Molecular Velcro for characterizing hydrogel coating adhesion

Rapid advances in hydrogel adhesion have enabled the development of hydrogel coating on various substrates for lubrication, drug delivery, and anti-fouling paints. The quality of adhesion between hydrogel coating and substrate is important in applications, but has been shown to be extremely challenging to assess. Here we develop a general approach to characterizing the adhesion of hydrogel coating, called the molecular Velcro. Such Velcro is a stiff layer, to which polymer chains are grafted through covalent interlinks, and adheres to hydrogel through non-covalent interlinks. We prepare a 35 μm-thick hydrogel coated on a rigid substrate, introduce a pre-cut crack to their interface, and use the molecular Velcro as backing layer to measure the adhesion toughness by 90-degree peel. As the non-covalent adhesion between the molecular Velcro and the hydrogel is instant and tough, and does not affect the mechanical properties of the hydrogel, the crack grows along the interface between the hydrogel and the substrate, or kinks into the hydrogel. Consequently, the adhesion between the hydrogel coating and the substrate is precisely characterized at various coating thicknesses, peel velocities, and crosslink densities. To show the robustness of our approach, we further measure the adhesion toughness of several types of 50 μm-thick hydrogels coated on the same substrate. This work will guide the characterization of adhesion of soft and wet coatings.