The effect of substrate rigidity on the assembly of specific bonds at biological interfaces

We present a thermodynamic model for the coupling between a flexible membrane and a compliant bioadhesive substrate. The local adhesion between the membrane and the substrate relies on the aggregation of transmembrane mobile receptors and their binding to the immobilized ligands on the substrate. The model predicts that the substrate hampers the energetic driving force for bond aggregation and entropic repulsion between ligand-receptor bonds becomes increasingly more dominant as the substrate rigidity decreases. On very compliant substrates, the rigidity-dependent distance between the nearest neighboring bonds may exceed the characteristic size of the crosslinking proteins (e. g., talin) connecting the cytoplasmic ends of clustered integrins. This can prevent the stabilization and reinforcement of the adhesion sites and lead to development of immature focal adhesions on very compliant substrates, as observed in experiments.