Electrokinetic preconcentration and separation of proteins in microfluidic devices for biochemical analysis

Sample preconcentration is a critical operation required for the determination of trace amounts of analytes of interest for which the concentration in the original solution is lower than the detection limits of the instrumentation. In this dissertation, two novel techniques, embodied in microdevices, are created that achieve large factors of preconcentration; a major advantage of these devices is the inexpensive production enabling use as simple single-use clinical devices.; First, a simple PDMS device for protein preconcentration is described. The advantages of this device are its simple fabrication process and high and fast concentration. Upon application of electric fields, sample concentration is enhanced 103∼106-fold in 30 minutes at a specific location where two microchannels are positioned in close proximity to each other. Furthermore, subsequent separation of two different concentrated proteins is easily achieved. It is hypothesized that a weakly-bonded interface between PDMS and glass at the proximal point serves as a cationic selective filter by ion exclusion enrichment effect (EEE). This hypothesis is verified with supporting experiments.; Second, a simple PDMS device using Temperature-Gradient Focusing (TGF) is described. This device delivers rapid and repeatable focusing of model analytes using relatively low-power compared to current TGF approaches, because Joule heating in the buffer itself generates the temperature gradient. High electric potential applied to the device induces a temperature gradient within the microfluidic channel due to the channel's variable-width, and analytes are focused at a specific location as a result of temperature-dependent species mobility. This device also shows simultaneous separation and concentration capability of a mixture of two sample analytes in less than 10 minutes. An experiment combining Joule heating with external heating/cooling further supports the hypothesis that temperature is indeed the dominant factor in achieving focusing.; This thesis also describes a theoretical study that provides the governing equations depicting the electrokinetic transport phenomena along a variable-width channel supporting a temperature gradient. Those equations are then implemented into a quasi-1D multiphysics code, along with empirical data for various temperature-dependent parameters, to predict temperature distributions, analyte transport speeds, focusing locations, rates, and sample band widths. Numerical simulation results show good agreement with experimental results.