Nanosensors Seek Self-Empowerment

While nanosensors are good candidates for many medical device applications because of their sensitivity and low power consumption, their batteries and integrated circuits are difficult to miniaturize. Zhong Lin Wang, a materials scientist at Georgia Tech, is hoping to overcome that limitation by developing minuscule piezoelectric generators that will enable nanosensors to power themselves.In 2005, Wang succeeded in using the probe of an atomic-force microscope to bend zinc oxide nanowires, demonstrating that the piezoelectric effect--in which crystalline materials under mechanical stress produce an electrical potential--can operate at the nanoscale. By flexing and returning to their original shape, the zinc and oxide ions produced an electrical current. While the current was minimal, Wang postulated that he could design a practical nanoscale power source by harnessing ubiquitous vibrations such as sound waves, wind, or the flow of blood over an implanted device. These vibrations, he thought, could bend nanowires and thus generate electricity.Since then, Wang has been able to embed zinc oxide nanowires in a layer of polymer that, when flexed, produce 50 mV of electricity--an important step toward developing self-powering nanosensors. Although 200 mV of power are necessary to charge the batteries used in such devices as iPods, Wang believes that with further engineering, his invention can achieve higher power output levels.Wang's nanopiezotronic technology capitalizes on zinc oxide nanowires' piezoelectric and semiconductor properties. As piezoelectric devices, the sensors produce an electrical response to mechanical stress. As semiconductors, they can form the basic components of integrated circuits such as transistors and diodes. Unlike traditional electronic components, nanopiezotronic parts do not require an external source of electricity. They generate their own power when exposed to the kinds of mechanical stresses that power nanogenerators.By generating their own power, nanosensors may someday find their way into medical implants such as hearing aids and bone-loss monitors. A nanopiezotronic hearing aid powered by a nanogenerator might use an array of nanowires to convert sounds into electrical signals and process them so that they can be conveyed directly to neurons in the brain. More sensitive than traditional hearing aids, nanopiezotronic devices will not need to be removed from the body for battery changes. In future bone-loss monitors, a mesh of piezoelectric nanowires could monitor mechanical strain indicating bone loss. Elevated levels of bone stress would generate an electrical current in the wires, causing the device to sound an alarm.Wang recently detailed his research on nanogenerators and potential medical applications for the technology in MPMN's Webcast event, "Emerging Technologies Showcase." View Wang's presentation in our Webcast archives to learn more.