Fundamentals of Microfluidics
For Biochemical Analysis:A Tutorial
MONDAY, OCTOBER 29, 2007
4:15 p.m. • Smith Hall 331

Abstract: Microfluidics has emerged as an active area of research over the past 15 years. Every day new instruments and applications using this approach are described leading us closer to the reality of a “Lab on a Chip”. What are the features of a microfluidic system that have created such interest? In this talk, background on microfluidic systems, their fabrication, properties, and how such properties are exploited for chemical analysis, especially electrophoresis, are described. It will be shown that laminar flow, small diffusion distances, high surface area to volume ratios, and precise control over fluidic elements are the main features that are of interest. Such devices have been used to fabricate basic elements of a chemistry lab such as mixers, valves, dispensers, and separation chambers. Integration of these basic elements is feasible and has led to many devices from DNA sequences to on-demand chemical synthesizers.

Ion channels, like enzymes, have their specific substrates: potassium, sodium, calcium, and chloride channels permit only their namesake ions to diffuse through their pores. Potassium channels exhibit a remarkable ability to discriminate between potassium and sodium—by a factor of nearly 10,000—even though these ions are similar in size (1.33 and 0.95 Å, respectively). Such exquisite selectivity is impressive when one considers that potassium flows though the pore at a rate approaching the diffusion limit. My laboratory has determined the atomic structures of several potassium channels. The architecture of the pore allows potassium ions to remain hydrated at the center of the membrane, where the dielectric barrier to ion flow is expected to be greatest. The selectivity filter coordinates dehydrated potassium ions, but not sodium ions, thus accounting for ion selectivity. Structural and thermodynamic data on the binding of various ions to the selectivity filter will be discussed. A detailed conduction mechanism in which two potassium ions adopt two configurations within four binding sites is hypothesized to account for near diffusion-limited conduction rates.