We examine the process of expansion of a focal adhesion complex by which a biological membrane containing mobile binders adheres to a substrate with complementary binders. Attention is focused on the situation, common among living cells, in which the mean mobile binder density is insufficient to overcome generic resistance to close approach of the membrane to its substrate. For the membrane to adhere, binders must be recruited from adjacent regions to join an adhesion patch of density adequate for adhesion, thereby expanding the size of the patch. The specific configuration examined is the expansion of a circular adhesion zone for which diffusive binder transport driven by a chemical potential gradient is the mechanism of binder recruitment. An aspect of the process of particular interest is the stability of the circular shape of the expanding front. It is found that the adhesion front radius increases as ͌ t, where t is the time elapsed since nucleation, and that the circular shape becomes unstable under sinusoidal perturbations for radii large compared with the nucleation size, as observed in recent experiments.adhesion receptors ͉ biomembrane ͉ cell adhesion A dhesion of cells with other cells and surfaces plays an important role in processes such as cell migration, spreading, differentiation, and growth. The interactions that are responsible for cell adhesion include nonspecific electrostatic and repulsive steric forces as well as specific binding between receptors and ligands. In a series of early and important papers, Bell and coworkers (1-3) have developed quantitative models for cell adhesion based on equilibrium thermodynamics; their work shows that adhesion of cells involves a competition between the nonspecific repulsive forces due to the presence of glycocalyx and the specific bonding interactions between ligand-receptor pairs. The changes in shapes of adhered cells due to the deformation of the cell membrane and the role of ligand-receptor bond kinetics on attachment and separation of cells has been considered by Evans (4, 5) and Dembo and coworkers (6). Advances in experimental techniques based on atomic force microscopes, optical tweezers, and flexible transducers has led to considerable improvement in the understanding of ligand-receptor interactions that are responsible for cell adhesion (7-9).Recent experiments have shown that cell adhesion to surfaces which carry a large concentration of receptors that are complementary to the binders in the cell membrane results in the formation of localized regions of tight adhesion (sometimes referred to as focal adhesion complexes). Examples include spreading of blood platelets (10), vesicles (11,12), and different types of cells including fibroblasts, melanocytes, osteoblasts, lymphoblasts, and red blood cells (13-16) on substrates functionalized with receptors. On the basis of experiments on adhesion of a variety of cultured cells on arrays of patterned gold nanodots with a single bound integrin per dot, Arnold and coworkers (16) have proposed that the c...