PASSIVE MECHANICAL-BEHAVIOR OF HUMAN NEUTROPHILS - POWER-LAW FLUID

The mechanical behavior of the neutrophil plays an important role in b oth the microcirculation and the immune system. Several laboratories i n the past have developed mechanical models to describe different aspe cts of neutrophil deformability. In this study, the passive mechanical properties of normal human neutrophils have been further characterize d. The cellular mechanical properties were assessed by single cell mic ropipette aspiration at fixed aspiration pressures. A numerical simula tion was developed to interpret the experiments in terms of cell mecha nical properties based on the Newtonian liquid drop model (Yeung and E vans, Biophys. J., 56:139-149, 1989). The cytoplasmic viscosity was de termined as a f unction of the ratio of the initial cell size to the p ipette radius, the cortical tension, aspiration pressure, and the whol e cell aspiration time. The cortical tension of passive neutrophils wa s measured to be about 2.7 x 10(-5) N/m. The apparent viscosity of neu trophil cytoplasm was found to depend on aspiration pressure, and rang ed from approximately 500 Pa.s at an aspiration pressure of 98 Pa (1.0 cm H2O) to approximately 50 Pa.s at 882 Pa (9.0 cm H2O) when tested w ith a 4.0-mum pipette. These data provide the first documentation that the neutrophil cytoplasm exhibits non-Newtonian behavior. To further characterize the non-Newtonian behavior of human neutrophils, a mean s hear rate gamma(m) was estimated based on the numerical simulation. Th e apparent cytoplasmic viscosity appears to decrease as the mean shear rate increases. The dependence of cytoplasmic viscosity on the mean s hear rate can be approximated as a power-law relationship described by mu = mu(c)(gamma(m)/gamma(c))-b, where mu is the cytoplasmic viscosit y, gamma(m) is the mean shear rate, mu(c) is the characteristic viscos ity at characteristic shear rate gamma(c), and b is a material coeffic ient. When gamma(c) was set to 1 s-1, the material coefficients for pa ssive neutrophils were determined to be mu(c) = 1 30 +/- 23 Pa-s and b = 0.52 +/- 0.09 for normal neutrophils. The power-law approximation h as a remarkable ability to reconcile discrepancies among published val ues of the cytoplasmic viscosity measured using different techniques, even though these values differ by nearly two orders of magnitude. Thu s, the power-law fluid model is a promising candidate for describing t he passive mechanical behavior of human neutrophils in large deformati on. It can also account for some discrepancies between cellular behavi or in single-cell micromechanical experiments and predictions based on the assumption that the cytoplasm is a simple Newtonian fluid.