Adhesion Mechanics\nand Detachment Dynamics of Vanishing\nSurface Layers on Hydrogels

Cross-linked hydrogel surfaces exhibit reduced stiffness\nwhen polymerized\nagainst polymeric hydrophobic surfaces. As such, these layers play\na critical role in contact mechanics, particularly exhibiting strong\nrelative adhesion with colloidal probes when the contact area is small.\nThis prevents the use of continuum models of adhesive soft contact.\nTo connect mechanisms of stretch to the force response, depth-controlled\nnanoindentation experiments were conducted on polyacrylamide (pAAM)\nhydrogel samples using colloidal probe atomic force microscopy (AFM).\nThe pAAM sample had a high water content of >90% and was molded\nagainst\npolyoxymethylene (POM) to create a more dilute surface layer with\nthickness ∼0.5 μm. Indentations to multiple depths between\n50 nm and 1.25 μm were repeated 10 times each. First, the force\ndrops during the unloading, and separation segments of each indentation\nwere characterized. This described the detachment progression for\nincreasing areas of contact, revealing that the pull-off force for\na single chain was in the single-pN range. Second, the stretched polymer\nnetwork was modeled as an array of parallel, linear springs. Assuming\na constant areal chain density of α = 100 chains/μm<sup>2</sup>, the maximum force of adhesion was plotted versus the volume\nof chains stretched upward, and the average chain stiffness was calculated\nfrom a linear fit to be 22.8 × 10<sup>–6</sup> N/m. A\nWeibull distribution analysis of detachment events revealed a dependence\nof chain stiffness on maximum indentation depth (<i>d</i><sub>max</sub>), with higher stiffness at shallower depths approaching <i>k</i><sub>chain</sub> ≈ 20 × 10<sup>–6</sup> N/m. These findings on adhesion mechanics between a vanishing hydrogel\nsurface and probe can guide the development of multifunctional hydrogels\nfor various biomedical applications.