Increased Piezoresistive Sensitivity Could Enhance Performance of Catheter Flow SensorsIncreased Piezoresistive Sensitivity Could Enhance Performance of Catheter Flow Sensors

In piezoresistive materials, mechanical stress influences the movement of charge available for conduction. The application of pressure to such materials translates into an easily measurable change in conductivity, making piezoresistives suitable for use in pressure and flow sensors. Chee Chung Wong and colleagues from the A*STAR Institute of Microelectronics and Nanyang Technological University (Singapore) have now demonstrated a method to increase the piezoresistive sensitivity of silicon nanowires using an applied electric field. As presented in "Electrically Controlled Giant Piezoresistance in Silicon Nanowires," a paper published in Nano Letters, this technology is being integrated into flow sensors used in catheters and related applications.

To measure the piezoresistive properties of silicon nanowires, Wong and his colleagues embedded the nanowires within the base of an 80-µm-long cantilever made of silicon dioxide. The cantilever and nanowires can be bent by a stylus, simplifying the measurement process, which otherwise would require complicated instruments such as atomic force microscopes. By combining the nanoscale piezoresistive response of the nanowires with the experimentally accessible macroscale properties of a large cantilever, the researchers did not have to sacrifice the properties inherent to either scale.

They also applied an electric field to the nanowires using a conducting substrate beneath the cantilever. By applying a positive voltage, they reduced the concentration of positive carriers from the cantilever and nanowires, leaving only a narrow funnel of available charge carriers near the cantilever base. This in turn greatly increased the magnitude of the piezoresistive effect on the conductivity of the nanowires.

As a result, the change in nanowire conductivity per unit stress increased from an approximate factor of 50 to more than 5000. The higher sensitivity may allow these hybrid nanowire-cantilever devices to be used in force-activated nanoelectromechanical switches, or hybrid mechanical-electrical logic circuits. The externally tunable sensitivity of the device may also allow for a greater degree of design flexibility. High-sensitivity switching devices could be combined with low-sensitivity conventional sensors using the same underlying device design.

The research was the result of a lucky coincidence, Wong recounts. "We discovered the giant piezoresistance effect by chance while trying to develop small-footprint pressure sensors for biomedical applications." Real applications may soon be on hand: "Our colleagues from the Institute of Microelectronics are already integrating these nanoscale devices into flow sensors for health care applications such as catheters," Wong says.