Microfluidic Mixing For Biosensors

Among other factors, the performance of an affinity-based biosensor is dependent on the rate at which analyte is transported to, and captured by, its active sensing surface. The efficiency of analyte delivery can be increased via the use of microfluidics, albeit not without detraction, as microfluidic biosensors are often subjected to severe diffusion limitations when used for the detection of biologically relevant analytes. Such conditions lead to the formation of a boundary layer, void of analyte, which acts to resist the rate at which analyte is captured. It is often proposed to mix the fluid in the sensing chamber, where the exchange of depleted solution with fresh analyte can potentially increase sensor performance. The nature of analyte transport in a mixed channel is complex, however, and simply mixing the contents of a microchannel does not guarantee success. In this chapter, we review developments in the characterization (and prediction of) analyte transport in both mixed and unmixed channels. Our discussion focuses on the conditions under whichmixingwill (and will not) be beneficial and furthermore, the magnitude of performance increase that can be expected. Special attention is given to flow in the staggered herringbone mixer (SHM): a passive chaotic micromixer often used to enhance the performance of a biosensor. We review relevant experimental works on the topic and compare the results from several studies with the behavior expected from theory. Finally, we note several challenging aspects regarding the detection of circulating tumor cells which, due to their large size, are subject to additional transport mechanisms with respect to smaller analytes.