Lipid Membrane\nDomains Control Actin Network Viscoelasticity

The mammalian cell membrane is embedded with biomolecular\ncondensates\nof protein and lipid clusters, which interact with an underlying viscoelastic\ncytoskeleton network to organize the cell surface and mechanically\ninteract with the extracellular environment. However, the mechanical\nand thermodynamic interplay between the viscoelastic network and liquid–liquid\nphase separation of 2-dimensional (2D) lipid condensates remains poorly\nunderstood. Here, we engineer materials composed of 2D lipid membrane\ncondensates embedded within a thin viscoelastic actin network. The\nnetwork generates localized anisotropic stresses that deform lipid\ncondensates into triangular morphologies with sharp edges and corners,\nshapes unseen in many 3D composite gels. Kinetic coarsening of phase-separating\nlipid condensates accelerates the viscoelastic relaxation of the network,\nleading to an effectively softer composite material over intermediate\ntime scales. We dynamically manipulate the membrane composition to\ncontrol the elastic-to-viscous crossover of the network. Such viscoelastic\ncomposite membranes may enable the development of coatings, catalytic\nsurfaces, separation membranes, and other interfaces with tunable\nspatial organization and plasticity mechanisms.