How Nanogenerators Work

In "The Matrix," computer-based life forms had enslaved humans on Earth and used their bodies as a power source. The humans served as the computers' limited-life batteries (batteries which can be recharged, but not indefinitely), similar to the way we'd use AA batteries in a TV remote control. Though "The Matrix" is fiction, researchers developing nanogenerators are finding reality in harnessing the body's energy to power electronic devices.

Dr. Zhong Lin (Z.L.) Wang of the Georgia Institute of Technology (Georgia Tech) is a leading researcher in nanogeneration [source: Nano]. For more than a decade, Wang and his team have been working to produce incredibly small circuits which can generate electrical current. Nanotechnology projects like the ones developed by the Georgia Tech team are so tiny that researchers must use microscopes to see what they're working on and instruments that can create and manipulate microscopic electronic components and measure their output [source: Illinois, Ravindran].

Dr Wang's team develops their nanogenerators entirely within their lab. Wang and his colleagues start by creating the materials they need and then fabricates, analyzes, measures and packages each nanogenerator prototype. This ground-breaking research group has successfully powered a light-emitting diode [LED], alaser diode and a liquid crystal display [LCD] using nanogenerated power exclusively [source: GeorgiaTech].

Different groups have experimented with different piezoelectric materials during their nanogenerator exploration. Researchers Michael McAlpine at Princeton University and Prashant Purohit at the University of Pennsylvania have been using lead zirconate titanate, or PZT. Though PZT is extremely brittle, McApline and Purohit discovered how to shape the material so it could stretch up to ten percent without breaking [source: Berger]. In 1999, Wang became the first researcher using zinc oxide (ZnO) as the piezoelectric material used in the nanogenerators [source: GeorgiaTech]. In a 2009 publication, Wang's team credits the use of ZnO for their continued success in improving nanogenerators [source: Lu, et al.].

We know piezoelectric material is the key component of a nanogenerator. Now, let's take a closer look inside to see just how it generates electricity.

A nanogenerator consists of an integrated circuit, with components made from silicone and a piezoelectric ceramic, etched onto a flexible surface, called a substrate. While the strength and other properties of the substrate are important in engineering the nanogenerator, the magic happens in the circuitry. On the surface, using the naked eye, we can see a series of lines and boxes that appear as a flat, two-dimensional image. However, a microscopic look beneath the outer layers of the flexible chip reveals a completely different three-dimensional picture.

The electricity is generated in the piezoelectric material. As mentioned before, Wang's team has used ZnO to develop nanowires. Each nanowire measures between 100 and 300 nanometers in diameter (the width of the wire). Each nanowire's length is about 100 microns; one micron = 100,000 nanometers. To put this in perspective, note that the length of the wire (not the width) is about the same as the width of two human hairs.

In their nanogenerators, Wang's team attaches an array of nanowires to the substrate and places a silicone electrode at the other end of the wires. The electrode has a zigzag pattern on its surface. When a small physical pressure is applied to the nanogenerator, each nanowire flexes and generates an electrical charge. The electrode captures that charge and carries it through the rest of the nanogenerator circuit. The entire nanogenerator might have several electrodes capturing power from millions of nanowires [sources: Greenemeier, Science Daily].

In their own research groups, McAlpine and Purohit have taken a different approach to nanogenerators, using PZT to create nanoribbons. Each nanoribbon is about 10 micron wide and 250 to 500 nm thick. They first form the nanoribbons on a magnesium oxide surface and then remove them using phosphoric acid. Then, they fix the nanoribbons to a pre-stretched silicone rubber surface that, when relaxed, causes the nanoribbons to buckle without breaking. When the nanoribbons are bent, their movement generates electricity without breaking them away from the surface [source: Berger].

Building on the basic premise of forming flexible wires from piezoelectric material, researchers have studied ways to get more power out of each generator. For example, Wang's lab has improved both the nanowires and the circuitry in each successive design. Wang reports that over the last decade, he's seen output improved to over a billion times better than when he started.

So far, you've seen how the nanogenerator works from the inside. Now, let's examine where you might find nanogenerators at work within the next few years.

Doctors need dependable technology to power devices implanted in patients to regulate or monitor the patient's health. Examples of such devices are pacemakers and continuous glucose monitoring systems. Implant devices come with an inherent challenge: They can wear out over time or require replacement batteries or a cumbersome external power source.

By using nanogenerators, doctors could implant a new generation of these devices with the capacity to stay powered and last a long time with minimal body invasion [source: Medgadget]. Such devices would harness the energy of involuntary movement like a heartbeat or lung expansion. In short, you could be using your body to keep alive a device that helps keep you alive in return. In addition, by using non-toxic materials like ZnO as the piezoelectric material, there is a better chance of implanting a nanogenerator without harming the body [source: Greenemeier].

So what's beyond medicine for nanogenerators? Researchers believe that nanogenerators could soon be powering your iPod or smartphone. Because nanogenerators are so small, they could easily be embedded in the cloth of a T-shirt or hoodie, so your iPod could use your pulse to keep its internal battery charged. Wang expects the nanogenerators his group has developed to be part of such products and available for purchase within five years [source: FoxNews].

A side benefit of using nanogenerators is their potential positive impact on the environment. Nanogenerators use a renewable resource: kinetic energy from body movement. They're created from more environmentally friendly materials than batteries, and they have the potential to reduce the waste associated with battery production and disposal. Still, the impact is small, literally, due to the size and limited power of nanogenerators. Time will tell if the nanogenerators will be viable in powering larger devices such as laptops.

Nanogenerators probably won't replace batteries, at least not in the near future. You still need battery backups for devices with which you're not in regular physical contact, such as alarm clocks. You also want to ensure that some devices continue to run idly even when you're not using or touching them, such as your smartphone. Perhaps in the future, manufacturers will pair nanogenerators with some type of rechargeable battery system to create a dependable power source with reduced environmental impact.

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