Synthetic design of force-responsive hydrogels with ring-forming catch bonds

Abstract

Catch bonds are interactions whose lifetimes increase under mechanical load, a counterintuitive behaviour that underlies diverse biological processes. Translating this mechanism to synthetic materials offers the potential to create systems that are compliant at low stress but stiffen under applied force, with applications ranging from impact-responsive materials to dynamic tissue scaffolds. However, engineering materials with tunable, force-dependent interactions remains challenging, and existing conceptual designs are limited. Here, we present a minimal synthetic framework for catch bond behaviour in dynamic hydrogels, based on reversible ring-forming polymers. Using coarse-grained molecular dynamics simulations, we show that hydrogels with such a chemistry undergo fewer bond-breaking reactions as the stress increases and can even display a non-monotonic dependence of the strain rate on the applied stress. Our results highlight the potential of reversible ring formation as a versatile platform for designing mechanically adaptive materials with tunable durability and responsiveness.

Figures (11)

• Figure 1: (a) Coarse-grained visualisation of a reversible ring-forming polymer. Dotted lines schematically indicate that the connected sites may be separated by an arbitrary number of beads in the coarse-grained model. Going from the bottom to the top configuration, is referred to as ring-opening (RO), while going from the top to the bottom configuration is referred to as ring-closing (RC). The reaction occurs when the reactive (blue) groups are in close proximity. (b) The higher the force $f$, the longer it takes for these reactive groups to encounter each other, thereby increasing the average lifetime $\tau_\text{RC}$ before ring-closing. Hence, this system represents a catch bond. At very high forces, bonds between individual atoms would break, but this is outside the regime of interest in this work. (c) An example of reversible ring formation in the form of the bisamide exchange reaction van2021reprocessing. Here, a polymer strand contains two amide groups, which can react to form a cyclic imide and a free amine.
• Figure 2: To mimic a click chemistry, star polymers with two different end groups (black / grey) are placed in a simulation box. By diffusion, when the ends are in close proximity, they irreversibly bind or "click". After the gelation process, the reversible ring-forming reaction is turned on (see Fig. \ref{['fig:Fig1']}). A distinction is made between local and nonlocal ring-closing. A local reaction refers to a reaction initiated by two reactive groups on the same chain, which in our model corresponds to groups separated by exactly six beads, while a nonlocal reaction occurs between any other pair of reactive groups.
• Figure 3: (a) Formation of the network by click chemistry. At the end of the simulation, on average there are $262.21 \pm 0.38$ out of 300 possible crosslinks or clicks formed. (b) Relaxation of the network with reversible reactions. On average, 20 rings are formed after relaxation, out of which 18 are local. After the relaxation protocol, unit probability for percolation in all spatial directions was observed.
• Figure 4: Tensile test illustration with two catch bond manifestations: (a) shows the microscopic manifestation, with fewer reactions occurring with increasing stress, while (b) shows the macroscopic manifestation, where the strain rate exhibits a non-monotonic dependence on applied stress.
• Figure 5: Microscopic catch bond behaviour under applied uniaxial stress. (a) Number of RC reactions as a function of time during a tensile test for fixed target uniaxial stress. Increasing the stress leads to a reduced number of reactions, showing a catch bond effect. (b) Again the number of RC reactions, but now distinguishing between strictly local reactions (solid lines), and reactions without the locality constrained (dotted lines). The stress dependence is markedly stronger for strictly local reactions, indicating that the microscopic catch bond effect predominantly originates from local reactions.