Fig. 5. Pull-and-hold model of ECM-triggered mechanosensing.

(A) Stress relaxation at 50 kPa of H10, H25, and H25′ materials. Data obtained from three specimens of each hydrogel from independent cross-linking reactions and two purification batches. (B) Pull-and-hold substrate straining model. Initially, cells strain the substrate monotonically leading to an increase in force (pull phase). When a force threshold is reached (black dashed line), cells enter length-clamp mode (hold phase). During hold phase, the force generated by a viscoelastic substrate drops due to energy dissipation. Green indicates the region where talin unfolding, and therefore mechanosensing, is most probable. (C) Single-chain molecular clutch. A single integrin binds to a two-element generalized Maxwell-Wiechert substrate. Integrin detachment occurs according to a K_off rate. Inside the cell, talin attached to integrins can be in a folded or unfolded state according to K_unfold and K_fold rates. Vinculin binds unfolded talin given a K_bind rate. (D) Iterative Monte Carlo algorithm to estimate adhesion reinforcement. Substrate generates force according to a given strain rate ($\stackrel{·}{\mathrm{\epsilon }}$). Simulations resulting in integrin detachment are considered adhesion failures. If no detachment occurs, and if vinculin binds to talin, then the run is considered a successful reinforcement event. If there is no detachment nor vinculin binding, then a new Monte Carlo iteration starts with updated strain values. This cycle is repeated until adhesion failure or reinforcement occurs. (E) Simulated reinforcement probability for elastic substrates increases with substrate stiffness. (F) Left: Heatmap of simulated reinforcement probability according to elastic long-term stiffness (K_L) and additional viscous stiffness (K_A). The white dashed arrow highlights regions of maximum reinforcement probability for substrates with equal K_L. Right: In the low long-term stiffness regime, K_A increases overall stiffness ensuring reinforcement. In the high long-term stiffness region, viscosity leads to stress relaxation after reaching the force threshold, blunting mechanosensing.