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For the more than 100 million people worldwide who have diabetes, testing their blood glucose level is the only way to be sure that it is within normal range and to adjust the insulin dose if it is not. The current method for monitoring blood glucose requires poking a 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 any desired interval, 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 health care.

Nanosensors are already under intensive 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 toward 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, until recently, innovations for delivering nanoscale energy sources to power these devices have been almost nonexistent. The energy to be fed into a nanogenerator is likely to be mechanical energy that is converted into electric energy that will then be used to power nanodevices without using a battery. Examples of 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.

Let us revisit Zhong Lin Wang and his nano power generators, whom we met in Chapter 6.1. Wang's group has also developed a DC nanogenerator that is driven by ultrasonic waves. "The basic principle is to use piezoelectric and semiconducting coupled nanowires, such as zinc oxide, to convert mechanical energy into electricity," explains Wang. "This nanogenerator has the potential to convert hydraulic energy in the human body, such as blood flow, heartbeat, and contraction of blood vessels, directly into electric energy. Our nanogen-erator is able to generate electricity in biocompatible fluid as driven by ultrasonic waves."

''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 atomic force microscope for the mechanical deformation of the nanowires, so that the power generation can be achieved by an adaptable, mobile, and cost-effective approach on a larger scale. Secondly, all of the nanowires are required to generate electricity simultaneously and continuously, and all the electricity must be effectively collected and output. Finally, the energy to be converted into electricity has to be provided in the form of waves or vibration from the environment, so the nano-generator can operate independently and wirelessly.''

Using their innovative approach, the Georgia Tech scientists have made their nanogenerator work in biofluid. By generating electricity in liquid, this work sets a platform for developing self-powering nanosystems with important applications in implantable in vivo biosensing.

As an ultrasonic wave travels through the fluid, it triggers the vibration of the electrode and nanowires to generate electricity. The size of the nanogenerators used in these studies was ~2mm2. There are more than 1 million nanowires in each of these generators. Wang and his team kept the ultrasonic wave on for 4 h without interruption and the nanogenerator remained active and generated electricity continuously. ''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.

In future, Wang and the team will try to optimize the growth of the nanowire arrays in terms of 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 to enhance the efficiency of energy generation. The final goal is to test and improve the power generation at low frequency.

Featured scientist: Zhong Lin Wang

Organization: Center for Nanostructure Characterization, Georgia

Institute of Technology, Atlanta, GA (USA) Relevant publication: Xudong Wang, Xudong Wang, Jin Liu, Jinhui Song, Zhong Lin Wang: Integrated nanogenerators in biofluid, Nano Lett., 7, 2475-2479.

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