Design of Christmas-Tree-like Microfluidic Gradient Generators for Cell-Based Studies

Microfluidic gradient generators (MGGs) provide a platform for investigating how cells respond to a concentration gradient or different concentrations of a specific chemical. Among these MGGs, those based on Christmas-tree-like structures possess advantages of precise control over the concentration gradient profile. However, in designing these devices, the lengths of channels are often not well considered so that flow rates across downstream outlets may not be uniform. If these outlets are used to culture cells, such non-uniformity will lead to different fluidic shear stresses in these culture chambers. As a result, cells subject to various fluidic stresses may respond differently in aspects of morphology, attachment, alignment and so on. This study reports the rationale for designing Christmas-tree-like MGGs to attain uniform flow rates across all outlets. The simulation results suggest that, to achieve uniform flow rates, the lengths of vertical channels should be as long as possible compared to those of horizontal channels, and modifying the partition of horizontal channels is more effective than elongating the lengths of vertical channels. In addition, PMMA-based microfluidic chips are fabricated to experimentally verify these results. In terms of chemical concentrations, perfect linear gradients are observed in devices with modified horizontal channels. This design rationale will definitely help in constructing optimal MGGs for cell-based applications including chemotherapy, drug resistance and drug screening.