Aspiration of human neutrophils: analysis of non-Newtonian fluid models

The dynamics of human neutrophils during micropipet aspiration have been rationalized in terms of mechanical models which describe the cell as a "slippery droplet" of viscous fluid. A number of different fluids have been proposed to approximate the fluid cytoplasm. This study focuses on two specific schemes: a shear thinning fluid with a constant surface tension and an interpenetrating mixture of a high viscosity network and a low viscosity solvent. We have implemented both representations using a low Reynolds number hydrodynamics code based on finite element method. Furthermore, aspiration experiments were conducted to test the validity of each of these schemes. Qualitatively both the shear thinning and interpenetrating models are consistent with the time course of aspiration. However, neither model quantitatively reproduces the observed decrease in the apparent viscosity with increasing pressure of the neutrophil. Overall, both models capture many features of cell aspiration; however, they also fall short in several instances.