INFLUENCE OF DIRECTION AND TYPE OF APPLIED FORCE ON THE DETACHMENT OFMACROMOLECULARLY-BOUND PARTICLES FROM SURFACES

In biological adhesion experiments, cells use surface receptors to att ach to ligand-coated substrata, and forces, such as centrifugation or shear forces, are used to remove cells. Receptors bind with a high-aff inity lock-and-key mechanism, and their bonds are weaker than covalent bonds. The magnitude of the force at which the cell relents is quoted as the strength of adhesion. However, the character or direction of t he force, which depends on the adhesion assay, can affect the results of an adhesion assay, even if forces of precisely the same magnitude a re applied in the different assays. We demonstrate this principle by s imulating the detachment of receptor-coated hard spheres from ligand-c oated surfaces using normal, tangential, and shear forces after the pa rticles are allowed to bind to steady state. For a single bond, a 20-f old greater force is required to detach the particle if a normal, rath er than tangential, force is applied. At high receptor densities, tang ential forces can be as much as 56 times more disruptive than normal f orces in removing cells from surfaces. The higher sensitivity to tange ntial forces is because body forces exerted tangentially are focused o n bonds at the back edge of contact and because the ratio of bond leng th to bead radius is small, which constrains the bonds in an orientati on near to normal to the substrate and results in large axial bond ten sions. Hydrodynamic shear, and with its associated torque, is only sli ghtly more disruptive than a tangential force of the same magnitude. F orces applied at an angle to the substrate from 0 degrees (tangent) to 80 degrees are as effective as a tangential force at detaching a part icle. Our simulations provide a rational means for comparing the resul ts between different adhesion assays.