Enhance the debonding resistance of hydrogel by large-scale bridging

The application of hydrogels has recently expanded markedly owning to the achievement of strong adhesion. In characterizing adhesion, a hydrogel is often subjected to 90° peel, during which the peel force increases, maximizes then drops to a plateau at steady state. The steady state peel force determines adhesion toughness. The maximum peel force determines a debonding resistance that is higher than adhesion toughness, which, however, has been largely unheeded before. This paper studies the mechanics pertaining to the maximum peel force and describes a method to enhance the debonding resistance by invoking the large-scale bridging mechanism. We achieve, by varying the bending stiffness, an increment of debonding resistance from 185 to 856 N/m for a single-network polyacrylamide hydrogel and from 486 to 2054 N/m for a double-network Ca-alginate/PAAm hydrogel on a glass substrate. The increment of debonding resistance depends on the thickness of the hydrogel and the bending stiffness of the backing. As a proof-of-concept deployment of the method, we fabricate a bilayer consisting of a passive hydrogel 2 and a responsive (PAAc/Ca(Ac)) hydrogel 1. The PAAc/Ca(Ac) hydrogel is soft at 25 °C (E ~ 0.5 MPa) but stiffens dramatically at 75 °C (E ~ 100 MPa), serving as the stiff backing to elicit large-scale bridging mechanism to improve the debonding resistance by one order of magnitude. We establish a theoretical model to probe the peel behaviors based on the cohesive-zone model and solve the resultant boundary value problem numerically. Theoretical predications satisfactorily agree with experimental results. We discuss the importance of maximum peel force and the potentials of large-scale bridging mechanism in improving debonding resistance for soft materials.