Cellular adhesion is controlled by the balance of dispersive hydrodynamic forces and receptor-mediated adhesive interactions. On a molecular level, these interactions are characterized by kinetic constants for association (K_on) and dissociation (K_off). The probability of adhesion is regulated by the ligand density on the substrate as well as the receptor density presented by the cell. This probability also depends on the residence time that a cell spends in contact with a ligand presenting patch. For a patch of length L in a shear environment with hydrodynamic velocity U, the residence time T_res= L/U. Patch sizes can be controlled on micron length scales and used to infer K_on.

Data are presented for LS174T colon carcinoma cells rolling on micro-patterned surfaces presenting immobilized P-selectin. Micropatterned surfaces with patches 10 μm wide (which corresponds to a cell diameter) and length in the direction of flow varying from 10-80 μm are presented. As the hydrodynamic velocity U is decreased from 700 to 300 μm/s, cellular adhesion thresholds become apparent, with cells adhering first only to the longest patches, and then to progressively smaller patches as U is decreased. K_on can be inferred as 1/T_res for the threshold events in these experiments, corresponding to a value of K_on of 10-15 s^-1. These data are consistent with preliminary results obtained by SPR.