Ultra-Low-Powered Aqueous Shear Stress Sensors Based on Bulk EG-CNTs Integrated in Microfluidic Systems

Novel aqueous shear stress sensors based on bulk carbon nanotubes (CNTs) were developed by utilizing microelectrical mechanical system (MEMS) compatible fabrication technology. The sensors were fabricated on glass substrates by batch assembling electronics-grade CNTs (EG-CNTs) as sensing elements between microelectrode pairs using dielectrophoretic technique. Then, the CNT sensors were permanently integrated in glass-polydimethylsiloxane (PDMS) microfluidic channels by using standard glass-PDMS bonding process. Upon exposure to deionized (DI) water flow in the microchannel, the characteristics of the CNT sensors were investigated at room temperature under constant current (CC) mode. The specific electrical responses of the CNT sensors at different currents have been measured. It was found that the electrical resistance of the CNT sensors increased noticeably in response to the introduction of fluid shear stress when low activation current (Lt1 mA) was used, and unexpectedly decreased when the current exceeded 5 mA. We have shown that the sensor could be activated using input currents as low as 100 muA to measure the flow shear stress. The experimental results showed that the output resistance change could be plotted as a linear function of the shear stress to the one-third power. This result proved that the EG-CNT sensors can be operated as conventional thermal flow sensors but only require ultra-low activation power ( ~ 1 muW), which is ~ 1000 times lower than the conventional MEMS thermal flow sensors.