(371e) Concentration Gradient Generation Using Induced Charge Electro-Osmosis

Bio-molecule gradients play an important role in the understanding of various biological processes, e.g. in wound healing, cancer metastasis etc. Typically, biological cells are exposed to linear and non-linear bio-molecule concentration gradients and their response is studied for understanding cell growth, cell migration and cell differentiation mechanisms. Traditionally concentration gradients are generated using hydrogels, micropipettes, Boyden chamber, etc. However, these techniques have limitations in terms of gradient stability, reproducibility, predictability and controllability. Although recent studies have demonstrated the use of microfluidic devices for precise, reproducible and stable concentration gradient generation, most of the reported devices are geometrically complex and lack dynamic controllability, which is the ability to alter the gradient characteristics during experimentation.

In this work, a novel microfluidic gradient generator is presented which utilizes induced charge electro-osmosis (ICEO) by introducing conducting obstacles in the microchannel. In comparison with the other reported designs, the proposed method is geometrically simpler and has the unique feature of inducing transverse flows which can be controlled by the magnitude of the applied field. By employing a range of conducting protrusions as obstacles (with different shapes and sizes), transverse convection of varying magnitude can be introduced at different spatial locations in the channel, which can in turn be used to create non-linear as well as asymmetric gradients. The characteristics of the developed concentration gradients are dependent upon the interplay between fixed charge electro-osmotic (FCEO) and ICEO flows. It is shown that the proposed device can switch between linear and non-linear gradients by simply altering the applied electric field, demonstrating the superior controllability of the proposed design. For analysis, both full PNP (Poisson-Nernst-Planck) model and correction method based on slip-velocity approach are used to model the steady ICEO flow. Finally, the formation of user-defined concentration profiles (linear, convex and concave) is demonstrated by varying the conducting obstacle size.