The team has developed two types of self-powered sensor, One can measure pH, the second can detect incident ultraviolet light. [Wang et al., Nature Nanotechnol (2010) 10.1038/nnano.2010.46]

Zhong Lin Wang and colleagues have built arrays of zinc oxide nanowires to form a nanogenerator that can convert mechanical energy into a 1.2 V electrical output. Their fabrication method uses a low-temperature (less than 100 Celsius) chemical process that can use flexible polymer substrates and is inexpensive to manufacture.

To make a nanogenerator, the team starts by growing zinc oxide nanowires on a gold-coated surface using a wet chemical method. They then spin coat this with poly-methylmethacrylate to completely cover the nanowires. They then use oxygen plasma etching to clean the tips of the nanowires on which a wafer of platinum-coated silicon can then be placed. A current is produced by applying a strain to the piezoelectric nanowire array to flex it. A strain rate of less than two percent per second is adequate to produce 1.2 V at a strain of 0.19%. The peak power density they measured was 2.7 milliwatts per cubic centimetre.

Wang's team has so far produced two tiny sensors that are based on zinc oxide nanowires and powered by the nanogenerators. By measuring the voltage change across the device in different liquids, they can obtain pH. Similarly, ultraviolet exposure affects the voltage too.

“We have demonstrated a robust way to harvest energy and use it for powering nanometre-scale sensors,” explains Wang. “We now have a technology roadmap for scaling these nanogenerators up to make truly practical applications.” The devices might find application as remote environmental sensors that exploit mechanical vibration, tides, wind, or even body movements rather than batteries to power the devices.

“Building devices that are small isn't sufficient,” Wang says. “We must also be able to power them in a sustainable way that allows them to be mobile. Using our new nanogenerator, we can put these devices into the environment where they can work independently and sustainably without requiring a battery.”

Ultimately, the same approach could be used to develop portable music players that get their power from the flexing of an exerciser's muscles or a medical monitor that tests blood pressure and blood glucose using the power of a heartbeat.