A living sheet of cells resembles a crowd on an escalator. In a calm state, everyone moves smoothly, but as soon as a crush occurs, movement comes to a halt. Similarly, cell layers under load can suddenly stiffen, turning from a pliable mass into an elastic scaffold.
Biologists and physicists studied how model cell monolayers (thin sheets one cell thick) respond to constant stress. In the rigid regime, the tissue holds its shape up to a certain limit, then begins to flow like thickening honey. But in the flowing state, it can do the opposite: resistance sharply increases, and the layer solidifies. This effect is called shear jamming. The space fills with chains of stressed cells that span the entire sheet.
The most surprising thing is that the viscosity in all these regimes follows the same mathematical law. Only the external conditions change, while the underlying physics remains common. This links living tissues to foams, emulsions (mixtures of immiscible liquids), and granular media, where particle shape dictates the transitions from liquid to solid. Cells connected at their vertices create topological constraints — geometric obstacles that replace familiar chemical signals.
The same mechanism likely operates during wound healing: the wound edges pull together, and the tissue temporarily becomes stronger to prevent reopening. And in embryonic development, local cell compaction triggers the formation of folds and organs. Life uses a universal trick borrowed from non-living matter. And once again we see the crowd on the escalator: once the crush disappears, movement returns — only this time it is movement toward healing.
The jamming effect was first described for traffic jams: when there are too many cars, the flow freezes, even though each car is in working order.
In Peter Watts's novel Blindsight, alien tissue changes stiffness on command — terrestrial biology turned out to be closer to this science fiction than we thought.