LAUSR

Significance Micrometer-scale small active systems, ranging from cells migrating through viscoelastic tissues to self-propelled droplets in microfluidic platforms, are often subject to environmental confinement. Growing evidence suggests that such confined environments are an essential factor to generate propulsive force, but its physical basis is still poorly understood. Here, by creating a simplified model of cell migration with active cytoskeleton enclosed in cell-sized droplets under confinement, we demonstrate that a propulsive force can be generated by the physical interaction between contracting active cytoskeleton and inner droplet surface. Furthermore, we show experimentally and theoretically that the force balance between propulsive force and confinement-induced viscous drag determines the migration speed, revealing a physical mechanism of the active cytoskeleton-based motility that utilizes environmental mechanical constraints.