Tribological behavior of shape-specific microplate-enriched synovial fluids on a linear two-axis tribometer

Nano- and micro-particles are being increasingly used to tune interfacial frictional properties in diverse applications, from friction modifiers in industrial lubrication to enhanced biological fluids in human osteoarthritic joints. Here, we assessed the tribological properties of a simulated synovial fluid enriched with non-spherical, poly lactic-co-glycolic acid (PLGA) microparticles (μPL) that have been previously demonstrated for the pharmacological management of osteoarthritis (OA). Three different μPL configurations were fabricated presenting a 20 μm × 20 μm square base and a thickness of 5 μm (thin, 5H μPL), 10 μm (10H μPL), and 20 μm (cubical, 20H μPL). After extensive morphological and physicochemical characterizations, the apparent Young’s modulus of the μPL was quantified under compressive loading returning an average value of ~ 6 kPa, independently of the particle morphology. Then, using a linear two-axis tribometer, the static (µ_s) and dynamic (µ_d) friction coefficients of the μPL-enriched simulated synovial fluid were determined in terms of particle configuration and concentration, varying from 0 (fluid only) to 6×10⁵ μPL/mL. The particle morphology had a modest influence on friction, possibly because the μPL were fully squeezed between two mating surfaces by a 5.8 N normal load realizing boundary-like lubrication conditions. Differently, friction was observed to depend on the dimensionless parameter Ω, defined as the ratio between the total volume of the μPL enriching the simulated synovial fluid and the volume of the fluid itself. Both coefficients of friction were documented to grow with Ω reaching a plateau of µ_s ~ 0.4 and µ_d ~ 0.15, already at Ω ~ 2×10^-3. Future investigations will have to systematically analyze the effect of sliding velocity, normal load, and rigidity of the mating surfaces to elucidate in full the tribological behavior of μPL in the context of osteoarthritis.