TITLE: Micropipette Suction and the Mechanical Behavior of Single Cells

SPEAKER:

TIME: Thursday Oct. 28, 1999 at 4 PM

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

The mechanical behavior of living cells is studied with micropipette suction in which the surface of a cell is aspirated into a small glass tube while tracking the leading edge of its surface. Such edges can be tracked in a light microscope to an accuracy of ± 25 nm and suction pressures as small as 0.1 - 0.2 pN/mm² (0.1 - 0.2 Pa) can be imposed on the cell. Both soft cells, such as neutrophils (the most common white cell in humans) and red cells, and more rigid cells, such as chondrocytes and endothelial cells, are studied with this technique. Interpretation of the measurements with basic continuum models leads to values for a cell's elastic and viscous properties. In particular, neutrophils are found to behave as a liquid drop with a total membrane (surface) tension of about 30 pN/mm (0.03 mN/m) and a viscosity on the order of 100 Pa·s. On the other hand chondrocytes and endothelial cells behave as solids with an elastic modulus on the order of 500 pN/mm² (0.5 kPa). Recent work has focused on the adhesion between the lipid membrane and the underlying f-actin-rich cytoskeleton of neutrophils. The adhesion energy per unit area is determined by measuring the force required to form lipid membrane tethers. Adhesion energies on the order of 130 pN/mm are calculated, which means that the tension in the cortex is opposite in sign to that of the membrane; i.e., 30 - 130 pN/mm = -100 pN/mm. This means that the f-actin-rich cortex wants to expand while overall, the cell wants to contract.