Neutron Reflectometry and Compression of Graded Hydrogel Surfaces

Polyacrylamide hydrogels with depth‐wise gradients in polymer density (i.e., surface gel layers) are ideal synthetic models to understand stress modulation in hierarchical, compositionally‐graded biological tissues, including articular cartilage, in part, due to their similarities in water content and network structure. This work investigated surface gel layer thickness and crosslinker mobility (e.g., covalent crosslinks vs. physical entanglements) in polyacrylamide hydrogels and their impact on mechanical properties via confocal microscopy, indentation and compression measurements, neutron reflectivity, and mesoscale modeling. Hydrogels polymerized against oxygen‐permeable polydimethylsiloxane exhibited thicker surface gel layers and significantly lower elastic modulus compared to hydrogels polymerized against oxygen‐impermeable glass. Physical entanglements lowered the hydrogel elastic modulus within the surface gel layer and throughout the bulk. Neutron reflectivity revealed the collapse of near‐surface polymer networks under compressive loads, in good agreement with dissipative particle dynamics (DPD) simulations. Our results suggested that both the hydrogel elastic modulus and crosslinker mobility set the load required to collapse the polymer network. Furthermore, the collapse of thicker surface gel layers resulted in lower polymer network density than thinner surface gel layers. This work points to polymer density and crosslinker mobility as key design parameters in the design of stress‐modulating and tissue‐like materials.