Droplets as Cell Models: Chemical Gradient-Induced Directional Filopodia Formation

Cells are complex chemical systems capable of sensing and responding to environmental cues by dynamically reshaping themselves, e.g., by forming arm-like protrusions such as filopodia. Recapitulating cellular behavior in artificial systems is a long-standing goal in understanding the matter-to-life transition and designing responsive soft materials. Here, we use oil-in-water emulsions that mimic cellular environmental sensing and form directional arm-like filopodia in response to external chemical cues. Our work analyzes the step-by-step process involved in the formation of artificial filopodia, and we engineer ways to direct filopodia growth through different chemical gradients. The process is driven by asymmetric surfactant partitioning across the oil-water interface, followed by ordering at the interface to form lamellar structures, which are projected out as filopodia. We observe filopodia growing away from the source of kosmotropic anions and toward the source of chaotropic anions from the Hofmeister series. Significantly, these systems also respond to amino acid gradients, similar to cells: tryptophan gradients favor growth toward the source, while lysine and arginine gradients cause growth away from the amino acid source. Our findings open new avenues for fabricating life-like materials that sense and grow in response to external signals.