Characterization of the chemical gradient generation and neutrophil sorting for the KOALA platform. (A) COMSOL simulations of the diffusion of a chemoattractant loaded in a hydrogel sphere in the lid of the assay, dipping into a microfluidic channel in the base. (B) Fluorescent microscopy images of an AlexaFluor dye diffusing into the microchannel in the base of the KOALA platform. (C) Comparison of the concentration profiles between the experiments and the numerical simulations. (D) COMSOL Version 3.3 simulation of the shear stress in microchannels of different heights and comparison with the efficacy of the whole blood washing. (B,D) Scale bars represent 100 μm. Phase-contrast images acquired using Slidebook Version 5.0.0.14 software with an Olympus IX-81 microscope using 4× (NA = 0.13; B) and 10× (NA = 0.30; D) objectives at 37°C. Additional details on imaging are given in “Microscopy.”