Modification of sliding mechanisms in polyacrylamide hydrogels through nanoparticle incorporation.

Nanomedicine platforms are increasingly utilized in biomedical applications as direct therapeutics, delivery vehicles, and hydrogel nanocomposites. As many of these applications occur in dynamic biological environments, understanding how nanomaterials influence soft material sliding behavior is critical for their successful implementation. While nanoparticle composition and surface chemistry have been shown to affect frictional behavior in some systems, the influence of nanoparticle capping ligand chemistry under different modes of therapeutic use remains understudied. To compare these effects, gold nanoparticles (Au NPs) capped with citrate, cetyltrimethylammonium bromide, polyvinylpyrrolidone (PVP), and poly(acrylic acid) (PAA) were examined both as solution additives, representing direct nanomedicine injection, and as additives forming polyacrylamide (PAM) hydrogel nanocomposites. Friction measurements were conducted using a rheometer with a tribology adapter to quantify sliding behavior over time. As solution additives, Au NP capping ligand chemistry produced minimal differences in hydrogel frictional behavior. In contrast, incorporation of polymer-based ligands (PVP and PAA-capped Au NPs) into PAM hydrogel nanocomposites produced pronounced time-dependent frictional responses, with increasing friction during sliding. Comparison with pH-dependent sliding indicates that this friction evolution cannot be fully explained by hydrolysis-driven mechanisms. Instead, the results suggest that polymeric NP ligands introduce charge-mediated interfacial effects that emerge during sliding. These findings demonstrate that NP incorporation can alter the physicochemical mechanisms governing soft-material friction, highlighting the importance of interfacial chemistry in dynamic hydrogel systems.