Influence of Nanoparticle Chemical Composition on <i>In Situ</i> Hydrogel Friction

Nanoparticles are promising candidates as direct therapeutics and delivery systems for osteoarthritis treatments, primarily via intraarticular injection, but little is known about the impact on sliding behavior for a soft material surface like cartilage that would be encountered in a joint. Nanoparticle additives have primarily been studied in the context of hard material interfaces, such as metals or metal oxides, where different lubricating or anti-wear mechanisms depend sensitively on chemical composition, size, and concentration. To understand what nanoparticle parameters influence in situ (in a fluid environment) frictional behavior of soft materials, polyacrylamide (PAM) hydrogels were used as a model soft material platform. Friction tests were conducted in a rheometer with a tribology adapter, with PAM hydrogels molded in a petri dish and immersed in different nanoparticle containing fluid environments. A range of nanoparticle compositions were selected to compare broad categories: gold (metal) with a citrate capping ligand, nanodiamond (carbon), and zirconium dioxide (metal oxide). Comparing surface chemistry, concentration, and degree of aggregation, both nanoparticle surface chemistry and nanoparticle solution viscosity were found to modulate in situ hydrogel friction.