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Microscopic-physical determinants in biological adhesion.

Adhesion processes in biology are dynamic events and seldom "ideal" in character. Adhesion embodies both initiation, and subsequent separation ("fracture"), of contact. With few exceptions in vivo, active cell motility is the universal process by which cells spread on other cells and artificial substrates. Although colloidal attraction is ever present, it is usually too weak (because of steric hindrance and local electrostatic repulsion) to overcome mechanical stiffness of the cell membrane and cortical cytoskeleton in order to advance contact. Cells simply "stick" at a few points where they touch with little affinity (adhesion energy) to promote contact. On the other hand, after cell contact has been made (either by active spreading or mechanical impingement), separation is often opposed by tenacious intersurface attachments, and a large mechanical force (e.g., membrane tension) is required to "peel" the contact apart. The physical determinants of biological cell adhesion are complex because of "strong coupling" between cellular biochemistry, structure, and interface bonding at the microscopic level.