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* Biomedical Engineering, Boston University, Boston, Massachusetts 02215 USA; and Departments of Pathology and Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 2B5, Canada
Correspondence: Address reprint requests to Evan A. Evans, E-mail: evans{at}physics.ubc.ca.
We have used a biomembrane force probe decorated with P-selectin to form point attachments with PSGL-1 receptors on a human neutrophil (PMN) in a calcium-containing medium and then to quantify the forces experienced by the attachment during retraction of the PMN at fixed speed. From first touch to final detachment, the typical force history exhibited the following sequence of events: i), an initial linear-elastic displacement of the PMN surface, ii), an abrupt crossover to viscoplastic flow that signaled membrane separation from the interior cytoskeleton and the beginning of a membrane tether, and iii), the final detachment from the probe tip by usually one precipitous step of P-selectin:PSGL-1 dissociation. In this first article I, we focus on the initial elastic response and its termination by membrane separation from the cytoskeleton, initiating tether formation. Quantifying membrane unbinding forces for rates of loading (force/time) in the elastic regime from 240 pN/s to 38,000 pN/s, we discovered that the force distributions agreed well with the theory for kinetically limited failure of a weak bond. The kinetic rate for membrane unbinding was found to increase as an exponential function of the pulling force, characterized by an e-fold scale in force of 
17 pN and a preexponential factor, or apparent unstressed off rate, of 1/s. The rheological properties of tether growth subsequent to the membrane unbinding events are presented in a companion article II.
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