Contribution of Depletion Effect for Size-Selective Target Cell Acquisition in Cup-Shaped Microstructures

Abstract

Purification of target cells from a cell complex is an important preparation step in life science studies. Magnetic cell separation (MACS) is one useful method for the purification of target cells depending on specific antigen-antibody recognition; however, this method requires known ligand molecules expressed on the target cell surfaces. One challenge for the improvement of MACS is target cell recognition by other physical parameters instead of molecular recognition, and we developed a method of target cell purification depending on their sizes using cup-shaped superparamagnetic hemispheres, “magcups”. The magcups were fabricated by coating magnetic elements such as Ni on polystyrene microsphere templates and removing the templates by burning. Cells were size-dependently trapped to inner concaves of magcups, and notably, a kind of target particles having close diameter with that of magcups was frequently acquired in comparison with other particles, which suggests depletion effect is one promoter of conjugations between targets and magcups. To evaluate size-specific acquisition mechanisms between targets and magcups, two different diameters of microbeads (10 and 20 µm) were simultaneously mixed with 10- to 50-µm magcups with and without nanoparticles (NPs, 55nm) in order to evaluate whether the presence of NPs promote target-magcup conjugations caused by the depletion effect. In results, (i) the frequency of 10-μm bead capture increased 3.0 times for magcups smaller than 20 µm, and (ii) the frequencies of 10-μm beads capture increased as almost the same level with that of 20-μm bead for magcups larger than 20 µm by the presence of NPs in reaction solvents. These results suggest that depletion effect is one promoter for size-selective target acquisition into concave microstructures.