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Generation of linear and non-linear concentration gradients along microfluidic channel by microtunnel controlled stepwise addition of sample solution.

The ability to generate stable chemical gradients in microfluidics has important applications, since such gradients are useful in both chemical and biological studies. Growing evidence reveals that many cellular responses are specific to non-linear spatial gradients, hence a need to control complex concentration gradient profiles with and within microfluidics. In this paper, we present a structure-based approach to generate linear and non-linear chemical gradients, with profiles controlled by microtunnels fabricated alongside two main channels. Using single-step photolithography, microtunnels and main channels were fabricated at different heights thus having different fluidic resistance. Through these microtunnels, sample solutions were stepwise dispensed into the buffer stream to generate a chemical gradient profile. By varying the lengths of microtunnels that dictated the volume of sample solutions being dispensed, complex gradient profiles were generated. We have successfully demonstrated the formation of linear, convex and concave gradient profiles and a simple mathematical expression was established to approximate the profiles produced in our microfluidic gradient-generators.