Electrothermal and buoyancy driven microstirring to enhance scalar transport

An AC electrothermal microstirring device and two buoyancy driven microstirring devices were developed. These devices enhance temporal performance in heterogeneous immuno-sensors by nearly an order of magnitude, and enhance the spatial environment within hematopoietic and human embryonic stem cell cultures, respectively. AC electrothermal flow and buoyancy-driven flow are exploited to generate a circular stirring fluid motion that removes inhibitory depletion regions and augments passive transport of diffusion-limited molecules, such as proteins and growth factors. This allows for more binding opportunities between suspended antigens and immobilized antibodies within heterogeneous immunoassays, and between suspended nutrients and cell surface receptors within stem cell cultures.;The development and characterization of all the devices followed the same sequence of steps. First, numerical simulations aided in device design and optimization. The electrothermal stirring device was optimized to enhance binding rates in heterogeneous immunoassays. The buoyancy-driven devices were optimized to create a stirring motion throughout the medium, while maintaining reasonable temperature fields (∼36--37°C) within stem cell cultures. The fluid velocity was measured experimentally using muPIV for various voltages and locations in the devices and compared with the numerical simulations. In the devices that exploit Joule heating as a result of an applied AC potential---the electrothermal flow device and the human embryonic stem cell buoyancy-driven device---the measured velocity is significantly less (≥ 10x) than expected. However, an effective voltage is introduced, such that the fluid experiences an effective voltage that is 38% of the applied voltage. Using the effective voltage to calibrate the numerical model, the numerical model results come into agreement with the measured velocities in both devices. The velocity measurements within the other buoyancy-driven device for hematopoietic stem cell cultures, which functions through DC resistive heating, matches the numerical simulations to within a few percent.;With the characterized electrothermal flow device, microstirring was generated within a biotin-streptavidin assay and demonstrated increased binding by nearly an order of magnitude. The results demonstrate the ability for electrothermal stirring to reliably improve the response time and sensitivity within a given time limit for microfluidic diffusion-limited sensors.;With the characterized buoyancy-driven devices, microstrirring was generated within hematopoietic and human embryonic stem cell cultures, and maintained cell viability and enhanced cell behavior (CD34 and Ki67 expression) for the stirred cultures. The devices' biocompatibility and functionality demonstrate new small-scale (∼100 mul) technologies for stirring stem cell cultures, which positively influence their microenvironment and growth.