Poster AA1 - Synovial fluid nanofilm formation on model articular cartilage surfaces

Introduction: The extracellular articular cartilage (AC-ECM) matrix is a complex network of biomolecules crucial for properly functioning synovial joints, which, combined with synovial fluid, form energy-dissipating and lubricating thin films. Various components, such as collagen type II (COL-II), collagen type I (COL-I), and fibronectin (FN), are present on the surface of AC-ECM. However, their specific roles in controlling synovial fluid components' adsorption, film assembly, and retention to form a protective film remain unclear. This study introduces a simple model of articular cartilage surface using COL-II, COL-I, and FN nanofilms immobilized to quartz substrates to investigate how these precursor nanofilms interact with synovial fluid. The formation of synovial fluid nanofilms on these precursor films was quantified by quartz crystal microbalance with dissipation (QCM-D), an acoustic shearing technique, monitoring in real time the formation of synovial fluid nanofilms, as well as its dissipation properties. Using QCM-D data, we calculated the adsorbed mass and the shear dependence compliance of the nanofilms. We find that synovial fluid nanofilms formed on FN and COL-II precursor films are similar in amount and compliance but not on COL-I, highlighting the importance of the articular cartilage surface composition.

Materials and

Methods: Full-length bovine FN, bovine COL-II, and rat-tail COL-I were immobilized to gold-coated quartz substrates by flowing solutions of either 10, 20, 30, 50, 75, and 100 ug/mL in phosphate-buffered saline (PBS) to substrates pre-treated with a cysteamine hydrochloride self-assembled monolayer and activated with glutaraldehyde mounted onto a QCM-D fluidic cell. Substrates with grafted FN, COL-II, or COL-I were then exposed to a blocking buffer, followed by exposure to bovine synovial fluid diluted to 25% v/v using PBS to prevent clogging of the QCM-D lines. Synovial fluid nanofilms were finally rinsed with PBS to remove loosely bound components. Adsorbed mass and film compliance were then estimated using the Sauerbrey mass and Johansmann’s model for thin film compliance in Newtonian liquids. The QCM-D system is capable of monitoring several harmonics during each experiment sequentially. Quartz substrates consist of 14 mm AT-cut quartz crystals with fundamental nominal resonance frequencies of 5 MHz.

Results, Conclusions, and Discussions: Preliminary results suggest that both COL-II, COL-I, and FN precursor films are independent of the starting bulk concentration. However, the precursor film shear-dependent compliance, based on the thin film in the Newtonian media model, is different for each precursor film. Interestingly, synovial fluid amounts adsorbed onto COL-II and FN were very similar, with almost identical shear-dependent compliances. Surprisingly, synovial fluid did not adsorb onto COL-I precursor films. These results indicate that the composition of the articular cartilage surface play a vital role in mediating the formation of thin films, with probable consequences in tribological performance. Overall, this proposed AC-ECM surface model enables a quantitative comparison of how major cartilage components contribute to the arrangement of synovial fluid.

Acknowledgements (Optional): We are grateful for support from the U.S. National Science Foundation, HRD-1547848 and CAREER CMMI-2239665 to RCAE.