Many microfluidic devices operate with cells suspended in buffer solutions. Researchers who work with large cell types in such devices often run into problems with gravitational cell settling in the injection equipment and in the device itself. A method for reducing this problematic settling is discussed in this paper using tumor cell lines as an example. Microfluidic circulating tumor cell (CTC) isolation devices (MCIDs) are benchmarked using buffer solutions spiked with in-vitro tumor cell lines prior to validation with clinical samples (i.e. whole blood). However, buffer solutions have different rheological properties than whole blood. Here we describe the use of alginate in PBS buffer solutions to mimic blood rheology and reduce cell settling during preliminary validation experiments. Because alginate increases the viscosity of a solution, it helps to maintain cells in suspension. We report that vertical equipment configurations are important to further mitigate the effects of cell settling for MDA-MB-468 carcinoma cells. We also report that alginate does not disrupt the specific binding interactions that are the basis of carcinoma cell capture in MCIDs. These results indicate that vertical equipment configurations and the addition of alginates can be used to reduce cell settling in buffer based MCID testing and other applications involving large cells suspended in buffer solution.