Optically Transparent Microfluidic Systems Integrated with Carbon Nanotube Sensors

In this paper, we present carbon nanotube (CNT) based thermal shear stress sensors integrated inside optically transparent Polymethylmethacrylate (PMMA) microfluidic systems. The sensors were fabricated on PMMA substrates by batch assembling multi-walled carbon nanotubes (MWNTs) as sensing elements between microelectrode pairs using AC dielectrophoretic (DEP) technique. PMMA chambers were fabricated using SU-8 molding/hot-embossing technique. Then, the PMMA substrate with a micro chamber and vortex micropump was bonded to the other PMMA substrate embedded with the MWNT sensor array to form a closed flow chamber. Experiments showed that the CNT sensors could detect volumetric air flow rate in the order of 10-8m3/s inside this microchannel system. We have also proved that upon exposure to constant liquid (DI-water) flow, the electrical resistance of the CNT sensor was found to increase linearly at low activation current of 100μA. And a linear relation between the change of output resistance and one-third power of flow rate was observed for flow rate from 0.3 to 2.3m/s. This result proved that the CNT sensors work with the same principle as conventional MEMS based thermal shear stress sensors, but only require ultra-low activation power (~μW) to achieve comparable sensitivity, which is three orders of magnitude lower than conventional MEMS polysilicon based flow sensors