  Tissues are characterized by layers of functional units such as cells and
extracellular matrix (ECM). Nevertheless, how dynamics at interlayer interfaces
help transmit cellular forces in tissues remains overlooked. Here, we
investigate a multi-layer system where a layer of epithelial cells is seeded
upon an elastic substrate in contact with a hard surface. Our experiments show
that, upon a cell extrusion event in the cellular layer, long-range wave
propagation emerges in the substrate only when the two substrate layers were
weakly attached to each other. We then derive a theoretical model which
quantitatively reproduces the wave dynamics and explains how frictional sliding
between substrate layers helps propagate cellular forces at a variety of
scales, depending on the stiffness, thickness, and slipperiness of the
substrate. These results highlight the importance of interfacial friction
between layers in transmitting mechanical cues in tissues in vivo.
