Stability analysis of micropipette aspiration of neutrophils

During micropipette aspiration, neutrophil leukocytes exhibit a liquid-drop behavior, i.e., if a neutrophil is aspirated by a pressure larger than a c ertain threshold pressure, it flows continuously into the pipette. The poin t of the largest aspiration pressure at which the neutrophil can still be h eld in a stable equilibrium is called the critical point of aspiration. Her e, we present a theoretical analysis of the equilibrium behavior and stabil ity of a neutrophil during micropipette aspiration with the aim to rigorous ly characterize the critical point. We take the energy minimization approac h, in which the critical point is well defined as the point of the stabilit y breakdown. We use the basic liquid-drop model of neutrophil rheology exte nded by considering also the neutrophil elastic area expansivity. Our analy sis predicts that the behavior at large pipette radii or small elastic area expansivity is close to the one predicted by the basic liquid-drop model, where the critical point is attained slightly before the projection length reaches the pipette radius. The effect of elastic area expansivity is quali tatively different at smaller pipette radii, where our analysis predicts th at the critical point is attained at the projection lengths that may signif icantly exceed the pipette radius.