The application of nanotechnology to biomedical sensors is receiving an increasing amount of attention.   In particular, one of foremost applications is wireless monitoring of vital signs.   A major problem preventing the research and development of such devices is power supply.   Conventional batteries cannot be used in the body due to size and possible contamination.   Researchers from Georgie Tech have developed a nanogenerator which converts mechanical forces such as pressure from blood flow or vibration to DC electricity.   At the heart of the tiny generator are zinc oxide nanowires.   Zinc oxide was chosen by the developers because they have piezoelectric and semiconducting properties.   Zinc oxide also does not have any toxicity issues with the body.   When a one-dimensional zinc nanowire is bent, a piezoelectric effect occurs.   Upon bending, charges separate radially (positive on the outside and negative on the inside of the wire).   The nanowire, then, acts like the gate of a transistor.   Instead of activating upon a voltage, however, the nanowire is activated on bending.   Researchers at Georgia Tech created a "zig-zag electrode" to transfer charge from a zinc oxide nanowire array.   Under normal conditions where the wires are not bent, the nanowires do not make contact with the tips of the electrode.   Charges are only transferred when the nanowires bend enough to make contact with the electrode.  

Top: image showing the nanogenerator components. Bottom: output produced from nanogenerator when an ultrasonic wave was being toggled. Image courtesy of Zhong Lin Wang of Georgia Tech