Complex structures and devices, both natural and manmade, are often constructed sequentially. From crystallization to embryogenesis, a nucleus or seed is formed and built upon. Sequential assembly allows for initiation, signaling, and logical programming, which are necessary for making enclosed, hierarchical structures. Although biology relies on such schemes, they have not been available in materials science. Here, we demonstrate programmed sequential self-assembly of DNA functionalized emulsions. The droplets are initially inert because the grafted DNA strands are pre-hybridized in pairs. Active strands on initiator droplets then displace one of the paired strands and thus release its complement, which in turn activates the next droplet in the sequence, akin to living polymerization. Our strategy provides time and logic control during the self-assembly process, and offers a new perspective on the synthesis of materials.

Cell-cell contacts in tissues are continuously subject to mechanical forces due to homeostatic pressure and active cytoskeleton dynamics. In the process of cellular adhesion, the molecular pathways are well characterized but the role of mechanics is less well understood. To isolate the role of pressure we present a dense packing of functionalized emulsion droplets in which surface interactions are tuned to mimic those of real cells. By visualizing the microstructure in 3D we find that a threshold compression force is necessary to overcome electrostatic repulsion and surface elasticity and establish protein-mediated adhesion. Varying the droplet interaction potential maps out a phase diagram for adhesion as a function of force and salt concentration. Remarkably, fitting the data with our theoretical model predicts binder concentrations in the adhesion areas that are similar to those found in real cells. Moreover, we quantify the dependence of the area of adhesion on the applied force and thus reveal adhesion strengthening with increasing external pressure even in the absence of active cellular processes. This biomimetic approach reveals a physical origin of pressure-sensitive adhesion and its strength across cell-cell junctions.cell mechanics | protein emulsion | lipid emulsion C ell-cell adhesion is important in biology because it underlies the structure of tissues and their dynamic reorganization during processes as important as morphogenesis (1, 2), cell locomotion (3, 4) and signaling (5, 6). In addition to the high level of complexity in the identified biochemical pathways, it has recently become clear that mechanical effects also play an important role. For example, pushing cells together or increasing their contractile forces by changing the substrate stiffness reinforces the strength of contacts (7-9). Furthermore, because homeostatic pressure arising from the balance of cell division and cell death is important in achieving the mechanical integrity of tissues (10) it should also affect cell-cell adhesion. Despite these important observations, the physical origin of force-sensitive adhesion remains an open question. In fact, theoretical models are derived from the behavior of simplified model membranes that lack mechanical resilience (11). Although these models successfully describe the kinetics and energetics of adhesion in the absence of rigidity (12, 13), they cannot address the effects of force. In an individual cell, the cytoskeleton scaffold coupled to the membrane gives rise to a cortical tension of ≈0.035 mN∕m (14). Moreover, the interplay between cortical tension and adhesive interactions with surrounding neighbors gives rise to a different surface tension in cellular aggregates (15, 16) and measures ≈1-20 mN∕m (17, 18).To mimic the dense packing of cells in tissue with a homeostatic pressure of ≈10 kPa (19, 20), we use a 3D assembly of biomimetic emulsion droplets under an external compression. In the emulsion system the compression of individual droplets with an interfacial tension of ≈10 mN∕m ...

Using confocal microscopy, we measure the packing geometry and interdroplet forces as a function of the osmotic pressure in a 3D emulsion system. We find that the nonlinear elastic response of the pressure with density is not a result of the anharmonicity in the interaction potential, but of the corrections to the scaling laws of the microstructure away from the critical point. The bulk modulus depends on the excess contacts created under compression, which leads to the correction exponent α = 1.5. Microscopically, the nonlinearities manifest themselves as a narrowing of the distribution of the pressure per particle as a function of the global pressure.