Flow rate independent gradient generator and application in microfluidic free-flow electrophoresis

Microfluidic gradient generators have been employed in several works in the literature. However, these are typically application specific and especially limited in the range of flow rates that result in the required concentration gradient outputs. Here, a flow rate independent gradient generator designed as a modified Christmas tree-like microfluidic channel network including micromixers at each channel branch is demonstrated. The device was characterized theoretically, modeled using finite element analysis and tested experimentally. Input flow rates up to 200 μl/min, resulting in a maximum speed of about 333 mm/s, for the generation of linear and mirrored linear gradients were demonstrated. As an application example, the gradient generator was monolithically integrated with microfluidic free-flow electrophoresis for the separation/concentration of fluorophores using a novel E-field gradient free-flow electrophoresis mode. The separation of fluorophores, having different charge stages, showed concentration factors of up to 10 fold. In addition, an extended theoretical description of the realizable concentration gradients and the electric field gradient is presented as supplementary information.