Expansion channels for low-pass filtering of axial concentration gradients in microfluidic systems.

Chemical gradients that run axially in a microfluidic channel often contain undesirable high-frequency concentration variations, or noise, that results from mechanical and thermal fluctuations in the system. In this paper, we describe a passive microfluidic component called an 'expansion channel' (EC), that removes high frequency noise through axial dispersion. We show that the behavior of the filter can be modeled analytically, using an expression for the transfer function of the microfluidic channel, derived by Xie et al. (Y. W. Xie, L. Chen and C. H. Mastrangelo, Lab Chip, 2008, 8, 907-912). The use of ECs to remove noise from gradients formed in enyzmatic assays in a microfluidic channel is demonstrated. The resulting data quality is improved which enables better fits to chemical models and more accurate analysis. ECs should be very effective in removing noise from axial concentration gradients found in many microfluidic applications, e.g. liquid chromatography, biochemistry, and chemotaxis studies.