Figure 5.

Test methods for the adhesion of hydrogel coatings. (a) The 90 degree peel test. A hydrogel coating is peeled off the substrate by a backing attached at an angle of 90 degrees to the substrate. (b) The simple stretch test. A stretchable substrate coated with hydrogel coating is loaded by uniaxial stretch, resulting in debonding of the coating under a critical load. (c) The scratch test. A rigid stylus is drawn across the surface of the coating, accompanied by a synchronous movement in the thickness direction of the coating. (d) The probe pull test. A flat tipped indenter is bonded to a hydrogel coating and then pulled away. (e) The double cantilever beam test (left) and contoured double cantilever beam test (right). A hydrogel coating is sandwiched between two cantilever beams. The two beams are pulled apart, resulting in fracture of the hydrogel coating.