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Posted: Jul 06, 2007

Nanoscale power plants

(Nanowerk Spotlight) For the over 100 million people worldwide who suffer from diabetes, testing blood glucose is the only way to be sure that it is within normal range and allows them to adjust the insulin dose if it is not. The current method for monitoring blood glucose requires poking your finger to obtain a blood sample. The equipment needed to perform the blood test includes a needle device for drawing blood, a blood glucose meter, single-use test strips, and a log book. Now imagine this scenario: your doctor implants a tiny device the size of a rice grain under your skin. This device automatically and accurately measures your blood glucose levels at whatever intervals, even constantly if required. It transmits the data to an external transceiver. If any abnormality is detected, the device warns you and automatically transmits the data to your doctor's computer. This scenario is one of the many promises of nanomedicine where in-situ, real-time and implantable biosensing, biomedical monitoring and biodetection will become an everyday fact of normal healthcare. Nanosensors are already under intense development in laboratories around the world. One of the important components for implantable nanosensors is an independent power source, either a nanobattery or a nanogenerator that harvests energy from its environment, so that the sensor can operate autonomously. Not only has such a nanogenerator now been developed, but a new prototype has been demonstrated to effectively generate electricity inside biofluid, e.g. blood. This is an important step towards self-powered nanosystems.
Huge research efforts go into the development of nanoscale sensing devices for applications ranging from medical and biosensing to environmental monitoring to military use. In comparison, innovations for delivering nanoscale energy sources to power these devices are almost non-existent. The energy to be fed into a nanogenerator is likely to be mechanical energy that is converted into electric energy that then will be used to power nanodevices without using a battery. Examples for mechanical energy are body movement or muscle stretching, vibration energy such as acoustic/ultrasonic waves, and hydraulic energy such as body fluid and blood flow.
"We have developed a direct-current nanogenerator that is driven by ultrasonic waves" Dr. Zhong Lin Wang explains to Nanowerk. "The basic principle is to use piezoelectric and semiconducting coupled nanowires, such as zinc oxide, to convert mechanical energy into electricity. This nanogenerator has the potential to directly convert hydraulic energy in the human body, such as blood flow, heart beat, and contraction of blood vessels, into electric energy. Our latest design of this nanogenerator is able to generate electricity in biocompatible fluid as driven by ultrasonic waves."
Wang, Regents' Professor, COE Distinguished Professor, and Director, Center for Nanostructure Characterization, at Georgia Tech, and his group have previously demonstrated the concept of a nanogenerator by deflecting aligned nanowires using a conductive atomic force microscopy (AFM) tip, and the output signal came from one nanowire in a form of short pulses ("Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays").
"In order for this nanogenerator to have practical uses in real life applications, however, we had to come up with an innovative design to drastically improve its performance with regard to the following aspects" says Wang: "First, we must eliminate the use of an AFM for making the mechanical deformation of the nanowires so that the power generation can be achieved by an adaptable, mobile and cost-effective approach over a larger scale. Secondly, all of the nanowires are required to generate electricity simultaneously and continuously, and all the electricity can be effectively collected and output. Finally, the energy to be converted into electricity has to be provided in the form of wave/vibration from the environment, so the nanogenerator can operate independently and wirelessly."
Wang's group has solved these problems using an innovative approach published in a recent issue of Science ("Direct-Current Nanogenerator Driven by Ultrasonic Waves").
Schematic of nanogenerator
Schematic shows the direct current nanogenerator built using aligned ZnO nanowire arrays with a zigzag top electrode. The nanogenerator is driven by an external ultrasonic wave or mechanical vibration and the output current is continuous. (Image: Georgia Tech)
In the most recent development, the Georgia Tech scientists now have made their nanogenerator work in biofluid and the performance has improved 30-fold compared to what they reported in the paper mentioned above. By generating electricity in liquid, this new report ("Integrated Nanogenerators in Biofluid") sets a platform for developing self-powering nanosystems with important applications in implantable in vivo biosensing.
The ultrasonic wave transported through the fluid and triggered the vibration of the electrode and nanowires to generate electricity. The size of the nanogenerators used in these studies was about 2 mm2. There are more than one million nanowires in each of these generators. Wang and his team kept the ultrasonic wave on for 4 hours without interruption and the nanogenerator remained active and generated electricity without stop. "We expect the lifetime of the nanogenerator is much longer than the time we have tested" says Wang.
The nanogenerators in this study also show the possibility of integrating multiple nanogenerators in biofluid for receiving high power output. This is a key step towards self-powering biosensing and medical applications.
Going forward, Wang and the team will try to optimize the growth of the nanowire arrays in size and height uniformity and their distribution on substrate as well as the design of the top electrode, so that most of the nanowires will generate electricity. The second goal will be to raise the output voltage to more than 0.5 V so that it can be used for practical applications. The third goal is to improve the packaging of the nanogenerator for enhancing efficiency of energy generation. The final goal is to test and improve the power generation at low frequency.
Maybe it's time to read Isaac Asimov's Fantastic Voyage again...
By , Copyright Nanowerk LLC

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