too low to be practical right now (power = current x voltage). But that should change soon, Zhang says. While the researchers have only tested a single nanowire device inside a rat, they have also built a device that integrates hundreds of nanowires in an array. This device, which the researchers recently reported in the journal Nature Nanotechnology, gives an output current of about 100 nanoamperes at 1.2 volts, producing 0.12 microwatts of power. Wang says the next step is to connect this higher-output nanogenerator to a nano sensor inside an animal.

Better piezoelectric materials than zinc oxide nanowires exist and are also being considered for biomedical applications. The most efficient piezoelectric material known is PZT, a compound of lead, zirconium, and titanium. It is 10 times more efficient than zinc oxide at converting mechanical stress into electric current, says Michael McAlpine, a mechanical engineering professor at Princeton University. By sandwiching PZT between silicone pieces, he has made a material that can harvest 80 percent of the energy applied when flexed. Like Wang, he is focusing on using the material to power medical implants.

McAlpine says the material gives 10 nanowatts of power from human finger tapping. Larger sheets could generate enough power to charge a pacemaker, but the material has not been tested in animals yet. Here, zinc oxide might have an advantage over PZT because it is biocompatible. The lead in PZT would require the device to be robustly encased in silicone or another biocompatible polymer.

The biggest challenge for both materials, however, will be getting higher power outputs, McAlpine says. "It's amazing that they could implant these devices in these animals and get power out," he says. "But we still have to go far with our devices to get a meaningful power output."