One of the major applications touted for nanotechnology is in human health where, somehow, magical small machines are going to diagnose and cure all manner of ills. Of course, the technology is lagging well behind the dream, but (and it is a big but) active nano-machinery is almost certainly in our health future—even though it probably won't match expectations. One of the hurdles standing between the dreamy faced Ph.D student and riches is a good method to get power to the machine.
In fact this hurdle is really more like a complete gymkhana, with an open-water jump. For example, if you choose to use a battery, then wires tether the nano-machine, and all those external connections add up to an increased risk of infection, not to mention the difficulty in connecting really big wires to something so very small. To overcome these issues, many scientists are focusing on methods of harvesting energy from the environment.
New research in Science* makes use of the piezoelectric effect in zinc oxide to generate electricity from tiny pressure differences. When a piezoelectric crystal is squeezed or stretched, electrons tend to gather towards one end of the crystal. Once the crystal is relaxed, the electrons return to their normal position. However, if the charge is harvested from the crystal, it can be used to do some work. To take advantage of this idea, the researchers grew zinc oxide nanowires on a metallic surface. Over the top of the nanowires, they placed a surface of roughened silicon, which is a semiconductor and forms a diode junction that only allows current to flow in one direction. The semiconducting upper surface and conducting lower surface were then connected via a current meter that was used to monitor the performance of the device. To generate electricity, the silicon surface was vibrated using ultrasound waves.
The generator produced currents in the nanoampere range, which sounds pretty low until you consider the scale of the devices they want to power with it. The total power was limited by the construction technique; the nanowires are not all the same length so they don't all contribute equally to the power output, and many don't contribute at all. From the pictures and the piezo electric characteristics of zinc oxide, the researchers estimate that a maximum of 1,000 wires from a total of 20,000 will contribute to the measured output.
Although these experiments used direct ultrasound excitation, there is no reason to think that similar designs could not harvest electricity from natural body movements—much like self-winding watches.
Now that the power source is sorted out, I want my fantastic voyage... Where did I place that magic device for shrinking stuff?
*First Author: Wang