Using Rheology to Probe the Mechanism of Joint Lubrication: Polyelectrolyte/protein interactions in Synovial Fluid
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Using Rheology to Probe the Mechanism of Joint Lubrication: Polyelectrolyte/protein interactions in Synovial Fluid Katherine M. N. Oates, Wendy E. Krause,1 and Ralph H. Colby Department of Materials Science and Engineering, The Pennsylvania State University University Park, PA 16802 1 Current address: Lynntech, Inc. 7610 Eastmark Dr. College Station, TX, 77840 ABSTRACT The outstanding lubricating properties of synovial fluid, found in freely moving mammalian joints, may be due to intermolecular associations between hyaluronic acid, an anionic polysaccharide, and the plasma proteins. A synovial fluid model comprised of hyaluronic acid and the plasma proteins albumin and γ-globulins, was constructed. Rheological measurements reveal a pronounced viscoelasticity with a strong shear history dependence for the synovial fluid model and the plasma protein solutions at low shear rates. The addition of the anti-inflammatory drug D-Penicillamine to the solution alters the rheology of the synovial fluid model. We present two ideas about the structural features of synovial fluid that may explain this viscoelasticity and suggest further experimental techniques that can be used to test these ideas. INTRODUCTION Synovial fluid is a biopolymer solution that serves as a lubricating boundary layer/fluid between parts of freely moving mammalian joints (elbows, knees). We contend that the lubrication ability arises from the polymers in synovial fluid: a high molecular weight anionic polysaccharide, hyaluronic acid (HA), and the plasma proteins albumin and γ-globulins. Although the components of synovial fluid are known, the mechanism of lubrication is still not fully understood. The conformation of hyaluronic acid in solution is also subject of debate. The conformation of HA in solution was proposed to be a two-fold, “tape-like” single helix[1,2]. The hydrophobic patches on the sugar rings were postulated to promote network formation and intermolecular aggregation of hyaluronic acid in solution. However, HA rheology is very sensitive to small protein contaminants. HA derived from bacterial sources (i.e. with
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