Simulations of the adhesion between molecularly bonded surfaces in direct force measurements

Biological materials often adhere via discrete cross bridges between reactive molecules on opposing surfaces. The macroscopic environment and surface properties as well as the detailed molecular characteristics of the cross bridges affect the adhesive properties of such systems. This work investigates the influence of these properties on force probe measurements of biological materials. This was done by simulating the dynamics of cross-bridge formation and rupture during a typical experiment with the surface force apparatus (SFA). These simulations show how the surface curvature of the force probe, the kinetic properties of the adhesion molecules, and the lateral mobility of the cross bridges influence the measured forces. The role of these properties in relating such microscopic quantities as the strength of individual cross bridges to macroscopically observed forces is also considered. Using the same computational methodology, we also analyzed the validity of using the Derjaguin approximation to relate measured macroscopic surface and interaction free energies to cross-bridge strengths. If the lateral motion of the cross bridges is negligible, these simulations show that this approach is valid for typical SFA experiments. For the experimentalist, our results provide a rationale for relating the strengths of individual adhesive bonds to macroscopic force probe measurements. (C) 1998 American Institute of Physics.