Turing patterns form through chemical reaction and diffusion — an activator that promotes its own production and an inhibitor that spreads faster, creating stripes, spots, and labyrinths from homogeneous initial conditions. The patterns are chemical. The substrate is passive — it provides the space but not the mechanism.
Nicholas and Fielding found that a vertex model of biological tissue produces spatial patterns through a different mechanism entirely: shear banding. A uniform tissue under deformation spontaneously partitions into coexisting fluid and solid regions. No chemical gradient. No reaction-diffusion. The patterning is purely mechanical — strain localization creates bands of tissue that flow adjacent to bands of tissue that resist flow, and the two coexist stably.
The character of the bands depends on a single parameter: whether dissipation is primarily internal (cell-cell friction) or external (friction against a substrate). When cells dissipate energy mainly through mutual contact, the bands have one morphology. When they dissipate mainly through substrate drag, the bands have another. The friction type determines the pattern type — not the tissue composition, not the cell biology, not the signaling network.
This is felting's mechanism, formalized. When a sheet of wool fibers is agitated — rolled, pressed, wetted, compressed — some regions lock while others remain mobile. The felt pattern is not designed. It emerges from the strain applied to the fibers and the friction between them. No fiber knows it is part of a pattern. The pattern is an emergent property of deformation and dissipation. The loom is the force applied; the weave is the response.
The implication for morphogenesis is direct. Tissue patterning in development has been attributed primarily to chemical signals — morphogen gradients that tell cells where to be and what to become. This paper shows that mechanical strain alone, without any chemical signal, can produce patterned coexistence of distinct tissue states. The chemistry may be operating on a substrate that has already been mechanically patterned. The form precedes the signal.