Rarely in Nature are single molecule interactions used for cell adhesion. Rather, several different types of molecules or structures interact in concert to build effective adhesion. To study how these multivalent adhesive interactions are utilized by biological systems, we are working to develop a tailorable, self-assembled biosensor that can probe the range of interactions from the single molecule level to the ensemble average of multiple interactions. We selectively self-assemble functionalized polymer linkers with complementary terminal groups onto patterned nanoparticles to probe ligand-receptor interactions on the surface of the membrane. However, multivalent interactions, resulting from ligand-receptor pairs, must partner with the membrane properties to result in fusion. Therefore in parallel, we are studying the composition and behavior of cell membranes, both in vitro and in vivo. In particular, we study intracellular vesicles produced from particles phagocytosed by J774 cells. By studying the phase behavior, composition and interfacial behavior of phagosomal extracts, we aim to devise an improved model cell membrane system. We propose that by developing a more sophisticated model of a cell membrane, a wider range of techniques may be effectively used to study critical biological processes, such as adhesion and fusion.