Nanowires power devices from ordinary movement

Advances in nanotechnology have paved the way for a wide array of wearable or implantable electronic devices aimed at solving important problems, but each of these new devices inherently presents a new issue: how to maintain a steady and abundant supply of power to keep them running. The reliable functioning of devices like implantable blood pressure sensors and trackers used in military operations can literally mean the difference between life and death.

Conventional batteries are not a great option. Anyone with an iPod can attest to their batteries’ tendency to run dry just when you need them the most. Plus, for applications within the human body, the idea of placing a battery filled with toxic materials is not a very pleasing proposition. Solar energy is another possibility, but besides being expensive to implement, it has the drawback of requiring the user to be under clear sunlight.

To eliminate these obstacles, a group led by Tech professor Zhong Lin Wang has developed a self-powering method of powering nano-scale electronics that works by scavenging energy from the environment. Such nanogenerators could be powered by vibration caused by the ordinary motion of the wearer’s body. Heartbeats or even ambient noise could also be tapped for power with these nanogenerators.

Wang, a professor in the School of Materials Science and Engineering, has been working on methods of converting mechanical energy to electricity since 2005. This discovery, representing his group’s latest findings, appeared in the Feb. 14 issue of Nature. It builds upon research that was previously reported in Science in April 2006 and April 2007.

The key to the nanogenerators is the combination of zinc oxide’s properties of being a semiconductor as well as piezoelectric. Piezoelectricity is a phenomenon where the application of force to a crystal results in the production of an electric charge. In Wang’s system, the effect was elicited in zinc oxide nanowires which were grown radially on fibers of Kevlar, forming a structure that Wang describes as resembling a forest without branches. One of the fibers is coated with gold to serve as the electrode in the system.

For one application of this technology, Wang visualizes the nanowires someday being incorporated into the fibers of everyday clothing. Therefore the wearer could power essential devices such as the aforementioned blood pressure sensors or military trackers just by walking around in a shirt. Charge would be collected over a long period of movement by the wearer and stored, with power being consumed only at specified intervals or as needed. The cost of purchasing such a nanowire-containing shirt, Wang expects, will eventually be only slightly higher than an ordinary shirt.

Several hurdles lie ahead before the full possibilities can be realized. Among them is that the efficiency and lifetime of the nanogenerators needs to be increased before they could ever be used to power a device with the power requirements of a cell phone or iPod. Also, for these wires to be woven into clothing to create so-called power shirts, they must be made to withstand a cycle in a washing machine. Currently they would simply dissolve, according to Wang, so his group is investigating ways to protect the nanowires from moisture.