Micro-Fluidics of Micro- and Nano-Colloidal Suspensions: Designing Future Miniature Diagnostic Devices

Miniature diagnostic devices promise to revolutionize biomedical and environmental monitoring systems by integrating biosensors with microfluidic components on a chip platform to produce complete lab-on-a-chip systems. However, successful chip-size devices, even for common applications such as human blood and water pathogen tests, are still not available because of some key technological bottlenecks involving microfluidics of micro- and nano-colloidal suspensions, such as anomalous bioparticle migration due to hydrodynamic forces, insensitivity of many biosensors to low pathogen concentrations and the lack of rapid pathogen concentration technologies. Many of these phenomena are unique to microfluidic devices because certain distinctive forces are presented only at the micro dimensions.

I have studied the failure of blood loading into microneedles by high-speed imaging microscopy. Radial migration of deformable blood cell in the micro-needle is found to be responsible for the formation of a concentrated slug at the wetting interface (meniscus) which eventually stops the penetration of the blood sample into the micro-needle. After overcoming this loading failure, I showed that the difference in the radial migration rate of rigid and deformable blood cells can be used to diagnose diseases that render the blood cells rigid. I also developed new technologies to enhance on-chip pathogen detection. Such techniques exploit classical dielectrophoresis (DEP) and the new phenomenon of AC electro-osmosis (ACEO), both of which utilize high-frequency AC fields that do not generate bubbles at the embedded electrodes. ACEO micro-mixers and micro-pumps I developed are integrated within the chip to enhance the detection speed and time. Dispersed nanowires are used to trap and transport submicron bulk pathogens in an AC field due to their large induced dipoles. Working as dispersed nano-electrodes, they greatly increase the local electrical field and enhance the docking rate between nanowires and pathogens. Nanowire-pathogen aggregates can then be collected using dielectrophoresis within minutes. The integrated micro-fluidic technologies can rapidly (<10 minutes) capture and detect low numbers of bacteria (<1000 cells/ml) and nano particles in ml-sized samples. Because the attractive fluorescent properties of semiconductor nanowires as a biomarker, I have also studied their dielectric properties under electrical field. These nanowires promise to allow selective single pathogen detection based on the difference in the photoconductivity and conductivity of the pathogen and the nanowire.