Elastic modulus dependence on the specific adhesion of hydrogels

Mechanosensitivity in biology, e.g., cells responding to material stiffness, is important for the design of synthetic biomaterials. It is caused by protein receptors able to undergo conformational changes depending on mechanical stress during adhesion processes. Here the elastic modulus dependence of adhesive interactions is systematically quantified using ligand–receptor model systems that are generally not thought to be mechanosensitive: biotin–avidin, mannose–concanavalin A, and electrostatic interactions between carboxylic acids and polycationic surfaces. Interactions are measured by microgel sensors of different stiffness adhering to surfaces presenting a corresponding binding partner. Adhesion is generally decreased for softer microgels due to reduced density of binding partners. Density-normalized data show that low-affinity carbohydrate ligands exhibit reduced binding in softer networks, probably due to increased network conformational entropy. However, in case of stronger interactions with large interaction range (electrostatic) and large lifetime (biotin–avidin) density normalized adhesion is increased. This suggests compensation of entropic repulsion for softer networks probably due to their increased mechanical deformation upon microgel adhesion and enhanced cooperative binding. In essence, experiments indicate that soft interacting polymer materials exhibit entropic repulsion, which can be overcome by strongly interacting species in the network harnessing network flexibility in order to increase adhesion