Relaxation kinetics controls cell transport in crowded active monolayers

Soft cancer cells embedded in stiffer epithelial layers migrate in bursts, which prior simulations have attributed to their deformability. In a dense layer, however, interfacial tension sets two things at once: the energy cost of a deformation and the rate at which it relaxes. Here we disentangle the two in an active phase-field model of a near-confluent monolayer in which relaxation rate and free energy can be varied independently. Crowding slows cells not by pushing them back bodily but through the relaxation of their own deformations against neighbours, which cancels most of their propulsion. Slowing this relaxation in a minority of cells, at unchanged energetics and propulsion, more than doubles their transport and speeds up the untouched majority by half. Conversely, the advantage of soft cells vanishes, and at high soft-cell fractions reverses, once their relaxation is made as fast as that of normal cells. Independent of composition, mixtures transport like a uniform layer with the geometric-mean interfacial tension. Passive relaxation kinetics thus emerges as an independent lever on transport, suggesting that the fluidity of heterogeneous tissues may depend on how quickly cells recover their shape, not only on how stiff they are.