Nanoparticles have applications as therapeutic agents for joint diseases such as osteoarthritis. But the role of nanoparticle diffusion in synovial fluid or the fluid inside the joint is incompletely understood. In a new report now published on Science Advances, Mythreyi Unni and a research team in chemical engineering and biomedical engineering in the U.S. used the Stokes-Einstein relationship to describe the rotational and translational diffusion of polymer-coated nanoparticles in quiescent synovial fluid and hyaluronic acid solutions. The outcomes provided insight to the diffusive behavior of polymer-coated inorganic nanoparticles in complex aggregates of biological environments that are typically present in the joint.